Negative shifts in the flatband potential by adsorption of iodide ions on surface-alkylated and Pt nanodotted n-Si(111) electrodes for improvement of solar cell characteristics

The flatband potential (UFB) and solar cell characteristics for surface-alkylated and Pt nanodotted n-Si(111) electrodes have been studied in I3- I- redox electrolytes using various alkyls as the surface-terminating group. It is found that the UFB for the surface-methylated and Pt nanodotted n-Si(111) electrodes shifts toward the negative with increasing I- concentration in the electrolyte, in parallel to the equilibrium redox potential Ueq (I3- I-), and thus the open-circuit photovoltage Voc remains nearly constant among various I3- I- redox electrolytes with different Ueq (I3- I-). The constant Voc is observed only for the I3- I- redox couples with varied I- concentrations and not for other redox couples, indicating that it is not caused by the Fermi level pinning via a surface state. The UFB shift with the I- concentration is explained in terms of the I- adsorption in the form of a Si-I⋯ I- complex at surface Si-I bonds, which are formed at nonmodified (naked) Si sites. The n-Si electrodes modified with long-chain alkyls show similar negative shifts in the UFB by iodine adsorption. The UFB measurements in the dark and under illumination have also shown that the Pt nanodots act as an efficient catalyst (gate) for interfacial electron transfer.