thinning of absorber layer of thin film cigs solar cell thesis -tesfaye belete

Friedrich-Schiller-Universität Jena

Physikalisch-Astronomische Fakultät

Abbe School of Photonics

Thinning of Thin-film Cu(In,Ga)Se2 Solar Cell Absorber Layer

Master Thesis

Tesfaye Belete

September 2014

Thinning of Thin-film Cu(In,Ga)Se2 Solar

Cell Absorber Layer

A thesis submitted to Abbe school of Photonics for partial fulfillment

of the requirements for degree of Master of Science in Photonics.

Submitted by: Tesfaye Tadesse Belete

Born on 9th of April 1981 Deder Ethiopia

Supervisor: Dr. Michael Oertel

First Examiner: Prof. Carsten Ronning

Second examiner: Dr. Udo Reislöhner

September 2014

Jena Germany

Acknowledgements

It is a pleasure to thank the many people who made this thesis possible. First I would like to

present my heartfelt thanks to Prof. Dr. Carsten Ronning for providing me with this thesis topic,

I would like to give my biggest thanks to my supervisor Dr. Michael Oertel for his patience in

helping me develop my thesis and guiding my research and important discussions, and my

friend Mr. Adebowale Anthony for scanning electron microscopy imaging.

Moreover I am indebted to Abbe School of Photonics that opened this full opportunity to attend

MSc in photonics Program in Friedrich-Schiller-University Jena, Jena, Germany. Further, as

several ideas related to this project were extracted from several articles listed in the Reference, I

am also grateful for the authors of those articles.

Last but not least, I want to thanks all Photovoltaics group for useful discussions during group

meeting.

Abbe School of Photonics Institut für Festkörperphysik Uni-Jena

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Table of Contents

Chapter 1: Introduction .................................................................................................................. 2

1.1: The Importance of Alternative Sources of Energy ............................................................. 2

1.2: The Solar Cell ..................................................................................................................... 2

1.3: Thin-film Solar Cells .......................................................................................................... 2

1.4: The Objective and Scope of This Research ........................................................................ 3

Chapter 2: Cu(In,Ga)Se2 Solar Cells ............................................................................................ 5

2.1: The Cu(In,Ga)Se2 Solar Cells Structure ........................................................................... 5

2.2: The Substrate ...................................................................................................................... 6

2.3: The Back Contact................................................................................................................ 6

2.4: The CIGS Absorber Layer .................................................................................................. 6

2.5: Deposition Methods of CIGS absorber ............................................................................... 8

2.5.1: Coevaporation of Cu(In,Ga)Se2 ................................................................................... 8

2.5.2: Two-Stage Sequential Processes ............................................................................... 10

2.5.3: Other Fabrication Methods of Absorber Layer.......................................................... 11

2.6: Chemical Bath Deposition (CBD) for CdS Buffer Layer ................................................. 11

2.7: Transparent Front Contact (ZnO/ZnO:Al) ........................................................................ 13

2.8: Solar Cell working Principle............................................................................................. 14

2.9: Energy band diagram of Cu(In,Ga)Se2 thin-film solar cells ............................................. 15

Chapter3: Electro-Optical Characterization of Thin-Film Solar Cells ........................................ 18

3.1: Current Density -Voltage (J-V) Characteristics of a Solar Cell ....................................... 18

3.2: External Quantum Efficiency (EQE) ................................................................................ 22

3.3: Spectral Response ............................................................................................................. 23

Chapter 4: Experimental ............................................................................................................. 25

4.1 Substrate Preparation ......................................................................................................... 25

4.2: DC Magnetron Sputtering ................................................................................................ 26

4.3: The Selenization Process .................................................................................................. 27

4.4: The Chemical Bath Deposition (CBD) Process ............................................................... 30

4.5: ZnO/ZnO:Al Front Contact Deposition by RF Sputtering ............................................. 30

4.6: Grid Deposition................................................................................................................. 31


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4.7: Current Density-Voltage (J vs.V) Characteristics Measurements .................................... 31

4.8: Quantum Efficiency Measurement ................................................................................... 32

4.9: Wet Chemical Etching ...................................................................................................... 33

Chapter 5: Results and Discussion ............................................................................................... 36

5.1: Electro-optical characterization of different absorber thickness ...................................... 36

5.1.1: J-V curve for 1st set of samples with 8 min selenization at 620 °C. .......................... 36

5.1.2: J-V curve for 2nd

set of samples with 8 min selenization at 620 °C. ......................... 38

5.1.3: J-V curve for 3rd

set of samples with 4 min selenization at 620 °C. .......................... 40

5.1.4: J-V curve for 4th

set of samples prepared after BM etching. ................................... 42

5.2: External Quantum Efficiency Results............................................................................... 43

5.2.1: External Quantum Efficiency of 1st set of samples.................................................... 43

5.2.2: External Quantum Efficiency of 2nd

set of samples ................................................... 44

5.2.3: External Quantum Efficiency of 3rd

set of samples. .................................................. 47

5.2.4: External Quantum Efficiency of 4th

set of sample prepared after etching. ................ 48

5.3: CIGS Absorber Etching in Bromine-Methanol (BM) solutions ....................................... 48

5.3.1: 1.0 V/V % Bromine in Methanol (BM) Etching ....................................................... 50

5.3.2: 0.2 V/V % Bromine in Methanol (BM) Etching ....................................................... 51

5.3.3: 0.13 V/V % Bromine in Methanol (BM) Etching ..................................................... 52

5.3.4: 0.025 V/V % Bromine in Methanol (BM) Etching ................................................... 52

5.4: Scanning Electron Microscopy (SEM) Surface Characterization of Etched Samples ..... 53

Summary ...................................................................................................................................... 58

Conclusion ................................................................................................................................... 61

Bibliography ................................................................................................................................ 62

List of Figures

Figure 1: Annual production of some of the elements relevant for photovoltaics. Logarithmic y

scale (14). ------------------------------------------------------------------------------------------------------ 3

Figure 2: Schematic substrate cross section of a typical Cu(In,Ga)Se2 solar cell. ------------------ 5

Figure 3: I-III-VI2 materials based solar cells (44). ----------------------------------------------------- 7

Figure 4: The unit cells of chalcopyrite lattice structure (a) (45), absorption spectra of various

semiconductors (b) (26). ------------------------------------------------------------------------------------ 8

Figure 5: Coevaporation method using effusion cells (48). -------------------------------------------- 9


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Figure 6: Schematic illustration of different coevaporation process (a-d), the same selenium flux

is applied (24). ----------------------------------------------------------------------------------------------- 10

Figure 7: Sequential process of CIGS absorber layer preparation in which Mo on soda lime

glass and CuGa/In multilayers are sputtered from their respective targets by DC magnetron

sputtering followed by selenization of precursors in selenium vapor at 620 °C to grow

polycrystalline Cu(In,Ga)se2 absorber. ------------------------------------------------------------------- 11

Figure 8: Set up for CBD deposition of CdS (56). ----------------------------------------------------- 13

Figure 9: Schematics of RF magnetron sputtering of ZnO:Al from ZnO and Al2O3 targets (58).

----------------------------------------------------------------------------------------------------------------- 14

Figure 10: Schematics of solar cell operation when illuminated by solar irradiation the excited

carriers cross the band gap of absorber material and charge is separated by p-n junction. ------- 15

Figure 11: Energy-band diagram for two isolated semiconductors (a) and ideal p-n anisotype

heterojunction at thermal equilibrium (b) (59). --------------------------------------------------------- 16

Figure 12: Energy band of MoSe2/Cu(In,Ga)Se2/CdS/ZnO heterojunction (25). ------------------ 17

Figure 13: Typical illuminated J-V characteristics (60). ---------------------------------------------- 18

Figure 14: Overview over the three basic recombination mechanisms for photogenerated excess

carriers in a semiconductor. The excess energy is either transferred to (a) a photon, (b) kinetic

energy of an excess electron or hole, or (c) phonons. (34). ------------------------------------------- 19

Figure 15: An equivalent circuit of a pn-junction solar cell consisting of series and shunt

resistance (34). ----------------------------------------------------------------------------------------------- 21

Figure 16: Schematic of a solar simulator for electrical characterization of a solar cell under

illumination with a spectrum resembling the standard AM 1.5, current and voltage source

controlled through computer (63). ------------------------------------------------------------------------ 21

Figure 17: Effect of series resistance which results in voltage drop (a) and shunt resistance

which results in decrease of current (b) on the current–voltage characteristic of a solar cell (12).

----------------------------------------------------------------------------------------------------------------- 22

Figure 18: The schematics of ideal solar cell EQE (a) and spectral response of ideal solar cell (in

red) and a silicon solar cell (in blue) under glass in which the response fall back to zero at

higher (65). In both (a) and (b), the point at which the curve fall to zero defined the band-gap of

the solar cell. ------------------------------------------------------------------------------------------------- 23

Figure 19: Soda lime glass thin side labeled with substrate holder (back side view). ------------- 25

Figure 20: The substrate heating and Se evaporation versus time for selenization process of

CuGa/In multilayer. The first process of substrate heating at 375 °C for 3 minutes, and the

second stage of heating at 620 °C changed from 3 to 8minuts ( not shown here) during process.

----------------------------------------------------------------------------------------------------------------- 27

Figure 21: Ternary phase diagram of the Cu–In–Se system. Thin-film composition is usually

near the pseudobinary Cu2Se–In2Se3 tie-line (3). ------------------------------------------------------- 29

Figure 22: Scheme of a monochromator-based setup in which the chopped monochromatic light

illuminates first the reference (during calibration) and then the sample (during measurement).

The current output from reference cell or sample is converted to voltage and then amplified with

a lock-in amplifier triggered by the chopper. Finally amplified signal of the lock-in amplifier is

read and displayed by a computer (34). ------------------------------------------------------------------ 32


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Figure 23: Beakers with deionized water, 0.025 V/V % Br2-CH3OH solutions, stop watch and

teflon used to handle sample during CIGS etching in Br2-CH3OH. When etching time was over,

the sample was bathed in to the two beakers with water in the left one after the other to

terminate etching. ------------------------------------------------------------------------------------------- 35

Figure 24: J vs.V curve of the best cells of the 1st set of samples of selected cell from different

absorber thickness selenized at 620 °C for 8 minutes. ------------------------------------------------- 37

Figure 25: The box chart of samples of different thickness parameters. (a) Short circuit current

density, (b) open circuit voltage, (c) efficiency, and (d) fill factor for 1st set of samples selenized

for 8 min at 620 °C. ----------------------------------------------------------------------------------------- 37

Figure 26: J vs. V curve of the best cells of the 2nd

set of samples of selected cell from different

absorber thickness selenized at 620 °C for 8 minutes. ------------------------------------------------- 38


density, (b) open circuit voltage, (c) efficiency, and (d) fill factor for 2nd

set of samples

selenized for 8 min at 620 °C. ----------------------------------------------------------------------------- 39

Figure 28: The thickness of CGIS/CdS/ZnO/ZnO:Al layers of 1st and 2

nd set of samples. ------- 40

Figure 29: J vs. V characteristic curve of standard and 0.5 of standard absorber thickness best

cells taken among samples selenized for 4 min at 620 °C. -------------------------------------------- 41


density, (b) open circuit voltage, (c) efficiency and, (d) fill factor of the 3rd

set of samples

selenized for 4 min at 620 °C. ----------------------------------------------------------------------------- 41

Figure 31: J vs.V characteristic curves of sample etched for 8 min (a) and 12 min (b). Only one

cell (12 min etched sample) had diode curve as shown in (b). --------------------------------------- 42

Figure 32: EQE of 1st set of samples of different absorber thickness measured under 11 % of

AM 1.5 global illumination over 300 nm -1200 nm wavelength. ------------------------------------ 43

Figure 33: EQE of 2nd

set of samples of different absorber thickness measured under 11 % of


Figure 34: EQE comparison of 1st and 2

nd set of samples according to their respective thickness.

