Applications of Photovoltaic Technologies

2

Summery of losses in Solar cell

Losses

ElectricalOptical

RecombinationOhmic

• Reflection

• Shadowing

• Non-absorbed radiation

• SC material

Base

Emitter

• Contact Material

Metal

Junction

• Emitter region

material, surface

• Base region

material, surface

• Space charge region

3

Summery of losses in Solar cell

Loss

ElectricalOptical

RecombinationOhmic

• Reflection

• Shadowing

• Not-absorbed radiation

• SC material

Base

Emitter

• Contact Material

Metal

Junction

• Emitter region

material, surface

• Base region

material, surface

• Space charge region

4

Typical cell parameters

• Substrate Typically Si, it can be different forms of Si, GaAs,

CdTe,• Si is abundance in nature, non-toxic, dominate the

micro-electronics industry but have low absorption

coefficient• Cell thickness Typically 200 to 500 m

• Thinner cell are useful but difficult to handle, surface

passivation becomes important

• Doping of base Typically 1 Ω-cm, P-type

• Higher doping reduces resistive losses but carrier

lifetime also decrease, reducing Voc

5

Typical cell parameters

• Optical losses Typically ARC coating & surface texturing

• about 80nm thick Si3N4 layer, Pyramid of about 4 to 5

m height

• Emitter N-type, thickness < 1 m, doping 1019 #/cm3, 50-100 Ω/ □

• N-type material have better surface quality, high doping

to reduce the emitter resistance, junction should be close

to the surface

• Grid pattern 20 to 200 m wide fingers, 2 to 5 mm apart

• Resistivity of Si is high, metal contact at the front and

rear side is required to collect the current

6

Design tradeoffs: Efficiency Vs cost

0

40

80

120

160

200

0 10 20 30 40 50 60

Cost of electricity (Rs / kWh)

Mod

ule

cost

(R

s / W

)

10%

15%20%25% • High cell efficiencies are obtained in the laboratory using State-of-the-art technology in the lab produces

•But commercially mass produced cell efficiency lies between 13 – 16% research techniques used in the laboratory are not suitable for commercial production

•With higher efficiency modules, the cost per unit area can be much higher for a given cost target of electricity in kWh. (With high efficiency module additional costs, land, material are less.)

7

Solar PV Technologies

• Wafer based Si solar cells

• Thin-film solar cells

• Production of Si

Semiconductor Fundamentals, P-N Junction, Solar cell Physics, Solar cell design

8

Contents- Production of Si

•Solar PV Chain

•Why Si for PV?

•Demand for Si feedstock

•Si wafer production process

• EG poly-Si (Siemens type, FBR)

• CZ & FZ process of ingot production

• wafer dicing

•Si feedstock from various sources

•Multi-crystalline Si wafers and ribbon Si

9

Si for PV

• Solar energy (PV) is a very fast growing market where the basic technology depends on availability of pure Si. This material is today in high demand and a shortage is expected.

• Most analysts assume that silicon will remain the dominant PV material for at least a decade.

•One of Shell’s energy scenario indicates that solar energy will be the single largest energy source within 2060. Solar PV would play important role in it

10

Why Silicon?

•At the time being it is almost the only material used for solar cell mass production

•Easily found in nature, Silicon oxide forms 1/3 of the Earth's crust

• It is non-poisonous, environment friendly, its waste does not represent any problems

• It is fairly easy formed into mono-crystalline form

• Its electrical properties with endurance of 125°C

• Si is produced with 99.9999999% purity in large quantities.

11

Solar PV market

•Crossing the GW-level:

•Last year alone worldwide solar cell production reached 1,256 MW (in 2004),

• 67 percent increase over the 750 MW output in 2003.

Worldwide production of Solar PV modules

• Solar PV industry has recorded a growth of 30% in the last decade

12

Contribution of Si in PV market

•Others include CdTe, CIGS, C-Si/a-Si (4.5%)

•Over 90% of solar cell are made of Si

13

Companies producing Si

Si Wafer Manufacturers• Hemlock (USA)• SEH, SUMCO•Wacker Chemie (Germany)•Tokuyama Soda (Japan)•ASiMi (USA)•MEMC Electronic Material Inc., (USA)

•Dedicated manufacturers for PV (wafers and cells)

•Kyocera (Japan),

•BP Solar (USA),

•Shell Solar (USA),

•Photowatt (France).

