Theoretical and experimental studies on some new aspects of silicon-methanol photoelectrochemical cells

Abstract

A theory for semiconductor-liquid junction solar cells has been developed in which the concept of the hole quasi-Fermi level is introduced to accommodate the slow reaction rate at the semiconductor-liquid interface and the slow diffusion velocity of the redox species between the bulk of the solution and the interface. In the dark, the hole quasi-Fermi level under small forward bias coincides with the redox level of the solution, while it moves up under large forward bias because of the lack of holes at the interface. In the presence of a photocurrent, the hole quasi-Fermi level becomes deeper than the redox level, and consequently the components of the dark current increase, leading to the degradation of the fill factor. The dark current due to the majority carrier transport from the semiconductor to the solution is suppressed by reducing the concentration of the oxidized (or reduced) species of the redox couple for n-type or (or p-type) semiconductors. The minority carrier dark cunent can also be reduced by using highly doped semiconductors. Thus, a high photovoltage can be attained by adopting a proper design of the photoelectrochemical cell. The effect of the Helmholtz layer on the n value is analysed, and it is proposed that, for highly doped minority carrier solar cells, an additional photovoltage is gained by the shift of the semiconductor band edge under bias, resulting in an increase in the n value. The experimentally observed I- V curve for an n-Si electrode immersed in a methanol solution and the high open circuit photovoltage of 695 mV obtained are well explained by the theory proposed.