Spatially Segregated Catalyst and Light-Absorption in Silicon Microwires for Enhanced Photoelectrochemical Hydrogen Evolution Reaction

As a photocathode material in photoelectrochemical (PEC) cells, crystalline Silicon (c-Si) has been widely used due to its relatively narrow bandgap of 1.12eV, non-toxicity, and high stability compared to conventional oxide-based materials. Despite these advantages of c-Si, the photocathode only with c-Si shows relatively-low Faraday efficiency because it requires a high overpotential to reduce CO₂ in specific products. To overcome the high overpotential of c-Si based photocathode, applying PEC catalysts (e.g., Ag, Au) is essential to reduce the high overpotential and significantly increase the Faradaic efficiency. Although the effective CO₂ reduction reactions through c-Si photocathodes can be induced by coating the PEC metal catalyst onto the c-Si photocathodes, it has to discuss the geometry of c-Si photocathodes to enhance their optical and electrical properties. Generally, when a planar c-Si substrate is used as the photocathode in the PEC cell, the planar c-Si photocathode generates low current density due to high reflectance from the metal catalyst and planar c-Si surface. To overcome the limit in low current density originating from the planar structure, c-Si microwires have been investigated for the Si photocathodes because of their superior light absorption and effective carrier separation. However, the conventional c-Si microwire-based photocathode is fabricated by coating the particle-shaped catalysts onto the entire surface of c-Si microwires, leading to increased electrical loss due to surface and interface recombination at the metal-Si contact. Additionally, the catalyst-coated area causes optical shading loss that incident light is reflected by the catalyst, resulting in a reduction in photocurrent due to the decreased light absorption. These electrical and optical losses must be minimized to improve PEC cell performance.<br/>One important parameter related to the electrical and optical losses caused by the catalyst is the proportion of the catalyst coated on the surface of the Si photocathode. When the proportion of catalysts coated on the Si surface increases, the electrical loss by recombination and shading loss by reflection also increases, resulting in the degradation of cell efficiency. Conversely, both types of losses would decrease by reducing the proportion of catalyst, improving the saturated current density. However, the diffusion length of photocarriers should be increased up to half the distance between the catalysts, leading to a higher probability of recombination before photocarriers move to the catalyst. Considering the diffusion length of photocarriers within the Si substrate and the losses caused by the catalyst, it is necessary to control the proportion and placement of the catalyst on the Si photocathode to maximize the efficiency of the PEC cell.<br/>Typically, c-Si-based optoelectronic devices can suppress surface recombination and improve the photocarrier lifetime by depositing a passivation layer on the c-Si surface. The improved lifetime means increasing the diffusion length of photocarriers from tens to hundreds of micrometers, enabling the PEC catalyst to be placed in localized areas on the c-Si photocathode instead of coating the catalyst onto the entire surface of the c-Si photocathode. In this abstract, we aimed to fabricate a Si photocathode with surface-passivated microwire arrays that segregate between the absorption region capable of absorbing light and the catalyst region coated with the catalyst. In the absorption region, we fabricated the tapered microwire arrays to minimize light reflection in terms of morphology, and confirmed that light absorption was maximized compared to other structures through optical simulation. The catalyst region was constituted by each single catalyst wire array and we controlled the shape of the tip at the catalyst wire for inclined catalyst coating and figured out that this shape effectively reduced the light reflection at the top of the catalyst wire.