Multicrystalline and Back Contact Buried Contact Silicon Solar Cells

Abstract

This thesis examines multicrystalline silicon (mc-Si) as well as back contact silicon solar cells applying the Buried Contact Solar Cell (BCSC) technology. In this metallisation approach, the electrical contacts are buried into the silicon wafer leading to low shadowing losses in conjunction with highly conducting contact fingers. Further features include a selective emitter structure as well as surface passivation. A baseline process for the manufacturing of BCSC was developed which includes new processing steps compared to the state-of-the art like P-Al co-diffusion and mechanical contact groove formation. Due to the intrinsic material properties of mc-Si with a higher density of impurities and crystal defects, bulk passivation techniques by gettering and by hydrogen passivation were applied and optimised to increase the bulk diffusion length of minority charge carriers. Single blade texturing for the reduction of optical losses as well as improved rear surface and bulk passivation led to a record efficiency of 17.5 % (Voc=628 mV, Jsc=36.3 mA/cm2, FF=76.8 %, cell area 144 cm2, confirmed measurement) for large area solar cells on mc-Si, which supplants the record held by Japanese companies since 1993. The applied technologies allow the direct transfer into industrial production lines. With back contact solar cells, a newer generation of crystalline silicon solar cells was investigated. Back contact solar cells have the potential for higher solar cell efficiency, easier module assembly and homogenous optical appearance. Three different device designs of back contact solar cells have been investigated on monocrystalline Cz-Si. For each design (Metallisation Wrap Around, Metallisation Wrap Through, Emitter Wrap Through) industrially applicable processing sequences were developed and the fabricated devices were characterised and analysed. Efficiencies up to 17.5% were demonstrated on medium substrate sizes.