Solar cell technologies • First Generation Solar cells

» Silicon wafer based solar cell technologies (300 to 350 μm)

• Second Generation Solar cells» Thin film based solar cell technologiesCdTe, CuInS2 and amorphous‐Silicon (5 to 10 μm) 

• Third Generation Solar cells»Organic and nanostructured based solar cell technologies

Production of Silicon• Most widely used semiconductor material for solar cell 

applications.

• 90% of world’s solar photovoltaic modules are produced in Si wafers.

• 1975 , world wide PV module production was about 2 MWp

2007,                                                 it has grown to 3000 MWp

• Earlier, it was using electronic grad siliconNow, PV industry is looking for solar grade silicon  

Various routes for making solar cells from silicon material

Production of Metallurgical grade Silicon (MGS)Arc furnace

98% pure Silicon

Energy consumed in the process is quite high It is in the range of 13 to 15 kWh/KgOut of this 5% only will be used for making electronic grade silicon

Typical impurity concentrations in metallurgical‐grade silicon 

Element Concentration  (particles per million)

aluminum 1000‐4350 manganese 50‐120

boron 40‐60 molybdenum < 20

calcium 245‐500 nickel 10‐105

chromium 50‐200 phosphorus 20‐50

copper 15‐45 titanium 140‐300

iron 1550‐6500 vanadium 50‐250

magnesium 10‐50 zirconium 20

Production of Electronic grade Silicon (EGS)

SiHCl3(g) + H2 (g) ----- Si (s) + HCl

(a) Production of high purity Silicon containing gases

Si(s) + 3HCl (g) --- SiHCl3 (g) + H2 +Heatat 300 C

SiHCl3 --- SiH4 + SiCl4 + H2

(b) Obtaining electronic grade solid silicon

Seimens reactor

Typical impurity concentrations in electronic‐grade silicon 

Element Concentration   (particles per billion)

arsenic < 0.001 gold < 0.00001antimony < 0.001 iron 0.1‐1.0boron ≤ 0.1 nickel 0.1‐0.5carbon 100‐1000 oxygen 100‐400

chromium < 0.01 phosphorus≤ 0.3

cobalt 0.001 silver 0.001copper 0.1 zinc < 0.1

Production of silicon wafers 

Atomic arrangements of Si atoms in EGS is not goodWhich contains large number of defects.This can be done by Silicon ingots preparationCalled Czochralski (CZ) and Float Zone (FZ) process

Czochralski(CZ) process

Float-Zone (FZ) process

Multicrystalline Silicon IngotsSeveral crystals of different orientations in an ingot

70% silicon solar cells now produced from this material

Cheaper block costing method

Directional solidification

There is a large growth of PV industry (nearly 30%)& relatively small growth in microelectronic industry (< 10%)

During this time Microelectronic industry can not supply theenough Electronic  grade silicon  for Silicon based solar cells 

What is the alternate ? • Solar grade (SoG) Silicon

refining process from MGS

HCl, HF, HNO3, H2SO4

Al, Fe, Cr, Mn, Ni, Ti

Vapour pressure difference

Segregationcoefficient

Impurities are high compared to EGS

Production cost of Silicon as function of impurity level

Process flow for commercial wafer-based solar cells

Alkaline NaOH

Phosphorous

Plasms enhanced chemicalVapour deposition method

SiNx-H

Metall contacts with Al and Ag

Modeled losses from an ideal solar cell

Useful energy29%

Thermalization32%

Sub-bandgaplosses21%

Other losses18%

Incident solar radiation100%

The most noticeable loss mechanism in solar energy conversion relates to the fact that the basic electronic excitation process in Photovoltaics

and also in photochemical processes & photobiological such as photosynthesis

Third GenerationSolar cellsWill address thisproblem

Book Reference

Solar Photovoltaics: Fundamentals, Technologies and Applications

By Chetan Singh Solanki