1. Applications of Photovoltaic Technologies

2. Summery of losses in Solar cell
Electrical
Optical
Recombination
Ohmic
Reflection
Shadowing
Non-absorbed radiation
SC material
Base
Emitter
Contact Material
Metal
Junction
Emitter region
material, surface
Base region
Space charge region

3. Summery of losses in Solar cell
Electrical
Optical
Recombination
Ohmic
Reflection
Shadowing
Not-absorbed radiation
SC material
Base
Emitter
Contact Material
Metal
Junction
Emitter region
material, surface
Base region
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
Emitter  N-type, thickness < 1 m, doping 1019 #/cm3, Ω/ □
N-type material have better surface quality, high doping
to reduce the emitter resistance, junction should be close
to the surface
Optical losses Typically ARC coating & surface texturing
about 80nm thick Si3N4 layer, Pyramid of about 4 to 5
m height
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
20%
15%
25%
High cell efficiencies are obtained in the laboratory using State-of-the-art technology in the lab produces
10%
Module cost (Rs / W) 
But commercially mass produced cell efficiency lies between 13 – 16%  research techniques used in the laboratory are not suitable for commercial production
Cost of electricity (Rs / kWh) 
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. Semiconductor Fundamentals, P-N Junction, Solar cell Physics, Solar cell design
Solar PV Technologies
Production of Si
Wafer based Si solar cells
Thin-film solar cells

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  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 % purity in large quantities.

11. Solar PV market
Worldwide production of Solar PV modules
Solar PV industry has recorded a growth of 30% in the last decade
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.

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 Shape Crystal type Circular
Pseudo square
Square
Crystal type
Single crystal Si wafers
Multi-crystal Si Wafers

15. Si Wafer Production High temp, Carbon HCl Separation and purification
Chlorosil-anes (g)
Separation and purification
Pure silanes (g)
Silica
MGS
(s)
top
tail
ingot
Single crystal growth
Wafer production
CVD, Solid silicon(s)

16. Solar cell – Silica to Si wafer
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
Casting
Surface 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 % 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
On chlorination of MG-Si
Si + 2Cl  SiCl4
The following reactions result in tri-chlorine-silane gas:
SiCl4 + HCl  SiHCl3
4 SiHCl3 SiH4+ 3 SiCl4+ 2 H2
SiF4+ NaAlH  SiH4 + NaAlF4
The following reactions result in silane gas

20. Poly Si- Siemens type reactor
Boiling point.: +32 C•
Waste gases
SiHCl3 +H2
Power supply
Quartz bell
Jar
Polysilicon deposition
Pure SiHCl3 in Gas Phase  Pure Si in Solid Phase
– Chemical Vapor Deposition (CVD) process
– Siemens type reactor
SiHCl3 + H2→Si + 3HCl
• Deposition process is slow
10 days/ton using 12 Siemens reactors
Generate by-products containing chlorine•
Wacker, Hemlock, Mitsubishi, Tokuyama, Sumitomo SiTiX, MEMC Italia

21. Poly-Si -Fluidized bed reactor (BFR)
SiH4
Granular Si
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
Continuous process
 considerably higher production rates and lower energy consumption
Yielding silicon of the highest purity

22. Wafer-manufactured process

23. Czochralski (CZ) process
Production of sc-Si
Czochralski (CZ) process
• 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)
Seed Holder
Seed
Crystal neck
Si melt
Shoulder
Thermal shield
Air + SiO2+Co

24. Czochralski (CZ) process

25. More expensive than Czochralski (Cz) material
Production of sc-Si
Float Zone (FZ)
Poly-crystal rod
RF heating coil
Molten zone
Single crystal Si
Rod of solid, highly purified but polycrystalline silicon is melted by induction heating
Single crystal is pulled from the molten zone.
This material is of exceptional purity because no crucible is needed
 Record efficiency solar cells have been manufactured with float zone
More expensive than Czochralski (Cz) material

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%
Diamond particles
Inner diameter sawing
Sawing of pseudo square wafer
wire sawing

27. Solar grade Si (SOG-Si)
Cost of electronic grade Si is $/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. Direction solidification
Production of mc-Si
Casting
Si melt
Heat exchanger
Direction solidification
Poly-Si
mc-Si ingot

29. Dicing of mc-Si
mc-Si Ingot
Dicing
Wire sawing
mc-Si wafer

30. Production of mc-Si Si ribbons Wafer thickenss < 250 m
Reducing material consumption by:
Producing thinner wafers
Reducing kerf loss
Si ribbons
EFG growth
Methods of producing Si ribbons
The edge defined film fed growth
process (EFG)
Ribbon growth on substrate (RGS)
Silicon sheets from powder (SSP)
SSP growth
Si sheet from powder
Wafer thickenss < 250 m
Very low kerf loss
Efficiency over 14%

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

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 Cell reported in 1941, Grown junction,
Efficiency much less than one percent
Grown Jn
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
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
Light
light
finger
“inverted” pyramids
rear contact
oxide
p- Si p+ n+ n
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

38. Features of High Efficiency Solar Cell
Solar cell efficiencies increased with technological development
Route to high efficiency solar cells
Low recombination
High carrier absorption
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;

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
Standard process
Wet Acidic Isotropic texturing
POCl3 Diffusion
Parasitic Junction Removal
PECVD SiNx:H ARC layer
Screen Printed Metallisation
Dry processes, plasma etching are used for front texturing, emitter formation, are being employed to avoide wet chemical disposal and to avoid the large use of DI water.
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
Co-firing
Solar cell performance: %

41. Choice of staring wafer
Starting wafer:
400 m thick,
area 10 X 10 cm2, or 12.5 X 12.5 cm2.
P-type doped with boron concentration of cm-3 p -
type (base)
high doping to reduce minority carriers concentration, &
low doping to increase minority carrier life time
Doping ~ 1016 #/cm3
lower the minority concentration lower forward bias diffusion current and higher is Voc,
lower the doping  higher minority carrier lifetime  higher is Voc,

42. Junction formation Saw damage removal
Starting 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)
Junction formation
Junction formation by heating the wafer at oC in phosphorous (n-type) atmosphere
 diffusion using gaseous source
diffusion using spin-on dopants
N-type (emitter) p -
type (base)

43. Si + HNO3 + 6HF → H2SiF6 + HNO2 + H2O + H2
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 X N2 O2
Dopant gas
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

45. Antireflection Coating Deposition
Edge isolation
The edge of the cells are removed by either laser cutting or plasma etching. highly reactive plasma gas (CF4+O2) is used
N-type (emitter) p -
type (base)
Antireflection coating deposition
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 , (Si =3.42) p -
type (base)
Refractive index of Si
Other options
TiO2 (70nm)
 TiO2 (70nm) + MgF2 (110 nm),

46. Commonly used gases for CVD

47. Solar cell fabrication: Screen-printing
emitter contact (Ag)
base contact (Al) p -
type (base)
Metallisation
Screen printing of front and back contact,
paste of silver and aluminium is used to print the contact. 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)