The measurements performed under 11 % of AM 1.5 global illumination over 300 nm -1200 nm

wavelength. --------------------------------------------------------------------------------------------------- 46

Figure 35: The spectral response of two set of samples of different thickness CIGS absorber. (a)

for 1st set and (b) 2

nd set. ----------------------------------------------------------------------------------- 46

Figure 36: EQE of 3rd

set of samples of different absorber thickness measured under 11 % of


Figure 37: EQE of a cell prepared after etching CIGS absorber with 0.025 V/V % BM for 12

min measured under 11 % of AM 1.5 global illumination over 300 nm -1200 nm wavelength. 48

Figure 38: Plot of measures surface profile of etched samples across the edge between the

etched an unetched part of the sample. To average out the etched height of each measurement a

quasi-mean value isdrawn. --------------------------------------------------------------------------------- 49

Figure 39: Etched thickness vs. etching time of CIGS absorber etched in 1.0 V/V % of BM

which reviled very fast etching capacity of the solution. ---------------------------------------------- 50

Figure 40: Etched thickness vs etching time (a) and average roughness vs. etching time (b) of

CIGS absorber layer in 0.2 V/V % of BM solution. --------------------------------------------------- 51


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Figure 41: Etched thickness vs etching time (a) and average roughness vs. etching time (b) of

CIGS absorber etched in 0.13 V/V % of BM solution. This solution is prepared by adding 50 ml

of methanol to 0.2 V/V % BM solutions. ---------------------------------------------------------------- 52

Figure 42: The evolution of etched thickness vs etching time and CIGS left on glass/Mo vs

etching time (a) and average roughness vs. etching time (b) of CIGS absorber layer in 0.025

V/V % of BM solution. ------------------------------------------------------------------------------------- 53

Figure 43: SEM micrograph of absorber layer after the front contact and CdS were etched in

HCl. (a) low magnification reviling how some part of the surface had particles of different sizes,

(b) particle ranging up to 10 µm long on the surface at higher magnification. --------------------- 54

Figure 44: SEM micrograph of CIGS surface after front contact and CdS etched in 5 % HCl in

right part and 20 seconds etched in 0.2 V/V % of BM in the right part. --------------------------- 54

Figure 45: SEM micrograph absorber etched in 0.2 V/V %BM for 10 se (a) and 90 se (b). For

samples etched at the same concentration from 10 to 90 se there saw no change in surface

morphology was observed. The variations in color of the pictures are due to contrast. While (c)

and (d) were for samples etched in 0.025 V/V %, for 8 min and 12 min respectively. ----------- 55

Figure 46: EDS spectra of particles investigated on samples surface after CdS was etched with

HCl both (a) and (b). ---------------------------------------------------------------------------------------- 56

Figure 47: EDS spectrum of particles investigated after 0.025V/V%BM etching for 8 min (a)

and 12min (b). ----------------------------------------------------------------------------------------------- 56

Figure 48: SEM Micrograph of absorber etched for 90 seconds in 0.2 V/V % BM for which the

CIGS crystals are almost completely etched (a) and the EDS spectra of the whole part that

confirmed the back contact reached from Mo peak. --------------------------------------------------- 57

Figure 49: SEM micrograph of CIGDS solar cell cross-section. (a). Standard sample, (b) and (c)

etched in 0.025 V/V % of BM for 8min and 12 min respectively. ----------------------------------- 58


2

Chapter 1: Introduction

1.1: The Importance of Alternative Sources of Energy

Nowadays science and technology, along with the progress of the society resulted in increasing

energy demand. Energy production and storage are the main questions and social challenges

with the impact on human health, environment and world economy. The conventional energy

sources like: coal, natural gas, and oil are non-renewable, diminishing in supply and they are

cause for environmental pollution by emission of greenhouse gas (CO2), while nuclear energy

poses environmental problems related to large-scale consequences of reactor accidents. In order

to meet this rising demand, new energy generation technologies must be used together with the

more conventional forms of renewable energy (1) (2). Renewable energies are gaining

considerable interest as alternative sources of energy, of which solar cell devices used for the

direct conversion of light into electricity are a viable option for a safe and sustainable future (3).

Unlike other energy sources, during operation there is no harmful emission or transformation of

matter (generation of pollutants), nor any production of noise or other by-products from solar

cells. Only production and recycling of devices are critical in their effects on the environment.

Photovoltaic (PV) is an excellently suitable solution for low power electricity supply in rural

and remote areas. Solar cells, in a general sense electronic device that generate electricity when

illuminated, operate through the photovoltaics effect discovered by Alexandre Edmond

Becquerel in 1839 (4).

1.2: The Solar Cell

One way to use solar energy is by converting it into electrical energy. Solar cells perform this

conversion based on the photovoltaics effect. Although the idea of converting light energy into

useful electricity dates back to the 19th century, it was in the 1950‟s at Bell labs that the first

6% efficient silicon solar cell was produced (5). Even though solar cell usage has clear

advantages over conventional energy sources there are challenges to its wider use. The main one

is cost. In order to lower the price and expand its utilization, many steps have been taken to

reduce the quantity of materials and energy used to manufacture.

1.3: Thin-film Solar Cells

Currently single crystal based silicon solar cell technologies are available commercially with

highest efficiency 24.5 % (6) that are also very stable. Silicon‟s availability is huge, but

crystalline Si cells require an expensive production process. Silicon is fragile, and it is an

indirect band-gap material. Both of these features require production of relatively thick cells. A


3

typical Si-cell thickness is approximately 300 µm, more than 100 times the thickness of a

typical polycrystalline thin-film cell. Because of those challenges several other new emerging

technologies based on thin-film (7), dye-sensitized (8), and organic materials (9) have attracted

the attention of researchers. The main motivation for interest in thin-film photovoltaics has

always been the possibility of lower cost, which allows for rational large scale production.

There are several technology systems in thin-film category, and two of them, based on the

absorber material are Cu(In,Ga)Se2 and CdTe. The advantages of thin-film solar cells CIGS

and CdTe over single crystal Si cells is that sunlight is absorbed much more efficiently in these

compounds and absorber thicknesses of a few micrometers are sufficient to absorb most of the

useful part of sunlight. The main drawback of CIGS and CdTe based thin-film solar cells are

environmental and scarcity issues associated with constituent elements Te, In, Ga, and Se (10)

(11). Recent research tends towards finding alternatives abundant and non-toxic elements

(Fig.1) like Zn, Sn and S to fabricate an alternative Cu(Zn,Sn)(S,Se)2 solar cell (12) (13).

Figure 1: Annual production of some of the elements relevant for photovoltaics. Logarithmic y-scale (14).

1.4: The Objective and Scope of This Research

The standard thickness of the industrial Cu(In,Ga)Se2 (CIGS) absorber layer in CIGS thin-film

solar cells is presently 1.5-2 µm (15) (16) while, in Institute of Solis State Physics University of

Jena the standard ranges from 2.5-3 µm. If this thickness could be reduced, with no or only

minor loss in performance, production costs could be lowered. In addition reducing the

thickness is key issue for further competitiveness of CIGS technologies (17). Decreasing the

CIGS layer would reduce the direct materials usage especially for indium and gallium.

However, reducing absorber is associated with different problems. Some of them are decrease in


4

amount of light absorbed in the CIGS despite high absorption coefficient (18), electrically

shunted (19) when CIGS layer decreased to around 0.5 µm, increase in the shunt conductance

(20), and high probability of recombination of electrons generated close to back contact (21).

The main aim of this research is to decrease the absorber layer to different thickness by: (i)

decreasing the number of layers of CuGa/In, (ii) decreasing time of sputtering, and (iii)

chemical etching of the standard CIGS absorber.


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Chapter 2: Cu(In,Ga)Se2 Solar Cells

In this section the history and development of Cu(In,Ga)Se2 thin-film solar cells are reviewed.

The state of the art including the solar cell structure and the deposition techniques for individual

thin-film layers are shortly introduced. In addition, some fundamentals are presented, which

provide a theoretical basis for the experimental findings in the upcoming chapters. Finally, an

electrical measurement method will be introduced, which is generally applied for determining

significant photovoltaics parameters of solar cells.

2.1: The Cu(In,Ga)Se2 Solar Cells Structure

The first report on chalcopyrite based solar cell was published in 1974 (22). The cell was

prepared from a p-type CuInSe2 (CIS) single crystal onto which a CdS film was evaporated in

vacuum. The combination of a p-type chalcopyrite absorber and a wide-gap n-type window

layer is the basic concept upon which current cell designs are based. Typically, the CIGS solar

cell has a soda lime glass/Mo/CIGS/CdS/TCO (transparent conducting oxides) configuration,

where sunlight comes from the TCO(ZnO:Al/ZnO) side (Fig.2) for which CuInSe2 crystal was

replaced by a polycrystalline thin-film of the more general composition Cu(In,Ga)Se2 (CIGS).

CIGS is a very promising photovoltaics material, with conversion efficiencies of up to 20.8 %

have been demonstrated on rigid glass substrates configuration (23).

Figure 2: Schematic substrate cross section of a typical Cu(In,Ga)Se2 solar cell.


6

2.2: The Substrate

The substrate is a passive component that should be mechanically stable and chemically inert

during device fabrication. It should also match the thermal expansion coefficients of the on top

deposited layers. Most commonly CIGS solar cells are grown in substrate configuration, this

configuration gives the highest efficiency owing to favorable process conditions and material

compatibility (24). In this configuration, the most common choice of substrate is soda lime

glass. The use of flexible substrates, like polymer films (25), and metal (Ti, stainless steel, Cu

etc.) has also been explored, but the major problem with the metal substrates is the undesirable

diffusion of impurities into the absorber at high process temperatures (26). The importance of

the soda lime glass substrate on Cu(In,Ga)Se2 film growth is that, it supplies sodium during the

absorber growth. Sodium diffuses through the Mo back. The other mechanism of incorporating

Na in the absorber layer is by post-deposition treatment (27). Different researchers studied the

effect of sodium which improves grain growth and cell performance (28) (29), increases the free

carrier density by at least one order of magnitude and this is associated with a lower number of

compensating donors (30), improvement in p-type conductivity due to an increase in the

effective hole carrier density and improved open circuit voltage and fill factor (31) (32). Sodium

placed on the grain boundaries rather than in the bulk (33) (27).

2.3: The Back Contact

The suitable back contact material for CIGS solar cell should be able to have high reflectivity in

order to reflect weakly absorbed light multiple times and good electrical contact (34). The

material of choice that fulfills those conditions as back contact is Molybdenum (Mo). Mo is

deposited on the glass substrate and it is ohmic in nature. Mo emerged as the dominant choice

for back contact due to its relative stability at the processing temperature, resistance to alloying

with Cu and In, and its low contact resistance to CIGS (35) (36) (24). Recent studies have

shown that the contact between Mo and the absorber is ohmic most probably due to the

formation of MoSe2 at the interface between CIGS /Mo (37) (38) (39), formed due to the

diffusion of Se into the CIGS film and the reaction with Mo above 440 °C (40). Nowadays, Mo

deposited by DC sputtering is the most commonly used back contact for CIGS solar cells.

2.4: The CIGS Absorber Layer

In CIGS solar cells, the absorber layer is center of energy conversion process. It has a steep rise

of the absorption coefficient above the band-gap, a high mobility and low recombination rates

(34).


7

CIS or CIGS belongs to the I-III-VI2 compounds (Fig.3).This crystal structure is chalcopyrite

structure (Fig.4a). CIS is a direct band-gap semiconductor material and has a large absorption

coefficient ( =105cm

-1) compared to known semiconductor materials (Fig.4b) for 1.4 eV and

higher photon energy. The fundamental absorption edge of a typical direct band-gap

semiconductor is given by equation 2.1 (41):

1.2 E

)EA(Eα

2

g

Where constant A depends on the density of states associated with the photon absorption and

Eg = 1.02 ± 0.02 eV (3) for CIS. When Gallium atom is placed in the sight of Indium in CIS

unit cell in certain ratio the CIGS unit cell is formed, and the CuIn1−xGaxSe2 have different

compositions from

x = Ga/(Ga + In) = 0 to 1 (42). Thus, the band-gap of CIGS is given by equation 2.2 (3):

2.2 x)x(10.167x0.6261.010Eg

S.-H.Wei (43) reported for substation of all In with Ga in Cu(In,Ga)Se2 an enlargement of the

band gap from 1.04 eV to 1.67 eV primarily due to the up-shift of conduction band. The CIGS

has higher band-gap than CIS which is very good to have higher open-circuit voltage during

solar cell operation since most of the incident sunlight is absorbed close to the p-n junction. As

a result, the cells based on CIGS can be made thin and with low cost. To produce efficient

devices, it is important to match the band-gap of the absorber layer to the solar spectrum. The

ideal band-gap for the solar spectrum is 1.4 eV. CIS has a lower-than-optimal band- gap of

1.04 eV and the theoretical maximum efficiency of CIS is 30 % (26).

Figure 3: I-III-VI2 materials based solar cells (44).


8

The band-gap can be increased by partial substitution of the In atoms with other group III

elements such as Al or Ga, or by replacement of Se with S. The electrical properties of CIGS

depend very strongly on the stoichiometry. Doping of CIGS comes from native defects, mainly

from VCu (copper vacancies) that act as shallow acceptors (3).

Figure 4: The unit cells of chalcopyrite lattice structure (a) (45), absorption spectra of various

semiconductors (b) (26).

2.5: Deposition Methods of CIGS absorber

In order to determine the most promising technique for deposition to be completed at low cost

while maintaining high deposition or processing rate with high yield and reproducibility,

different techniques has been used to deposit Cu(In,Ga)Se2 on top of the Mo back electrode as

the photovoltaic absorber material. The two common deposition approaches are vacuum

coevaporation and two-stage sequential processes (46).

