Effects of Porosity and Amount of Surface Hydroxyl Groups of a Porous TiO2 Layer on the Performance of a CH3NH3PbI3 Perovskite Photovoltaic Cell

Abstract

The structural and physicochemcal properties of a porous titanium oxide layer (pTiO2) in CH3NH3PbI3 perovskite 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 pTiO2 was 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 pTiO2 prepared at different temperatures showed a volcanic-like pattern depending on TpTO of pTiO2; the highest PCE was obtained by using pTiO2 prepared at TpTO of 550 °C. Structural analyses of the CH3NH3PbI3 perovskite part performed by X-ray diffraction indicated that formation of CH3NH3PbI3 perovskite was inhibited by the presence of a large amount of surface hydroxyl groups on pTiO2 prepared at relatively low TpTO (<550 °C). On the other hand, significant reduction of porosity of pTiO2 occurred when pTiO2 was prepared at relatively high TpTO (>550 °C) because of extinction of micropores and sintering between the TiO2 particles; such a structural alteration hindered the penetration of CH3NH3I into the pore channel of TiO2 filled by PbI2, resulting in a large amount of PbI2 remaining in the finally obtained photovoltaic cell. Hence, the optimum TpTO (550 °C) for fabrication of pTiO2 should be determined by its porous nature and sufficient removal of surface hydroxyl groups.