Domain Size, Temperature, and Time Dependence of Photodegradation in MAPbI₃ Probed by Raman Spectroscopy

Abstract

Organic–inorganic hybrid perovskites display remarkable photovoltaic properties, but instability arises from the material’s surface and photoexcitation. The photodegradation process of methylammonium lead triiodide (MAPbI3) investigated with above bandgap excitation Raman spectroscopy displays two stages of structural change in polycrystalline films of varying domain sizes and a single crystal. Subtle surface changes occur below an excitation density threshold; PbI2 formation scales linearly above this threshold in thin films. However, the rate of change with excitation density increases with more grain surfaces. Whereas with a moderate level of heating into the cubic phase, accelerated photodegradation occurs in a single crystal. From experimental kinetic modeling, PbI2 nuclei on the surface grow until they meet in the polycrystalline films, proceeding one-dimensionally inward from the surface. These results suggest that the surface acts as a location for photogenerated carrier accumulation, leading to degradation and PbI2 formation.

Mechanosynthesis of MAPbI3@Graphite Composites with Active Interfaces and Promising Photodetection Properties

“…Surprisingly, intense and better-resolved bands are observed in the Raman spectra of composites. For the sample with 1 wt % graphite, the two bands at 95 and 110 cm –1 and the weak band at 166 cm –1 are characteristic of PbI 2 , the main degradation product of MAPbI 3 . − The spectrum of the composite with 5 wt % graphite is very different from the previous one with no characteristic peaks of PbI 2 , and it matches quite well with Raman spectra reported for MAPbI 3 . , Four bands are identified: the one at 81 cm –1 is attributed to the asymmetric stretching mode of Pb–I and that at 135 cm –1 to the liberation mode of methylammonium; ,, the two weak and broad bands at 266 and 363 cm –1 might be associated with torsional modes of methylammonium as they are predicted to be present in the range of 200–400 cm –1 …”

“…Raman spectroscopy was also carried out in the range of 0–500 cm –1 for ground MAPbI 3 and two composites doped with 1 and 5% graphite (Figure ). The spectrum of MAPbI 3 alone displayed a low resolution as often reported, which could be due to the strong absorption of HP leading to decomposition or phase transformation and to the strong photoluminescence nature of hybrid perovskite. − Indeed, the photodegradation tendency of MAPbI 3 makes it difficult to obtain well-resolved Raman spectra, even if calculated Raman spectra reported well-resolved spectral features. , The very broad and weak band at ∼100 cm –1 could be attributed to the Pb–I stretching modes and the very broad and weak one at around 250 cm –1 to the torsion mode of the methylammonium cation . Surprisingly, intense and better-resolved bands are observed in the Raman spectra of composites.…”

“…76−78 The spectrum of the composite with 5 wt % graphite is very different from the previous one with no characteristic peaks of PbI 2 , and it matches quite well with Raman spectra reported for MAPbI 3 . 76,79 Four bands are identified: the one at 81 cm −1 is attributed to the asymmetric stretching mode of Pb−I and that at 135 cm −1 to the liberation mode of methylammonium; 75,78,79 the two weak and broad bands at 266 and 363 cm −1 might be associated with torsional modes of methylammonium as they are predicted to be present in the range of 200−400 cm −1 . 79 The observed photodegradation of the composite with 1 wt % graphite when no degradation is noticed with the composites with 5 wt % graphite suggested that 5% graphite allowed limiting the degradation of MAPbI 3 even under a higher laser power (4 mW compared with 0.04 mW for bare MAPbI 3 and the composite with 1 wt % graphite).…”

“…75−78 Indeed, the photodegradation tendency of MAPbI 3 makes it difficult to obtain well-resolved Raman spectra, even if calculated Raman spectra reported wellresolved spectral features. 76,79 The very broad and weak band at ∼100 cm −1 could be attributed to the Pb−I stretching modes and the very broad and weak one at around 250 cm −1 to the torsion mode of the methylammonium cation. 75 Surprisingly, intense and better-resolved bands are observed in the Raman spectra of composites.…”

Reaction Dynamics of C(NH2)3SnI3 Formation from Vacuum-Deposited C(NH2)3I and SnI2 Bilayer Thin Films Investigated by In Situ Infrared Multiple-Angle Incidence-Resolved Spectroscopy

“…This trend would reflect the Arrhenius-type activation step(s) involved in this reaction process: the mutual diffusion of GAI and SnI 2 from the initial bilayer structure and their intermolecular reactions. To understand this transformation kinetics during isothermal reaction, we applied the Avrami model, , a kinetic model based on nuclei growth, which is often used in the kinetic analysis of the transformation of metals and alloys, crystal growth, ,,,,,,, thermal/photodegradation, , and two-solid reactions, , to analyze the experimental data. The equation of the Avrami model is given as α = 1 − exp false[ prefix− ( k t ) n false] where α is the formation ratio, k is the kinetic rate constant, t is the reaction time, and n is the Avrami exponent.…”