High-Efficiency P-I-N Microcrystalline and Micromorph Thin Film Silicon Solar Cells Deposited on LPCVD Zno Coated Glass Substrates

The authors report on the fabrication of microcrystalline silicon p-i-n solar cells with efficiencies close to 10%, using glass coated with zinc oxide (ZnO) deposited by low pressure chemical vapor deposition (LPCVD). LPCVD front contacts were optimized for p-i-n microcrystalline silicon solar cells by decreasing the free carrier absorption of the layers and increasing the surface roughness. These modifications resulted in an increased current density of the solar cell but also in significantly reduced fill-factor (FF) and open-circuit voltage (V_oc). In order to avoid these reductions, a new surface treatment of the ZnO is introduced. It transforms profoundly the surface morphology by turning the typical V-shaped valleys of the LPCVD ZnO into U-shaped valleys and by erasing from the surface small-sized pyramids and asperities. As a result, for fixed deposition parameters, the p-i-n microcrystalline silicon solar cell efficiency increased from 3.3% to 9.2%. Further optimization of the microcrystalline silicon solar cell on this 'new' type of LPCVD ZnO front contact has led to an efficiency of 9.9%.