Band Tailing and Deep Defect States in CH3NH3Pb(I1–xBrx)3 Perovskites As Revealed by Sub-Bandgap Photocurrent

Sutter‐Fella, Carolin M.; Miller, David; Ngo, Quynh Phuong; Roe, Ellis T.; Toma, Francesca M.; Sharp, Ian D.; Lonergan, Mark C.; Javey, Ali

doi:10.1021/acsenergylett.6b00727

Cited by 121 publications

(157 citation statements)

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“…S2). 22,23 The Urbach energy, which provides a qualitative indication of the crystal lattice distortion caused by defects, dislocations, strain, and impurities, is found to be small, especially considering the fact that these samples are processed at low temperature with simple solution processing, and confirms high crystal quality. Figure 2.…”

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confidence: 89%

Cation-Dependent Light-Induced Halide Demixing in Hybrid Organic–Inorganic Perovskites

et al. 2018

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Mixed cation metal halide perovskites with increased power conversion efficiency, negligible hysteresis, and improved long term stability under illumination, moisture, and thermal stressing have emerged as promising compounds for photovoltaic and optoelectronic applications. Here, we shed light on photoinduced halide demixing using insitu photoluminescence spectroscopy and synchrotron Xray diffraction (XRD) to directly compare the evolution of composition and phase changes in CH(NH 2 ) 2 CsPbhalide (FACsPb) and CH 3 NH 3 Pbhalide (MAPb) perovskites upon illumination, thereby providing insights into why FACsPbhalides are less prone to halide demixing than MAPbperovskites. We find that halide demixing occurs in both materials.However, the formed Irich domains accumulate strain for the case of FACsPbperovskites but readily relax for the case of MAPbperovskites. The accumulated strain energy is expected to act as a stabilizing force against halide demixing and may explain the higher Br composition threshold for demixing to occur in FACsPbhalides. In addition, we find that while halide demixing leads to a quenching of the high energy photoluminescence emission from MA 2 perovskites, the emission is enhanced for the case of FaCsperovskites. This behavior points to a reduction of nonradiative recombination centers in FACsperovskites arising from the demixing process. FACsPbhalide perovskites exhibit excellent intrinsic material properties, with photoluminescence quantum yields that are comparable to MAperovskites. Since improved stability is achieved without sacrificing electronic properties, these compositions are better candidates for photovoltaic applications, especially as wide bandgap absorbers in tandem cells. , and high photoluminescence quantum yields 2,4 . Their general crystal structure is described by ABX 3 , typically comprising a monovalent organic cation A (e.g. Despite the importance of overcoming halide demixing for achieving stable perovskitebased photovoltaic devices, there remains uncertainty about the underlying mechanism(s) and most studies have focused on MAperovskites. Currently, strain or carrierinduced lattice distortion, 6 compositional inhomogeneity, 7 defectmediated halide migration, 6,8,9 and crystal domain size 10 are actively considered as contributing to halide segregation. In particular, Bischak et al. propose that halide demixing is a consequence of localized strain generated from the interaction of charge carriers with the lattice (polaron formation). 6In this respect, they find that the combination of 4 mobile halides, long charge carrier lifetimes, and significant electronphonon coupling are prerequisites for halide demixing. 6 In a different study, Barker et al. suggest that defectassisted halide ion migration away from the illuminated surface, with a slower hopping rate of iodide and a potential dependence on charge carrier generation gradients, results in formation of Irich regions at the surface. In this explanation, they argue that halide segregation in a single cr...

