Mixed-Halide Double Perovskite Cs2 AgBiX6 (X=Br, I) with Tunable Optical Properties via Anion Exchange

Multidimensional high-throughput screening for mixed perovskite materials with machine learning

Mixed halide inorganic perovskites exhibit exceptional stability and photovoltaic performance and are considered to be promising photovoltaic materials. However, the chemical diversity of these materials presents a vast screening space, making it challenging to efficiently identify high-performance materials solely through theoretical calculations or experiments. To address this challenge, in this work, we introduce a multidimensional high-throughput screening strategy that combines machine learning with first-principles calculations, specifically designed to identify MHIPs with optimal bandgap and light absorption properties. The bandgap and light absorption models have achieved determination coefficients (r2) of 0.9896 and 0.9833, with root mean square errors of 0.1890 eV and 0.2190 105 eV · cm-1, respectively, demonstrating the high precision and reliability of the models. In the present work, the generation of 306 521 candidate materials through mixed B-site elements is reported, leading to the successful identification of 295 materials with ideal characteristics for MHIPs via screening. Subsequently, an in-depth density functional theory validation is conducted on 20 of these materials. The research results demonstrate that Cs2AgBi0.5Sb0.25Ir0.25I6 and CsSn0.75Ge0.25I3 exhibit outstanding performance, making them the most promising candidate materials for practical applications. These results fully confirm the scientific validity and effectiveness of our screening strategy, laying a solid foundation for the exploration and optimization of high-performance perovskite solar cell materials.

Optimizing Cs2CuBiBr6 double halide perovskite for solar applications: the role of electron transport layers in SCAPS-1D simulations

Perovskite solar cells are commonly employed in photovoltaic systems because of their special characteristics. Perovskite solar cells remain efficient, but lead-based absorbers are dangerous, restricting their manufacture. Therefore, studies in the field of perovskite materials are now focusing on investigating lead-free perovskites. The SCAPS-1D simulator is used to simulate the impact of lead-free double perovskite as the absorber in perovskite solar cells. The research examines how the effectiveness of solar cells is impacted by a hole transport layer (CBTS) and several electron transports layers (WS2, C60, PCBM, and TiO2), using Ni and Al acting as the back and front contacts metal. This work explores the impact of a Cs2CuBiBr6 perovskite as a solar cell absorber. The effectiveness of these device structures depends on defect density, absorber thickness, ETL thickness, and ETL combination. With WS2, C60, PCBM, and TiO2, the device's power conversion efficiency (PCE) is 19.70%, 18.69%, 19.52%, and 19.65%, respectively. This research also highlights the impact of the absorber and HTL thickness. This investigation further included the analysis of the valence band offset (VBO) and conduction band offset (CBO). We also investigated the current density-voltage (J-V), quantum efficiency (QE), series and shunt resistance, capacitance-Mott-Schottky characteristics, and photocarrier generation-recombination rates and effective temperature. This study provides crucial structural design guidelines for a lead-free double perovskite device and highlights solar energy optoelectronic developments.

Exploring the structural, electronic, optical, transport, and photovoltaic properties of Rb2LiGa(Br/I)6 using DFT and SCAPS-1D simulations

Lead-free double perovskite halides are attracting considerable interest in the optoelectronics sector due to their remarkable electronic, optical, and transport properties. These materials are not only stable and easy to synthesize but also present a wide range of potential applications. This study investigates the fascinating characteristics of Rb₂LiGa(Br/I)₆, focusing on its structural, electronic, optical, transport, and photovoltaic attributes. Our findings indicate that Rb₂LiGaBr₆ and Rb₂LiGaI₆ have band gaps of 1.19 eV and 1.13 eV, respectively, highlighting their versatility for various applications. Both compounds exhibit exceptional optical properties, featuring high absorption coefficients and optical conductivity, along with low reflectivity throughout the UV-visible spectrum, positioning them as excellent candidates for solar cell technologies. Moreover, Rb₂LiGa(Br/I)₆ demonstrates impressive thermoelectric performance, with high figure-of-merit (ZT) values between 200 K and 800 K, indicating their potential as effective thermoelectric materials. Consequently, this study offers valuable insights for the development of efficient double perovskite-based solar cells. Encouraged by the outstanding absorption and optical conductivity of Rb₂LiGa(Br/I)₆, we simulated an Au/Cu₂O/Rb₂LiGa(Br/I)₆/TiO₂/FTO solar cell. Our results reveal that the modeled solar cell, Au/Cu₂O/Rb₂LiGaI₆/TiO₂/FTO, achieves an efficiency of 26.48%, surpassing previous reports. This research sets a new benchmark for high-performance double perovskite-based solar cells and lays the foundation for future advancements in this exciting area.