2.5.1: Coevaporation of Cu(In,Ga)Se2

In coevaporation process all the constituents, Cu, In, Ga, and Se, are simultaneously evaporated

onto a substrate heated at 400 °C to 600 °C and the Cu(In,Ga)Se2 film is formed in a single

process Fig.5. The evaporation temperatures for each metal will depend on the specific source

design; typical ranges are 1300 °C to 1400 °C for Cu, 1000 °C to 1100 °C for In, 1150 °C to

1250 °C for Ga, and 300 °C to 350 °C for Se evaporation (3). Evaporation rates from each

source are controlled by the source temperature. As a result during evaporation the film

composition and growth is determined by flux distribution and effusion rate, since Cu, In, and

Ga have higher sticking coefficient (3). Selenium has higher vapor pressure and lower sticking


9

coefficient (47); therefore it should be evaporated in excess since insufficient Se can result in a

loss of In and Ga in the form of In2Se or Ga2Se.

Figure 5: Coevaporation method using effusion cells (48).

In coevaporation by changing elemental flux over time different deposition variations have

been explored. The first process (Fig.6a) involves constant flux as well as substrate temperature

throughout the deposition process, the second process is known as Boeing or bilayer process

(Fig.6b), which has Cu-rich stage during the growth process and end up with an In-rich overall

composition in order to combine the large grains of the Cu-rich stage. The third process is

“three-stage process” (Fig.6c) which is the most efficient fabrication technique. It reduces the

junction area and reduces the number of defects at the junction. That also facilitates the uniform

conformal deposition of a thin buffer layer. Coevaportation allows the design of graded band-

gap structures by varying the Ga/In ratio during deposition (49)(Fig.6d).


10

Figure 6: Schematic illustration of different coevaporation process (a-d), the same selenium flux is applied

(24).

2.5.2: Two-Stage Sequential Processes

In the two-stage process typically Cu, Ga, and In are deposited using low cost and low

temperature methods that facilitate uniform composition, and followed by annealing the film in

Se atmosphere at 400 °C to 600 °C. This two- stage process approach was first demonstrated by

Grindle (50) who sputtered Cu/In layers and reacted with hydrogen sulfide to form CuInS2 and

also adapted for CuInSe2 by Chu (51). Chenene (52) fabricated Cu(In,Ga)Se2 thin-films by

H2Se selenization process.

The Sputtering of CuGa/In is a process using commercially available deposition equipment

which can provide good uniformity over large areas with high deposition rates. The

Cu(In,Ga)Se2 film formation then requires a second step, the selenization step, in which the

precursors are placed in Se vapor at 450 °C to 650 °C for 25 min to 32 min to form

Cu(In,Ga)Se2 absorber layer. This process is known as selenization in which the sequential

process is depicted in Fig7. The Substrate heating and selenium evaporation temperature versus

time graph during selenization is given in section 4.3 of Experimental.


11

Figure 7: Sequential process of CIGS absorber layer preparation in which Mo on soda lime glass and

CuGa/In multilayers are sputtered from their respective targets by DC magnetron sputtering followed by

selenization of precursors in selenium vapor at 620 °C to grow polycrystalline Cu(In,Ga)se2 absorber.

The chemical reaction for CIS and CIGS reported (12) as in equation 2.3 and 2.4 respectively:

3.2 CISInSeCuSe29SeIn2Cu 1911

2.4 CIGSSeSeGaInSeCuSe 32

Kushiya (53) reported for Se deficiency and excess, the In loss occurs by re-evaporation in the

form of In2Se at 450 °C. The excess selenium also induces the formation of thicker MoSe2 layer

in between CIGS and Mo. The delamination of CIGS/MoSe2/Mo from glass substrate was

attributed to the internal stress induced partly by the thick MoSe2 formation (volume expansion)

and partly by the densification of CIGS layer (54).

2.5.3: Other Fabrication Methods of Absorber Layer

In addition to the two methods discussed above there are other methods used for absorber layer

deposition, namely: sputtering in which Cu, In, and Ga are sputtered while Se is evaporated,

closed space sublimation, chemical bath deposition (CBD), laser evaporation, and spray

pyrolysis (12).

2.6: Chemical Bath Deposition (CBD) for CdS Buffer Layer

The basic function of the buffer layer is to transmit most of the light and form the p-n junction

so that a low interface recombination is obtained. It also protects the CIGS surface from

sputtering damage during further process. To achieve this, material that is of interest should:


12

Have large band-gap material for optical transmission

Completely cover CIGS surface with low thickness to prevent shunting

Have good lattice match to reduce interface trap density

Have adjusted conduction band offset to CIGS

Therefore, a thin layer CdS thin layer fulfills all the above conditions to be deposited on CIGS

absorber. The CdS layers have been prepared by several techniques such as spray, CBD,

sputtering, vacuum evaporation, MOCVD, and e-beam (12). Among these CBD is a simple,

easy, and inexpensive method. CBD is thin-film materials deposition technique from liquid

phase. This method has been used for deposition of CdS and CdSe as buffer layer for thin-film

solar cell form precursor compounds. For deposition of CdS buffer layers on Cu(In,Ga)Se2 the

constituents compounds used are a cadmium salt (CdSO4, Cd(CH3COO)2), NH4OH, and

a sulfur precursor SC(NH2)2 (thiourea). The used composition of the solution changes with the

components as well as with the laboratories. For the deposition of the buffer layer the

Cu(In,Ga)Se2 film is immersed in a bath containing the solution and deposition takes place in a

few minutes at a temperature of 60 °C to 80 °C (3) . The CBD process that involves deposition

of CdS can be described by series of chemical reactions in equation 2.5 to 2.8.

CdS formation is promoted by the reaction of cadmium salt dissolved in a basic ammonium

solution (equation 2.5), and complex ion [Cd (NH3)4]2+

is deposited on CIGS absorber as

Cd(OH)2 and further reacts with the thiourea. The addition of thiourea (equation 2.7) along with

controlled heating and mechanical stirring, initiates CdS deposition onto the surface of absorber

immersed within the chemical solution (55).

2.8 4NHCdS(s)S)Cd(NH

2.7 O2HHCNS2OH)CS(NH

2.6 )Cd(NH4NHCd

2.5 OHNHOHNH

343

222

2

2

2

433

2

234

22

2

.

Typical thickness of CdS ranges from 50 nm-70 nm. This nano scale thickness has great

advantage to maximize the light transmission into the active part of the device and result in a

current gain (3). If CdS buffer is omitted from the interface of ZnO and CIGS absorber, it

results in a strong decrease in device performance.


13

Figure 8: Set up for CBD deposition of CdS (56).

The common setup for CBD consists of a hot plate with magnetic stirring, a beaker holding the

solutions into which the substrate is immersed, and a thermocouple to measure bath

temperature, as it is showed in Fig.8. During CBD the native oxides on CIGS films are

subjected to chemical etching by ammonia, and CBD process cleans the CIGS surface and

enables the epitaxial growth of the CdS buffer layer (57) (3).

The growth of CdS occurs from ion by ion reaction or by clustering of colloidal particles. The

color of chemical bath changes from colorless to deep yellow during CBD deposition process.

Alternative buffer layer like mixed (CdZn)S which has a wider band-gap, allowing increased

optical transmission, and better lattice match to Cu(In,Ga)Se2 than CdS, ZnSe and ZnO are

suggested (3).

2.7: Transparent Front Contact (ZnO/ZnO:Al)

RF sputtering is the most commonly used low-temperature deposition method for ZnO/ZnO:Al

front contact. Intrinsic ZnO, which has high resistivity due to a low carrier concentration, was

used as shunt barrier. It is also used for optical matching layer and smoothening the spick in

hetrojunction solar cell. The common materials used for front contact layer are SnO2, In2O3: Sn

(ITO) and ZnO:Al which have a high conductivity for a good lateral current collection. Those

materials are used as displays and low-emission coatings on window glass panes. ZnO:Al is the

material of choice since it is cheaper than ITO. Intrinsic ZnO is deposited by radiofrequency

(RF) sputtering techniques from ZnO target. The other importance of ZnO buffer layer is to add


14

protection for the interface region from sputter damage induced during deposition of the TCO

layer which typically requires more harsh conditions (3). Like ZnO, ZnO:Al films are deposited

by RF magnetron sputtering from ceramic ZnO:Al2O3 and Al2O3 under Ar plasma (Fig.9). The

ZnO:Al films could be sputtered from Al/Zn alloy target by reactive DC sputtering under

mixture of Ar and O2 plasma (58). The drawback of reactive sputtering is it needs precise

process control because of hysteresis effect.

Figure 9: Schematics of RF magnetron sputtering of ZnO:Al from ZnO and Al2O3 targets (58).

An alternative ZnO deposition technique is chemical vapor deposition (CVD) which is used in

commercial manufacturer of Cu(In,Ga)Se2 solar cell (3). After TCO, metal grid of (Ni/AL,

Mo/Cu) with minimum shadow could be deposited by electron beam evaporation or DC

sputtering to complete the cell fabrication.

2.8: Solar Cell working Principle

Solar cells are produced by joining together p-type and n-type material into a p-n diode. The

position of the Fermi levels can be determined from equation 2.9 and 2.10 (59). For n-type

2.9 )/N(Nq

KTEcE CDfn

and for p-type

102. )/N(Nq

KTEE VAfpv

where k is the Boltzmann constant, T is the absolute temperature, Efn and Efp are the Fermi

levels, Ec is the energy level at the bottom of the conduction band, Ev is the energy level at the


15

top of the valence band, NC and NV are the effective density of states in the conduction and

valence band respectively, ND and NA are the donor and acceptor concentrations respectively.

The concept of semiconductor band-gap is essential to estimate how many photons contribute to

the electric current. Each semiconductor material has a unique band-gap. A material‟s band gap

is simply the energy at which a photon will dislodge an outer electron from its bond.

When the solar cell is illuminated (Fig.10) by photons of an energy greater than the band-gap,

photons enter the structure to excite electrons from the valence band to the conduction band and

create electron-hole pairs. Unless, the electron-hole pairs are separated by electric field and the

electrons and the holes leave the device at the opposite contacts as net photocurrent, the

generated carriers will recombine at the same rate at which they are generated (34).

In the p-type part of the device electrons diffuse and are driven by the electric field towards the

n -type part of the device. The opposite is true for generation in the n-type part of the device,

with holes diffusing and being driven towards the p-type side. This creates a charge separation,

which leads to a voltage difference between the two sides to drive current through an external

load and the charge carriers are circulated through the system. The power delivered to the

external load (Fig.10) is mainly governed by the irradiation and the load itself. The basic

transport equations governing the flow will be discussed in section 3.1.

Figure 10: Schematics of solar cell operation when illuminated by solar irradiation the excited carriers cross

the band gap of absorber material and charge is separated by p-n junction.

2.9: Energy band diagram of Cu(In,Ga)Se2 thin-film solar cells

The p-n junction is formed between the p-type CIGS absorber layer and the n-type CdS and

ZnO layers, which results in bending of the valence and conduction bands (Fig.11).


16

Figure 11: Energy-band diagram for two isolated semiconductors (a) and ideal p-n anisotype heterojunction

at thermal equilibrium (b) (59).

A junction is formed between two different semiconductor materials like in the

Cu(In,Ga)Se2/CdS solar cell is called hetrojunction. The two materials having different band-

gap (Eg), different permittivities (ε), different work functions (Φm) and different electron

affinities (χ) are called anisotype heterojunction (Fig.11a). When a p-n junction is formed

between the two materials, the Fermi level on both sides of the junction should coincide in

equilibrium (Fig.11b).

The p-type CIGS absorber layer has average band-gap energy of about 1.2 eV. The p–n junction

is formed with n-type CdS buffer layer that has band gap energy of around 2.4 eV. The front

contact consists of a bilayer of intrinsic and aluminum-doped ZnO that have a wide band-gap

over 3 eV (25). Because of its high band gap, almost all light passes through to the underlying

layers. Most of the incident light that passed through the wider band-gap window layer and CdS

is absorbed in the lower band-gap Cu(In,Ga)Se2 layer. The energy-band diagram of a MoSe2/

Cu(In,Ga)Se2 / CdS / ZnO heterojunction solar cell is shown in Fig.12.

Incoming light with higher energy than the CIGS band-gap is mostly absorbed within the first

micrometer of the absorber layer (25). To obtain maximum conversion efficiency, collection of

the minority charge carriers (electrons) from the CIGS absorber layer is essential.


17

Figure 12: Energy band of MoSe2/Cu(In,Ga)Se2/CdS/ZnO heterojunction (25).