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confidence: 89%

Cation-Dependent Light-Induced Halide Demixing in Hybrid Organic–Inorganic Perovskites

et al. 2018

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“…Alternatively, the organic–inorganic mixed‐halide perovskites have higher phase stability. So far, the cation used in organic–inorganic mixed‐halide perovskites are mainly formamidinium (FA + ) or methylammonium (MA + ),24,28–30 and the bandgap can be tuned by varying the iodide (I − ) and bromide (Br − ) ratio 29,31–33. According to the calculation results, the optimal energy bandgap for a front cell in double‐junction device is 1.7–1.9 eV 33.…”

Section: Device Performance Of Pvscs Prepared With Different Perovskimentioning

confidence: 99%

“…However, such high Br − content would cause phase impurity and high defect density issues in the resulting films, which are detrimental to the device performances 14,35,36. Sutter‐Fella et al reported that incorporating Br − will induce defects close to the mid‐gap of the perovskites using sub‐bandgap external quantum efficiency (EQE) measurement, and the defect density increases with increasing the Br − content 31. Several other reports have also shown light‐induced phase segregation into I − dominated and Br‐dominated domains in perovskite films with such high Br − content 37,38.…”

Section: Device Performance Of Pvscs Prepared With Different Perovskimentioning

confidence: 99%

FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

Xie

Zeng

et al. 2020

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Mixed‐halide wide‐bandgap perovskites are key components for the development of high‐efficiency tandem structured devices. However, mixed‐halide perovskites usually suffer from phase‐impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA+)‐based mixed‐halide perovskites, MAPb(I0.6Br0.4)3, the halide composition of the spin‐coated perovskite films is preferentially dominated by the bromide ions (Br−). Additional thermal energy is required to initiate the insertion of iodide ions (I−) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA+) in the precursor solution, it can effectively facilitate the I− coordination in the perovskite framework during the spin‐coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high‐crystallinity perovskite film with high Br− content. As a result, high‐quality MA0.9FA0.1Pb(I0.6Br0.4)3 perovskite film with a bandgap (Eg) of 1.81 eV is achieved, along with an encouraging power‐conversion‐efficiency of 17.1% and open‐circuit voltage (Voc) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.

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“…measured Urbach energies and photoluminescence quantum yield measurements do not imply any trends in structural distortion as a function of composition. [23][24] Given that the energy difference between the randomized and ordered structures for the x = 0.6 composition is only 0.08 eV per formula unit, we hypothesize that thermal motions dominate over any preferred static tilting for any halide composition. This small energy difference also indicates that there is little preference for ordered halides versus a random distribution, a conclusion supported by tests of a "clustered" structure for the x = 0.6 composition in which I and Br are grouped around different Pb atoms ( Figure S9).…”

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confidence: 96%

Determining Atomic-Scale Structure and Composition of Organo-Lead Halide Perovskites by Combining High-Resolution X-ray Absorption Spectroscopy and First-Principles Calculations

et al. 2017

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Abstract:We combine high energy resolution fluorescence detection (HERFD) X-ray absorption spectroscopy (XAS) measurements with first-principles density functional theory (DFT) calculations to provide a molecular scale understanding of local structure, and its role in defining optoelectronic properties, in CH 3 NH 3 Pb(I 1-x Br x ) 3 perovskites. The spectra probe a ligand field splitting in the unoccupied d states of the material, which lie well above the conduction band minimum and display high sensitivity to halide identity, Pb-halide bond length, Pb-halide octahedral tilting, as well as the organic cation. We find that the halides in these mixed compositions are randomly distributed, rather than having preferred octahedral sites, and that thermal tilting motions dominate over any preferred structural distortions as a function of halide composition. These findings demonstrate the utility of the combined HERFD XAS and DFT approach for determining structural details in these materials and connecting them to optoelectronic properties observed by other characterization methods.

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Band Tailing and Deep Defect States in CH₃NH₃Pb(I_1–xBr_x)₃ Perovskites As Revealed by Sub-Bandgap Photocurrent

Cited by 121 publications

References 39 publications

Cation-Dependent Light-Induced Halide Demixing in Hybrid Organic–Inorganic Perovskites

Cation-Dependent Light-Induced Halide Demixing in Hybrid Organic–Inorganic Perovskites

FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

Determining Atomic-Scale Structure and Composition of Organo-Lead Halide Perovskites by Combining High-Resolution X-ray Absorption Spectroscopy and First-Principles Calculations

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