Probing Transport Losses in Transition-Metal Oxide Photoanodes Using Robust Spatial Collection Efficiency Analysis

Maximizing the performance of photoelectrochemical (PEC) devices is of great importance for the viability of mass solar fuel production. Transition metal oxides are the materials of choice for PEC solar hydrogen production, chief among them hematite (a-Fe₂O₃) due to its non-toxicity, abundance, stability in alkaline conditions and suitable bandgap (~2 eV). However, hematite suffers from poor charge transport properties, which lead to extremely short charge carrier diffusion lengths which is detrimental for hematite-based devices. In addition, open d-shell materials such as hematite suffer from non-unity photogeneration yield (PGY), meaning that not every absorbed photon will generate a mobile electron-hole pair. The effects of the transport losses, or spatial collection efficiency (SCE) and PGY cannot be individually measured, and the proposed route to decouple them is through external quantum efficiency and optical measurements combined with computational analysis. Until now, SCE analysis has been carried out to probe single junction metal-oxide photoabsorbers with a homogenous photoactive region, but practical devices for solar fuels are often made in a multilayer configuration, utilizing either homojunctions or heterojunctions. In addition, a priori assumptions on the PGY and SCE profiles are often necessary and can lead to inaccurate results due to biasing. Herein, we present a new analysis technique to decouple the PGY and SCE losses in planar thin films employing multiple active layers with only assumption is the measurable charge transfer efficiency (SCE at the interface). As a first case study, hematite thin film photoanodes of different thicknesses and dopants were prepared using pulsed laser deposition. The thin film optical constants were extracted via spectroscopic ellipsometry and transfer matrix method simulations were then used to model the optical response of the thin film photoanodes. Using the optical simulations and measured EQE as inputs, an algorithm was developed to decouple the PGY and SCE losses. In addition, the algorithm is utilized to probe the SCE profile of hematite p-n homojunction thin films. I will present a detailed explanation of the algorithm workflow and showcase current results we have achieved using this algorithm.