Nanostructural and photo-electrochemical properties of solution spin-coated Cu₂ZnSnS₄-TiO₂ nanorod forest films with an improved photovoltaic performance

Cu₂ZnSnS₄ (CZTS), other than in standard p-n junction device architecture, can be employed as a broad light absorber upon coating onto a wide bandgap electron conducting TiO₂ film. Earlier CZTS sensitized TiO₂ films have yielded low photoconversion efficiency and V_oc. In this work, a water-ethanol solution spin coating approach has been applied to directly deposit CZTS nanocrystallites on rutile TiO₂ nanorods grown on an FTO substrate (TNR) for evaluation in a sensitized solar cell configuration. The FTO@TNR@CZTS photoanode following controlled annealing is shown to exhibit improved photovoltaic properties. Focused-ion beam cross-sections of CZTS nanocoating onto TiO₂ nanorod forest films have revealed nanoscale morphological details and electrochemical impedance spectroscopy helped identify TiO₂ nanorod film growth characteristics for a reduced charge recombination. The band alignment of CZTS and TiO₂ has been determined by XPS helping to explain the origin of V_oc deficit. An all-solid state device featuring spiro OMeTAD as HTM and CdS as a buffer layer has been designed with 2% efficiency.