Copper indium selenide water splitting photoanodes with artificially designed heterophasic blended structure and their high photoelectrochemical performances

Conventional p-CuInSe₂ absorbers for solar cells have been homogenously synthesized using multi-step process despite a narrow crystal phase region in the phase diagram and the existence of various secondary phases. In contrast, here we propose artificially-designed heterophasic blended copper indium selenide compounds for water splitting photoanodes using a simple one-step annealing synthetic process where the electrodeposited metal precursors were directly annealed with Se vapor injection and without additional intermediate steps. The resultant product is revealed to possess a novel “phase-blended structure” comprising two phases of p-type CuInSe₂ and n-type CuIn₃Se₅ crystals. The CuInSe₂ nanoparticles with a higher Cu fraction are three-dimensionally (3D) embedded in the n-type CuIn₃Se₅ matrix, which has been verified by various analysis methods such as X-ray diffraction, transmission electron microscopy, and capacitance-voltage curve. The average diameter of the CuInSe₂ nanoparticles is 66.8 nm and the interval between the nanoparticles in the CuIn₃Se₅ matrix is 67.6 nm. Consequently, the phase-blended structure photoabsorber exhibits a remarkably enhanced anodic photocurrent of 12.7 mA/cm² at 1.23 V versus the reversible hydrogen electrode. The considerably enhanced photocurrent gain of the phase-blended structure photoanode is attributed to the excellent charge separation facilitated by the built-in potential generated from the 3D p-n junction.