Photocurrent and photovoltage mechanisms in silicon Schottky barrier and MOS solar cells

We describe here the results of an experimental and theoretical investigation of metal-silicon and metal-SiO2-silicon solar cell structures as a function of the thickness of a controlled SiO2layer in the region of 8Å to 40Å. We first investigate the short wavelength response of near-ideal silicon Schottky barriers. It is shown that the major cause of reduced quantum efficiency in this range of λ is the collection by the metal of majority carriers photogenerated within the image force maximum. The bias-voltage dependence of the photocurrent in the near ultraviolet region is also in good quantitative agreement with the image force model. The open-circuit voltages, fill factors and conversion efficiencies of majority carrier and minority carrier MOS solar cells are compared experimentally for SiO2layers produced under identical conditions. We have found that Au-SiO2-nSi (majority carrier) cells have optimum AMI efficiencies of 9 - 10% as opposed to 11 - 12% for Al-SiO2-pSi (minority carrier) cells, but that the behavior of the majority carrier cells is less sensitive to SiO2thickness. Finally, the effective tunneling barriers of the ultrathin SiO2layers in the thickness range 20 - 40Å to electrons and holes tunneling between the semiconductor and the metal have been measured independently on the same MOS devices by a new experimental technique. The method combines dark measurements of current and capacitance vs. voltage with measurements of short-circuit photocurrent suppression under optical illumination. Analysis of the data shows consistently that the tunneling barriers for holes are much larger than those for electrons.