Flat-band voltage of a-Si pin solar cells from spectral characteristics

Experimental and simulation results on a-Si p-i-n solar cells are presented. The internal collection efficiency is shown to be a function of wavelength and voltage. The function shows a certain kind of symmetry that is not perfect, but reveals a plateau voltage U_p. A simple photocurrent model is used to explain the experimental results. The model assumes a uniform field through the i-layer. It neglects bulk recombination of photocarriers, but considers surface recombination at the p-i and i-n barriers. A particular voltage, the flatband voltage U_F, causes the field to vanish. A dominant front barrier yields U_pF. It is suggested that a symmetric cell should show a sharp center and U_p=U_F.