Effects of TiCl treatment on the structural and electrochemical properties of a porous TiO layer in CHNHPbI perovskite solar cells.

The effects of surface treatment with TiCl on the structural and electrochemical properties of a porous titanium oxide (pTiO) layer deposited on a fluorine-doped tin oxide (FTO)/glass substrate covered with a dense TiO layer (pTiO/dTiO/FTO/glass) were systematically investigated in order to obtain an optimum pTiO layer for use in CHNHPbI perovskite solar cells. As confirmed by thermal desorption spectroscopy (TDS) analyses, the amount of surface hydroxyl groups in pTiO varied when the pTiO/dTiO/FTO/glass sample was treated with solutions with different concentrations of TiCl (i.e., 20, 50, 80, and 100 mM). Photoelectrochemical (PEC) analyses of the pTiO/dTiO/FTO/glass samples after TiCl treatment showed significant increments of photocurrent densities compared to the pTiO/dTiO/FTO/glass sample without TiCl treatment regardless of the concentration of TiCl used in the solution. Electrochemical impedance spectroscopy (EIS) analyses of the TiCl-treated pTiO/dTiO/FTO/glass samples also indicated a lower recombination probability with an increase in TiCl concentration. The results suggest that TiCl treatment resulted in passivation of defect sites on the surface of the TiO nanoparticles as well as improvement of the interconnectivity between the TiO nanoparticles in pTiO. In contrast, the power conversion efficiencies (PCEs) and short circuit current densities of CHNHPbI perovskite solar cells based on these pTiO/dTiO/FTO/glass samples exhibited volcano-like patterns depending on the TiCl concentration used for the pTiO treatment: the highest PCE was obtained by using pTiO/dTiO/FTO/glass treated with 50 mM of TiCl solution. Structural analysis of the CHNHPbI perovskite part performed by X-ray diffraction (XRD) indicated that the formation of CHNHPbI perovskite was inhibited by the presence of surface hydroxyl groups in the pTiO film without TiCl treatment. TiCl treatment using TiCl solutions with concentrations up to 50 mM enhanced the formation of the CHNHPbI perovskite layer, whereas TiCl treatment using TiCl solutions with concentrations higher than 50 mM was detrimental due to the formation of nanoparticulate TiO aggregates that induce poor porosity and act as recombination sites.