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

CH 3 NH 3 PbI 3 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 °C. SPV is sensitive only to the PMMA/CH 3 NH 3 PbI 3 interface region where photogeneration and charge separation take place. The T dependencies of the Tauc gap ( E g-Tauc, equivalent to absorption measurements) and of the gap determined from the maximum slope ( E g-tp, almost at increased absorption) were analyzed on the basis of the in-phase SPV spectra. At 32 °C, the values of E g-Tauc and E g-tp were 1.540 and 1.560 eV, respectively. A jump of E g-Tauc and E g-tp by 10 meV at 40 °C was interpreted as the transition from the tetragonal to the cubic phase at T lower than values known from literature. E g-Tauc and E g-tp of the cubic phase decreased with increasing T . In contrast, E g-Tauc and E g-tp of the tetragonal phase decreased moderately with decreasing T to 1.528 and 1.546 eV at −182 °C, respectively. No signature has been observed in E g-Tauc and E g-tp for the transition from the tetragonal to the orthorhombic phase. Structural interactions at PMMA/CH 3 NH 3 PbI 3 interfaces seem important for phase transitions in CH 3 NH 3 PbI 3 layers.