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1. Phase segregation in inorganic mixed-halide perovskites: from phenomena to mechanisms

   https://doi.org/10.1364/PRJ.402411  

   Wang, Yutao; Quintana, Xavier; Kim, Jiyun; Guan, Xinwei; Hu, Long; Lin, Chun-Ho; Jones, Brendon Tyler; Chen, Weijian; Wen, Xiaoming; Gao, Hanwei; et al (October 2020, Photonics Research)

   Halide perovskites, such as methylammonium lead halide perovskites ( ${\mathrm{MAPbX}}_3$, $\mathrm{X}=\mathrm{I}$, Br, and Cl), are emerging as promising candidates for a wide range of optoelectronic applications, including solar cells, light-emitting diodes, and photodetectors, due to their superior optoelectronic properties. All-inorganic lead halide perovskites ${\mathrm{CsPbX}}_3$are attracting a lot of attention because replacing the organic cations with ${\mathrm{Cs}}^{+}$enhances the stability, and its halide-mixing derivatives offer broad bandgap tunability covering nearly the entire visible spectrum. However, there is evidence suggesting that the optical properties of mixed-halide perovskites are influenced by phase segregation under external stimuli, especially illumination, which may negatively impact the performance of optoelectronic devices. It is reported that the mixed-halide perovskites in forms of thin films and nanocrystals are segregated into a low-bandgap I-rich phase and a high-bandgap Br-rich phase. Herein, we present a critical review on the synthesis and basic properties of all-inorganic perovskites, phase-segregation phenomena, plausible mechanisms, and methods to mitigate phase segregation, providing insights on advancing mixed-halide perovskite optoelectronics with reliable performance. 

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2. In situ formation of pseudohalide anions induced by humid air and light passivates formamidinium‐based halide perovskites

   https://doi.org/10.1002/inf2.12643  

   Ye, Jiselle Y; Kerner, Ross A; Jiang, Qi; Yang, Fengjiu; Yang, Jonghee; Ahmadi, Mahshid; Harvey, Steven P; Steirer, Kenneth X; Kuciauskas, Darius; Berry, Joseph J; et al (February 2025, InfoMat)

   Abstract Metal halide perovskites based on formamidinium (FA), or FA‐rich compositions have shown great promise for high‐performance photovoltaics. A deeper understanding of the impact of ambient conditions (e.g., moisture, oxygen, and illumination) on the possible reactions of FA‐based perovskite films and their processing sensitivities has become critical for further advances toward commercialization. Herein, we investigate reactions that take place on the surface of the FA_0.7Cs_0.3, mixed Br/I wide bandgap perovskite thin films in the presence of humid air and ambient illumination. The treatment forms a surface layer containing O, OH, and N‐based anions. We propose the latter originates from formamidine trapped at the perovskite/oxide interface reacting further to cyanide and/or formamidinate—an understudied class of pseudohalides that bind to Pb. Optimized treatment conditions improve photoluminescence quantum yield owing to both reduced surface recombination velocity and increased bulk carrier lifetime. The corresponding perovskite solar cells also exhibit improved performance. Identifying these reactions opens possibilities for better utilizing cyanide and amidinate ligands, species that may be expected during vapor processing of FA‐based perovskites. Our work also provides new insights into the self‐healing or self‐passivating of MA‐free perovskite compositions where FA and iodide damage could be partially offset by advantageous reaction byproducts. image 

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3. Robust Molecular Dipole‐Enabled Defect Passivation and Control of Energy‐Level Alignment for High‐Efficiency Perovskite Solar Cells

   https://doi.org/10.1002/anie.202105512  

   Wang, Bing; Li, Hong; Dai, Qingqing; Zhang, Meng; Zou, Zhigang; Brédas, Jean‐Luc; Lin, Zhiqun (June 2021, Angewandte Chemie International Edition)

   Abstract The ability to passivate defects and modulate the interface energy‐level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine‐terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs. 

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4. Effects of size mismatch of halide ions on the phase stability of mixed halide perovskites

   https://doi.org/10.1088/1402-4896/ad1adb  

   Yang, Fuqian (January 2024, Physica Scripta)

   Abstract The phase stability of mixed halide perovskites plays a vital role in the performance and reliability of perovskite-based devices and systems. In this work, we incorporate the contribution of the strain energy due to the size mismatch of halideions in Gibbs free energy for the analysis of the phase stability of mixed halide perovskites. Analytical expressions of the chemical potentials of halide ions in mixed halide perovskites are derived and used to determine the critical atomic fractions of halide ions for the presence of spinodal decomposition (phase instability). The numerical analysis of CH₃NH₃PbI_xBr_3-xmixed halide perovskite reveals the important role of the mismatch strain from halide ions in controlling the phase instability of mixed halide perovskite, i.e., increasing the mismatch strain widens the range ofxfor the phase separation of mixed halide perovskites. To mitigate the phase instability associated with the strain energy from intrinsic size mismatch and/or light-induced expansion, strain and/or field engineering, such as high pressure, can be likely applied to introduce strain and/or field gradient to counterbalance the strain gradient by the mismatch strain and/or light-induced expansion. 

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5. Interfacial charge-transfer doping of metal halide perovskites for high performance photovoltaics

   https://doi.org/10.1039/C9EE01773A  

   Noel, Nakita K.; Habisreutinger, Severin N.; Pellaroque, Alba; Pulvirenti, Federico; Wenger, Bernard; Zhang, Fengyu; Lin, Yen-Hung; Reid, Obadiah G.; Leisen, Johannes; Zhang, Yadong; et al (October 2019, Energy & Environmental Science)

   The remarkable optoelectronic properties of metal halide perovskites have generated intense research interest over the last few years. The ability to control and manipulate the crystallisation and stoichiometry of perovskite thin-films has allowed for impressive strides in the development of highly efficient perovskite solar cells. However, being able to effectively modify the interfaces of metal halide perovskites, and to controllably p- or n-type dope the surfaces, may be key to further improvements in the efficiency and long-term stability of these devices. In this study, we use surface doping of the mixed-cation, mixed-halide perovskite FA 0.85 MA 0.15 Pb(I 0.85 Br 0.15 ) 3 (FA – formamidinium; MA – methylammonium) to improve the hole extraction from the perovskite solar cell. By treating the surface of the perovskite film with a strongly oxidizing molybdenum tris(dithiolene) complex, we achieve a shift in the work function that is indicative of p-doping, and a twofold increase in the total conductivity throughout the film. We probe the associated interfacial chemistry through photoelectron and solid-state nuclear magnetic resonance spectroscopies and confirm that charge-transfer occurs between the perovskite and dopant complex. The resulting p-doped interface constitutes a homojunction with increased hole-selectivity. With charge-selective layers, we show that this surface doping enhances the device performance of perovskite solar cells resulting in steady-state efficiencies approaching 21%. Finally, we demonstrate that a surface treatment with this dopant produces the same effect as the commonly employed additive 4- tert butylpyridine ( t BP), allowing us to achieve “ t BP-free” devices with steady-state efficiencies of over 20%, and enhanced thermal stability as compared to devices processed using t BP. Our findings therefore demonstrate that molecular doping is a feasible route to tune and control the surface properties of metal halide perovskites. 

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