On the Sigmoidal Shape of Current-Voltage Curves of Photoelectrochemical Solar Cells

Current-voltage (J-V) curves of n-GaAs/KOH(aq)-Se₂^2-/Se^2- and n-Si/acetone-LiC1O₄-Fc^+/° (ferrocenium ion/fer-rocene) solar cells exhibit sigmoidal shapes near the open-circuit potential. J-V and flatband potential measurements indicate that the deviation of the J-V curves from the ideal square-like shape to S shape is due to the light-induced displacement of the bandedges. Analytical expressions describing the dependence of light-induced bandedge shifts on the anodic photocurrent, the density of surface states, and the interfacial charge-transfer velocity are derived and verified experimentally. Application of the expressions to J-V data yields quantitative information on the kinetics of interfacial charge transfer.