EcoMat：原位技术探究钙钛矿太阳能电池降解机制

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成果简介

在过去十年中，钙钛矿太阳能电池 (PSC)的性能出现了惊人的提升。最先进的器件效率已超过25%，与硅太阳能电池相当。虽然PSC性能优异，但它们的寿命却落后于传统的硅技术。湿度、热量和光线等环境因素都会对PSC的性能产生不利影响并限制其寿命。系统地阐明和消除PSC降解途径对于该技术的成功至关重要。原位技术可以实时追踪结构、成分、形态和光电变化。鉴于此，萨斯喀彻温大学Timothy L. Kelly团队在EcoMat发表了题为“In situ studies of the degradation mechanisms of perovskite solar cells”的综述性文章，概述了钙钛矿降解过程，重点集中在原位研究。

内容详情

FIGURE 1 Overview of the various analytical tools used to study moisture, heat, and light-induced perovskite degradation pathways

FIGURE 2 A, In situ UV/vis spectra (acquired at 15 minutes intervals) of a MAPbI₃ film exposed to 98±2% RH, B, The proposed moisture-induced decomposition pathway of MAPbI₃, showing the 3D tetragonal structure of MAPbI₃, the 1D chain-like structure of the MAPbI₃·H₂O intermediate, and the 2D layered structure of PbI₂

FIGURE 3 SFM images and profiles of the MAPbI₃ films exposed to 80% RH (20°C) in the dark at t = A, 0 hour, B, 4 hours, C, 6 hours, D, 8 hours, E, after flushing with dry N₂ overnight (13 hours), and, F, 80% humidity for 144 hours.

FIGURE 4 A, LBIC EQE maps (at 532 nm) of PSCs when exposure to 50 ± 5% RH. B, Areal average LBIC EQE (at 532 nm) as a function of time after exposure to humidity. C, 3D reconstructed images of perovskite films at different exposure times, as obtained from spatially resolved ToF-SIMS analysis. The color map illustrates the molecular density from high (bright green) to low (blue).

FIGURE 5 A, Operando I–V curves (acquired at 1-minute intervals; t = 0 hour, purple; t = 1.6 hours, red) and, B, corresponding azimuthally-integrated GIWAXS data for an FTO/ZnO/MAPbI₃/P3HT/Ag PSC. C, Operando I–V curves (acquired at 1-minute intervals; t = 0 h, purple; t = 6 hours, red) and, D, corresponding azimuthally-integrated GIWAXS data for an FTO/ZnO/MAPbI₃/P3HT/Au PSC. E, Contour plots of in situ XRD data and corresponding device performance parameters for PSCs exposed to a nitrogen atmosphere with RH = 65% and T = 25°C.

FIGURE 6 TG-DTA traces (upper panel, dark green, and blue color, respectively.) and the m/z peaks registered simultaneously during the thermal degradation (heating rate of 20°C/min) of MAPbI₃. Dashed green lines in the m/z vs time contour plots are guides to the eye delimiting the mass loss steps in the TG/DTA graph.

FIGURE 7 Experimental in situ setup, temperature profile, and STEM imaging of FAPbI₃. A, To resolve the evolving microstructure and evolution of FAPbI3 we performed a controlled temperature study of a FA-based PSC inside a heated in situ gas cell from 50°C to 225°C while flowing inert argon gas. B, A specific temperature profile. C, To track the microstructure, atomic contrast STEM plane-view imaging was performed, tracking a specific region, outlined by a white box, zoomed-in micrographs from a to i are shown below. D, In addition, an accompanying series of normalized image histograms for micrographs are provided for comparison from grain interiors (blue curve) and areas containing multiple grain boundaries (red). The x-axis is the normalized grayscale from 0 to 1 and the y-axis is the number of pixels with that gray value taken from either an area containing a grain interior (blue curve) or boundary (red curve).

FIGURE 8 A, Variable-temperature (80°C to 320°C) XRD data for Cs_xFA_1 −xPbIyBr_3−y; the black, blue, and red curves indicate the temperature ranges in which perovskite, PbI₂, and δ-CsPbI₃−_zBr_z are the dominant phases, respectively. A, Expanded view of the pXRD data from, A, showing the shift in the position of the (100) peak as the temperature is varied from 80°C to 220°C. C, Decomposition kinetics with respect to change in the 2θ position the perovskite main peak (100) and the impurity ratio (PbI₂+δ-CsPbI₃-_zBr_z)/perovskite obtained via integrating the main XRD peak. D, Schematic illustration of the degradation process.

FIGURE 9 Heat-induced ion migration. HAADF images and EDX elemental maps for iodine and lead for FTO/compact TiO₂/mesoporous TiO₂/ MAPbI₃/spiro-MeOTAD/Au acquired after heating at different temperatures. The heating steps were carried out for 30 minutes up to 175°C, and for 15 minutes from 200°C owing to the faster sample dynamics. The same scale bar applies to all panels. Diffusion of iodine into the HTM is clearly visible at low temperature, and lead migration is triggered at higher temperature (∼175°C)

FIGURE 10 A, Decomposition kinetics of MAPbI₃ at 77°C under vacuum/light monitored by in situ XRD. B, Integrated intensity calculated from in situ XRD patterns of MAPbI₃ films at 77°C under vacuum/light. Selected peaks assigned to MAPbI₃ (2θ = 28.5°), PbI₂ (2θ = 39.5°), and Pb⁰ (2θ = 36.3°). 

FIGURE 11 Confocal microscopy images of a (PEA)₂PbI₄ flake at increasing illumination time under a 488 nm laser.

FIGURE 12 Light-induced halide phase segregation followed by different in situ tools. A, Photoluminescence spectra of a MAPb(Br_0.4I_0.6)₃ thin film over 45 seconds in 5 seconds increments under 457 nm, 15 mW cm⁻² light at 27°C. B, In situ GIWAXS of MAPb(I0.4Br0.6)3 (100) diffraction peaks before illumination, during, and after relaxation in the dark. C, Normalized 100 peak area under illumination over time. D, In situ EDX of the I 4d spectra measured with a photon energy of 120 eV and, E, Br 3d spectra measured with a photon energy of 139 eV before, during, and after laser illumination at 515 nm and a power of 0.52 mW. The black horizontal lines indicate where the laser was switched on (5 minutes) and off (35 minutes). Only one spin doublet is seen in each case, indicating that no formation of new I and Br species is observed in the solid state. F, Comparison of the intensities of Pb 5d, I 4d, and Br 3d vs time obtained by fitting the individual spectra. The intensities are normalized to the intensity before laser illumination for each core level. Linear fit lines are included during and after laser illumination.

FIGURE 13 Halide ion segregation monitored by in situ optical microscopy. Photographs of the photoluminescence of a mixed halide film kept under illumination at different time scales. The green and red emissions come from different areas in the sample

结论

卤化铅钙钛矿的功率转换效率创下历史新高，但它们的不稳定键和移动离子通常导致器件的长期稳定性较差。先进的表征技术以及原位测试方法有助于揭示水分、热量和光如何触发钙钛矿晶格的化学和物理变化，大大提高了我们对钙钛矿降解机制的了解。已有大量工作致力于系统地消除这些降解途径。现在已有许多既具有良好性能又具有较长寿命的PSC，希望未来PSC的发展和提升可以实现商业可行且极具竞争力的光伏技术。

文献信息

Soumya Kundu, Timothy L. Kelly,* In situ studies of the degradation mechanisms of perovskite solar cells, EcoMat. 2020;2: e12025.

原文链接：https://doi.org/10.1002/eom2.12025

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