•RWE Schott (USA/Germany)

14

Wafers for solar cells

Crystal type

•Single crystal Si wafers

•Multi-crystal Si Wafers

Shape

• Circular

• Pseudo square

• Square

15

Si Wafer ProductionHigh temp, Carbon

Silica

MGS

(s)

HCl

Chlorosil-anes (g)

• Separation and purification

Pure silanes (g)

CVD, Solid silicon(s)

Wafer production

top

tail

ingo

t

Single crystal growth

16

Solar cell – Silica to Si wafer

•Silica

•Metallic Silica

•Refining

•Tricholoro Silicane•Deposition

•Poly Si

Addition of B or P

•Single crystalline Si

•Single crystalline

Si Wafer

•Melting

•Slicing

• Multi-crystalline Si

• Multi-crystalline Si wafer

17

Solar PV Chain

• There are several steps from raw material to power systems

MG-Si

Silica

Purification CastingSurface treatment

Cell assembly

18

Metallurgical grade (MG) Si

• MG-Si is material with 98-99% purity,

• Produced in about 1 Million tons per year

• Produced in countries which cheap electricity and quartz deposits (USA, Europe, Brazil, Australia, Norway)

• Average price is 2 to 4 $/kg

• MG-Si is produced by reduction of SiO2 with C in arc furnace at 1800 oC.

SiO2 + C Si + CO2

• Application in producing chlorosilane for electronic grade Si production, production of Al and Steel

• Typical impurities are iron (Fe), aluminium (Al), calcium (Ca) and magnesium (Mg)

19

Electronic grade (EG-Si)

• Electronic grade (EG-Si), 1 ppb Impurities (i.e. 99.99999999% purities)• MG-Si EG-Si: impurities reduction by five order of magnitude is required convert MG-Si to gaseous chlorosilanes or silane, purified by distillation• For instance Trichlorosilane SiHCl3 and silane SiH4

4 SiHCl3 SiH4+ 3 SiCl4+ 2 H2

SiF4+ NaAlH SiH4 + NaAlF4

The following reactions result in silane gas

• On chlorination of MG-Si

• Si + 2Cl SiCl4

• The following reactions result in tri-chlorine-silane gas:

• SiCl4 + HCl SiHCl3

20

Poly Si- Siemens type reactor

• Deposition process is slow• 10 days/ton using 12 Siemens reactors

-Pure SiHCl3 in Gas Phase Pure Si in Solid Phase

– Chemical Vapor Deposition (CVD) process

– Siemens type reactor

SiHCl3 + H2→Si + 3HCl

•Generate by-products containing chlorine•

•Wacker, Hemlock, Mitsubishi, Tokuyama, Sumitomo SiTiX, MEMC Italia

•Boiling point.: +32 C•

•Waste gases

SiHCl3 +H2

•Power supply

•Quartz bell

•Jar

•Polysilicon

deposition

21

Poly-Si -Fluidized bed reactor (BFR)

•Continuous process

considerably higher production rates and lower energy consumption

•Yielding silicon of the highest purity

• Silicon seed particles are held in suspension by a gas mixture (H2 and SiH4)

• At 600°C gas phase decomposition takes place, causing the seed particles to grow up to 2 mm in size

• Big particles falls due to weight

• Si is collected from the bottom of the jar•SiH4

•Granular Si

22

Wafer-manufactured process

23

Production of sc-Si

•Seed Holder

•Seed

•Crystal neck

• Si melt

•Shoulder

•Thermal shield

•Air + SiO2+Co

•Air + SiO2+Co

• Poly-EGS is melted in a

quartz crucible (SiO2)

• Seed particle introduced to

begin crystallization

• Seed pulled to generate

desired wafer diameter

• Ingot is cooled

• Crucible is discarded

(warping and cracking)

•Czochralski (CZ) process

24

Czochralski (CZ) process

25

Production of sc-Si

•Rod of solid, highly purified but polycrystalline silicon is melted by induction heating

• Single crystal is pulled from the molten zone.

