Wavelength Dependent Photocurrent of Hematite Photoanodes: Reassessing the Hole Collection Length

TL;DR

This study reveals that the photocurrent in hematite photoanodes depends on wavelength and involves long-lived delocalized holes, challenging the traditional hole collection length model.

Contribution

It reassesses the hole collection length in hematite photoanodes by demonstrating wavelength-dependent photocurrent behavior and the role of delocalized holes.

Findings

Photocurrent observed with back illumination at wavelengths indicating holes traverse at least 700 nm.

Back illumination at 450 nm yields higher photocurrent than at 530 nm, despite deeper absorption.

Photocurrent depends on optical excitation wavelength, involving delocalized and localized holes.

Abstract

The photoelectrochemical behavior of a planar 1 cm2 thick Ti-doped hematite film deposited on F:SnO2 coated glass was studied with both front and back illumination. Despite low quantum efficiency, photocurrent was observed upon back illumination with low wavelengths, indicating that some photogenerated holes are able to traverse at least 700 nm across the hematite film and effectively oxidize water. This cannot be accounted for using the commonly accepted hole collection length of hematite based on fitting to the Gartner model. Furthermore, under back illumination, 450 nm excitation resulted in increased photocurrent as compared to 530 nm excitation despite most of the light being absorbed further away from the surface. These results demonstrate that the photocurrent is strongly dependent on the optical excitation wavelength, and related to both delocalized holes with long lifetime and localized excitations rather than only being dependent on the proximity of the absorption to the surface.