Nanoscale imaging of charge carrier transport in water splitting photoanodes

Johanna Eichhorn,Ian D Sharp,Adam M Schwartzberg,Dominik Ziegler,Francesca M Toma,Christoph Kastl,Jason K Cooper

doi:10.1038/s41467-018-04856-8

Abstract

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Here we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity-dependent current–voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. These findings support the fabrication of highly efficient bismuth vanadate nanostructures and provide insights into how local functionality affects the macroscopic performance.

Highlights

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales
We show that the low intrinsic bulk conductivity of BiVO4 limits electron current through the film, and that at large bias, the transport mechanism can be attributed to a space charge-limited current in the presence of trap states
Experiments were performed using spin-coated BiVO4 thin films on fluorine-doped tin oxide (FTO)-coated glass substrates (Supplementary Figs. 1 and 2) with photoelectrochemical performance, comparable to the best values reported for undoped BiVO4 photoanodes (Supplementary Fig. 3)[19,20]

Summary

Introduction

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. PEC systems must target high solar-to-hydrogen efficiency, low production cost, and long device lifetimes[3,4,5,6] In this context, a promising semiconductor light absorber is the monoclinic scheelite-phase bismuth vanadate (BiVO4)[7,8,9,10,11]. Conductive mapping is already an established technique to analyze local charge transport and optoelectronic properties of solar cell materials such as hybrid halide perovskites[26,27,28] and CdTe29 For this type of measurement, the solar cell top contact is replaced by a metal-coated AFM probe. To the best of our knowledge, the use of pc-AFM to investigate the functional heterogeneity and to understand the charge carrier transport limitations at the nanoscale in photoelectrochemical materials such as metal-oxide semiconductors still needs to be fully explored. For pc-AFM, the solid/liquid interface is substituted by a solid/solid interface, namely the semiconductor/probe contact

Methods

Results

Conclusion