Halide Mixing in Cs2AgBi(I x Br1-x )6 Double Perovskites: A Pathway to Tunable Excitonic Properties

Cs2AgBiBr6 is an emerging double perovskite semiconductor with robust stability. However, its potential for photovoltaics is limited by its indirect band gap and localized electronic structure featuring a resonant exciton with a large binding energy. Cs2AgBi(I x Br1-x )6 nanocrystals with iodide concentrations of up to 100% were recently demonstrated, but an atomistic understanding of how halide mixing affects the electronic and excited-state structure is missing. Here, we use first-principles GW and Bethe-Salpeter Equation calculations to show that halide mixing leads to a pronounced change in the band gap and character of optical excitations. Exciton binding energies are reduced by up to a factor of 5, with significantly more delocalized excitons in I-rich compounds. We further show that phase-pure bulk alloys with x ≤ 0.11 can be fabricated using mechanosynthesis and measure a red-shifted absorption in line with our calculations. Our study highlights that halide mixing in double perovskites can not only lead to significant band gap changes but may also be used for tuning excitonic properties.

Synthesis, Structure, and Properties of CuBiSeCl2: A Chalcohalide Material with Low Thermal Conductivity

Mixed anion halide-chalcogenide materials have recently attracted attention for a variety of applications, owing to their desirable optoelectronic properties. We report the synthesis of a previously unreported mixed-metal chalcohalide material, CuBiSeCl2 (Pnma), accessed through a simple, low-temperature solid-state route. The physical structure is characterized through single-crystal X-ray diffraction and reveals significant Cu displacement within the CuSe2Cl4 octahedra. The electronic structure of CuBiSeCl2 is investigated computationally, which indicates highly anisotropic charge carrier effective masses, and by experimental verification using X-ray photoelectron spectroscopy, which reveals a valence band dominated by Cu orbitals. The band gap is measured to be 1.33(2) eV, a suitable value for solar absorption applications. The electronic and thermal properties, including resistivity, Seebeck coefficient, thermal conductivity, and heat capacity, are also measured, and it is found that CuBiSeCl2 exhibits a low room temperature thermal conductivity of 0.27(4) W K-1 m-1, realized through modifications to the phonon landscape through increased bonding anisotropy.

Which Ion Dominates the Temperature and Pressure Response of Halide Perovskites and Elpasolites?

Halide perovskites and elpasolites are key for optoelectronic applications due to their exceptional performance and adaptability. However, understanding their crucial elastic properties for synthesis and device operation remains limited. We performed temperature- and pressure-dependent synchrotron-based powder X-ray diffraction at low pressures (ambient to 0.06 GPa) to investigate their elastic properties in their ambient-pressure crystal structure. We found common trends in bulk modulus and thermal expansivity, with an increased halide ionic radius (Cl to Br to I) resulting in greater softness, higher compressibility, and thermal expansivity in both materials. The A cation has a minor effect, and mixed-halide compositions show intermediate properties. Notably, thermal phase transitions in MAPbI3 and CsPbCl3 induced lattice softening and negative expansivity for specific crystal axes, even at temperatures far from the transition point. These results emphasize the significance of considering temperature-dependent elastic properties, which can significantly impact device stability and performance during manufacturing or temperature sweeps.

