P‐Type InP Nanopillar Photocathodes for Efficient Solar‐Driven Hydrogen Production

Abstract

Water splitting by using sunlight for the production of hydrogen yields a storable product, which can be used as a fuel. There is considerable research into H2 generation, namely the reduction of protons to H2 in aqueous solution using semiconductor photocathodes. To maximize the photoelectrochemical (PEC) performance, the selection of the active materials and device configurations should be carefully considered. First, the short-circuit current density (Jsc) should be maximized by choosing materials with high optical absorption coefficients and low carrier recombination rates, both in the bulk and at the surface. The reflectance should be minimized by using surface nanotexturing to further improve light absorption. The onset potential (Eos) of the PEC device versus the reversible H /H2 redox potential should be maximized. Finally, the surface energy needs to be controlled to minimize the accumulation of gas bubbles on the surface of the photoelectrode. Light absorbers with band gaps in the range of 1.1–1.7 eV provide both a good match to the terrestrial solar spectrum and a significant fraction of the 1.23 eV free energy required to split water. Overpotentials associated with the electron transfer to (solvated) protons in aqueous solution should be minimized by improving carrier transport from semiconductor to electrolyte by decorating the semiconductor with cocatalysts, tuning band edges, and decreasing contact resistance. p-Type Si has been extensively investigated as a photocathode for photochemical hydrogen production. Planar Si has relatively low short-circuit current densities under AM1.5 G illumination, approximately 10 mAcm 2 (reference [9]), compared to what can be achieved in a pn junction solar cell (> 35 mAcm ). Nanostructuring and incorporation of cocatalysts have been used to raise the short-circuit current density to over 30 mAcm . A recent study using np Si radial junction microwires reported an Eos value of 0.54 V and an Jsc value of 15 mA, leading to an overall efficiency near 6%. The onset potential observed to date for p-Si photocathodes is less than half of the value required for overall water splitting (1.23 V). This low onset potential limits the performance of tandem or “Z-scheme” approaches, which would function without external bias, as it limits the potential overlap required for spontaneous water splitting. An ideal photocathode for use in a solar-driven hydrogen production system without bias should have both a high current density and a favorable open-circuit potential versus the reversible H/H2 redox couple. Herein, we employ nanotextured p-InP photocathodes in conjunction with a TiO2 passivation layer and a Ru cocatalyst to increase both Jsc and Eos values under H2 evolution conditions. InP has a number of attractive attributes as a photocathode: 1) Its band gap of 1.3 eV is well-matched to the solar spectrum; InP-based solar cells have achieved AM1.5 G efficiencies of up to 22%. 2) The conduction band edge of InP is slightly above the water reduction potential, thus electron transfer is favorable in this system. 3) The surface-recombination velocity of untreated InP is low (ca. 10 cms 1 for n-type and 10 cms 1 for p-type), which is particularly important for nonplanar devices with high surface areas, such as those explored in this study. For these reasons, InP has been studied previously as a photocathode for both water splitting and CO2 reduction. [18–20] Specifically, Heller and Vadimsky reported attractive PEC performances with current densities up to 28 mAcm 2 and conversion efficiencies of approximately 12% in InP photocathodes. Motivated by these results, we use InP as a model material system to elucidate the role of surface nanotexturing on the PEC device performance. We find that nanotextured InP photocathodes exhibit drastically enhanced performances compared to our planar cells that were processed using identical conditions. We examine the various effects of nanotexturing [*] M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, J. W. Ager, Prof. A. Javey Material Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) E-mail: jwager@lbl.gov ajavey@berkeley.edu M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, Prof. A. Javey Electrical Engineering and Computer Sciences University of California, Berkeley, CA 94720 (USA) M. H. Lee, T. S. Matthews, J. W. Ager, Prof. A. Javey Joint Center for Artificial Photosynthesis Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)