Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6737−6746
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In Situ Observation of Light Illumination-Induced Degradation in Organometal Mixed-Halide Perovskite Films Rui-Peng Xu,† Yan-Qing Li,*,† Teng-Yu Jin,† Yue-Qi Liu,† Qin-Ye Bao,*,‡ Conor O’Carroll,§ and Jian-Xin Tang*,† †
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, P. R. China ‡ Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, P. R. China § School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland S Supporting Information *
ABSTRACT: Organometal mixed-halide perovskite materials hold great promise for next-generation solar cells, light-emitting diodes, lasers, and photodetectors. Except for the rapid progress in the efficiency of perovskite-based devices, the stability issue over prolonged light illumination has severely hindered their practical application. The deterioration mechanism of organometal halide perovskite materials under light illumination has seldom been conducted to date, which is indispensable to the understanding and optimization of photonharvesting process inside perovskite-based optoelectronic devices. Here, explicit degradation pathways and comprehensive microscopic understandings of white-light-induced degradation have been put forward for two organometal mixed-halide perovskite materials (e.g., MAPbI3−xClx and MAPbBr3−xClx) under high vacuum conditions. In situ compositional analysis and real-time film characterizations reveal that the decomposition of both mixed-halide perovskites starts at the grain boundaries, leading to the formation of hydrocarbons and ammonia gas with the residuals of PbI2(Cl), Pb, or PbClxBr2−x in the films. The degradation has been correlated to the localized trap states that induce strong coupling between photoexcited carriers and the crystal lattice. KEYWORDS: organometal halide perovskites, in situ characterization, light illumination, electronic structures, degradation
1. INTRODUCTION Organometal halide perovskites have been attracting tremendous attention in optoelectronic devices due to their excellent optical and electrical properties, such as large absorption coefficients, broad wavelength tunability, long carrier diffusion length, and ambipolar charge transport characteristics.1−3 These excellent properties enable perovskites to be promising for applications in solar cells,4−7 light-emitting diodes (LEDs),8−10 lasers,2 photodetectors,11 etc. Among various perovskite materials, methylammonium lead mixed-halides (MAPbX3, MA = CH3NH3, X = Cl, Br, or I) with two of Cl−, Br−, and I− halide anions (e.g., MAPbI3−xClx) are widely used as solar energy harvesters in the field of solar cells because their band gap can be continuously tuned over a large range of wavelengths and their carrier diffusion length can be greatly enhanced with the substitution of halide anions, delivering good device performance.12,13 Therefore, the development of MAPbX3-based solar cells has rapidly intensified, with a power conversion efficiency beyond 22%.14−16 Besides excellent photovoltaic properties, MAPbX3, particularly MAPbBr3−xClx perovskites, hold great promise in the fabrication of high© 2018 American Chemical Society
efficiency LEDs due to the desirable features such as direct band gap nature and high photoluminescence quantum efficiencies.17−19 Despite tremendous improvement in device efficiency that is comparable even with that of the conventional silicon-based ones, the operational lifetime and long-term stability of MAPbX3 perovskite-based optoelectronic devices are still low,20 which is the current benchmark for practical applications. Indeed, MAPbX3 perovskites are sensitive to moisture,21 oxygen,22,23 heat,24 light,25 and ion migration,26,27 which could cause severe property degradation and thus stability deterioration. Worse still, these factors not only work alone, but also intertwine with each other, thus doing harm to the film property and device performance. For instance, it was found that moisture could induce the formation of hydrated intermediate phases in MAPbI3 (e.g., MAPbI3·H2O and (MA)4PbI6·2H2O), which would ultimately decompose to HI, Received: December 3, 2017 Accepted: February 1, 2018 Published: February 1, 2018 6737
DOI: 10.1021/acsami.7b18389 ACS Appl. Mater. Interfaces 2018, 10, 6737−6746
Research Article
ACS Applied Materials & Interfaces
2. EXPERIMENTAL DETAILS
CH3NH2, and PbI2, as observed in a process typically accelerated by heat and light.28,29 Exposure of MAPbI3 films to light and oxygen could lead to the formation of reactive superoxide species O2−, which can deprotonate the MA cation and decompose the photoexcited perovskite to PbI2, H2O, CH3NH2, and I2.30 Clearly, these multiple factors make the stability issue extremely complicated, and further blur the mechanism of how single factor affects the decomposition of MAPbX3 perovskites. Therefore, a comprehensive understanding of the degradation pathway under one sole factor and the corresponding degradation mechanism for MAPbX3 perovskites has undoubtedly become the most imperative issue for future development of more robust perovskite materials and devices. The degradation factors of moisture, oxygen, and heat can be circumvented through optimized encapsulation schemes, whereas the issue of stability against light illumination is inevitable for MAPbX3 perovskite-based materials and devices (e.g., solar cells and photodetectors). It was initially believed that MAPbI3 films only degraded in the presence of both light and oxygen.31 Later, several reports have shown that fullspectrum light soaking in an inert atmosphere or under primary vacuum conditions also induces performance degradation in MAPbI3 perovskite solar cells.25,32−34 It has been demonstrated that high-energy laser irradiation causes partial decomposition of MAPbI3 films with the appearance of a new metallic Pb component, whereas the decomposition achieves a saturation level after several hours of continuous illumination.35 Recent work further revealed that significant changes in the electronic and chemical structures of perovskites films occur upon whitelight illumination with a formation of PbI2 defects.36 Regardless of these preliminary results on pure MAPbX3 perovskites, it is indispensable to unravel the exact process and pathways of light illumination-induced degradation in mixed-halide MAPbX3 perovskites with dual halide anions so as to explain the complex dynamic phenomena and ultimately obtain photostable perovskite optoelectronic devices. In this regard, only few pertinent studies have been conducted to date. Here, we investigate the dynamic degradation process and explore the underlying mechanism of two mixed-halide perovskite films (i.e., MAPbI3−xClx and MAPbBr3−xClx) under white-light illumination in an ultrahigh-vacuum (UHV) environment (base pressure
