Effects of TiCl4 treatment on the structural and electrochemical properties of a porous TiO2 layer in CH3NH3PbI3 perovskite solar cells

The effects of surface treatment with TiCl4 on the structural and electrochemical properties of a porous titanium oxide (pTiO2) layer deposited on a fluorine-doped tin oxide (FTO)/glass substrate covered with a dense TiO2 layer (pTiO2/dTiO2/FTO/glass) were systematically investigated in order to obtain an optimum pTiO2 layer for use in CH3NH3PbI3 perovskite solar cells. As confirmed by thermal desorption spectroscopy (TDS) analyses, the amount of surface hydroxyl groups in pTiO2 varied when the pTiO2/dTiO2/FTO/glass sample was treated with solutions with different concentrations of TiCl4 (i.e., 20, 50, 80, and 100 mM). Photoelectrochemical (PEC) analyses of the pTiO2/dTiO2/FTO/glass samples after TiCl4 treatment showed significant increments of photocurrent densities compared to the pTiO2/dTiO2/FTO/glass sample without TiCl4 treatment regardless of the concentration of TiCl4 used in the solution. Electrochemical impedance spectroscopy (EIS) analyses of the TiCl4-treated pTiO2/dTiO2/FTO/glass samples also indicated a lower recombination probability with an increase in TiCl4 concentration. The results suggest that TiCl4 treatment resulted in passivation of defect sites on the surface of the TiO2 nanoparticles as well as improvement of the interconnectivity between the TiO2 nanoparticles in pTiO2. In contrast, the power conversion efficiencies (PCEs) and short circuit current densities of CH3NH3PbI3 perovskite solar cells based on these pTiO2/dTiO2/FTO/glass samples exhibited volcano-like patterns depending on the TiCl4 concentration used for the pTiO2 treatment: the highest PCE was obtained by using pTiO2/dTiO2/FTO/glass treated with 50 mM of TiCl4 solution. Structural analysis of the CH3NH3PbI3 perovskite part performed by X-ray diffraction (XRD) indicated that the formation of CH3NH3PbI3 perovskite was inhibited by the presence of surface hydroxyl groups in the pTiO2 film without TiCl4 treatment. TiCl4 treatment using TiCl4 solutions with concentrations up to 50 mM enhanced the formation of the CH3NH3PbI3 perovskite layer, whereas TiCl4 treatment using TiCl4 solutions with concentrations higher than 50 mM was detrimental due to the formation of nanoparticulate TiO2 aggregates that induce poor porosity and act as recombination sites.