Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu2O photoelectrodes

Mario Borgwardt,Klaus Schwarzburg,Daniel Abou-Ras,Paul Plate,Stefan T Omelchenko,Christian Höhn,Marco Favaro,Rainer Eichberger,Nathan S Lewis,Roel Van De Krol,Dennis Friedrich,Harry A Atwater

doi:10.1038/s41467-019-10143-x

Abstract

Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development.

Highlights

CC (t0) Delay 180 fs Delay 100 ps Delay 1 ns0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Electron kinetic energy
Recently has its capabilities been utilized to study processes in emerging photovoltaic material systems, such as hot electron relaxation dynamics in hybrid metal–organic perovskite semiconductors[25,26,27]. We have extended these efforts to the group of metal oxide semiconductors by including Cu2O—one of the most promising metal oxide candidates for solar water splitting[28]
Timeresolved 2PPE is a pump–probe technique in which one of the beams is guided through an optical delay stage of variable length to generate an adjustable time delay between two femtosecond laser pulses (Fig. 1a)

Summary

Introduction

CC (t0) Delay 180 fs Delay 100 ps Delay 1 ns0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Electron kinetic energy (eV). Color maps of the transient photoemission signal as a function of the electron kinetic energy and the pump–probe time delay for the reconstructed Cu2O (100) surface (a), and for Pt-covered Cu2O (c). The definition of a single binding energy scale representing all signals is not feasible and the directly measured electron kinetic energy scale was chosen. The Pt-covered Cu2O sample exhibited a similar steady-state energy structure to that of the reconstructed Cu2O surface, except that the Pt adlayer provided an extension of the signal up to the Fermi level, providing a clear signature of the metallic character of the adlayer. XPS, as well as SEM measurements were indicative of a non-conformal structure of the Pt adlayer The island-like growth of the Pt adlayer is consistent with a non-uniform distribution of pinholes at the reconstructed surface

Methods

Results

Conclusion