18

Chapter3: Electro-Optical Characterization of Thin-Film Solar Cells

In this section the characterization methods to determine the response of a solar cell to optical

and electrical excitation will be discussed. There are varieties of solar cell device

characterization methods namely: current (current density) measurement as a function of

voltage, illumination intensity, wavelength of monochromatic illumination, and sample

temperature (34). In this thesis, the characterization method used to determine the efficiency of

the energy conversion process is measuring current verses voltage curves.

3.1: Current Density -Voltage (J-V) Characteristics of a Solar Cell

Electrical characterization of solar cell devices is necessary to determine the sources of

performance losses and suggest ways to minimize them. The current-voltage curve of an ideal

solar cell in the dark follows the exponential Shockley diode equation (in blue Fig.13 and

equation 3.1 (59)), and when the cell is illuminated (in red Fig.13), the curve is shifted

downwards by an amount referred to as the light generated current JL.

Where J0 is the diode saturation current, A is diode quality factor (greater than or equal to 1 for

ideal diode case), q is elemental charge, k is Boltzmann constant, and T is temperature. The

important parameters for conversion efficiency are short-circuiting current density (jsc), open-

circuit voltage (Voc), and fill- factor (FF).

Figure 13: Typical illuminated J-V characteristics (60).

3.1 J1AKT

qVexpJJJJ L0LD


19

The short-circuit current (Isc), at V=0, depends on a number of photogenerated carriers and

their collection efficiency. The number of generated carriers can be maximized by minimizing

the area taken by contact grids and by sufficiently thick absorbers, which allow all of the

photons with sufficient energy to be absorbed. Open-circuit voltage is the voltage at zero

current, when the forward current balances the photogenerated current. From the diode

equation, Voc is given by equation (3.2) (59):

2.3 1J

Jln

q

AKTV

o

Loc

M.A. Green (61) reported at the pn-junction, the open-circuit voltage changes with mobility due

to increased surface recombination at high mobilities. Furthermore he explained the fact that Voc

of any solar cell is considerably lower than its radiative limit, implying that nonradiative

recombination mechanisms like Auger recombination or recombination via defects occur.

The three recombination mechanisms in semiconductors (Fig.14) are (a) radiative

recombination: recombination of electron–hole pair that results excess energy transferred into a

photon. (b) Auger recombination the excess energy serves to accelerate a third charge carrier

(electron or hole), which thermalizes rapidly by emitting phonons. (c) Shockley–Read–Hall

recombination via states in the forbidden gap and the excess energy is also transferred to

phonons leading to an increase in the lattice temperature (34).

Figure 14: Overview over the three basic recombination mechanisms for photogenerated excess carriers in a

semiconductor. The excess energy is either transferred to (a) a photon, (b) kinetic energy of an excess

electron or hole, or (c) phonons. (34).


20

The saturation current density (J0) depends on the material properties and the cell structure. It is

limited by recombination coming from several different recombination mechanisms:

recombination in the bulk CIGS, in the space-charge region, and at the CdS/CIGS interface.

The power extracted from the cell is a product of current and voltage. The point at which

current-voltage curve product has a maximum value is the maximum-power point (MPP), and

the corresponding current density and voltage are the maximum power current (Jmpp) and the

maximum-power voltage (Vmpp) (Fig.13). In addition to Voc and jsc, the maximum power

depends on the fill-factor (FF) which is given by equation 3.3 and 3.4 (59). Despite the fact that

solar cell device operation at jsc and Voc is instructive, it is not intended to be operational at

either of these points. When device is short circuited, V=0 V. Thus Power is zero. Similarly at

open circuit voltage P=0 W, since J = 0 mA/cm². Therefore at neither of these points there is no

of the sun‟s power converted into electricity.

3.3 .JV

.JVFF

scoc

mppmpp

The efficiency that includes all parameter is:

.43 P

.FF.VJ

P

Pη

in

ocsc

in

out

Pin is the incident light power on the cell. It is commonly taken to be 100 mW/cm2 for standard

solar illumination. This illumination is referred to as Air Mass (AM) = 1.5 and it is equivalent to

sunlight passing through 1.5 times the air mass of vertical illumination. To measuring solar cells

parameters artificial illumination source that matches the conditions of sunlight is required. The

standard solar cell testing conditions are: Air mass 1.5, intensity of 100 mW/cm2 (1 kW/m

2,

one-sun of illumination), cell at room temperature and four point probe to remove the effect of

cell contact resistance. Emery (62) suggested the ideal illumination source would have a spatial

non uniformity of less than 1 %, a variation in total irradiance with time of less than 1 %,

filtered for a given reference spectrum to have a spectral mismatch error of less than 1 %, and

with these requirements an accuracy of better than 2 % could be obtained. The most common

light source used in solar simulator (Fig.16) is a Xenon arc lamp with filters installed to

approximate the AM 1.5 spectrum.

The above equation (3.1) is the ideal diode equation. The real solar cells have additional losses

mechanisms like series resistance (Rs) and shunt resistance (Rsh). The Rs comes from the bulk


21

semiconductor and the contact resistances. While Rsh, is from parallel paths for current flow.

The equivalent circuit of a solar cell with these resistances is shown in Fig.15.

Figure 15: An equivalent circuit of a pn-junction solar cell consisting of series and shunt resistance (34).

Figure 16: Schematic of a solar simulator for electrical characterization of a solar cell under illumination

with a spectrum resembling the standard AM 1.5, current and voltage source controlled through computer

(63).

Therefore, when the Rs and Rsh are added to the ideal diode equation it becomes;

3.5 JR

JRVJRV

AKT

qexpJJJJ L

sh

ss0LD

For better solar cell operation very large Rsh and negligible Rs are favored. The series

resistance may originate from the finite conductivity of the absorber layers themselves or from


22

the front and back contacts. Increase in series resistant results in voltage drop which is given by

IxRs, while decrease in shunt resistance results in drop in current (V/Rsh) as depicted in Fig.17a

and Fig.17b respectively.

Figure 17: Effect of series resistance which results in voltage drop (a) and shunt resistance which results in

decrease of current (b) on the current–voltage characteristic of a solar cell (12).

3.2: External Quantum Efficiency (EQE)

In today‟s advancing researches of solar cells technology characterization, the quantum

efficiency is one of the most important parameters for the research and development of new

materials and devices; but also for production and quality control. In an ideal solar cell every

photon with energy greater than equal to band-gap of semiconductor leads to one electron–hole

pair that is collected from the solar cell. In reality, this is not the case because of loss

mechanisms that are responsible for not every photon in the solar spectrum contributes to Jsc.

The external quantum efficiency is affected by factors „external‟ to the diode, such as

reflections, and absorption in different layers of the solar cell, while the internal quantum

efficiency considers only the collection of photons incident on the junction. The external

quantum efficiency (EQE) is defined as the ratio of the number of carriers collected by the solar

cell per the number of photons of a given energy irradiating solar cell which is mathematically

represented by equation 3.6 (64). EQE might be represented either as a function of the

wavelength or energy. Theoretically if all photons are absorbed with in certain wavelength and

the minority carriers are also collected, then EQE value is unity (or 100 %) (65). In CIGS solar


23

cell EQE is controlled by the band gap of the Cu(In,Ga)Se2 absorber layer, the CdS and ZnO

window layers, and a series of loss mechanisms (3).

3.6 (E)d

(E)dj

q

1EQE(E) sc

Where dΦ(E) is the incident photon flux in units of (Φ)=cm_2

s_1

in the (photon) energy interval

dE that leads to the short-circuit current density djsc.

3.3: Spectral Response

In addition to EQE, the second frequently used quantity to characterize solar cell is spectral

response. The spectral response is current produced per unit optical power incident on the solar

cell with unit ampere per watt (64). It is commonly used to characterize the ability to collect

charge carriers generated by different wavelengths of the sun spectrum (66). The ideal spectral

response is limited at long wavelengths by the inability of the semiconductor to absorb photons

with energies below the band gap. This limit is the same for external quantum efficiency curves.

Unlike the square shaped EQE curves (Fig.18a), the spectral response curve decreases at small

photon wavelengths (Fig.18b). As light of different colors is absorbed at differently depths in

the solar cell, the spectral response provides a depth resolution of the recombination processes

(66).

Figure 18: The schematics of ideal solar cell EQE (a) and spectral response of ideal solar cell (in red) and a

silicon solar cell (in blue) under glass in which the response fall back to zero at higher (65). In both (a) and

(b), the point at which the curve fall to zero defined the band-gap of the solar cell.


24

According to (65)and (64), the quantum efficiency of solar cell could be obtained from the

spectral response by replacing power of illumination source at a particular wavelength with the

photon flux for that wavelength and spectral response and quantum efficiency are related by

equation 3.7 (64).

3.7 EQE(E)E

q

E

1

(E)d

(E)djSR sc

For photons with energy above the band gap of the semiconductor, the EQE and SR is zero and

it would rather results in heating of the solar cell. The inability of a cell to utilize all sunlight at

high energies and the inability to absorb low energies of sunlight represents a significant power

loss in solar cells consisting of a single pn- junction (64).


25

Chapter 4: Experimental

In this chapter experimental procedures of Soda lime glass/Mo/CIGS/CdS/ZnO/ZnO:Al/Mo/Cu/Mo

thin-film solar cell preparation is reported and discussed. The basic art of preparation of CIGS thin-

film solar cell have been discussed in the previous chapter. For this experiment the Mo back contact,

In/CuGa layers, and Mo/Cu/Mo grids are deposited by DC magnetron sputtering from respective

targets. The p-type absorber layer (CIGS) is processed by a sequential process in which the In/CuGa

sputtering is followed by a reactive annealing step in selenium atmosphere. The second step takes

place in the HV chamber. The n-type buffer layer (CdS) is deposited by Chemical bath deposition

(CBD), and i-ZnO/ZnO:Al windows are deposited by RF sputtering techniques. Thereafter

characterization will be explained.

4.1 Substrate Preparation

The experimental procedure is started with cutting 10 cm x 10 cm soda lime glass into

5 cm x 5 cm and identifying the tin (Sn) side (here the opposite side is used during sputtering)

followed by randomly labeling the corners of each substrate for Mo, CIG (In/CuGa), Se, ZnO,

ZnO:Al on the tin side. The corner with respective labeling will be at the center of the substrate

holder during corresponding manufacturing process. For example see (Fig.19).

Figure 19: Soda lime glass thin side labeled with substrate holder (back side view).

Since the substrate should be clean, the glass substrate is cleaned in warm water with soap by

ultrasonic bath and by hand. It is finished with deionized water and dried by nitrogen air jet.


26

Finally, the substrate is transferred to sputtering chamber for Mo and CuGa/In sputtering. The

chamber holds total of 16 samples (4 sample holders each with 4 samples).

4.2: DC Magnetron Sputtering

Before sputtering the Mo back contact, the substrates are etched with a mixture of Ar and 2 %

O2 plasma for 120 seconds and plasma current of 120 mA. Two different layers of Mo were

sputtered as back contact under Ar plasma. The first layer was sputtered under high pressure to

have better adhesion to the glass, while the second layer is sputtered at lower pressure to have

better conductance. The sputter parameters of the two layers are shown in Table 1.

Table 1: Parameters used for Mo sputtering.

Mo Layer Flux Φ

in(sccm) Pcpt(mbar) Power(kW) Time(s) Thickness(nm)

1 166.7 0.0200 0.25 100 100

2 37.7 0.005 0.80 210 400

The next step was to sputter In and CuGa layers from the respective targets. The CuGa target

has an atomic Cu/Ga ratio of 7/3. Before the In/CuGa deposition, the HV sputtering chamber

was opened to put the CIG corners of the substrate at the center of substrate holder.

Table 2: Parameters used to sputter different layers of In/CuGa by DC magnetron sputtering.

Sample layers and

thickness

Samples

rename

tIn=tCuGa (s)

(for each

layer )

Total time

(s)

Flux Φ

in(sccm)

Pcpt(mbar)

Standard: 10 layers

of

CuGa and In each

Standard

20

400

95.3

0.0130

¾ of standard 10

layers of CuGa and

In each

¾ thick

15

300

95.3

0.0130

½ of standard:10

layers of CuGa and

In each

½ thick

10

200

95.3

0.0130

½ of standard:5

layers of CuGa and

In each

½ thick *

20

200

95.3

0.0130

From this point onwards the samples name as re-named in table 2 will be used throughout this

thesis. For the standard absorber, 10 layers of In and CuGa were sputtered, respectively each for

20 seconds. To decrease the thickness of absorber layer either the number of layers or the

sputtering time of each layer was decreased. The parameters used to sputter different thickness


27

layers are given in Table 2. The power supply for In and CuGa sputtering is 0.22 kW and

0.21 kW respectively.

4.3: The Selenization Process

To complete the preparation of the p-type CIGS absorber layer, the In and CuGa precursor

layers were selenized under Se vapor in HV chamber. Before selenization the selenium rate was

calibrated with vapor pressure of Se to be 6.5x10-5

mbar equivalent to the deposition rate of

200 nm/min and 264 nm/min according to equation 4.1 and 4.2 respectively. For a single

process, four samples from different sputtered In/CuGa layers were selenized. Selenium

evaporation temperature and substrate heating temperature was as depicted in Fig.20.

