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

Abstract

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.

Inhibition of a structural phase transition in one-dimensional organometal halide perovskite nanorods grown inside porous silicon nanotube templates

“…This mechanism seems to be consistent with our findings of phase transition inhibition in 1D and the decreasing phase transition temperature in 2D, as well as providing a mechanical origin to the structural phase transition in 3D. 69 Notice also a recent report about the possibility of phase transition inhibition in thin films of perovskites stabilized with PMMA, 70 a result that might also be attributed to interfacial stress.…”

Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr3 Single Crystals

“…However, the linear dependencies in the inset and their tangents are similar to those of other perovskite materials, e.g. polycrystalline tetragonal and cubic phases of MAPI 3 , MAPbI 3-x Br x and MAPbI 3-x Cl x [17,22], powders of orthorhombic, tetragonal, and cubic phases of MAPbBr 3 and FAPbBr 3 [23], and polycrystalline films of CsSnI 3 [24]. …”

Investigation of Structural and Electronic Properties of CH3NH3PbI3 Stabilized by Varying Concentrations of Poly(Methyl Methacrylate) (PMMA)

“…Furthermore, the phase shifted by 90 • signal was smaller in magnitude compared to in-phase signal. This suggests that trapping and de-trapping of charge carriers may lead to a slow response of the SPV spectra [18]. Since in-phase and phase shifted by 90 • signals had the same signs, charge separation was caused by the two mechanisms of charge separation and that relaxation of SPV signals was due to recombination processes [24].…”

“…Recently, Dittrich et al reported on the temperature dependence of the band gap of CH 3 NH 3 PbI 3 stabilized with PMMA using SPV and observed a jump in the band gap at 40 • C corresponding to transition from the tetragonal to cubic phase. In contrast, there was no signature in the band gap for transition from the tetragonal to orthorombic phase and concluded that there were structural interactions between CH 3 NH 3 PbI 3 and PMMA interfaces [18]. Charge transport properties and structural properties between perovskite and PMMA were investigated using modulated surface photovoltage spectroscopy (SPV), photoluminescence (PL), and grazing incidence X-ray diffraction (GIXRD).…”

“…Compared to other methods, modulated SPV spectroscopy can give information on onset energies, change of sign, defect states below the band gap [22], and can also be applied to measure diffusion length using the method developed by Goodmann [23]. Furthermore, SPV is very sensitive at low light intensities where charge separation takes place [18]. SPV can be used to study exponential defect states below the band gap of CH 3 NH 3 PbI 3 and previous studies showed that the addition of a small amount of ammoniumvaleric acid iodide (AVAI) into solution containing CH 3 NH 3 I and PbCl 2 leads to a reduction in the value of defects below the band gap, and hence a reduction in the disorder in the material [24].…”