Electron Transport Layer Design Considerations for Si-based Photocathodes for CO₂ Reduction

Use of charge-selective, passivating contacts is responsible much of the recent progress in improving the efficiency of Si-based photovoltaic cells [1]. Designing such contacts for the photoelectrochemical (PEC) environment (i.e. electron transport layers for photocathodes and hole transport layers for photoanodes) has some commonalities with PV but also brings additional challenges and constraints [2]. Focusing on ETLs, materials used for this purpose much be stable in the PEC environment and either be catalytic for the desired reaction (e.g. hydrogen evolution, HER, or CO₂ reduction, CO₂R) or be compatible with a co-catalyst that is. For Si photocathodes used for HER, TiO₂ is widely used: it is stable in the low pH conditions which are typically employed and provides a conductive path to typically used catalysts such as Pt [3].<br/>However, although TiO₂ ETLs appear to be stable at the near-neutral pH conditions used in PEC CO₂R, they also appear to be highly active for the competing HER reaction, which motivates a search for materials which will be more catalytically inert. Consideration of thermodynamic stability rules out many candidates that are used in PV applications (e.g. ITO). We will show that TaO_x synthesized by pulsed laser deposition and RF sputtering has promise as an ETL for PEC CO₂reduction. Control of the oxygen vacancy concentration enables tuning its conductivity and hence improving its photocurrent densities under CO₂reduction conditions. Initial work show that while the photovoltage of p-Si/TaO_x junctions is smaller compared to p-Si/TiO₂, TaO_xis catalytically less active for HER. As a result, p-Si/TaO_x/Ta/Au and p-Si/TaO_x/Cu photocathodes produce CO and C1 and 2 products (faradaic efficiencies &gt; 50%), as expected.<br/>As many PV absorber materials are themselves thermodynamically unstable under the strong reducing conditions of PEC CO₂ reduction, investigation of chemically compatible ETLs which also provide surface passivation is warranted. In this context, materials selection strategies, including the use of engineered and multifunctional layers (inspired by those used for PEC HER [4]), will be discussed.<br/> <br/> <br/>[1] Allen, T. G.; Bullock, J.; Yang, X.; Javey, A.; De Wolf, S. Passivating Contacts for Crystalline Silicon Solar Cells. <i>Nat. Energy</i> <b>2019</b>, <i>4</i>, 914–928.<br/>[2] Hu, S.; Lewis, N. S.; Ager, J. W.; Yang, J.; McKone, J. R.; Strandwitz, N. C. <i>J. Phys. </i><i>Chem. C</i> <b>2015</b>, <i>119</i>, 24201–24228.<br/>[3] Lin, Y.; Battaglia, C.; Boccard, M.; Hettick, M.; Yu, Z.; Ballif, C.; Ager, J. W.; Javey, A. <i>Nano Lett.</i> <b>2013</b>, <i>13</i>, 5615–5618.<br/>[4] Paracchino, A.; Laporte, V.; Sivula, K.; Grätzel, M.; Thimsen, E. <i>Nat. Mater.</i> <b>2011</b>, <i>10</i>, 456–461.