Figure 20: The substrate heating and Se evaporation versus time for selenization process of CuGa/In

multilayer. The first process of substrate heating at 375 °C for 3 minutes, and the second stage of heating at

620 °C changed from 3 to 8minuts ( not shown here) during process.

The main purpose of this thesis is to prepare CIGS solar cells of different thickness under

different second phase selanization time and compare the results of each thickens with the

corresponding standard sample as stated in chapter one. During the experiments since the

process system was changed there are two batches of samples for which the selenium rate is

given by equation (4.1) and equation (4.2).The corresponding samples of each batch with the


28

specific precursors are named in table 3 and table 4 respectively. The first batch of samples had

only 8 min selenization time at 620 °C as displayed in table 3.

4.2 nm/min 2mbar10

ΔP41

.14 nm/min 33mbar10

ΔP36

5

se

5

se

Table 3: Set of samples prepared with first selenium rate (equation 4.1) for 8 min during 2nd

phase of selenization.

Set of

process Sample

2st selenization

time (min)

# of CuGa/In

layers each

Time of

sputtering(s)

Sample

thickness

1 472A 8 10 20 Standard

473B 8 10 15 ¾ thick

474C 8 10 10 ½ thick

475D 8 5 20 ½ thick*

For the second set of experiments different sample thickness had been selenized at 620 °C for 4,

6 and 8 minutes, twice for 6 min at different Se rate.

Table 4: Set of samples prepared with second selenium rate (equation 4.2) for 3 to 8 min during

2nd phase of selenization.

Set of

process Sample

2st

selenization

time (min)

# of

CuGa/In

layers

each

Time of

sputtering(s) Sample thickness

Delaminat

ed after

CBD

510A 8 10 20 Standard No

2 511A 8 10 15 ¾ thick No

512A 8 10 10 ½ thick No

513A 8 5 20 ½ thick* No

510B 4 10 20 Standard No

3 511B 4 10 15 ¾ thick Yes

512B 4 10 10 ½ thick Yes

513B 4 5 20 ½ thick* No

510C 6 10 20 Standard Yes

4 511C 6 10 15 ¾ thick Yes

512C 6 10 10 ½ thick Yes

513C 6 5 20 ½ thick* Yes

510D 6 10 20 Standard Yes

5 511D 6 10 15 ¾ thick Yes

512D 6 10 10 ½ thick Yes

513D 6 5 20 ½ thick* Yes

Standard =2.75 ± 0.35 µm thick absorber

During process control the first step was to heat the selenium source to a temperature of 325 °C

to 350 °C depending on fill level of the source. For process soon after refill, temperature around


29

325 °C is used. The main point is to attain the necessary selenium flux pressure (6.5x10-5

mbar)

which takes 10 to 12 min to reach the respective temperature. The next step was to open the

selenium and the substrate shutter for the first phase of the selenization for 3 min and to start

heating up of the substrate from room temperature to T1= 375 °C with a ramping time of 3 min.

During the selenization process the samples were rotated to insure that the selenium is

incorporated uniformly in the CuGa/In multilayer precursors. During this event the selenium

will react on the surface and start to produce binary phases with In/CuGa precursors. Based on

the ternary phase diagram of the Cu–In–Se system (Fig.21), phases like Cu2Se and In2Se3 were

most probably formed. The binary phase In2Se3 at the end point of the pseudobinary tie-line can

be alloyed to form (InGa)2Se3 (3) are utilized during the proceeding process.

Figure 21: Ternary phase diagram of the Cu–In–Se system. Thin-film composition is usually near the

pseudobinary Cu2Se–In2Se3 tie-line (3).

The second step of selenization was done by heating the substrate fromT1= 375 °C to T2= 20 °C

within 2 min ramping time and kept constant for 3 to 8min for different samples. During this

process selenium flux pressure was greater than 6.5x10-5

mbar. Finally, the selenium source and

substrate heating power was turned off, while selenium shutter kept open till the substrate

temperature decreased to 350 °C (not show in Fig.20).

In the second step of selenization the binary phases and more selenium formed the

Cu(In,Ga)Se2 absorber. As diffusion is material transport mechanism by atomic motion;

inhomogeneous materials can become homogeneous by diffusion. For an active diffusion to

occur; temperature should be high enough to overcome the energy barriers of atomic motion. In

addition, the concentration gradient of selenium and other components are often called the


30

driving force for diffusion. Therefore, high temperatures (620 °C) favor the diffusion of

selenium from source and also the sodium from the soda lime glass into the In/CuGa layers to

form the p-type Cu(In,Ga)Se2 absorber. Since the GGI ratio of high performance CIGS absorber

ranger from 0.2 to 0.3 (67), there could be probability of CuInSe2 and CuGaSe2 formed during

this high temperature growth and finally the CuInSe2 alloyed in any proportion with CuGaSe2,

thus forming Cu(In,Ga)Se2 (3). When the process time is too short there will be a high chance

of an unfinished reaction that results in the loss of Indium by re-evaporation in the form of

In2Se (53), while too much time (greater than 8 min for standard (2.75 ± 0.35) µm absorber)

induces the formation of thicker MoSe2 layer in-between CIGS and Mo. The drawback of thick

MoSe2 is it results in delamination of CIGS/MoSe2 from the Mo/glass substrate.

4.4: The Chemical Bath Deposition (CBD) Process

The basics principles of chemical bath deposition (CBD) of the n-type CdS buffer layer were

discussed in section 2.6. The chemical reagents used for this CdS chemical bath deposition are:

Thiourea (SC(NH2)2), CdSO4, and. To prepare the solution for CBD, 3.400 gm of thiourea is

dissolved in 90 ml of deionized water and filtered by filter pap, and 0.200 gm of CdSO4 is

dissolved in 24 ml deionized water and 40 ml NH4OH. The whole solution of thiourea, CdSO4,

and NH4OH are mixed with additional 400 ml deionized water in a beaker directly before the

deposition was started. The selenized samples were adjusted on a holder with teflon and

immersed in the prepared chemical bath. The beaker with the chemical bath is placed in another

big beaker with water heated to 65 °C and the time for CBD process was started. The bath

temperature of 65 °C was kept constant throughout the process while stirring the solution and

the water bath with magnetic stirrers. The color of the chemical bath changeed through the

progressive time of CBD process from colorless to yellow and deep yellow in 11.5 min

deposition time. When the CBD process was finished, a thin layer of CdS of a thickness around

50 nm is deposited on the CIGS absorber. Finally, the samples were taken out of the chemical

bath and washed by deionized water and dried with nitrogen air jet.

4.5: ZnO/ZnO:Al Front Contact Deposition by RF Sputtering

The intrinsic ZnO layer was deposited on the CdS by the use of Rf sputtering of ZnO target in

mixture of Ar and 2 % O2 plasma. The Ar +2 % O2 mixture was used to avoid oxygen vacancies

in the deposited layers which are donors and induce free charge carriers (68). Like ZnO, the

aluminum-doped zinc oxide (ZnO:Al) was deposited by the same technique in a pure Ar

plasma. The used target was a mixture of ZnO with 2 wt % Al2O3. The sputtering parameters

used for ZnO and ZnO:Al front contact layers are given in table 5.


31

Table 5: Parameters used for RF sputtering of ZnO and ZnO:Al.

Sputtered

Layer Plasma

Flux Φ

in

(sccm)

Pcpt(mbar) Power(kW) Bias

Voltage(V)

Time

(min)

Thickness

(nm)

ZnO Ar/O2 6.00 2.95x10-3

0.15 45 13 70

ZnO:Al Ar 6.00 2.15x10-3

0.20 51 20 300

RF Sputtering time to sputter ZnO and ZnO:Al is long compared to DC magnetron sputtering

since RF sputtering rate is generally quite low and the electron flux on the substrate is much

higher, which may cause significant heating. RF sputtering is used since the targets (ZnO and

Al2O3) are insulating material.

4.6: Grid Deposition

The final process to complete the solar cell was to deposit a metal grid as electrical contact to

measure J-V curves. The common materials used for this purpose are Ni and Al. In this thesis

Mo and Cu layers are deposited on the ZnO:Al by DC magnetron sputtering after putting masks

on the samples. The parameters used for sputtering are given in Table 6.

Table 6: Grid deposition parameters by DC magnetron sputtering.

Layer Material Flux Φ in

(sccm) Pcpt(mbar) Power(kW) Time(s)

Thickness

(nm)

1 Mo 14.00 0.002 0.80 100 200

2 Cu 71.00 0.010 0.40 300 800

3 Mo 14.00 0.002 0.80 50 100

4.7: Current Density-Voltage (J vs.V) Characteristics Measurements

In order to measure the solar cell, the stability of light source that closely matches the

conditions of sunlight intensity and spectrum is critical. Measurement is carried out by using a

solar simulator (WACOM ELECTRIC CO., LTD) equipped with a Xenon (Xe) arc lamp, optic

filters and lenses developed to create an optical system capable to provide optimized artificial

light for evaluating the output of photovoltaic devices under standard testing conditions. The

sours measurement unit was controlled by a computer with LabView software. The calibration

of the solar simulator intensity was done by adjusting the lamp current until the output current

of a silicon standard cell reached 144mA. When illuminated by solar simulator. Finally the

measurements were performed by the scaling voltage ranging from -0.5 to 0.8 V with 131 data

points. During the measurement the LabView software extracts values of Isc (short-circuit


32

current), Voc (open-circuit voltage), Impp (current at maximum power point), Vmpp (voltage of

maximum power point), η (efficiency), and FF (fill factor). During data analysis in addition to

those parameters; series resistance, shunt resistance, ideality factor, maximum voltage,

minimum voltage and others are extracted by fitting the data points of measured J-V

characteristic curve with the theoretical series of equation by using a Mathematica program

written by member of the Pothovoltaics group of Institute of Solid State Physics, University of

Jena.

4.8: Quantum Efficiency Measurement

To measure the quantum efficiency for the solar cells, a monochromator- based setup was used.

The equivalent set up is depicted in Fig.22. The part and parcel of this EQE measurement

system is the monochromator. It selects light of a specific wavelength by deflecting the light of

a Xenon arc lamp at a grating. In addition, accurate quantum efficiency measurements of solar

cells require a light source for biased measurements.

Figure 22: Scheme of a monochromator-based setup in which the chopped monochromatic light illuminates

first the reference (during calibration) and then the sample (during measurement). The current output from

reference cell or sample is converted to voltage and then amplified with a lock-in amplifier triggered by the

chopper. Finally amplified signal of the lock-in amplifier is read and displayed by a computer (34).

The white light from a Xe-arc lamp equipped with a light intensity controller is chopped before

entering the monochromator. The chopper is an essential part of the set-up to obtain a periodic

signal, which a lock-in amplifier can use. The current output from the reference cell or the

sample is converted to voltage and then amplified with a lock-in amplifier triggered by the

chopper. Finally the amplified signal of the lock-in amplifier is read and displayed by a

computer. Since EQE measurement requires the calibrated reference solar cell, for the results


33

presented in this thesis a single crystalline silicon solar cell was used as reference. In order to

characterize losses responsible for difference in current of the cells, the absolute spectral

response measurements were executed under chopped illumination and a lock-in technique. The

illumination of 11 % of AM 1.5 global illumination was used during calibration and

measurements. Since light-generated current was the integral of the product of external quantum

efficiency and illumination spectrum, the reference silicon solar had light-generated current of

1.943 mA at this illumination. All measured cells had current less than this value. The

maximum light-generated current of 1.62 mA was obtained from cells of standard samples of 1st

set. During EQE measurement up to 3 cells of each sample were measured. To measure the

current response of the reference silicon solar cell as function of wavelength, the LabView

software was set to scan wavelength from 300 nm to 1200 nm with 5 nm steps. During

measurement the current of the reference device which was stored in computer program is

compared to the current of the sample to carry out measurement.

4.9: Wet Chemical Etching

Wet chemical etching is a mechanism that uses liquid chemicals or etchant to remove material

and leave specific patterns on the material of interest or completely remove. The specific patters

are defined by mask that is used during etching process so that materials parts that are not

protected by the masks are selectively etched away by liquid chemicals (69). These masks are

deposited or patterned on the sample prior to etching. The etching process involves immersion

of the sample in a pure or mixture of etchants of intended concentration for a given amount of

time. The effect of etching depends on time of etching, concentration of etchant, temperature of

etchant, chemical nature of etchant, and oxygen content of solution as well as mechanical

stirring (57). A wet etching process involves multiple chemical reactions that consume the

etchant and material required to be etched, and produce chemical composition in the solution.

