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

The structural and physicochemcal properties of a porous titanium oxide layer (pTiO(2)) in CH3NH3PbI3 perovskite photovoltaic cells were systematically investigated by Raman spectroscopy, nitrogen sorption, and temperature desorption spectroscopy analyses. When the heat treatment temperature (T-pTO) during the fabrication of pTiO(2) was changed from 400 to 700 degrees 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 pTiO(2) prepared at different temperatures showed a volcanic-like pattern depending on T-pTO of pTiO(2); the highest PCE was obtained by using pTiO(2) prepared at T-pTO of 550 degrees 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 pTiO(2) prepared at relatively low T-pTO (<550 degrees C). On the other hand, significant reduction of porosity of pTiO(2) occurred when pTiO(2) was prepared at relatively high T-pTO (>550 degrees 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 T-pTO (550 degrees C) for fabrication of pTiO(2) should be determined by its porous nature and sufficient removal of surface hydroxyl groups.