Synthesis and Characterization of High-Photoactivity Electrodeposited Cu2O Solar Absorber by Photoelectrochemistry and Ultrafast Spectroscopy

We present a systematic study on the effects of electrodeposition parameters on the photoelectrochemical properties of Cu2O. The influence of deposition variables (temperature, pH, and deposition current density) on conductivity has been widely explored in the past for this semiconductor, but the optimization of the electrodeposition process for the photoelectrochemical response in aqueous solutions under AM 1.5 illumination has received far less attention. In this work, we analyze the photoactivity of Cu2O films deposited at different conditions and correlate the photoresponse to morphology, film orientation, and electrical properties. The photoelectrochemical response was measured by linear sweep voltammetry under chopped simulated AM 1.5 illumination. The highest photocurrent obtained was −2.4 mA cm−2 at 0.25 V vs RHE for a film thickness of 1.3 μm. This is the highest reported value reached so far for this material in an aqueous electrolyte under AM 1.5 illumination. The optical and electrical properties of the most photoactive electrode were investigated by UV−vis spectroscopy and electrochemical impedance, while the minority carrier lifetime and diffusion length were measured by optical-pump THz-probe spectroscopy. ■ INTRODUCTION Cuprous oxide (Cu2O) is one of the few oxides that naturally shows p-type conductivity and is attractive for solar energy conversion thanks to its direct band gap of 2 eV, which upon integration of the AM1.5 spectrum would correspond to a theoretical photocurrent of 14.7 mA cm−2. In recent years, it has been explored as a photocatalyst for solar-driven water splitting and H2 generation, 4−11 as an electrode for lithium ion batteries and as a p-type semiconductor in junction with ntype ZnO for photovoltaic applications. The p-type character is attributed to Cu vacancies that form acceptor levels several hundred meV above the valence band. The hole mobility depends on the preparation method, but values as high as 100 cm V−1 s−1 have been reported for single crystals, making it attractive from a majority carrier transport perspective. For solar water splitting, Cu2O possesses favorable energy band positions, with a predicted flat band potential approximately 0.2 V positive of the hydrogen evolution reaction based on the Mulliken electronegativities of the constituent atoms. There are only a few studies of Cu2O photocathodes for hydrogen evolution in the literature, and the measured photocurrents are much smaller than the theoretical maximum, with values typically reported to be less than 1 mA cm−2. Also, the reported photocurrents have been measured with varying illumination intensities and spectral distributions, making comparisons difficult. The presence of the redox potentials for the reduction and oxidation of Cu2O within the bandgap, at −0.365 V vs NHE and +0.220 V vs NHE, respectively, has not encouraged the photoelectrochemical investigation of this material in aqueous electrolytes. While single crystal Cu2O decomposes 23,24 into Cu when used as photocathode in aqueous solution, in some cases electrodeposited polycrystalline Cu2O has been reported to be stable. This contradictory behavior has been explained in terms of exposed crystal faces, with O2−-terminated faces allowing H species to adsorb and initiate the Cu2O reduction reaction. The electrodeposited sample reported by De Jongh et al. presented octahedral grains exposing the {111} faces, which can be either Cu-terminated or O2−-terminated, and would be more stable if the surface were Cu-terminated. The single crystal reported by Takeushi et al. had exposed {211} and {311} faces, which are always O2−-terminated, and therefore photocathodic degradation was observed. Received: February 6, 2012 Published: March 8, 2012 Article