Avinash P. Nayak (70) described wet chemical etch process by three basic steps. (1) Diffusion

of the liquid etchant to the structure that is to be removed. (2) The reaction between the liquid

etchant and the material being etched away. (3) Diffusion of the byproducts in the reaction from

the reacted surface.

The experimental methodologies of the wet chemical etching employed during the experimental

procedures will be disclosed. To lay the foundation for the CIGS etching process, initially the

grids, CdS and i-ZnO/ZnO:Al etching were carried out on larger sample (5 mm x 5 mm area)

using 5 % HCl for 10 to 15 minutes at room temperature. The samples were then cleaned in

deionized water and finally dried with nitrogen.


34

To give more emphasis about what eventuate during HCl etching, some literatures are reviewed.

Kirt R. William (71) reported aqua regia (mixture of HCl and HNO3), HCl, and H3PO4 are

etchants used for Mo and Cu etching. According to (72), HCl is used not only as a

macrostructure etchant to reveal grain boundaries but also as a dislocation etchant to reveal

dislocations in materials during etching. For ZnO etching numerous wet etchants have been

reported that it is soluble in most single acids like (HNO3, HCl, H3PO4, H2SO4, HCl and HF),

mixed acids, alkalis and ammonium chloride (73) (74) (75). The reaction of HCl and ZnO takes

place by proton ions attacking the oxygen forming water and soluble salt in aqueous solution

given by chemical equation 4.3 (76).

4.3 OH(aq)Zn(aq)2HZnO(s) 2

2

Bromine in aqueous solution is recognized as an efficient oxidizing agent on III–Vs or II–VIs

(77). Studies (78) and (79) demonstrated that bromine solution of (0.001 – 1.0 M) is a suitable

etching agent for CIGS in which KBr is added to the solution to help with the dissolution of

bromine. M. Bouttemy (80) investigated etching of CIGS absorber in HBr (0.25 M)/Br2

(0.02 M)/H2O at 20 ± 0.5 °C while rotating at 40 rpm. For the quantification of the Ga, In and

Cu dissolved during the etching treatment, graphite furnace atomic absorption spectrometry

(GF-AAS) was used and found out 0.17 µm/min etching. Canava (78) reported about detail

etching mechanism. Accordingly, there are two consecutive oxidation steps that involved

overall etching process of CIGS absorber in aqueous bromine solution. When etching process

starts the process leaves a selenium rich surface which can be explained according to oxidation

equation 4.4 leading to the preferential dissolution of the metals.

.44 5Br2SeGaInCuBr2

5SeGaIn,Cu 332

22

The rate determining second step of oxidation of Se is represented by equation 4.5:

4.5 4BrSe2BrSe 4

2

In this manner the selenium and bromine ions washed out in aqueous solution. Since methanol

was used instead of water which is hydrophilic and good solvent similar etching mechanism

could probably resulted in etching.

The approach used in this thesis is to begin etching from the standard glass/Mo/CIGS structure

by wet chemical etching from the top using bromine-methanol (BM) solution. Since etching did

not reach the back contact, from this conceptualization it is practicable to engineer thinner CIGS

layers while keeping exactly the same back contact configuration. In order to find out the

appropriate etching conditions that will result in etching of half and three-fourth of the standard


35

absorber thickness, volume fraction of 1.0, 0.2 and 0.025 V/V % of bromine in methanol were

tested for different times. The grids, front contacts and CdS are etched first as aforementioned.

Thereafter parts of each samples is covered by “pecien”, the black wax that dissolves in

trichloromethane (CHCl3), and dried so that the uncovered part of the absorber will be etched.

In the result chapter the effect of etching in 1.0, 0.13, 0.2 and 0.025 V/V % Br2 in CH3OH for

different time will be reported. In addition CIGS absorber etched after selenization down to

thickness approximately ¾ thick and ½ thick for 8 min and 12 min respectively in 0.025 V/V %

solutions was used to build solar cell by depositing 50 nm of CdS by CBD soon after etching;

along with evolution of surface morphology will be addressed further. The raw materials used

with different concentration of bromine and etching time are given in table 7.

Table 7: Chemicals, concentration and duration of etching of CIGS absorber.

Bromine

(ml)

Methanol

(ml)

Bromine

(V/V %) in

Br2-CH3OH

solution

Time of etching

(se) at room temperature

Mechanical

(hand)

rotation

1.0 100 1.0 5,10,15,20,25,40 No

1.0 100 1.0 5,10,1520,25,40, No

0.2 100 0.20 10,20,30,40,50,60,90 Yes

0.2 150 0.13 30,60,90,120 Yes

0.1 400 0.025 2.0,2.5,3.0,3.5,4.0,5.0,6.0,7.0,8.0,12.0* Yes

*Unit of time in minutes

To terminate etching process in the above given table, the sample is withdrawn from Br2 in

CH3OH solution and immersed into beakers with deionized water (Fig.23).

Figure 23: Beakers with deionized water, 0.025 V/V % Br2-CH3OH solutions, stop watch and teflon used to

handle sample during CIGS etching in Br2-CH3OH. When etching time was over, the sample was bathed in

to the two beakers with water in the left one after the other to terminate etching.


36

Chapter 5: Results and Discussion

In this chapter the results of electro-optical characterizations of samples with standard

thickness will be compared to samples of three-fourth and half of the standard thicknesses are

presented. Bromine in methanol (BM) etching and morphological investigations of etched

samples by SEM will be discussed in detail.

Furthermore the Scanning Electron Microscopy (SEM) micrographs, Energy-Dispersive X-ray

spectroscopy (EDS) spectra, etched thickness and roughness versus time measured with Dektak

3030 Profilometer of the chemically etched standard samples in Br2-Metanol of different

concentration will be presented.

5.1: Electro-optical characterization of different absorber thickness

In this section the electro-optical characterizations namely: J-v curve and external quantum

efficiency of samples of different thickness will be compared with standard. The 2nd

and 3rd

sets

were prepared in the same manner as the 1st set except for the selenium rate changed from as

given in equation 4.1 (200 nm/min) to equation 4.2 (264 nm/min) for 2nd

and 3rd

sets after the

selenization process chamber was turned off for some time. The 3rd

set samples were prepared

by selenizing CuGa/In multilayer precursors for 4 min at 620 °C. Finally, parameters of samples

prepared after 0.025 V/V % BM etching will be reported.

5.1.1: J-V curve for 1st set of samples with 8 min selenization at 620 °C.

In this sub-section the J-V curves of best cells with highest efficiency from each sample

thickness will be presented. The graph in Fig.24 showed increase in open circuit voltage (Voc),

decrease in short circuit current density (jsc) and efficiency with decrease in thickness of

absorber. The ½ thick* cell had the lowest Voc and jsc than the other cells.


37

Figure 24: J vs.V curve of the best cells of the 1st set of samples of selected cell from different absorber

thickness selenized at 620 °C for 8 minutes.

Figure 25: The box chart of samples of different thickness parameters. (a) Short circuit current density, (b)

open circuit voltage, (c) efficiency, and (d) fill factor for 1st set of samples selenized for 8 min at 620 °C.


38

In order to compare the whole set of samples jsc, Voc, FF and efficiencies of all cells are given in

Fig.25 as a box plot. As investigated for the best cells, also the overall short circuit current

density decreased from the standard thickness to1/2 thick* (Fig.25a) where the two samples

with 1/2 thick and ½ thick* absorbers had relatively equal current density. The open circuit

voltage and FF increased with the decrease in absorber thickness, while efficiencies of all the

samples were comparable. However, the ½ thick samples had the Voc, FF and efficiency lower

than the values of sample with equivalent thickness. Measurements of the sample‟s thickness,

shown in Fig.28, showed that the samples with ½ thick* had the smallest thickness of all

samples Therefore, decrease jsc could not be assigned to the reduction of efficiency.

Theoretically, the ½ thick samples were expected to facilitate better mixing of the materials

during the sputtering process. This seems to change the reaction kinetics of the selenization in

the sample was selenized for too short or long. This would explain the decrease in the solar cell

performance.

5.1.2: J-V curve for 2nd

set of samples with 8 min selenization at 620 °C.

Similar to prior section, now the results of the electrical characterization of 2nd

set of samples

are presented. The J-V curves for this set of samples of the best cells from each thickness are

drawn in Fig.26. Unlike 1st set of samples most of cell parameters are inferior.

Figure 26: J vs. V curve of the best cells of the 2nd

set of samples of selected cell from different absorber

thickness selenized at 620 °C for 8 minutes.


39

In Fig.26 the Voc of the cells increased with decrease in thickness of absorber, while the jsc

decreased with decrease in thickness. More loss in jsc was observed for ½ thick* sample. In

Fig.27 all solar cell parameters again are showed as box plot of all cells of the samples against

the absorber thickness for better comparison. The short circuit current density increased slightly

from standard to ¾ thick samples. While the ¾ thick and ½ thick samples had equivalent values.

The ½ thick* sample had lower jsc value than others. When the Voc values as showed in Fig.27b

were compared, like in the 1st set of samples the Voc increased with decrease in CIGS absorber

thickness. The sample with ½ thick* had largest Voc. Unlike the 1st set of samples, the efficiency

increased slightly with decrease in absorber thickness (Fig.27c), where the highest efficiency

was achieved for the sample with ½ thick. The fill factor also increased from the standard to

½ thick samples (Fig.27 d). However, the standard and ¾ thick samples had equivalent FF.


open circuit voltage, (c) efficiency, and (d) fill factor for 2nd

set of samples selenized for 8 min at 620 °C.


40

The measured thickness of the samples (CGIS/CdS/ZnO/ZnO:Al) of the 2nd

set are also shown

in Fig.28. All samples of the 2nd

set of samples were thicker than the corresponding ones of the

1st set. This could be because more selenium was provided by the system.

Figure 28: The thickness of CGIS/CdS/ZnO/ZnO:Al layers of 1st and 2

nd set of samples.

5.1.3: J-V curve for 3rd

set of samples with 4 min selenization at 620 °C.

Among samples selenized for 4 min at 620 °C, only two did not delamination during CBD

process. The lower selenization time was intended to find out how the selenium incorporation in

the different thick precursors manifested in CIGS absorber crystal growth process. As the

standard sample had largest efficiency at 8 min selenization, if the thickness was half, the time

of selenization could be half of standard. Too much selenium during the process could result in

the formation of a thick layer of MoSe2 at the Mo/CIGS interface that end up with delamination

of absorber from back contact.

However, it was found out in 1st and 2

nd set of samples reported in previous sections, samples

with half of the standard thickness selenized for 8 min did not detached, while samples in 3rd

set selenized for 4 min had delaminated during CBD process except one the standard and other

with ½ thick*. The J-V curves of the best cells are shown in Fig.29.


41

Figure 29: J vs. V characteristic curve of standard and 0.5 of standard absorber thickness best cells taken

among samples selenized for 4 min at 620 °C.


open circuit voltage, (c) efficiency and, (d) fill factor of the 3rd

set of samples selenized for 4 min at 620 °C.


42

The four fundamental parameters measured from the two samples are given in Fig.30. The

values of jsc, Voc, efficiency, and FF given in Fig.30 (a-d) respectively showed that the sample

with ½ thick* selenized for 4 min worked the same way as selenized for 8 min. In contrast, the

standard sample is not working. This can be explained by the fact that 4 min of selenization was

not enough to finish the formation of the CIGS absorber layer and /or the MoSe2 layer.

5.1.4: J-V curve for 4th

set of samples prepared after BM etching.

In this section the result of J-V measurement of standard sample selenized at 620 °C for 8 min

and then etched in 0.025 V/V % BM for 8 min and 12 min will be reported.

Figure 31: J vs.V characteristic curves of sample etched for 8 min (a) and 12 min (b). Only one cell (12 min

etched sample) had diode curve as shown in (b).

Generally the J-V curve shown in Fig.31 do not show a diode character anymore, neither for

8 min etched sample nor for the 12 min etched sample. Even if they show more or less linear

behavior, they are shifted by a small amount on the J-axis and also on the V-axis. Therefore a

small photo activity existed. In addition the curves showed the absence of pn-junction which is

responsible for schottky diode J-V curve. Fortunately, a single cell prepared after 12 min

etching had diode curve (Fig.31b) with very negligible solar cell parameters (jsc, Voc, FF and

efficiency). From practical point of view, etching was only responsible for thinning of absorber

to the expected thickness depending on etchant concentration and etching time. If the pn-

junction which was critical to the device performance was missed, there are two possible

explanations for that. The first one could be the residue after etching distorted the formation of

the pn-junction acting as defect which results in recombination center. The second explanation


43

is according to (81). The surface of the CIGS had a significant amount of Cd when CdS was

deposited by CBD. This Cd is much more strongly bound to the CIGS than the Cd in the bulk of

the CdS and Cd replaced the Cu atoms or had filled Cu vacancies, leading to a highly n-type

surface layer on the CIGS. Therefore, if that was the case under normal condition, BM etching

could have removed the Cu vacancies filled for the surface inversion that could lead to an

increase of interface defects

5.2: External Quantum Efficiency Results

The experimental results of EQE will be reported in similar sequence of J-V measurements

from 1st set to 4

th set of samples in this section.

