Nanostructured semiconductor materials are generating considerable interest for application in photoelectrochemical cells (PEC) for solar water splitting. A key challenge is improving the long-term chemical and operational stability of semiconductor electrodes. Our research in PEC devices focuses on utilizing atomic layer deposition (ALD) as a means to engineer the semiconductor-liquid interface of photoelectrodes. ALD modification can both impart chemical stability and tune the electronic band structure at the semiconductor’s surface. P-type silicon photocathodes are a model PEC system capable of high photocurrents (>10 mA/cm2) in aqueous electrolytes under AM 1.5 illumination. Here we will detail our efforts to improve the reproducibility of silicon photocathode fabrication and to use TiO2 ALD coatings for band engineering that permits planar catalyst integration.

This talk will discuss silicon photocathodes fabricated from p-type Si (100) wafers with ~1 cm2 functional area. Electrodes were tested in a three-electrode electrochemical cell containing sulfuric acid electrolyte (0.5M, pH ~0), a Pt mesh (>5 cm2) counter electrode, and a Ag/AgCl reference electrode. To ensure similar dopant profiles, experiments were run using a range of silicon samples from the same wafer. We will first discuss the effects of varying the processing schemes for forming an ohmic back contact. We find a large and distinct effect on both the photocurrent saturation value and the photocurrent onset potential with the size and composition of this back contact. Through contact optimization, series resistance of the back contact can be reduced by 5x, as measured by impedance spectroscopy.

The second portion of our talk will describe our results using ALD TiO2 thin films to engineer the electronic band structure at the photocathode/electrolyte interface. Deposition of a coalesced Pt thin film catalyst layer directly on p-type silicon is well known to form an Ohmic contact that pins the silicon’s Fermi level in a nearly flat band state. Without the internal bias caused by surface carrier depletion, photoelectrode activity is eliminated. However, by inserting an interfacial TiO2 layer with sub-nanometer thickness control, a p-n junction can be formed which generates the necessary electric field for photoelectrode operation. Here, we will demonstrate how uniform ALD layers are capable of providing the necessary electronic band engineering to form completely planar p-Si/TiO2/Pt structures with photocurrents exceeding 10 mA/cm2 with no applied bias.