Heterojunctions for polycrystalline silicon solar cells

We optimize amorphous/crystalline silicon heterojunctions on single crystalline silicon and multicrystalline silicon substrates. Heterojunctions are fabricated by depositing ultrathin amorphous silicon by plasma enhanced chemical vapor deposition at temperatures below 200 degrees C. These can therefore be used as charge separating junctions for polycrystalline silicon films on glass substrates. Solar cells with a simple design reach a conversion efficiency of 12.5% on monocrystalline silicon wafers and 10.2% on coarse-grained multicrystalline silicon with mm grain sizes. Current-voltage measurements and quantum efficiency analyses serve to analyse the electrical properties of the solar cells. From IQE measurements we deduce an effective minority diffusion length L-eff = 317 mu m for monocrystalline of L-eff = 79 mu m for multicrystalline silicon substrates. Experimental open circuit voltages of about 580 mV compare well with theoretical values.