5.2.1: External Quantum Efficiency of 1st set of samples

The EQE of the 1st set of samples of different absorber thickness measured from 300 nm –

1200 nm wavelength under 11 % of AM 1.5 global illumination depicted in Fig.32.

Figure 32: EQE of 1st set of samples of different absorber thickness measured under 11 % of AM 1.5 global

illumination over 300 nm -1200 nm wavelength.


44

There are different optical loss mechanism that contributed for current loss in the cells, namely:

The reflection from Cu(In,Ga)Se2/CdS/ZnO/ZnO:Al interface which affected the effective light

intensity penetrating the absorber and shadowing by the grid (represented Fig.32 in yellow),

absorption in the front contact (ZnO/ZnO:Al) and CdS buffer layer (Fig.32 in blue) ranges from

300 nm-550 nm (visible wavelengths) and increases up to near IR region (λ > 875 nm) where

free-carrier absorption becomes significant. Additional loss result out of incomplete absorption

in CIGS layer near its band-gap (1100 nm) Fig.32 in black. The loss in each part of the EQE

increased as the thickness of the absorber decreased from standard (in red) to ½ thick* (in aqua)

and ½ thick (in blue) through ¾ thick (in green). The reduction of absorbed light due to the

thinning of the absorber lead to more pronounced loss in the wavelengths range of 550 nm to

1100 nm. For a cell with ½ thick (in blue), decrease in the quantum efficiency in the wavelength

range from 600 nm to 1000 nm is observed. This can be explained by the higher penetration

depth of light for higher wavelengths, because the generated free carriers are close to the back

contact and have a high probability to recombine there. Another possibility is that part of

transmitted light do not generated free carriers at all and is absorbed at the back contact. The

spectra extended beyond 1100 nm proved the CIGS absorber had smaller band gap. Here the

decrease in EQE of a cell ½ thick was not related to the thickness of the absorber, as the sample

was thicker than ½ thick* (Fig.28). The zigzagging EQE line in the wavelength range of

850 nm to 950 nm was due to spectra from calibration caused by measurement itself.

In General the drop of the short circuit current (Fig.25a) with decreasing layer thickness

represented by the overall decrease of the quantum efficiencies of the samples. The primary

assessments indicate that this decrease started at the short wavelength range 550 nm mainly

caused by reduced absorption due to the thinning of the absorber layer. The ripple in the

wavelength range of 550 nm to 850 nm in the EQE graph is due to interference effect of the

incident light in the window layers.

5.2.2: External Quantum Efficiency of 2nd

set of samples

The EQE of the 2nd

set of samples of different absorber thickness measured from 300 nm-

1200 nm wavelength under 11 % of AM 1.5 global illumination is depicted in Fig.33. In similar

manners as in the 1St

set, the effect of absorption in different layers of the CIGS thin-film solar

cell holds. A comparison of the cells shows a slight decrease in EQE with decreasing absorber

thickness. The cell with ½ thick* shows even larger losses in the range from 500 nm to


45

1200 nm. This loss in EQE was manifested in the same way in jsc box chart shown in Fig.27a

for which a great loss is visible in the sample with ½ thick* shown in aqua Fig.33.

Figure 33: EQE of 2nd

set of samples of different absorber thickness measured under 11 % of AM 1.5 global


The EQE of 1st and 2

nd set of samples of different thickness were compared according to their

corresponding thickness as in Fig.34 (a-d). In the graph all 1st set samples in red, while the 2

nd

set in black.

Accordingly, for standard sample the 1st set had slightly higher EQE then 2

nd set Fig.34a. For

the ¾ thick both samples up to 950nm EQE is comparable, thereafter the 2nd

set shows much

higher EQE up to 1200 nm. This indicates strongly a lower band-gap of the 2nd

set Fig.34.b.


46

Figure 34: EQE comparison of 1st and 2

nd set of samples according to their respective thickness. The

measurements performed under 11 % of AM 1.5 global illumination over 300 nm -1200 nm wavelength.

For the ½ thick the 2nd

set had better EQE by absorbing more light in the range above 850 nm

especially than the 1st set. Again much higher EQE in the range above 1100 nm indicate lower

band-gap for the 2nd

set (Fig.34.c).

Figure 35: The spectral response of two set of samples of different thickness CIGS absorber. (a) for 1st set and (b) 2nd set.


47

Finally for the ½ thick* sample the 1st set had better EQE by absorbing light in the range of

500 nm-1050 nm wavelength, here the band gap seems to be similar for both sets. The spectral

response of the two set of sample showed decrease in light generated current per power with

decrease in thickness for 1st set of sample Fig. 35a and for 2

nd set the spectra response did not

change much (Fig.35b) except for ½ thick sample in 1st set and ½ thick* in 2

nd set sample as

shown in Fig.33.

5.2.3: External Quantum Efficiency of 3rd

set of samples.

In this section the EQE of two samples selenized for 4 min at 620 °C with the standard

thickness (in red) and ½ thick* (in black) were measured in similar ways as the aforementioned

sample sets.

Figure 36: EQE of 3rd

set of samples of different absorber thickness measured under 11 % of AM 1.5 global


Even though the J-V curve of those two samples in Fig.29, the standard one had very low jsc,

Voc, FF and efficiency, the EQE showed in Fig.36 unfold the standard sample had batter value

by absorbing longer wavelength in the range of 600 nm to 1200 nm than the ½ thick* sample.

The short time selenization for standard sample resulted in smaller band gap in which the EQE

ranges beyond 1200 nm in Fig.36. The main reason was that the gallium in the precursor was


48

not reacted with other components to form CIGS absorber due to selenium deficiency during

process.

5.2.4: External Quantum Efficiency of 4th

set of sample prepared after etching.

A single cell found in a sample prepared by etching the absorber in 0.025 V/V % of BM had its

EQE measured likewise as above analyzed samples. Its EQE curve had response as laid out in

Fig.37. This cell had larger absorption due to front contact and thereafter the spectra started

falling till the band-gap of CdS (2.4 eV); then drops which shows the higher wavelength light

was not absorbed by the CIGS. Despite the thickness of CIGS was enough for the cell

operation, the cell failed due to defects and residues after etching that made bad pn-junction

formed at the CdS/CIGS interface.

Figure 37: EQE of a cell prepared after etching CIGS absorber with 0.025 V/V % BM for 12 min measured

under 11 % of AM 1.5 global illumination over 300 nm -1200 nm wavelength.

5.3: CIGS Absorber Etching in Bromine-Methanol (BM) solutions

In this section the result of bromine –methanol (BM) etching of CIGS absorber are presented.

The etching rate of different concentrated solutions and the sample surface roughness before

and after etching were determined by using a Dektak Profilometer. In order to determine the

etching rate and the step height by BM etching different parts of the samples were scanned (6 to

10 measurements) Fig.38. Because the surfaces were too rough, the measured profiles were

plotted and a single line was drawn as a quasi-mean value to obtain the approximate height


49

etched in each measurement. The step between etched and unetched surface were not like step

function, rather there were groves in between as shown in each plots of Fig.38. Finally the

average and standard deviations were calculated for each samples etched for different duration

of time to plot etched thickness and surface roughness as shown in Fig.39-Fig.42.

Figure 38: Plot of measures surface profile of etched samples across the edge between the etched an

unetched part of the sample. To average out the etched height of each measurement a quasi-mean value is

drawn.

For roughness measurement the program of the Dektak Profilometer calculated the roughness

parameter (Ra) which is the arithmetic mean of the deviations in height from the profile mean

value Zavg given by equation 5.1. Measurements were performed with in selected ranges of

scanned length where there were no big spikes.


50

5.1 ZZN

1R

N

1i

avgia

Where

N

i

iavg ZN

Z1

1

The etched thickness of different samples etched in 1.0, 0.2, 0.13 and 0.025 % V/V of Br2 in

methanol will be discussed next.

5.3.1: 1.0 V/V % Bromine in Methanol (BM) Etching

Here two sets of samples were etched for the same time interval after one set was finished. The

CIGS layer was etched for duration of 5 to 40 seconds without rotating the sample during

etching in the BM solution; this could lead to a local saturation of the solution which might

explain the relatively small rate of etching after 15 seconds in Fig.39.

Figure 39: Etched thickness vs. etching time of CIGS absorber etched in 1.0 V/V % of BM which reviled

very fast etching capacity of the solution.

Unfortunately at 40 seconds it was difficult to measure the etched thickness and the values are

not included in the etched thickness verses time plots in Fig.39. In this concentration of the


51

solution, the dissolution of absorber was so vigorous that the measurement of these samples was

difficult which is indicated by the big error bars in Fig.39.


Etching of CIGS absorber in 0.2 V/V % of BM while rotating the sample with hand was found

to be more appropriate than etching in 1.0 V/V %. The etching rate increased nearly linearly in

the first 50 seconds, while the average roughness decreased Fig.40b. The reader should notice

that the etched thickness unit in µm while roughness is in nm. At 60 seconds the values of the

etched thickness decreased suddenly, whereas the roughness increased despite increase in

etching time. However, the values are reasonable since decrease in etching is manifested with

increase in roughness. This could be the concentration of etching solution was changed while

etching the samples for different time in different order, or the initial surface of the sample was

different. More or less in Fig.40 the etched thickness and the average roughness of the samples

increased and decreased respectively as expected. For this concentration almost all the absorber

layer was etched at 2 min and it was not possible to perform measurement while aligning level

in Dektak meter. In Fig.40b etching time zero second was surface roughness measured after

5 % HCl etching of grids, front contact and CdS layers.

Figure 40: Etched thickness vs etching time (a) and average roughness vs. etching time (b) of CIGS absorber

layer in 0.2 V/V % of BM solution.


52


In this experimental section, the 0.13V/V % solution was missed by adding 50 ml of methanol

to the 0.2 V/V % solution used in the former experimental result to explore further effect of

decreasing the concentration.

Figure 41: Etched thickness vs etching time (a) and average roughness vs. etching time (b) of CIGS absorber

etched in 0.13 V/V % of BM solution. This solution is prepared by adding 50 ml of methanol to 0.2 V/V %

BM solutions.

The etched thickness increased while time of etching increased, similarly the roughness

decreased with time, despite at 60 seconds the average roughness increased as shown in

(Fig.41 b).


Since the main point of wet chemical etching was to reveal the rightful BM solution

concentration and etching time to thin the standard thick absorber to ¾ and ½ of the standard,

BM solution of 0.025 V/V % etching was investigated. The main advantage of a very low

concentration was to had the opportunity of etch more samples for longer time while controlling

the etching conditions, in such a way that a few seconds did not affect the etched thickness too

much as seen for in higher concentrations. As depicted in Fig.42, the etching times were in

minutes and more samples were etched. In Fig.42a the thickness of the absorber remained on

top of the glass/Mo was measured for samples etched for more than 5 min. The remained

absorber thickness decreased while the etched thickness increased, and it matches with the

concept of etching. Theoretically, at the point where the two lines (black and red) meet, half of

standard thickness is etched. However, attaining this point was not easy since all standard


53

samples did not have equivalent thicknesses. Fig.42b shows that the surface roughness was

decreased strongly till 4 min then slightly increased from 4 to 6 min, and after this decreased

again.

Figure 42: The evolution of etched thickness vs etching time and CIGS left on glass/Mo vs etching time (a)

and average roughness vs. etching time (b) of CIGS absorber layer in 0.025 V/V % of BM solution.

From this 0.025 V/V % of BM concentration etching result, etching durations of 8 and

12 minutes were selected to prepare solar cell with thickness of ¾ thick and ½ thick. An

important thing that should be noticed at this particular point is that, to build cells by etching a

standard sample in 0.025 V/V %, a just selenized sample was taken. Whereas for the result

shown in the above sections, HCl etched samples were taken. For this reason it is not easy to

comment on if etching could be similar or not. After etching the standards for 8 min and 12 min

in 0.025 V/V % solution, CdS buffer and window layer were deposited as mentioned in

section 4.4 and 4.5 respectively. Additional results of sample etched in different concentration

of BM and prepared after etching were given in SEM surface characterization section.

5.4: Scanning Electron Microscopy (SEM) Surface Characterization of Etched

Samples

To investigate the evolution of the surface morphological due to etching, SEM micrographs of

the samples were taken after front contact and CdS were etched in 5 % HCl for absorber etched

in 0.2 and 0.025 V/V % of BM . The surface of the absorber after HCl etching have particles on


54

their surface as shown in Fig.4, despite repeated cleaning in deionized water and drying by

nitrogen air after HCl etching.

