Utilizing Band Diagrams to Interpret the Photovoltage and Photocurrent in Photoanodes: A Semiclassical Device Modeling Study

The photovoltage and photocurrent both serve as important design metrics when assessing the performance of photoanodes within photoelectrochemical cells. However, to date, wide disagreement persists (even in the recent literature) regarding how the photovoltage should be physically interpreted; this lack of consensus is further coupled to physical interpretations of the photocurrent. In this work, we utilize state-of-the-art device modeling to help clarify the physical origins of both the photovoltage and photocurrent in photoanodes. Our methodology is based on directly solving the governing electron and hole continuity equations, coupled self-consistently with Poissons equation, with appropriate boundary conditions. Through a systematic examination of a model photoanode, hematite, we correlate directly measurable current-voltage characteristics with operational band diagrams. It is shown that, by directly mapping specific operating points of either equal current or equal voltage (both illuminated and in the dark) to band diagram plots, one is able to obtain substantial insights into the physical nature of both the photocurrent and photovoltage. Throughout this analysis, the fundamental distinction between electrostatic and electrochemical perturbations under arising illumination is underscored. By aiding the community-wide effort to arrive at a consensus on these concepts, we aim to further enable the design of higher-efficiency photoanodes.