The thermal evaporation of Cu2ZnSnS4 (CZTS) nanoparticles synthesized by changing the sulfur ratio in the precursor solution was used to investigate the critical role of sulfur concentration in the photoelectrochemical (PEC) water splitting reactions of the CZTS photocathode. X-ray diffraction, transmission electron microscopy, UV–visible spectroscopy, impedance spectroscopy, capacity measurement, and PEC performance measurement were used to investigate the effect of precursor sulfur concentration on the structural, morphological, optical, and PEC performance of films. The half-cell solar-to-hydrogen efficiency of CZTS photoelectrodes increased as the sulfur content increased. The power conversion efficiency of the CZTS cathode constructed with CZTS nanoparticles synthesized with a 0.8 to 1.0 M precursor sulfur concentration was 0.32% at 0 V versus RHE under AM 1.5 illumination. The increase in crystallite size and charge carrier concentration is observed with increasing precursor sulfur concentration, and the maximum crystallite size and charge carrier concentration are observed at 1.0 M sulfur concentration. By carefully adjusting the crystallite size and thus the charge carrier concentration of the CZTS material, the PEC water splitting efficiency of the CZTS photocathode can be improved.
A photoanode consisting of hydrothermally synthesized TiO2 nanotubes (TNT) and TiO2 nanoparticles (TNP) was designed for efficient charge collection in dye-sensitized solar cells. TNT and TNP films were fabricated on a conductive glass substrate by using electrophoretic deposition and doctor-blade methods, respectively. The TNP, TNT, and TNT/TNP bi-layer electrodes exhibit solar cell efficiencies of 5.3, 7.4, and 9.2%, respectively. Solar cell performance results indicate a higher short-circuit current density (Jsc) for the TNT/TNP bi-layer electrode when compared to a TNT or TNP electrode alone. The open-circuit voltages (Voc) of TNT/TNP and TNT electrodes are comparable while the Voc of TNP electrode is inferior to that of the TNT/TNP electrode. Fill factors of TNT/TNP, TNT, and TNP electrodes also exhibit similar behaviors. The enhanced efficiency of the TNT/TNP bi-layer electrode is found to be mainly due to the enhancement of charge collection efficiency, which is confirmed by the charge transport parameters measured by electrochemical impedance spectroscopy (EIS). EIS analyses also revealed that the TNT/TNP incurs smaller charge transport resistances and longer electron life times when compared to those of TNT or TNP electrodes alone. It was demonstrated that the TNT/TNP bi-layer electrode can possess the advantages of both rapid electron transport rate and a high light scattering effect.
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