Design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes

Abstract

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported so far for water-splitting silicon photoanodes. The loss mechanism is systematically probed in metal-insulator-semiconductor Schottky junction cells compared to buried junction p(+)n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A leaky-capacitor model related to the dielectric properties of the protective oxide explains this loss, achieving excellent agreement with the data. From these findings, we formulate design principles for simultaneous optimization of built-in field, interface quality, and hole extraction to maximize the photovoltage of oxide-protected water-splitting anodes.

Understanding Photovoltage in Insulator-Protected Water Oxidation Half-Cells

“…In this particular system, research has shown that the conduction mechanism through the SiO 2 is a tunneling process 5,9 and through TiO 2 films greater than ∼2 nm thickness is bulk, trapmediated conduction. 6,7 In principle, for n number of distinct insulator layers in the stack and k parallel conduction pathways, there are n k permutations of conduction mechanisms. With two pathways and a bilayer insulator (e.g.…”

“…Unfortunately, the underlying physical properties that control corrosion stability and photovoltaic efficiency of semiconductors are linked, such that Type 0 structures do not generally achieve stable and efficient solar-driven electrolysis. 5,7 The protection layers discussed herein and reported in the literature have permitted the use of highly efficient photovoltaic materials with enhanced stability that allow for the advancement of solar fuel and solar chemical technology. The simplest thin insulator-protected structure that achieves high efficiencies and stabilities as a photoanode is the nSi/SiO 2 /TiO 2 /Metal device utilizing TiO 2 for protection and the surface metal layer as both an OER catalyst and a contact for the resulting Schottky junction.…”

“…7,10 In this case, the only difference between the p + nSi photoanode and the p + Si anode reference will be the difference between hole and electron transport pathways affecting the reductive forward bias, as shown in Figure 5. For anodic operation, the two are now essentially identical.…”

“…The same effect can also be observed in the water oxidation measurements for this particular kind of loss and is characteristic of a charge extraction barrier. By subtracting Equation 4 from Equation 7, we obtain a different, and more easily measured photovoltage. This difference between the E 1/2 values for the Type 1 nSi photoanode and reference p + Si systems is:…”

Engineering Interfacial Silicon Dioxide for Improved Metal–Insulator–Semiconductor Silicon Photoanode Water Splitting Performance

“…The lower dielectric constant of SiO2 relative to TiO2 means that any increase in SiO2 thickness will also incur a much greater photovoltage penalty than the same increase in TiO2 thickness. 11 This strong dependence of both photovoltage and series resistance on SiO2 thickness makes reducing SiO2 thickness one of the most effective methods for improving silicon MIS photoanode performance.…”