Post-annealing treatment of Cu2ZnSnS4¬-based multilayer photocathodes for enhancing photocurrent stability during photoelectrochemical water reduction

Kesterite Cu2ZnSnS4 (CZTS) thin film is technologically attractive as a photoabsorber for photovoltaic and as a photocathode for photoelectrochemical (PEC) water reduction applications due to its independence on critical raw materials. In this contribution, we propose that by using non-critical raw material and environmentally benign Zn(O,S) film, a photocathode consisting of multilayer CZTS/Zn(O,S)/Nb-doped TiO2/Pt demonstrates a promising performance for PEC water reduction with typical photocurrent of ~14 - 16 mA/cm2 at 0 VRHE using 0.2 Na2SO4 electrolyte with a pH of 10. The multilayer photocathodes were fabricated by sulfurization of DMSO-based molecular precursor films on Molybdenum-coated glass substrates at 620 °C, followed by subsequent sputtering deposition of Zn(O,S), Nb-doped TiO2 and Pt overlayers at room temperature. Despite its high photocurrent, however, this particular multilayer photocathode suffers from a photocurrent instability to reduce water which can partially be attributed to the detrimental defect-related problem in the CZTS/Zn(O,S) junction or in the bulk photoabsorber CZTS. In this regard, to study what causes the photocurrent instability, a set of systematic post-annealing experiments was performed on (i) CZTS photoabsorber, (ii) CZTS/Zn(O,S) heterojunction and (ii) the entire multilayer CZTS/Zn(O,S)/Nb-doped TiO2/Pt photocathode at moderate temperatures (200 – 330 °C) under various atmospheres (ambient air and N2). It was revealed that post-annealing of the CZTS/Zn(O,S) heterojunction in N2 at 330°C for 10 minutes provides superior photocurrent stability, denoted by much slower photocurrent decay within 15 minutes of chronoamperometry stability test. This result provides an understanding that the CZTS/Zn(O,S) heterojunction quality is the decisive factor for kesterite CZTS-based photocathode application in long-term PEC water reduction.