Role of TiO2 Thickness on Depletion Properties of TiO2/CH3NH3PbI3 Heterojunction

It is well recognized that the thickness of TiO2 film serving as hole blocking layer (bl-TiO2) in planar n-i-p structured perovskite solar cells is required to be thin for the purpose of reducing charge transporting resistance. However, the TiO2 film thickness should be multi-dimensional considering that a too thin bl-TiO2 film might not achieve full coverage on the rough surface of substrate. Moreover, the TiO2/perovskite heterojunction properties could also be strongly affected by the bl-TiO2 thickness, which has rarely been investigated as far as we know. Herein, we deposited the different thickness of bl-TiO2 layer through a multi-spin-coating process with a mildly acidic solution of titanium isopropoxide in ethanol. The thickness of bl-TiO2 layer, roughness of FTO/bl-TiO2 surface, coverage of bl-TiO2 on FTO and the transmission variation of FTO/bl-TiO2 were systematically reported. Additionally, the depleted degree of TiO2/perovskite heterojunction was found to be strongly affected by n-side bl-TiO2 thickness, which was analytically discussed by the photoluminescence (PL) measurement. The PL emission spectra incident from different sides of FTO/bl-TiO2/perovskite structure, excitation power dependent PL spectra and PL excitation spectra at different detected wavelength were comprehensively conducted to reveal the band bending effect and depletion thickness across the heterojunction.