The effect of surface plasmon on hot electron flows generated in Ag/TiO2 nanodiodes

Abstract

Over the last several decades, innovative light-harvesting devices have evolved to achieve high efficiency in solar energy transfer. Understanding the mechanism of plasmon resonance is very desirable to overcome the conventional efficiency limits of photovoltaics. The influence of localized surface plasmon resonance on hot electron flow at a metal-semiconductor interface was observed with a Schottky diode composed of a thin silver layer on TiO2 and subsequent characterization of interface layer of Ag/TiO2 nanodiode influencing on Schottky barrier height by means of x-ray photoelectron spectroscopy (XPS). The photocurrent is generated by photoexcited electrons when they have enough energy to travel over the Schottky barrier formed at the metal-semiconductor interface. We observe that the photocurrent can be enhanced by the optically excited surface plasmons. When the surface plasmons are excited on the corrugated Ag metal surface, they decay into energetic hot electron-hole pairs, contributing to the total photocurrent. The observed abnormal resonance peaks in the IPCE (incident photons to current conversion efficiency) can be attributed to the surface plasmon effects. We observe that the enhancement of the photocurrent due to the surface plasmon is closely related to the corrugation (or roughness) of the metal surface. Even though the photocurrent measured on Ag/TiO2 exhibits the surface plasmon peaks, the photocurrent on Au/TiO2 does not show any peaks even at the frequency of Au surface plasmon energy, presumably because of the smoothness of gold surface. We modified the thickness and morphology of a continuous Ag layer by electron beam evaporation deposition and heating under gas conditions and found that the morphological change and thickness of the Ag film are key factors in controlling the internal photoemission efficiency.