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

The presence of TiO2 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. The hydrolysis of TiCl4 is one of the most widely used routes for forming TiO2 layer in solar cells. In this study, the role of TiCl4 precursor concentration in the growth process of TiO2 layer on FTO substrate for HTM-free carbon-based perovskite solar cells is examined. Based on the analysis of the Ti(IV) intermediate states for different TiCl4 concentrations from Raman spectra, 2 M TiCl4 precursor exhibits moderate nucleation and growth kinetics without generating too many intermediates which occurs in 3 M TiCl4 precursor, yielding ~300 nm-size spherical TiO2 agglomerates with a rutile phase. In the aspect of devices, although different TiCl4 precursor concentrations show a similar film coverage and bandgap, the HTM-free carbon-based perovskite solar cells fabricated using 2 M TiCl4 precursor delivers a conversion efficiency beyond 17%, higher than that based on 1M and 3M TiCl4 precursor, which may be attributed to the reduced defect in compact TiO2 layer.&#xD.