Quantifying Thermal, Photovoltage, and Defect Contributions to Transient Absorption of Ta$_{3}$N$_{5}$ Photoanodes

TL;DR

This study dissects the various contributions to transient absorption in Ta$_{3}$N$_{5}$ photoanodes, revealing how thermal, electrostatic, and defect effects influence carrier dynamics crucial for solar water splitting.

Contribution

It introduces a combined spectroscopic approach to separately quantify thermal, electrostatic, and defect contributions in transient absorption spectra of Ta$_{3}$N$_{5}$.

Findings

Identification of three key band structure points in Ta$_{3}$N$_{5}$.

Discovery of a new photo-induced absorption at 2.80 eV linked to lattice heating.

Quantitative analysis of thermal and photovoltage dynamics in transient absorption.

Abstract

Ta$_{3}$N$_{5}$ is among the most intensively studied photoanode materials for solar-driven water oxidation, yet its performance often remains limited by short carrier lifetimes and defect mediated recombination. Although transient absorption spectroscopy is widely used to probe carrier dynamics in photoelectrodes, spectral assignments are frequently ambiguous due to overlapping contributions. Here, microsecond-to-second transient absorption of Ta$_{3}$N$_{5}$ thin films is combined with complementary optical spectroscopies to disentangle contributions from lattice heating, electrostatics, and defect states. Photoreflectance reveals three critical points in the Ta$_{3}$N$_{5}$ band structure, including two anisotropic near-edge transitions at 2.14 eV and 2.27 eV and a higher-lying transition near 2.80 eV, all closely aligned with dominant transient absorption features. A previously unreported photo-induced absorption at 2.80 eV is attributed to pump-induced lattice heating, while potential-dependent measurements reveal that near-edge bleach features arise from pump-induced band flattening and subsequent surface photovoltage relaxation. Fitting transient absorption spectra with independently measured thermal and electrostatic components enables quantification of both thermal and photovoltage dynamics, while the sub-bandgap response provides insight into the redistribution of defect charge states. Thus, this approach to quantifying thermal, electrostatic, and defect-mediated contributions to microsecond-to-second transient absorption provides broadly applicable insights into photoexcitation and relaxation mechanisms in functional semiconductor photoelectrodes.