new materials for solar cells applications

New Materials for solar cells applications

Prof. Theodore Ganetsos –

Dr. Kyriaki Kiskira

University of West Attica

Funded by the Horizon 2020 Framework Programme of the European Union under Grant Agreement n. 837854

Energy transition

Learning outcomes

▪ Recall the history of Solar Cells

▪ Identify the importance of Solar Energy

▪ Describe Solar cells technology

▪ Recall the operation of solar cells

▪ Describe the Production of solar cells

▪ List thin films solar cells

▪ Describe the polymer solar cells

▪ Define Methodology and Importance of materials characterization

▪ List the Characterization techniques

▪ Describe the optical measurements

▪ Identify materials properties and characterization

▪ Define implement Solar Energy Spectrum and the Necessity of Band Gap Tuning

▪ Recognize the relationship of the profession of Industrial Design and Production Engineering and the renewable resources of energy

and their interdependence.

▪ Ability to apply that knowledge in his/her business life.


Crystalline silicon solar cells

A holistic and Scalable Solution for research, innovation and

Education in Energy Transition


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Table of Contents• Silicon solar cells: How do silicon solar cells work, crystalline silicon solar cells,

mono- and multi-crystalline solar cells.

• Thin-film solar cells: a potential solution to the significant problem associated with

silicon solar cells: namely energy payback time, knowledge on the use of thin-film

solar cells.

• Thin-film technologies: Thin film solar cell production, working principle of CIGS

solar cells, production of CIGS solar cells.

4(Source: Vivint Solar Developer, 2020)



Silicon solar cells

5



• Invented in the 1950s.

• The first of commercially available solar cell technology.

• Still dominate the market.

• Used in the early space applications which really carried the developments of them.

Silicon solar cells

6



• Why do we use silicon solar cells?

• Silicon is the most abundant semiconductor material on Earth.

• It's the second most abundant material in the earth's crust after oxygen.

• The bandgap of silicon as a semiconductor is quite appropriate for portable tanks for

solar cells.

• A big overlap between the electronics industries.

• Use of silicon in transistors and integrated circuits.

How do silicon solar cells work

7



• Photocurrent generation at different depths of the cell.

• Absorption in the space charge region (photon 1).

• Photon 1 is absorbed within the space charge region.

• The field prevailing in the space charge region separates the

generated electron-hole pair and drives the two charge

carriers in different directions.

• The hole must travel a relatively long way through the base

to the plus contact, however, as it is in the p-region during

this movement, the probability of a recombination is small.

• Therefore, almost all generated electron-hole pairs

generated in the space charge region can be used for the

photocurrent.

(Source: Introduction to Solar Energy, Offered by Technical University of Denmark (DTU))

Cross section of a crystalline silicon solar cell. The

four different electron-hole pairs indicated have

different probabilities of contributing to the

photocurrent.


8



• Absorption within the diffusion length (photon 2).

• Photon 2 is absorbed deep in the solar cell, but within the

diffusion length for the electron.

• The generated electron is not situated in an electrical field

but diffuses somewhat randomly throughout the crystal.

• If, by chance, it arrives at the edge of the space charge

region it is drawn to the n-side by the prevailing field where

it can flow to the contact.

• As the electron was generated within the diffusion length,

the probability that it reaches the space charge region is

relatively high.





photocurrent.


9



• Absorption in the emitter (photon 3).

• Photon 3 is absorbed in the highly doped emitter.

• Because of the high degree of doping, the diffusion

length is extremely small.

• Therefore, the probability for the generated hole to

recombine before reaching the space charge region is

high.

• The highly doped upper edge of the emitter is

occasionally referred to as the dead layer in order to

emphasize that this is where the highest recombination

probability is situated.





photocurrent.


10



• Absorption outside the diffusion length (photon 4).

• Photon 4 is absorbed helplessly deep inside the lower region of

the solar cell.

• Although the electron diffuses through the p-base, it

recombines with a hole before it can reach the space charge

region.

• Thus, although an electron-hole pair is formed due to light

absorption, no contribution to the photocurrent is made.

• The diffusion length is depending on the crystal quality and

therefore highly pure silicon crystals are needed for solar cells,

so that absorbed infrared light rays can be used deep in the cell.





photocurrent.


