TiCl4 precursor affecting the performance of HTM-free carbon-based perovskite solar cell

Abstract The presence of TiO 2 used as an efficient electron transport layer is crucial to achieving high-performance solar cells, especially for a hole transport material (HTM)-free carbon-based perovskite solar cell (PSC). The hydrolysis of TiCl 4 is one of the most widely used routes for forming TiO 2 layer in solar cells, which includes the stock solution preparation from TiCl 4 initial precursor and the thermal hydrolysis of the stock solution. The second thermal hydrolysis step has been extensively studied, while the initial hydrolysis reaction in the first step is not receiving sufficient attention, especially for its influence on the photovoltaic performance of HTM-free carbon-based devices. In this study, the role of TiCl 4 stock solution in the growth process of TiO 2 layer is examined. Based on the analysis of the Ti(IV) intermediate states for different TiCl 4 concentrations from Raman spectra, 2 M TiCl 4 precursor exhibits moderate nucleation and growth kinetics without generating too many intermediates which occurs in 3 M TiCl 4 precursor, yielding ∼300 nm size spherical TiO 2 agglomerates with a rutile phase. In the aspect of devices, the HTM-free carbon-based PSCs fabricated using 2 M TiCl 4 precursor deliver a conversion efficiency beyond 17%, which may be attributed to the reduced defect in compact TiO 2 layer.