Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Abstract

Current-voltage-temperature ( I- V- T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I- V- T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density ( J sc) and open-circuit voltage ( V oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J sc and V oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J sc which improved by an order of magnitude. The treatment also affected the lower temperature I- V- T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.