11



• Absorption outside the diffusion length (photon 4).

• Photon 4 is absorbed helplessly deep inside the lower region of

the solar cell.

• Although the electron diffuses through the p-base, it

recombines with a hole before it can reach the space charge

region.

• Thus, although an electron-hole pair is formed due to light

absorption, no contribution to the photocurrent is made.

• The diffusion length is depending on the crystal quality and

therefore highly pure silicon crystals are needed for solar cells,

so that absorbed infrared light rays can be used deep in the cell.





photocurrent.

Thin film solar cells

12



• Thin film solar cells are made by depositing one or more thin layers, or thin films of

photovoltaic material on a substrate, such as glass, plastic or metal.

• In contrast to silicon solar cells, thin film solar cells use direct bandgap materials,

allowing for thinner absorbing layers.

• Film thickness varies from a few nanometers to tens of micrometers, much thinner

than crystalline silicon solar cells which uses wafers with thicknesses of 200 µm.

• This thinness allows thin film solar cells to be flexible, and lower in weight.



13



(Source: Wikipedia)

Rigid CdTe panels mounted on a supporting structure

CIGS solar cell on a flexible plastic backing

Flexible amorphous silicon cells being installed onto a roof


14



• Copper indium gallium selenide (CIGS) is one of three mainstream thin-film PVtechnologies, the other two being cadmium telluride (CdTe) and amorphous silicon.

• With all three materials, the absorbing layers are thin enough to be flexible, allowingthem to be deposited on flexible substrates.

• However, all three technologies normally use high-temperature deposition techniquesand therefore the best performances comes from cells deposited on glass.

Efficiency of thin film solar cells

15



• The efficiency of thin film solar cells aregenerally less than conventional solar cells,especially for commercial solutions.

• The world records for amorphous silicon,cadmium telluride, and CIGS are 14%, 22.1%,and 22.6% respectively.

• This places both cadmium telluride and CIGSabove the experimental efficiencies of multi-crystalline silicon solar cells.

• The market share of thin film technologiesmarket-share has never reached more than20% in the last two decades and has beendeclining in recent years to about 9% ofworldwide photovoltaic installations in 2013.

• Despite the competition from conventionalsilicon solar cells, the thin film technologieshold many unique promises, allowing solarcells to be produced with much lower energypayback times, and with much lowermaterial consumption.

(Source: Wikipedia)

Reported timeline of solar cell energy conversion efficiencies since 1976 compiled by the National Renewable Energy Laboratory. Thin film technologies are represented in green.

Thin film technologies

16



• Optimum materials• Semiconductors• A specific banker• Easy to produce

• Very important→ The magnitude of the energy gap in thesemiconductors.

• If the gap is very small → a high anti photon, most of itsenergy for heating up the cell.

• Optimum magnitude for the energy cap.

• Of course, it also depends on the source, which is the sun.

• There is maximum eV.

• These two factors in agreement.

• Most the cells have an energy cap between 1.7 and 1 eV.

(Source: Wikipedia)


17



• Optimum materials

• Silicon: 1.1 eV

• CdTe, GaAs: 1.4 eV

• CIGS: 1.1 – 1.7 eV

(Source: Wikipedia)


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• Cadmium Telluride (CdTe)

• Copper indium gallium selenide (CIGS)

• Amorphous silicon

(Source: Wikipedia)




Cadmium Telluride (CdTe)

19



• Developed 30 years ago

• 22% efficiency

• Simple to produce→ cadmium telluride liked to be formeda second pound, which is just CdTe → simple to grow.

• Drawbacks

• Cadmium is toxic and scarce

• Telluride is rare

• No chance that cadmium telluride ever can play animportant role for the complete energy harvest from thesun for the Earth.

(Source: Wikipedia)


CIGS

20



• Copper indium gallium selenide (CIGS)

• Developed in the beginning of the 70s → 14% efficiency

• But gradually, it developed so that we now have an efficiencyabout 22%.

• Commercially available

• But the cell → quite complicated.

• In the bottom, you have a molybdenum electrode on sulfurlime glass, then you have the absorber.

• Absorber is a mixture of copper, indium, gallium, andselenide.

