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1. Influence of intrinsic defects on the structure and dynamics of the mixed Pb–Sn perovskite: first-principles DFT and NAMD simulations

   https://doi.org/10.1039/d1ta09027e  

   Liu, Qi; Li, Akang; Chu, Weibin; Prezhdo, Oleg V.; Liang, WanZhen (December 2021, Journal of Materials Chemistry A)

   The mixed tin (Sn) and lead (Pb) perovskite compositions have shown great potential in perovskite photovoltaic devices due to the significantly enhanced material stability and prolonged carrier lifetime, compared to the pure Sn halide perovskites. In spite of the increasing interest, the behaviors of photo-generated charges and of the intrinsic point defects, such as the metal cation vacancies (V Sn and V Pb ) and the interstitial halogen (i I ), have not been well understood in this class of materials. We report first-principles density functional theory (DFT) calculations combined with ab initio non-adiabatic molecular dynamics (NAMD) simulations on the static and dynamic structures of MA 2 SnPbI 6 with and without these intrinsic defects. We discuss the nature of the defect states and unveil the influence of the intrinsic point defects on the structure, optoelectronic properties, and charge carrier dynamics of MA 2 SnPbI 6 . The i I defect significantly shortens the carrier lifetime by creating mid-gap states that provide new recombination pathways. In comparison, the vacancy defects have much weaker influence on the carrier lifetime. Both V Sn and V Pb produce the defect states just below the valence band maxima (VBMs), and do not alter the band gap. They affect the carrier lifetime through changing the energy dispersions of VBMs and the conduction band minima (CBMs). We suggest that excess cations should be used in the synthesis of perovskites to avoid the appearance of interstitial halogen defects. 

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2. Small‐Band‐Gap Halide Double Perovskites

   https://doi.org/10.1002/anie.201807421  

   Slavney, Adam H.; Leppert, Linn; Saldivar Valdes, Abraham; Bartesaghi, Davide; Savenije, Tom J.; Neaton, Jeffrey B.; Karunadasa, Hemamala I. (August 2018, Angewandte Chemie International Edition)

   Abstract Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs₂AgTlX₆(X=Cl (1) and Br (2)) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound2displays the lowest band gap for any known halide perovskite. Unlike in A^IB^IIX₃perovskites, the band‐gap transition in A^I₂BB′X₆double perovskites can show substantial metal‐to‐metal charge‐transfer character. This band‐edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B‐ and B′‐site metal frontier orbitals. Calculations reveal a shallow, symmetry‐forbidden region at the band edges for1, which results in long (μs) microwave conductivity lifetimes. We further describe a facile self‐doping reaction in2through Br₂loss at ambient conditions. 

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3. Light soaking in metal halide perovskites studied via steady-state microwave conductivity

   https://doi.org/10.1038/s42005-020-0350-2  

   Watts, C. Lowell; Aspitarte, Lee; Lin, Yen-Hung; Li, Wen; Elzein, Radwan; Addou, Rafik; Hong, Min Ji; Herman, Gregory S.; Snaith, Henry J.; Labram, John G. (April 2020, Communications Physics)

   Abstract The light-soaking effect is the observation that under constant illumination the measured power conversion efficiency of certain solar cells changes as a function of time. The theory of the light-soaking in metal halide perovskites is at present incomplete. In this report, we employ steady-state microwave conductivity, a contactless probe of electronic properties of semiconductors, to study the light-soaking effect in metal halide perovskites. By illuminating isolated thin films of two mixed-cation perovskites with AM1.5 solar illumination, we observe a continual increase in photoconductance over a period of many (>12) hours. We can fit the experimentally observed changes in photoconductance to a stretched exponential function, in an analogous manner to bias-stressed thin-film transistors. The information provided in this report should help the community better understand one of the most perplexing open problems in the field of perovskite solar cells and, ultimately, lead to more robust and predictable devices. 

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4. Impact of cesium on the phase and device stability of triple cation Pb–Sn double halide perovskite films and solar cells

   https://doi.org/10.1039/c8ta06391e  

   Tosado, Gabriella A.; Lin, Yi-Yu; Zheng, Erjin; Yu, Qiuming (January 2018, Journal of Materials Chemistry A)

   Triple cation Cs/methylammonium (MA)/formamidinium (FA) and double halide Br/I lead perovskites improved the stability and efficiency of perovskite solar cells (PVSCs). However, their effects on alloyed Pb–Sn perovskites are unexplored. In this work, perovskite thin films with the composition Cs x (MA 0.17 FA 0.83 ) 1−x Pb 1−y Sn y (I 0.83 Br 0.17 ) 3 are synthesized utilizing a one-step solution process plus an anti-solvent wash technique and deployed in PVSCs with an inverted architecture. All films show a cubic crystal structure, demonstrating that compositional tuning of both the tolerance factor and crystallization rate allows for dense, single phase formation. The band gaps, affected by both lattice constriction and octahedral tilting, show opposite trends in Pb-rich or Sn-rich perovskites with the increase of Cs for fixed Sn compositions. The Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb 0.25 Sn 0.75 (I 0.83 Br 0.17 ) 3 PVSCs achieve a power conversion efficiency (PCE) of 11.05%, a record for any PVSC containing 75% Sn perovskites, and the Cs 0.10 (MA 0.17 FA 0.83 ) 0.90 Pb 0.75 Sn 0.25 (I 0.83 Br 0.17 ) 3 PVSCs reach a record PCE of 15.78%. Moreover, the triple cation and double halide alloyed Pb–Sn perovskites exhibit improved device stability under inert and ambient conditions. This study, which illustrates the impact of cation and halide tuning on alloyed Pb–Sn perovskites, can be used to further eliminate Pb and improve device performance of high Sn PVSCs and other optoelectronic devices. 

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5. Tailoring ultrafast carrier dynamics in GeS and GeSe via Cu intercalation

   https://doi.org/10.1109/IRMMW-THz57677.2023.10299227  

   Khanmohammadi, Sepideh; Friedman, Kateryna Kushnir; Tran, Catherine; Kastuar, Srihari; Colin-Ulloa, Erika; Ekuma, Chinedu; Koski, Kristie J; Titova, Lyubov V (September 2023, IEEE)

   Germanium sulfide (GeS) and germanium selenide (GeSe) are layered 2D van der Waals materials that belong to a family of group-IV monochalcogenides. These semiconductors have high carrier mobilities and moderate band gaps in the near infrared. Additionally, we have demonstrated that above gap photoexcitation results in ultrafast surface photocurrents and emission of THz pulses due to a spontaneous ferroelectric polarization that breaks inversion symmetry in the monolayer. Beyond the sub-picosecond time scales of shift currents, photoexcited carriers in both materials result in long-lived transient conductivity. We find that 800 nm excitation results in longer lived free photocarriers, persisting for hundreds of picoseconds to several nanoseconds, compared to tens to hundreds of picoseconds lifetimes for 400 nm excitation. Here, we report on tailoring the free photoexcited carrier lifetimes by intercalation of zero-valent Cu into the van der Waals gaps of GeS and GeSe. Density functional theory calculations predict that Cu atoms introduce mid-gap states. We demonstrate that intercalating only ∼3 atomic % of zero-valent Cu reduces the carrier lifetime by as much as two-to-four-fold, raising the prospects of these materials being used for high-speed optoelectronics. 

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