Carrier collection in fine-grained p-n junction polysilicon solar cells

We investigate the factors which determine carrier collection in fine-grained p-n junction polysilicon solar cells. Si layers obtained with chemical vapor deposition at high temperature on oxidized Si wafers have been used in solar cell experiments. Lowering the doping level and increasing the thermal budget of the emitter diffusion increases the collection depth. Increasing the grain size from 2 to 7 mum surprisingly does not result in any improvement in the red response of the cells. Analysis of these results shows that carrier collection is mainly determined by the depth of dopant spikes created by enhanced diffusion at grain boundaries. The recombination velocity at grain boundaries appears to be very high (>> 3 x 10(4) cm/s). Active use of preferential doping in a columnar polysilicon layer could nevertheless lead to photovoltaic devices with satisfactory performance. The best result so far is a solar cell efficiency of 5.2 % obtained in a layer with an average grain size of 7 mum.