Optimizing external quantum efficiency in photocatalysts via first-principles optics and carrier transport modeling

Abstract

Photocatalytic water splitting with particulate semiconductors offers a scalable route to solar-to-hydrogen (STH) conversion, yet efficiency is limited by optical losses and carrier recombination. We present a predictive framework that integrates first-principles optical spectra, carrier diffusion, multilayer optics, and effective-medium theory. Applied to the visible-light-responsive photocatalyst Gd₂Ti₂O₅S₂, the model quantitatively reproduces apparent quantum efficiency (AQE) spectra and enables extraction of carrier diffusion lengths. The calculated absorptance is consistent with Kubelka–Munk theory and its diffuse-reflectance assumptions. Notably, particles larger than the diffusion length can still enhance STH performance via improved light harvesting. However, radiative photon escape, especially near shallow absorption edges, can outweigh recombination losses even in stacked configurations. These results underscore the need for simultaneous optimization of optical and electronic transport properties and challenge the assumption that maximizing photocatalyst loading alone ensures optimal efficiency once particle size is tuned. We also apply the framework to explain the gradual decline in AQE with increasing wavelength observed for particulate SrTiO₃:Al photocatalysts that exhibit near-unity AQE.