Synthesis and Optimization of Cs2B'B″X6 Double Perovskite for Efficient and Sustainable Solar Cells

Hybrid perovskite materials with high light absorption coefficients, long diffusion lengths, and high mobility have attracted much attention, but their commercial development has been seriously hindered by two major problems: instability and lead toxicity. This has led to lead-free halide double perovskite becoming a prominent competitor in the photovoltaic field. For lead-free double perovskites, Pb2+ can be heterovalent, substituted by non-toxic metal cations as a double perovskite structure, which promotes the flexibility of the composition. However, the four component elements and low solubility in the solvent result in synthesis difficulties and phase impurity problems. And material phase purity and film quality are closely related to the number of defects, which can limit the photoelectric performance of solar cells. Therefore, based on this point, we summarize the synthesis methods of Cs2B'B″X6 double perovskite crystals and thin films. Moreover, in the application of solar cells, the existing research mainly focuses on the formation process of thin films, band gap adjustment, and surface engineering to improve the quality of films and optimize the performance of devices. Finally, we propose that Cs2B'B″X6 lead-free perovskites offer a promising pathway toward developing highly efficient and stable perovskite solar cells.

Combined DFT, SCAPS-1D, and wxAMPS frameworks for design optimization of efficient Cs2BiAgI6-based perovskite solar cells with different charge transport layers

In this study, combined DFT, SCAPS-1D, and wxAMPS frameworks are used to investigate the optimized designs of Cs2BiAgI6 double perovskite-based solar cells. First-principles calculations are employed to investigate the structural stability, optical responses, and electronic contribution of the constituent elements in Cs2BiAgI6 absorber material, where SCAPS-1D and wxAMPS simulators are used to scrutinize different configurations of Cs2BiAgI6 solar cells. Here, PCBM, ZnO, TiO2, C60, IGZO, SnO2, WS2, and CeO2 are used as ETL, and Cu2O, CuSCN, CuSbS2, NiO, P3HT, PEDOT:PSS, spiro-MeOTAD, CuI, CuO, V2O5, CBTS, CFTS are used as HTL, and Au is used as a back contact. About ninety-six combinations of Cs2BiAgI6-based solar cell structures are investigated, in which eight sets of solar cell structures are identified as the most efficient structures. Besides, holistic investigation on the effect of different factors such as the thickness of different layers, series and shunt resistances, temperature, capacitance, Mott-Schottky and generation-recombination rates, and J-V (current-voltage density) and QE (quantum efficiency) characteristics is performed. The results show CBTS as the best HTL for Cs2BiAgI6 with all eight ETLs used in this work, resulting in a power conversion efficiency (PCE) of 19.99%, 21.55%, 21.59%, 17.47%, 20.42%, 21.52%, 14.44%, 21.43% with PCBM, TiO2, ZnO, C60, IGZO, SnO2, CeO2, WS2, respectively. The proposed strategy may pave the way for further design optimization of lead-free double perovskite solar cells.

Entropy-Driven Stabilization of Multielement Halide Double-Perovskite Alloys

Currently, a major obstacle restricting the commercial application of halide perovskites is their low thermodynamic stability. Herein, inspired by the high-stability high-entropy alloys, we theoretically investigated a variety of multielement double-perovskite alloys. First-principles calculations show that the entropy contribution to Gibbs free energy, which offsets the positive enthalpy contribution by up to 35 meV/f.u., can significantly enhance the material stability of double-perovskite alloys. We found that the electronic properties of bandgaps (1.04-2.21 eV) and carrier effective masses (0.34 to greater than 2 m₀) of the multielement double-perovskite alloys can be tuned over a wide range. Meanwhile, the parity-forbidden condition of optical transitions in the Cs₂AgInCl₆ perovskite can be broken because of the lower symmetry of the configurational disorder, leading to enhanced transition intensity. This work demonstrates a promising strategy by utilizing the alloy entropic effect to further improve the material stability and optoelectronic performance of halide perovskites.

Three-Dimensional CA-LBM Numerical Model and Experimental Verification of Cs2AgBiBr6 Perovskite Single Crystals Grown by Solution Method

A three-dimensional cellular automata-lattice Boltzmann (CA-LBM) coupling model is established to simulate the facet growth process and the controlled cooling growth process of Cs2AgBiBr6 perovskite single crystals. In this model, the LBM method is used to calculate the real-time solute field, the CA method is used to simulate the crystal growth process driven by supersaturation of solute, and the geometric parameter g related to the adjacent grid is introduced to reduce the influence of grid anisotropy. The verification of the model is achieved by comparing the simulation results with the experimental results. The comparison results show that a smaller cooling rate is helpful for the growth of large-size single crystals, which verifies the rationality and correctness of the model.