Applications of Photovoltaic
Technologies
Summery of losses in Solar cell
Losses
Electrical
Optical
Ohmic
• Reflection
• Shadowing
• Non-absorbed
radiation
• SC material
Base
Emitter
• Contact Material
Metal
Recombination
• Emitter region
material, surface
• Base region
material, surface
• Space charge region
Junction
2
Summery of losses in Solar cell
Loss
Electrical
Optical
Ohmic
• Reflection
• Shadowing
• Not-absorbed
radiation
• SC material
Base
Emitter
• Contact Material
Metal
Recombination
• Emitter region
material, surface
• Base region
material, surface
• Space charge region
Junction
3
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
4
Typical cell parameters
• 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
• 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
5
Module cost (Rs / W) 
Design tradeoffs: Efficiency Vs cost
25% 20% 15%
200
• High cell efficiencies are
obtained in the laboratory
using State-of-the-art
technology in the lab
produces
160
10%
120
80
40
0
0
10
20
30
40
50
Cost of electricity (Rs / kWh) 
60
•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.)
6
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
7
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
8
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
9
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.
10
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.
11
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
12
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)
13
Wafers for solar cells
Crystal type
Shape
•Single crystal Si wafers
• Circular
•Multi-crystal Si Wafers
• Pseudo square
• Square
14
Si Wafer Production
High temp,
Carbon
Silica
HCl
MGS
(s)
• Separation and
purification
Chlorosilanes (g)
Pure
silanes
(g)
ingot
top
Wafer
productio
n
tail
Single
crystal
growth
CVD,
Solid
silicon(s)
15
Solar cell – Silica to Si wafer
•Silica
•Metallic Silica
•Refining
•Tricholoro Silicane
•Deposition
• Multicrystalline
Si
•Poly Si
Addition of B or P
•Single crystalline Si
•Melting
•Slicing
• Multicrystalline
Si wafer
•Single crystalline
Si Wafer
16
Solar PV Chain
• There are several steps from raw material to power systems
Silica
MG-Si
Purification
Casting
Surface
treatment
Cell assembly
17
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)
18
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
•
On chlorination of MG-Si
• Si + 2Cl  SiCl4
• The following reactions result in tri-chlorine-silane gas:
• SiCl4 + HCl  SiHCl3
The following reactions result in silane gas
4 SiHCl3 SiH4+ 3 SiCl4+ 2 H2
SiF4+ NaAlH  SiH4 + NaAlF4
19
Poly Si- Siemens type reactor
•Quartz bell
•Jar
•Polysili
con
depositio
n
•Waste
gases
•Boiling point.: +32 C•
-Pure SiHCl3 in Gas
Phase  Pure Si in Solid
Phase
SiHCl3 + H2→Si + 3HCl
– Chemical Vapor
Deposition (CVD)
process
– Siemens type reactor
•Power supply
SiHCl3 +H2
• Deposition process is slow
• 10 days/ton using 12 Siemens
reactors
•Generate by-products containing chlorine•
•Wacker, Hemlock, Mitsubishi, Tokuyama, Sumitomo SiTiX, MEMC Italia
20
Poly-Si -Fluidized bed reactor (BFR)
•Gran
ular 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
•SiH4
• 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
21
Wafer-manufactured process
22
Production of sc-Si
•Czochralski (CZ) process
•Seed
Holder
• Poly-EGS is melted in a
quartz crucible (SiO2)
•Seed
• Seed particle introduced to
begin crystallization
•Crystal
neck
• Si melt
•Shoulder
•Air +
SiO2+Co
•Air +
SiO2+Co
•Thermal
shield
• Seed pulled to generate
desired wafer diameter
• Ingot is cooled
• Crucible is discarded
(warping and cracking)
23
Czochralski (CZ) process
24
Production of sc-Si
Poly-crystal rod
•Float Zone (FZ)
•Rod of solid, highly purified but
polycrystalline silicon is melted by
induction heating
• Single crystal is pulled from the
molten zone.
RF heating coil
Molten zone
Single crystal
Si
• 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
25
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
wire sawing
Sawing of pseudo square wafer
26
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
27
Production of mc-Si
•Casting
Si melt
Heat exchanger
Poly-Si
Direction
solidification
mc-Si ingot
28
Dicing of mc-Si
mc-Si Ingot
Dicing
Wire sawing
mc-Si wafer
29
Production of mc-Si
Reducing material consumption by:
1.
Producing thinner wafers
2.
Reducing kerf loss
•Si ribbons
EFG growth
Methods of producing Si ribbons
1.
1.
2.
The edge defined film fed growth
process (EFG)
Ribbon growth on substrate (RGS)
Silicon sheets from powder (SSP)
• Wafer thickenss < 250 m
Si sheet from powder
• Very low kerf loss
• Efficiency over 14%
SSP growth
30
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 microelectronics industry
31
Efficiency
Optimum efficiency vs bandgap
32
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
large area production
for
• Good PV conversion efficiency
• Long-term stability
33
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
34
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
35
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
36
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
light
Light
• mostly used under concentrated sunlight
“inverted”
pyramids
finger
• Highest efficiency Si cell structure reported
till now (24.7%)
• PESC with both front and rear side
Passivation
n
+
oxide
n
p
+
rear contact
p- Si
p
+
p
+
• Local diffusion at rear side to make low
resistance contact
oxide
37
Features of High Efficiency Solar Cell
• Route to high efficiency solar cells
Low recombination
 High carrier absorption
Solar cell efficiencies
increased with
technological
development
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;
38
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%.
39
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
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:
12 - 16%
40
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 1016 1017 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,
41
Junction formation
Starting wafer surface is damaged due to
wafer sawing damage,
To remove surface damage a strong
alkaline solution is used
 surface also gets textured
Saw damage removal
p-type (base)
Junction formation
• Junction formation by heating the wafer
at 800-1000oC in phosphorous (n-type)
atmosphere
N-type (emitter)
p-type (base)
• diffusion using gaseous source
•diffusion using spin-on dopants
42
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
43
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
X X
Dop N2
ant
gas
O2
• 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
44
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 1.8 -2.0, (Si =3.42)
p-type (base)
•Other options
n1 
n0 * n2
• TiO2 (70nm)
•  TiO2 (70nm) + MgF2 (110 nm),
45
Commonly used gases for CVD
L32- 46
Solar cell fabrication: Screen-printing
emitter contact (Ag)
Metallisation
•Screen printing of front and back
contact,
•paste of silver and aluminium is used to
print the contact.
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
p-type (base)
base contact (Al)
emitter contact (Ag)
p-type (base)
base contact (Al)
47