Solar cell fabrication technology Vítězslav Benda, CTU Prague, Faculty of Electrical Engineering

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

Preparing semicondutor silicon

Polycrystalline silicon fabrication SiHCl 3 + H 2 → 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-pullingslim-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 + HNO 3 + CH 3 COOH) 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 D (x j ;t diff ) = N A

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 - Si 3 N 4 ARC - Ag/Al print screen BS - Ag print screen FS - firing of contact pastes - edge grinding - measuring and sorting

Crystalline Si solar cells mono-crystallinemulti-crystalline   17%   16% (area up to 400 cm 2 )

rubber sealing hardened glass EVA solar cells back 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 3SiH 4 + 3NH 3 → Si 3 N H 2 deposition polysilicon layers SiH 4 → Si + 2H 2.

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 SiH 4 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 volume Very expensive equipment Module lifetime ≤ 10 years

Present market development

1 2 3 Implementation Progr. Technology Transfer Research & Dev Funding Mechanism s as driver Crystalline Si Thin Films High Eta, Lowest Cost

Fabrication stepABC Ingot growing0,37 0,73 Wafering0,000,290,24 Solar cell fab.0,15 0,19 Module fab.0,430,400,37 Factory Cost0,951,211,53 Manufacuring cost in EUR/W p for different crystalline Si manufacturing for production of 500 MW p per year A silicon ribbon B multicrystaline Si C 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/m kWh/m 2