Solar cell fabrication technology

Vítězslav Benda, CTU Prague, Faculty of Electrical Engineering

Market share by solar cell technologies

At present, nearly 90% of solar cell production is made from crystalline silicon

Preparing semicondutor silicon

arc furnace

silicon grinding Cl2 - drying

electrolysis

HCl - production

H2 - purification

condensation polysilicon processing

polysilicon deposition with doping

high purity distillation

distillation

condensation

fluid-bed reactor, SiHCl3 - production

NaCl

NaOH

SiCl4

CD

C SiO2

Polycrystalline silicon fabrication

SiHCl3 + H2 → Si + 3HCl

Monocrystalline silicon fabrication (Czochralski method)

- diameter up to 450 mm

- weight up to 300 kg

Single-crystal fabrication

polycrystalline silicon processing

FZ-pulling CZ-pulling slim-rod pulling

single crystal rod shaping

Wafer fabrication

crystalline silicon shaped rod

sawing

lapping

etch rounding

cleaning

polishing

microelectronics grade quality wafer

sawing

etching and cleaning

solar cell grade quality wafer

Multicrystalline rod fabrication

Wafer fabrication

isotropic etching (HF + HNO3 + CH3COOH)

texturing by anisotropic etching

~40% of material is lost during crystalline rod cutting (sawing)

Ribbon silicon

EFG (Edge-defined Film-fed Growth)

method

(~ 300 µm thick polycrystalline silicon sheets)

The ribbons are prepared in a form of a hollow octagonal tube (5 m in length) with eight 100 – 125 mm wide faces.

Diffusion technology

PN junction is usually realised by phosphorous diffusion into P-type basic material

N x t N erfcx

Dt( , ) 0

N x tQ

Dt

x

Dt, exp

2

4

ND(xj;tdiff) = NA

kT

WDTD exp)( 0

The structure of a high efficient solar cell (PERL) made from monkrystalline silicon (efficiency 24%)

very expensive technology

FZ starting material

microelectronics quality wafers

photolihography

anisotropic etching

diffusion

AR coating

photolithography

contact deposition

To decrease the fabrication cost….

•CZ quality material•Wire-cut wafers•Chemical surface processing (texturing)

•To avoid expensive fabrication techniques like photolithography and vacuum deposition techniques

- etching monocrystalline (1,0,0) Si in KOH

- acid etching in the case of other crystallographic orientation of Si

Standard mass production (c-Si cells)

• chemical surface texturing

• SiN(H) antireflection surface coating and passivation

• contact grid realised by the screen print technique

Fabrication of c-Si solar cells

c-Si wafer

- etching of damaged layer- texturing- N-type (P) diffusion

- Si3N4 ARC- Ag/Al print screen BS- Ag print screen FS

- firing of contact pastes

- edge grinding

- measuring and sorting

Crystalline Si solar cells

mono-crystalline multi-crystalline

17%

16%

(area up to 400 cm2)

rubber sealing

hardened glass

EVA

solar cellsback covering foil (tedlar)Al frame

solar cell

PV module

PV module technology

Module lifetime

> 20 years

- between two glass sheets

- sealing compound application

- hind side from non-transparent material

- laminate foil application

Alternative module constructions

Thin film solar cell technology

A) Vacuum deposition

Filament evaporation

Electron-beam evaporation

Flash evaporation

Sputtering

TCO for „light trapping effect“

ZnO sputtered and etched in HCl

ZnO prepared by CVD (Chemical Vapour Deposition)

B) CVD (Chemical vapour deposition ) technique

CVD technology is the formation of a stable compound on a heated substrate by the thermal reaction or decomposition of gaseous compounds

• Reaction chamber• Gas control section• Timing and sequence control• Heat source for substrates• Effective handling.

Atmosphere CVD

Low pressure CVD (LPCVD)

LPCVD is used for deposition of silicon nitride 3SiH4 + 3NH3 → Si3N4 + 12H2

deposition polysilicon layers SiH4 → Si + 2H2.

Plasma enhanced CVD (PECVD)

RF electrode and substrate create the capacitor structure. In this space the plasma and incorporated deposition of material on substrate takes place

The deposition rate is higher than in the case of LPCVD, but layer quality is lower

Hot wire chemical vapour deposition (HWCVD)

This technique relies on the catalytic decomposition of SiH4 by metal.

A filament is a basic component in this system.

The gas in presence of a heated filament (the filament material is W or Ta) is decomposed in radicals that diffuse to and are deposited on the substrate

The deposited layer structure depends on the gas composition and substrate temperature

dilution ratio rH = ([H2] + [SiH4])/[SiH4]. rH < 30, amorphous silicon growth

rH > 45, crystalline layers are formed

Tandem solar cell – „micromorph“ (microcrystal + amorphous)

Differences between crystalline Si cells and thin film cells

Crystalline Si thin film

structure n+-p(-p+) p+-i-n+

FS contact „fingers and busbars“ all-area TCO contact

thickness 300 m 0.3 až 3 m

BS contact „not important“ back reflector

antireflection texturing TCO light trapping effect

Illumination from „n+- side“ from „p+- side“

Technology n+-diffusion into substrate plasmatic processes

Thin film modules

The module area is limited by the reaction chamber volumeVery expensive equipment

Module lifetime ≤ 10 years

Present market development

0,01

0,1

1

10

100

10 100 1000 10000 100000

Cumulative Quantity Produced

Pri

ce

pe

r U

nit

Slope of Scale-Up: Half the Cost with 10 times more Volume

2002

1

2

3

Implementation Progr.

Technology Transfer

Research & Dev

Funding Mechanisms as driver

Crystalline Si

Thin Films

High Eta, Lowest Cost

Fabrication step A B C

Ingot growing 0,37 0,37 0,73

Wafering 0,00 0,29 0,24

Solar cell fab. 0,15 0,15 0,19

Module fab. 0,43 0,40 0,37

Factory Cost 0,95 1,21 1,53

Manufacuring cost in EUR/Wp for different crystalline Si manufacturing for production of 500 MWp per year

A silicon ribbonB multicrystaline SiC monocrystalline Si

Concentrator systems

The Sun tracking is necessary

heatsink

cells

parabolic mirror

Reliability problems• Environmental effect and aging results in a decrease of

efficiency– a decrease of glass transparency

– an increase of series resistance

– degradation of individual layers of the cell structure

A target: increase module lifetime > 30 years

900 kWh/m2

1800 kWh/m2