Multiple-Stage Structure Transformation of Organic-Inorganic Hybrid Perovskite ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$

By performing spatially resolved Raman and photoluminescence spectroscopy with varying excitation wavelength, density, and data acquisition parameters, we achieve a unified understanding towards the spectroscopy signatures of the organic-inorganic hybrid perovskite, transforming from the pristine state (${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$) to the fully degraded state (i.e., ${\mathrm{PbI}}_2$) for samples with varying crystalline domain size from mesoscopic scale (approximately 100 nm) to macroscopic size (centimeters), synthesized by three different techniques. We show that the hybrid perovskite exhibits multiple stages of structure transformation occurring either spontaneously or under light illumination, with exceptionally high sensitivity to the illumination conditions (e.g., power, illumination time, and interruption pattern). We highlight four transformation stages (stages I–IV, with stage I being the pristine state) along either the spontaneous or photoinduced degradation path exhibiting distinctly different Raman spectroscopy features at each stage, and point out that previously reported Raman spectra in the literature reflect highly degraded structures of either stage III or stage IV. Additional characteristic optical features of partially degraded materials under the joint action of spontaneous and photodegradation are also given. This study offers reliable benchmark results for understanding the intrinsic material properties and structure transformation of this unique category of hybrid materials, and the findings are pertinently important to a wide range of potential applications where the hybrid material is expected to function in greatly different environment and light-matter interaction conditions.

Popular Summary

A solar-cell absorber material, $({\mathrm{CH}}_3{\mathrm{NH}}_3){\mathrm{PbI}}_3({\mathrm{MAPbI}}_3)$, has demonstrated significant increases in its efficiency in less than ten years. However, ensuring the structural stability of this material is key during light-matter interactions in solar-cell operation and fundamental research. Here, we investigate the stability of this organic-inorganic hybrid perovskite using Raman spectroscopy and, for the first time, reveal multiple stages of structural transformation.

By performing Raman and photoluminescence measurements on four classes of perovskite samples with crystal domain sizes ranging from 100 nm to centimeter scale, we observe characteristic peaks in the resulting Raman and photoluminescence spectra at different stages. Our experiments were conducted at room temperature in the presence of ${\mathrm{N}}_2$. Photodegradation can occur based on various illumination conditions such as the power, illumination time, and interruption pattern. We characterize multiple stages of photodegradation or spontaneous degradation: Stages I–IV, where stage I corresponds to the pristine material and stage IV refers to a severely photodegraded sample. We use these stages to refer to the structure transformation of the hybrid perovskite from pristine ${\mathrm{MAPbI}}_3$ to ${\mathrm{PbI}}_2$. We find that this hybrid structure is extremely unstable under light illumination: Its photodegradation threshold is approximately 4 orders of magnitude lower than that of a typical semiconductor such as Si or GaAs. Additionally, the photodegradation threshold increases with crystalline domain size. Furthermore, our results reveal that the previously reported Raman spectra of ${\mathrm{MAPbI}}_3$ were in fact the spectra of severely or fully degraded hybrid structures (i.e., stage III or IV), which makes it difficult to accurately infer the properties of these materials.

The conclusions of this work are universally valid for small-domain polycrystalline to single-crystal samples and highlight an efficient way for monitoring material photodegradation.

Figure 1

SEM images of ${\mathrm{MAPbI}}_3$ hybrid perovskite samples: (a) a SKKU-p sample (with a large domain shown as an inset), (b) a NREL-p sample, (c) a degraded SKKU-p sample, (d) a SNU-c sample.

Figure 2

Raman and PL spectra of SKKU samples at different stages of structure transformation. (a),(b) Representative Raman spectra at different stages of structure transformation from ${\mathrm{MaPbI}}_3$ to ${\mathrm{PbI}}_2$, measured under a very low excitation density D4 (SKKU-p-3 was measured with the $50\times \mathrm{L}$ lens, but multiplied by the laser density ratio between the $100\times$ and $50\times \mathrm{L}$ lens). (c) PL spectra measured at D4 from the same location as in (a) and (b) before the Raman measurement. (d) Raman excitation density dependence of the most pristine sample SKKU-p-1. (e) Low-excitation density (D4) Raman spectra for two large-domain SKKU-L samples. (f) The corresponding PL spectra of (e), measured under D4 before the Raman measurement. The inset shows the spectral region near the ${\mathrm{PbI}}_2$ band gap.

Figure 3

Raman and PL spectra of NREL-p samples measured at different conditions. (a) D4 Raman spectra measured with different interruptions. (b) Raman spectra measured under different excitation density D4–D2. (c) $\mathrm{D}4/800\mathrm{s}/1\mathrm{s}/4\mathrm{s}$ measurements before and after the measurements shown in (b). (d) PL spectra measured before and after all the Raman measurements, using both a 442- and a 532-nm laser.

Figure 4

Raman and PL spectra of SNU-c samples measured at different conditions. (a)–(c) Low-excitation density (D4) Raman spectra measured before and after different higher excitation density measurements. (d) D4 PL spectra measured before the Raman measurement shown in (a) and after the Raman measurements shown in (b) and (c). (e)  Raman spectra measured at a moderately high excitation density D2 under different data collection conditions. (f) Comparison of the D4 PL spectra for four types of samples, SKKU-p, SKKU-L, NREL-p, and SNU-c, and an absorption spectrum of a SNU-c sample. The inset shows a room-temperature GaAs PL spectrum.

Figure 5

Effects of multiple consecutive Raman measurements in SKKU-p samples under D2 excitation. (a) Sixteen consecutive measurements with $\sim 20\mathrm{s}$ in between. (b) PL spectra measured after the Raman measurements shown in (a). (c) Eight consecutive measurements with $\sim 20\mathrm{s}$ in between, then waited for 10 min before the last one (no. 9). (d) Twenty-two consecutive measurements with $\sim 20\mathrm{s}$ in between, then waited for 10 min before the last one (no. 23).

Figure 6

Effects of multiple consecutive Raman measurements in SNU-c samples under D2 excitation. Panels (a) and (b) show spatial inhomogeneity. Panels (c) and (d) show the effect of long illumination time.

Figure 7

Photo-stability of the PL spectra. (a) and (b) for two SKKU-p samples, low excitation density PL spectra before and after a higher power flash ($\sim 1\mathrm{s}$). (c) and (d) for two SKKU-L samples, PL spectra measured before and after the Raman measurements shown in Fig. 2 and 2. (e) Consecutively measured PL spectra of a SKKU-p sample under very low excitation density. (f) Consecutively measured PL spectra of a SNU-c sample under very low excitation density.

Figure 8

Raman and PL spectra of partially or fully degraded samples. (a) Comparison of partially photodegraded SKKU sample between large and small domain (the inset shows the spectra in a larger spectral range). (b) Nearly fully degraded SKKU samples (including two distinctively different large domains), and the inset shows the spectra in a smaller spectral range. (c) PL spectra of the samples shown in (b). (d) The effect of photoinduced structure transformation in an already partially degraded sample due to the course of nature, showing a common Raman peak at around $250{\mathrm{cm}}^{-1}$. (e) PL spectra of a partially degraded SKKU-L sample measured before and after high-excitation density Raman measurement (the inset shows the corresponding Raman spectra). (f) Raman and PL spectra of an atypical location in a SKKU-p sample.