Figure 43: SEM micrograph of absorber layer after the front contact and CdS were etched in HCl. (a) low

magnification reviling how some part of the surface had particles of different sizes, (b) particle ranging up to

10 µm long on the surface at higher magnification.

The sizes of the particles range from 10 µm to 50 µm. In contrast to CIGS surface where the

buffer and window layers were etched by HCl (left part of Fig.44), the 20 second BM etched

CIGS surface show no residues (right part of Fig.44).

Figure 44: SEM micrograph of CIGS surface after front contact and CdS etched in 5 % HCl in right part

and 20 seconds etched in 0.2 V/V % of BM in the right part.


55

It is also important to note that the etched surface morphology did not change with etching

duration as the SEM micrograph displayed. A close look to the topography of the etched sample

revealed a preferential etching along the grain boundaries. This confirmed why the surface was

not smoothed with the progressive etching. For instance the sample etched in 0.2 V/V % BM

for 10 se (Fig.45a) and for 90 se (Fig.45b) had the same surface morphology. Similarly for

samples etched in 0.025 V/V % solutions for 8 min (Fig.45c), and 12 min (Fig.45d). Therefore,

the absence of the spectacular reduction of surface roughness during etching processes were due

to much faster etching in the valleys than peaks. Despite the fact that Dektak meter

measurement revealed relative decrease in roughness, from SEM micrographs the expected

further smoothing of the surface was not observed from Fig.45. This demonstrates that the

preparation of flat surfaces was impossible with BM etching procedure.

Figure 45: SEM micrograph absorber etched in 0.2 V/V %BM for 10 se (a) and 90 se (b). For samples etched

at the same concentration from 10 to 90 se there saw no change in surface morphology was observed. The

variations in color of the pictures are due to contrast. While (c) and (d) were for samples etched in

0.025 V/V %, for 8 min and 12 min respectively.


56

After 5 % HCl etching of ZnO/ZnO:Al/CdS, and BM etching the surface of etched absorber

had particles left on the surface even though the samples had been cleaned in deionized water

and CHCl3 respectively and finally samples were dried with nitrogen air jet. To further

investigate etching process of CIGS in BM solution associated with particles residues, the

etched samples were analyzed by SEM and Energy Dispersive X-ray Spectroscopy (EDS or

EDX) technique. This technique is widely used for the elemental analysis in material

characterization and surface compositions. HCl etched surface had particles as shown in Fig.43

before.

Figure 46: EDS spectra of particles investigated on samples surface after CdS was etched with HCl both (a)

and (b).

Figure 47: EDS spectrum of particles investigated after 0.025V/V%BM etching for 8 min (a) and 12min (b).


57

The EDS spectra of those particles showed strong reflection of carbon. This strong carbon in

spectrum also appeared on samples after 8 and 12 min etching in (Fig.47 a-b) of CIGS in BM

solution. The EDS spectrum of a particle found on 8 min etched sample (Fig.47a) exhibited the

presence of different elements like carbon, oxygen, chlorine and bromine as the chemical

composition. The possible source of carbon could be from the mask used, methanol or

trichloromethane during etching processes, while oxygen from surface oxidation. Furthermore

chlorine and bromine can traced back to HCl, CHCl3 and Br2 used during etching. Since Cl and

Br are chemically active elements in aqueous solution they had reacted with “pecien”, a black

wax used as mask, and reacted to form particle that appeared on the etched surface. For sample

etched in 0.2 V/V % BM for 90 se, some parts had different morphology (Fig.48a) unlike

shown in Fig.45b. The EDS spectrum taken from the whole picture in Fig.48a showed reflection

for Mo. This proved at 90 se some part of the sample was etched to the back contact.

Figure 48: SEM Micrograph of absorber etched for 90 seconds in 0.2 V/V % BM for which the CIGS

crystals are almost completely etched (a) and the EDS spectra of the whole part that confirmed the back

contact reached from Mo peak.

In general the etched surface had different compositions from the standard bulk composition

since the CIGS sample had different gallium content towards the front and the back contact. In

addition due to the exposition to air and etching with the formation of SeO and In and Ga oxides

(81) could change composition. The SEM cross-sections of the standard sample (Fig.49a),

etched in 0.025 V/V % of BM for 8 min (Fig.49 b) and 12 min (Fig.49c) were taken for

comparison. The measured thicknesses of the absorber layers were found to be 2.82 µm,

1.52 µm and 1.82 µm respectively. At this point sample etched for 8 min should have larger


58

thickness than the 12 min etched one. The possible reason behind this effect could be either the

original sample etched for 8 min was thinner than the 12 min etched or the side from which the

samples were taken could had effect since during sputtering and selenization process each

corner of the samples had relatively different flux of materials deposited. The SEM micrographs

in Fig.49 did not show the sharp interface for CIGS/CdS/i-ZnO/ZnO:Al because of bad

resolution of SEM, furthermore the thickness of CdS buffer layer and front contacts were too

thin.

Figure 49: SEM micrograph of CIGDS solar cell cross-section. (a). Standard sample, (b) and (c) etched in

0.025 V/V % of BM for 8min and 12 min respectively.

Summary

The detail work of this thesis aimed toward finding the effect of thinning of the absorber layer

of CIGS thin-film solar cell by investigating the loss in main parameters that characterize solar

cells like efficiency, fill-factor, open circuit voltage and short circuit current density.

For preparation of different thickness of CIGS absorbers layers were prepared from multilayers

of CuGa/In precursors by DC magnetron sputtering as described in experimental section and

selenized in HV for different time interval under selenium vapor while heating sample at

620 °C. The thin layer (50 nm) of CdS was also deposited by CBD and front contacts were

sputtered by RF sputtering. Finally the grids deposited to finish the sample and electrical

measurements were carried out. As stated in result section, the samples of 1st set to 4

th set were

characterized.


59

The 1st set of samples selenized for 8 min at 620 °C have jsc decreased, Voc increased, FF

increased and the efficiency relatively did not changed with decrease in thickness of absorbed.

This shows that by decreasing the standard thickness to half, the solar cell performance was not

deteriorated and it is indication that thinner absorber could be produced, while saving raw

materials, cost of production and time. In order to decreasing the absorber thickness and obtain

better devise performance, the experimental result depicter it is good idea to decrease the

number of layers of CuGa and In precursors, not the time of sputtering. The EQE measurements

showed absorption of higher wavelength light decreased as with decrease in absorber thickness

which is reasonable. Similarly for the 2nd

set of sample selenized for equal duration of time and

at equal temperature, some changes are observed. The main reason for change was the

difference in selenium rate during precursor‟s selenization process. In principle when selenium

vapors are incorporated in to the CuGa/In multilayers through time and heating the

thermodynamics of the materials change and polycrystalline CIGS well be formed. At this point

pressure and temperature and flux of selenium are the main parameters that plays role for

absorber formation, thus change in each will have dramatic effect on the formation of absorbed.

As a matter of fact this was what happened during the 2nd

set of process. Relative to the 1st set,

all 2nd

set of samples have thicker CIGS/CdS/ZnO/ZnO:Al layers measured by Dektak

profilometer. Samples ½ thick have got higher jsc, Voc, FF and efficiency, than the standard and

¾ thick samples. This is not what was seen in the 1st set of sample. The EQE of those samples

in 2nd

set did not changed too much unlike the 1st set of samples. From those two experimental

results the solar cell is working quite well, in spite of small changes.

For the case of 3rd

set of samples of different thickness that are selenized at 620 °C for 4 min,

literally samples with lower thickness were expected to form absorber that could work quite

well too. The reason is that for CIGS absorber growth there should be certain proportion of raw

materials. By decreasing the precursors and the time of selenium deposition by half CIGS

absorber of half of the thickness would be formed as long as temperature and pressure ate

maintains to the standard. All the samples except two of them delaminated from the back

contact soon CBD process started. As discussed in the experimental section, delamination of the

absorber occur when more selenium atoms are incorporated in the absorber and started to react

and form thick layer of MoSe2 in between CIGS/Mo interface. If this could be the case it should

happen to the 2nd

set of samples with half of standard selenized for longer time. Other

experiments run for 6 min selenization time also failed. At this point it is good idea to perform

further experiments and study process mechanism to identify the possible reasons. The result of


60

the two samples (standard and ½ thick*) in 3rd

set showed that the 1/2 thick* has better values

of four parameters (jsc, Voc, FF and efficiency) than the standard. This is promising result that

not only by decreasing the precursors but also by decreasing selenium, it is possible to produce

solar cell that is cost effective.

The final set of samples are prepared by etching the standard absorber after selenization in BM

of 0.025 V/V % for 8 min and 12 min to achieve ¾ thick and ½ thick samples respectively. The

etching mechanism of CIGS have been reported in different literatures as discussed in wet

chemical etching section of experiment and results of this thesis. In those references the

etchants were bromine in aqueous solution and also in HBr and KBr. The etching rates have

been determined in different ways, and the surface morphology evolved are different. In this

thesis, BM of different concentrations has been employed to find out appropriate etching

concentration and time of etching. Initially 1.0, 0.2, 0.13 V/V % were used to etch the absorber

of a finished samples at room temperature in which the grid/front contact/CdS were etched in

5% of HCl. Eventually etching at those concentration was fast (in less than a min the whole

absorber was etched) and found out to be challenging to control the etched thickness. Finally,

very lower concentration (0.025 V/V %) was found to be the best to control etching time. For

building the sample by etching this concentration was used. But the etching of finished sample

and selenized sample was quite different and thickness of the absorber lower than finished

samples were measured for sample etched after selenization. From the finished samples only a

single cell had diode curve, the rest of them did not work. The main reason was the inadequacy

of the pn-junction formation between CIGS and CdS. This could be the surface residues

induced. The surface investigations performed on samples etched in BM showed etching do not

smoothen the surface and the preferential etching at grain boundaries was observed.


61

Conclusion

This master thesis repost has given details techniques employed on the thinning of

Cu(In,Ga)Se2 absorber layer by different methods. Thin-film solar cell of Cu(In,Ga)Se2 of

different thickness of absorbers were successfully prepared on soda lime glass substrate by

sputtering Mo and multilayers of CuGa and In from their respective targets by DC magnetron

sputtering under Argon plasma. The CuGa/In multilayer precursors were selanized in selenium

vapor with deposition rate of 200 nm/min and 264 nm/min. The sample substrate temperature

was kept at 375 °C for 3 min during first phase and further for 4 min to 8 min at 620 °C to form

polycrystalline p-type CIGS absorber layer in HV chamber upon incorporation of selenium in

precursors during second phase. After salinization thin layer (50 nm) of n-type CdS was

deposited on the p-type absorber by chemical bath deposition process from (Thiourea

(SC(NH2)2), CdSO4, and NH4OH), in deionized water at 65 °C for process time of 11.5 min.

The front contact window (i-ZnO and ZnO:Al) is deposited by RF sputtering. Finally grids of

Mo/Cu evaporated. The electrically and optical characterize were performed on the samples.

The results depicted decrease in short circuit current density (jsc) and increase in open circuit

voltage (Voc) with decrease in thickness of absorber, while the fill-factor (FF) and efficiency (η)

did not change to much for 1st set of sample and all except jsc increased for the 2

nd set of

samples which has relatively thicker absorber. The EQE also decreased with decrease in

thickness of absorber which explains decrease in jsc. From samples selenized for 4 min during

second phase of heating substrate (3rd

set) delamination of absorber have been observed.

However, among samples that did not delaminated, the ½ thick* was found to have better

values of jsc, Voc, FF and efficiency relative to standard.

In general thinning absorber of CIGS showed promising result, that the loss in efficiency was

very slight, while jsc dropped with decrease in thickness. Decrease in the absorber thickness had

additional importance toward thin-film CIGS solar cell operation by providing higher Voc. In

addition, samples prepared under 4 min selenization and half of absorber thickness has proved

that high possibility of fabricating thinner absorbers that help in saving cost of production, and

time. Samples prepared after chemical etching did not work at all. Finally the surface of CIGs

investigated with SEM showed preferential etching of MB at grain boundaries and etched

surfaces were rough. Different etching concentrations investigated have their etching rate for the

CIGS material.

******************************** The End ************************************


62

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Declaration

I hereby declare that the thesis entitled “Thinning of Thin-film Cu(In,Ga)Se2 Solar Cell

Absorber Layer” has been carried out at Institute of Solid State Physics, Friedrich-Schiller-

University Jena, Germany under supervision of Dr. Michael Oertel. This work is original and

written without unauthorizised third-party support, incorporating only that literature cited in the

work. All citations/quotes from other works have been properly referenced.

Tesfaye T. Belete

29th

September 2014, Jena Germany

thinning of absorber layer of thin film cigs solar cell thesis -tesfaye belete

Education

film cuin

film solar cells

gase2 solar cells structure

photonics program

jena germany

university jena

cigs absorber layer

coevaporation of cuin