Poly-crystal rod

RF heating coil

Molten zone

Single crystal

Si

•More expensive than Czochralski (Cz) material

• This material is of exceptional purity because no crucible is needed

Record efficiency solar cells have been manufactured with float zone

•Float Zone (FZ)

26

Wafer dicing• Inner diameter (ID) saw where diamond particles are imbedded around a hole in the saw blade Si is hard material

• Almost 50% of the material is lost with ID sawing

• Using wire sawing thinner wafers can be produced and sawing losses are reduced by about 30%

Inner diameter sawing

wire sawing

Diamond particles

Sawing of pseudo square wafer

27

Solar grade Si (SOG-Si)

• Cost of electronic grade Si is 30-45 $/kg too high for solar cells (area related)

•Production with process modifications with relaxed specification allowing the silicon materials industry to produce at lower cost while meeting the requirements

•Earlier approaches in 1980 did not work did not work

Low production volume, insufficient purification

Present efforts to produce solar grade Si• by purifying metallurgical-grade (MG) silicon • Modifying Seimens reactor process and fluidized bed reactor process• REC +ASiMi produced 2000 tons of Solar grade Si in 2003

28

Production of mc-Si

mc-Si ingot

Si melt

Heat exchanger

Direction solidificationPoly-Si

•Casting

29

Dicing of mc-Si

Wire sawing

mc-Si Ingot

mc-Si wafer

Dicing

30

Production of mc-Si

Reducing material consumption by:

1. Producing thinner wafers

2. Reducing kerf loss

• Wafer thickenss < 250 m

• Very low kerf loss

• Efficiency over 14%

EFG growthMethods of producing Si ribbons

1. The edge defined film fed growth

process (EFG)

1. Ribbon growth on substrate (RGS)

2. Silicon sheets from powder (SSP)

SSP growth

Si sheet from powder

•Si ribbons

31

What is the best material for PV?

• According to solid state physics Si in not the best material

• 90% absorption of spectrum requires 100 µm of Si while only 1 µm of GaAs Si indirect bandgap material

• Larger thickness also demand for higher quality material, generated carrier needs to diffuse longer

• Diffusion length should be double of wafer thickness, at least 200 µm

• Si still is material of choice due to well developed micro-electronics industry

32

Optimum efficiency vs bandgap

Effi

cie

ncy

33

Ideal solar cell material

• Bandgap between 1.1 to 1.7 ev

•Direct band structure

•Consisting of readily available, non-toxic material

• Easily reproducible deposition techniques, suitable for large area production

• Good PV conversion efficiency

• Long-term stability

34

Early Si solar cells

Grown Jn

•Cell reported in 1941, •Grown junction, •Efficiency much less than one percent

•Cell reported in 1952, •Implanted junction •Efficiency about one percent

•Cell reported in 1954, Bell Labs•High temperature diffused junction •Single crystal, CZ method

• 6% cell efficiency

35

Early Si solar cells

• In 1960s solar cell were used only for space craft applications

• Cell design as shown here

• cell efficiencies up to 15%

• In 1970 cell design was changed (COMSAT labs)• Thinner emitter and closed spaced metal fingers (improved blue response)• Back surface field• so called “violet cell” due to lower wavelength reflection

•Further improvement in cell efficiencies have been obtained due to anisotropic texturing

These approached improved the current collection ability of solar cells

36

High efficiency solar cells• In 1980s it was clear that cell surface Passivation is key to obtain high open circuit voltage• Passivated emitter solar cell (PESC) exceeded 20% efficiency in 1985• Passivation was obtained by thin thermally grown oxide layer• use of photolithography to have small contact area and high aspect ratio

• Buried contact solar cells

• New feature incorporating laser grooving and electroplating of metal to avoid photolithography

• Oxide layer is also used as a mask for diffusion in groves and metallization

• High metal aspect ratio

37

High efficiency solar cells

finger “inverted” pyramids

rear contact oxide

oxide

p- Sip+p

+p+

n+ n

light

•Rear point contact solar cell demonstrated 22% efficiency in 1988

• Both contacts are made at rear surface no shadowing due to metal contact

• design is feasible only when high quality of Si is used

• mostly used under concentrated sunlight

• Highest efficiency Si cell structure reported till now (24.7%)

• PESC with both front and rear side Passivation

• Local diffusion at rear side to make low resistance contact

Light

38

Features of High Efficiency Solar Cell

Techniques for highest possible efficiencies:

• lightly phosphorus diffused emitters, to minimize recombination losses and avoid the existence of a "dead layer" at the cell surface;

