Design Predictions of n–n Heterojunction Based Photoanode for Efficient Unbiased Overall Solar Water Splitting

Abstract

There are multiple requirements for maximizing the efficiency of photoelectrochemical overall water splitting using a single n‐type semiconducting photoanode. Many visible‐light‐responsive photoanode materials have been developed but overall water splitting by such materials without external bias has hardly been demonstrated. Herein, the aim is to scale the impact of performance improvement by an n–n heterojunction photoanode comprising an n‐type thin modification surface layer over an n‐type photon absorber, using a finite element method. For the n‐type photon absorber, the semiconductor properties are initially set to those of a well‐studied visible–light‐absorbing Ta3N5, and key properties are varied. It is demonstrated that an appropriate donor density and thickness of the thin surface layer induce adequate energy band bending in the photon absorber, resulting in a dramatic reduction of the onset potential of the photoanode. Thus, even though the quality of the photoanode is poor (e.g., the donor density is high (≈1 × 1019 cm−3)), the optimized surface layer enables the n–n heterojunction photoanode to facilitate bias‐free solar water splitting. These exhaustive investigations could provide design guidelines for obtaining effective photoanodes using an n‐type surface modification layer, such as cocatalysts and hole collection layers.

Interfacial Design of a Ta3N5 Thin-Film Photoanode for Highly Stable Oxygen Evolution over a Wide pH Range

“…On the other hand, Ta 3 N 5 with the thinner IrO x layer showed the most negative onset potential, 0.7 V versus RHE. Our previous study reported that the onset potential of the OER shifted negatively as the electrocatalyst layer thickness decreased because of favorable upward band bending at the modification layer/ photoelectrode interface, 39 consistent with the current experimental result. Nevertheless, the onset potential is still substantially more positive compared to a flatband potential of ∼0 V versus RHE.…”

Elucidating the Role of Surface Energetics on Charge Separation during Photoelectrochemical Water Splitting

“…The fundamental mechanism governing charge separation in a photoelectrode is generally discussed using a one-dimensional (1D) energy-band model (Figure S1a). − In this model, the Schottky junction at the semiconductor/electrolyte interface and the ohmic contact at the semiconductor/substrate interface give rise to the energetics that drive charge separation. Such energetics are denoted as “bulk energetics” because they drive the migration of charge carriers across the bulk of a semiconductor.…”

Numerical modeling investigations of the impact of a thin p-type cocatalyst modifier on an n-type photon absorber for unbiased overall solar water splitting

“…[52][53][54] For example, Ta3N5-based n-n heterojunctions have been developed by many groups. 48,[55][56][57] Recently, our group reported a guideline for the design of Ta3N5-based n-n heterojunctions. 58 However, there is little research on Ta3N5based p-n heterojunctions.…”

“…48,55–57 Recently, our group reported a guideline for the design of Ta 3 N 5 -based n–n heterojunctions. 58 However, there is little research on Ta 3 N 5 -based p–n heterojunctions. 59,60…”

“…48,[55][56][57] Recently, our group reported a guideline for the design of Ta 3 N 5 -based n-n heterojunctions. 58 However, there is little research on Ta 3 N 5 -based p-n heterojunctions. 59,60 This study investigates the numerical modeling of the photoanodic performance of a p-n heterojunction.…”