Solution-Deposited F:SnO₂/TiO₂ as a Base-Stable Protective Layer and Antireflective Coating for Microtextured Buried-Junction H₂-evolving Si Photocathodes

Abstract

Protecting Si photocathodes from corrosion is important for developing tandem water-splitting devices operating in basic media. We show that textured commercial Si-pn(+) photovoltaics protected by solution-processed semiconducting/conducting oxides (plausibly suitable for scalable manufacturing) and coupled to thin layers of Ir yield high-performance H2-evolving photocathodes in base. They also serve as excellent test structures to understand corrosion mechanisms and optimize interfacial electrical contacts between various functional layers. Solution-deposited TiO2 protects Si-pn(+) junctions from corrosion for ∼24 h in base, whereas junctions protected by F:SnO2 fail after only 1 h of electrochemical cycling. Interface layers consisting of Ti metal and/or the highly doped F:SnO2 between the Si and TiO2 reduce Si-emitter/oxide/catalyst contact resistance and thus increase fill factor and efficiency. Controlling the oxide thickness led to record photocurrents near 35 mA cm(-2) at 0 V vs RHE and photocathode efficiencies up to 10.9% in the best cells. Degradation, however, was not completely suppressed. We demonstrate that performance degrades by two mechanisms, (1) deposition of impurities onto the thin catalyst layers, even from high-purity base, and (2) catastrophic failure via pinholes in the oxide layers after several days of operation. These results provide insight into the design of hydrogen-evolving photoelectrodes in basic conditions, and highlight challenges.

A Flexible Web‐Based Approach to Modeling Tandem Photocatalytic Devices

“…(B) Bottom cell H 2 evolving photoelectrodes using Refs. . (C) Top cell H 2 evolving photoelectrodes using Refs.…”

Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

“…Thin amorphous materials (< 4 nm) possess the passage of high current densities (> 1 A cm −2 ) 7 for efficient photoelectrodes, but, are not stable in this case. To the best of my knowledge, amorphous oxides for stabilizing the Si-based photocathodes in alkaline solutions is less than 100 h 25 . Increasing the thickness of the protective layer cannot solve the problem, as the carrier tunneling probabilities decay exponentially with the layer thickness.…”

Single‐Crystal Semiconductors with Narrow Band Gaps for Solar Water Splitting

“…[20] Reproducedw ith permissions. [18] TiO 2 with the conduction-band electron-transport mechanism was also applied to protect amorphous Si based photocathodes.J avey and colleagues deposited 80 nm of TiO 2 using reactive sputtering on top of 8nms puttered Ti layer to protect amorphous Si PEC cathodes with Ni-Mo catalytic layer and buried p-i-n junction (Ni-Mo cocatalyst and p-i-n junction will be discussed in Section 3.1.3 and 4.1, respectively). [19] Thet hin Ti layer was introduced to prevent the damage of amorphous Si during the TiO 2 sputtering process.T he resulting Si/Ti/TiO 2 /Ni-Mo electrode showed as table photocurrent of 11 mA cm À2 at 0V versus RHE (pH 4, AM1.5G), with less than 5% decay over 12 h. The amorphous Si photocathode without the TiO 2 protective layer decayed to 10 %ofits original photocurrent after 12 h.…”

“…Although they are rare and suffer from high cost, this group of metals may serve as archetypical catalysts to overcome the sluggish reaction kinetics,s ot hat other performance-determining factors can be identified and optimized, such as the effectiveness of the anti-corrosion layer (using Pt, [15,16] Ir, [18] and Ru [20] ), or the improvement of photovoltage from buried junctions (using Pt [11a] ). Although highly catalytically active, the high work function of Pt (5.7 eV) should be considered when contacting with p-type semiconductors, [13] as discussed in Section 4.3.…”

“…Ther esulting planar (Figure 5b)a nd microwire-array (data not shown) pn + -Si/Pt electrodes both exhibited stable and reproducible photovoltages of 0.55 V, with energy conversion efficiencies of 9.6 and 5.8 %, respectively (pH 2, AM1.5G), which is among the highest for Si-based photocathode.B yu sing at extured commercial pn + -Si substrate coated with F:SnO 2 /TiO 2 as the anti-corrosion layer and an Ir cocatalyst (2 nm thick), the textured pn + -Si photocathode reached an energy conversion efficiencyof10.9 %(1m KOH, AM1.5G). [18] Similar planar [11b] and microwire-based [42] p-type Si with top n + emitter layers were also used to fabricate highperformance photocathodes by loading MoS x and Ni-Mo alloy cocatalysts.…”