Temperature Dependence of the Band Gap of CH3NH3PbI3 Stabilized with PMMA: A Modulated Surface Photovoltage Study

CH3NH3PbI3 layers chemically stabilized with poly(methyl methacrylate) (PMMA), relevant for photovoltaic applications, have been investigated by modulated surface photovoltage (SPV) spectroscopy at temperatures (T) between -182 and 60 degrees C. SPV is sensitive only to the PMMA/CH3NH3PbI3 interface region where photogeneration and charge separation take place. The T dependencies of the Tauc gap (Eg-Tauc, equivalent to absorption measurements) and of the gap determined from the maximum slope (Eg-tp, almost at increased absorption) were analyzed on the basis of the in-phase SPV spectra. At 32 degrees C, the values of Eg-Tauc and Eg-tp were 1.540 and 1.560 eV, respectively. A jump of Eg-Tauc and Eg-tp by 10 meV at 40 degrees C was interpreted as the transition from the tetragonal to the cubic phase at T lower than values known from literature. Eg-Tauc and Eg-tp of the cubic phase decreased with increasing T. In contrast, Eg-Tauc and Eg-tp of the tetragonal phase decreased moderately with decreasing T to 1.528 and 1.546 eV at -182 degrees C, respectively. No signature has been observed in Eg-Tauc and Eg-tp for the transition from the tetragonal to the orthorhombic phase. Structural interactions at PMMA/CH3NH3PbI3 interfaces seem important for phase transitions in CH3NH3PbI3 layers.