• closely spaced metal lines, to minimize emitter lateral resistive power losses;

• very fine metal lines, typically less than 20 µm wide, to minimize shading losses;

• Route to high efficiency solar cells

Low recombination

High carrier absorption

Solar cell efficiencies increased with technological development

39

Features of High Efficiency Solar Cell

Techniques for highest possible efficiencies:

• top metal grid patterning via photolithography;

• low metal contact areas and heavy doping beneath the metal contact to minimize recombination;

• use of elaborate metallization, such as titanium/palladium/silver, that give very low contact resistances;

• good rear surface passivation, to reduce recombination;

• use of anti-reflection coatings, which can reduce surface reflection from 30% to well below 10%.

40

Generic industrial mc-Si Cell Process

Wafer Cutting

Wet Acidic Isotropic texturing

POCl3 Diffusion

Parasitic Junction Removal

PECVD SiNx:H ARC layer

Co-firing

Screen Printed Metallisation

Standard process

Process simplifications

• Mono-Si block-cast mc-Si wafers

Si ribbons to avoid kerf losses

• Double layer ARC single layer ARC

• Photolithographic finger patterns screen printing

Solar cell performance: 12 - 16%

41

Choice of staring wafer

p-type (base)

Starting wafer:• 400 m thick, • area 10 X 10 cm2, or 12.5 X 12.5 cm2.

• P-type doped with boron concentration of 1016 -

1017 cm-3

• high doping to reduce minority carriers concentration, &

• low doping to increase minority carrier life time

• lower the minority concentration lower forward bias diffusion current and higher is Voc,

• lower the doping higher minority carrier lifetime higher is Voc,

Doping ~ 1016 #/cm3

42

Junction formation

Junction formation

• Junction formation by heating the wafer at 800-1000oC in phosphorous (n-type) atmosphere

diffusion using gaseous source

diffusion using spin-on dopants

N-type (emitter)

p-type (base)

Saw damage removalStarting wafer surface is damaged due to wafer sawing damage, To remove surface damage a strong alkaline solution is used surface also gets textured

p-type (base)

43

Etching & texturing

• Wet chemical etching & Dry chemical etching

• Isotropic etching & an-isotropic etching

• Applications

Saw damage removal

for reducing reflection

•For semiconductor materials, wet chemical etching usually proceeds by oxidation

(oxidant, usually HNO3), accompanied by dissolution (etchant usually HF) of the oxide.

isotropic etching or anisotropic depending on the concentration

Si + HNO3 + 6HF → H2SiF6 + HNO2 + H2O + H2

• Anisotropic etching in alkaline solution, KOH or NaOH at 70 to 80oC

44

Phosphorous diffusion

• Phosphine (PH3) or POCl3 is used for diffusion n-type layer

• Dopant gas, PH3, reacts with O2, and formed P2O5

• Si with oxygen formes SiO2

• Phosho-Silicate glass is formed at the surface which acts as a source of Phosphorous a “dead layer” may form, high diffusion coefficient at high concentration

• Two step diffusion to avoid formation of dead layer, step-1: Predeposition and step-2: drive in

True Guassian profile is obtained in this way

XX X

N2 O2Dopant

gas

45

Antireflection Coating Deposition

The edge of the cells are removed by either laser cutting or plasma etching. highly reactive plasma gas (CF4+O2) is used

Edge isolation

N-type (emitter)

p-type (base)

•SiNx is deposited as an anti-reflection coating of layer thickness about 80 nm (done by PECVD)•SiNx also passives the emitter surface revolutionary process• refractive index 1.8 -2.0, (Si =3.42)

Antireflection coating deposition

•Other options

• TiO2 (70nm)

• TiO2 (70nm) + MgF2 (110 nm),

201 *nnn

p-type (base)

46

Commonly used gases for CVD

L32-

47

Solar cell fabrication: Screen-printing

Metallisation

•Screen printing of front and back contact, •paste of silver and aluminium is used to print the contact.

emitter contact (Ag)

base contact (Al)

p-type (base)p-type (base)

Firing of contacts

• Annealing of contacts at high temperature for making metal contact with semiconductor

• cells are placed in a furnace with higher temperature(~700 oC), metal diffuse through to make contact with the silicon

emitter contact (Ag)

base contact (Al)

p-type (base)