• On top of that → a buffer layer, which is cadmium sulfide.

• Then you have optical layers → typically zinc oxide andAluminum-doped Zinc Oxide (AZO)

• Drawback → Indium is rare

• We'll not be able to cover the world’s energy needs with CIGSsolar cells.

(Source: Wikipedia)

Structure of a CIGS solar cell.

Amorphous silicon

21



• Use of amorphous silicon is that in the amorphousstate, silicon a direct band gap material and has reallyhigher absorption coefficient.

• Really thin material and dark, almost black.

• Compared to a more traditional silicon solar cell,where the material is not as black

• The main drawback of amorphous silicon → efficiencyis not that high → around 10%

• Also some stability issues.

• Less enthusiastic about amorphous silicon.

(Source: Wikipedia)



22



• Sum up

• Three main categories of thin film solar cells that are worth talking about from sort ofa commercial aspect

• Cadmium telluride

• Copper indium gallium selenide

• Amorphous silicon

• And these technologies have their advantages and disadvantages

• Amorphous silicon typically not that stable and lacks quite a lot on efficiency

• Cadmium telluride and CIGS→ high efficiency, at least for laboratory efficiencies

• However, both share the same problem → have elements that are rare

• Problematic → to scale the technology

Thin film solar cells production

23



• Thin-film modules involves depositing thin layers of semiconductor material on asurface made of glass, metal, or plastic whereby only about a fraction of thesemiconductor material used in silicon wafers is required along a significantly lowerenergy need.

• A clear advantage of this production process is that cells do not have to be madeindividually, rather, they connected as an intrinsic part of the layer structure.

• First the molybdenum back contact is deposited, commonly by sputtering. In additionto serving as a contact the molybdenum layer reflects most unabsorbed light back intothe CIGS absorber.

• After deposition, the molybdenum layer is scribed by a laser to define the cell area.


24




Production steps of a CIGS solar cell.


25



• Following the molybdenum deposition, the p-type CIGS absorber layer is grown by oneof several unique methods.

• A thin n-type buffer layer (typically CdS) is deposited via chemical bath deposition ontop of the absorber.

• Then the buffer layer and the absorber layer is patterned before the buffer is overlaidwith a thin, intrinsic zinc oxide (ZnO) layer capped by a thicker, aluminum doped ZnOlayer.

• The intrinsic ZnO layer is used to protect the buffer layer and the absorber layer fromsputtering damage while depositing the alluminium doped ZnO window layer, which isusually deposited by DC sputtering (a damaging process).

• Finally, the front contact is patterned and the entire stack is encapsulated.


26




Production steps of a CIGS solar cell.

Absorber layer

27



• There are two main approaches to creating the absorber (CIGS) layer: one-stepproduction or two-step production.

• In the single-step process, all four materials (Cu, In, Ga, and Se) are deposited at once.

• In the two-step process the Copper, Indium, and Gallium are first deposited in a rough form(not the final desirable crystal structure).

• Then Selenium is added in a selenization step where the deposited layer is recrystallized to thefinal desirable form.

• Co-evaporation, or co-deposition, is the most prevalent one step CIGS fabrication technique.

• Disadvantages of evaporation include uniformity issues over large areas and the relateddifficulty of co-evaporating elements in an inline system.

• The high growth temperatures increase the thermal budget and costs.

• Additionally, co-evaporation is plagued by low material utilization (deposition on chamber wallsinstead of the substrate) and expensive vacuum equipment.

Absorber layer

28



• The idea behind the two-step process is that the first step could be done cheaply,avoiding the costlier single-step process.

• This first step is called the precursor step.

• Nanoparticle printing is one method to achieve the first step.

• The second step is the selenization steps where selenium is added and the layer isrecrystallized to the final form.

• The advantage of the two-step process is that it is much simpler and does not require ahigh degree of control.

• However, films obtained through a two-step process is generally inferior to thatobtained with the co-evaporation technique.


29



• No need for wafers

• Fabricated directly on a substrate

• Lower energy payback times

(Source: Wikipedia)





Crystalline silicon solar cells

A holistic and Scalable Solution for research, innovation and

Education in Energy Transition



new materials for solar cells applications

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