Effects of Porosity and Amount of Surface Hydroxyl Groups of a Porous TiO2Layer on the Performance of a CH3NH3PbI3Perovskite Photovoltaic Cell

The structural and physicochemcal properties of a porous titanium oxide layer (pTiO2) in CH3NH3PbI3perovskite photovoltaic cells were systematically investigated by Raman spectroscopy, nitrogen sorption, and temperature desorption spectroscopy analyses. When the heat treatment temperature (TpTO) during the fabrication of pTiO2was changed from 400 to 700 °C, its porosity and amount of surface hydroxyl groups were varied without alteration of the crystalline structure (anatase). Power conversion efficiencies (PCEs) of solar cells based on pTiO2prepared at different temperatures showed a volcanic-like pattern depending on TpTOof pTiO2; the highest PCE was obtained by using pTiO2prepared at TpTOof 550 °C. Structural analyses of the CH3NH3PbI3perovskite part performed by X-ray diffraction indicated that formation of CH3NH3PbI3perovskite was inhibited by the presence of a large amount of surface hydroxyl groups on pTiO2prepared at relatively low TpTO(<550 °C). On the other hand, significant reduction of porosity of pTiO2occurred when pTiO2was prepared at relatively high TpTO(>550 °C) because of extinction of micropores and sintering between the TiO2particles; such a structural alteration hindered the penetration of CH3NH3I into the pore channel of TiO2filled by PbI2, resulting in a large amount of PbI2remaining in the finally obtained photovoltaic cell. Hence, the optimum TpTO(550 °C) for fabrication of pTiO2should be determined by its porous nature and sufficient removal of surface hydroxyl groups.