An analytical approach for modeling of high-efficiency crystalline silicon solar cells with homo–hetero junctions

Abstract

An analytical model based on charge neutrality principle and energy band diagram analysis is investigated to model a new structure of silicon heterojunction solar cell, which encompasses both homo and heterojunctions. In the analysed structure, a thin (p+)c-Si and a thin (n+)c-Si layers are used at front and back surfaces of (n)c-Si substrate respectively. The purpose of incorporating these layers is to reduce the solar cell output parameters sensitivity to a-Si:H/c-Si interface defect densities, increase the open circuit voltage (Voc) and increase the fill factor (FF). Since (n)c-Si silicon bulk becomes quasi neutral in the mentioned structure, charge neutrality equation can be separated as two independent equations for front and back surfaces. Solving charge neutrality equations, result in a-Si:H/c-Si interface potential, and subsequently charge concentration, electric field, surface recombination velocity, and open circuit voltage (Voc) calculation. Then, by adjusting doping concentration of the two additional layers, their effect on surface potential and energy band bending is studied. It is observed that when doping concentration of (p+)c-Si layer is increased from 1×1018cm−3 to 5×1019cm−3, the Voc drops by 15mV, and FF increases by 2.5%, while Voc and FF sensitivity to interface defects density is improved considerably. On the other hand, by (n+)c-Si layer insertion at back surface, in addition to decreasing sensitivity to interface defect densities in comparison to conventional SHJ silicon solar cell, Voc and FF are increased due to combination of high conductivity and enhancement of field effect passivation at back surface of solar cell. Moreover, when doping concentration of (n+)c-Si layer increases from 1×1018cm−3 to 5×1019cm−3Voc and FF increases from 680 mV and 71% to 740 mV and 82% respectively.