Lead-free double perovskites Cs2InCuCl6 and (CH3NH3)2InCuCl6: electronic, optical, and electrical properties

Numerical simulation to optimize power conversion efficiency of an FTO/GO/Cs2AgBiBr6/Cu2O solar cell

Efficient conversion of solar power to electrical power through the development of smart, reliable, and environmentally friendly materials is a key focus for the next-generation renewable energy sector. The involvement of degradable and toxic elements present in hybrid perovskites presents serious concerns regarding the commercial viability of these materials for the solar cell industry. In this study, a solar cell with a stable, nondegradable, and lead-free halide-based double perovskite Cs2AgBiBr6 as the absorber layer, Cu2O as a hole transport layer, and GO as the electron transport layer has been simulated using SCAPS 1D. The thickness of the absorber, electron transport, and hole transport layers are tuned to optimize the performance of the designed solar cell. Notably, perovskite solar cells functioned most efficiently with an electron affinity value of 4.0 eV for Cu2O. In addition, the effect of variation of series resistance and temperature on generation and recombination rates, current density, and quantum efficiency has been elaborated in detail. The findings of this study provide valuable insight and encouragement toward the realization of a non-toxic, inorganic perovskite solar device and will be a significant step forward in addressing environmental concerns associated with perovskite solar cell technology.

Alloying Bi-Doped Cs2Ag1-xNaxInCl6 Nanocrystals with K+ Cations Modulates Surface Ligands Density and Photoluminescence Efficiency

We show how, in the synthesis of yellow-emissive Bi-doped Cs₂Ag_1-xNa_xInCl₆ double perovskite nanocrystals (NCs), preventing the transient formation of Ag⁰ particles increases the photoluminescence quantum yield (PLQY) of the NCs from ∼30% to ∼60%. Calculations indicate that the presence of even a single Ag⁰ species on the surface of a NC introduces deep trap states. The PL efficiency of these NCs is further increased to ∼70% by partial replacement of Na⁺ with K⁺ ions, up to a 7% K content, due to a lattice expansion that promotes a more favorable ligands packing on the NC surface, hence better surface passivation. A further increase in K⁺ lowers the PLQY, due to both the activation of nonradiative quenching channels and a lower oscillator strength of the BiCl₆→AgCl₆ transition (through which PL emission occurs). The work indicates how a deeper understanding of parameters influencing carrier trapping/relaxation can boost the PLQY of double perovskites NCs.

Sb-Doped Metal Halide Nanocrystals: A 0D versus 3D Comparison

We synthesize colloidal nanocrystals (NCs) of Rb₃InCl₆, composed of isolated metal halide octahedra ("0D"), and of Cs₂NaInCl₆ and Cs₂KInCl₆ double perovskites, where all octahedra share corners and are interconnected ("3D"), with the aim to elucidate and compare their optical features once doped with Sb³⁺ ions. Our optical and computational analyses evidence that the photoluminescence quantum yield (PLQY) of all these systems is consistently lower than that of the corresponding bulk materials due to the presence of deep surface traps from under-coordinated halide ions. Also, Sb-doped "0D" Rb₃InCl₆ NCs exhibit a higher PLQY than Sb-doped "3D" Cs₂NaInCl₆ and Cs₂KInCl₆ NCs, most likely because excitons responsible for the PL emission migrate to the surface faster in 3D NCs than in 0D NCs. We also observe that all these systems feature a large Stokes shift (varying from system to system), a feature that should be of interest for applications in photon management and scintillation technologies. Scintillation properties are evaluated via radioluminescence experiments, and re-absorption-free waveguiding performance in large-area plastic scintillators is assessed using Monte Carlo ray-tracing simulations.

A2AgCrCl6 (A = Li, Na, K, Rb, Cs) halide double perovskites: a transition metal-based semiconducting material series with appreciable optical characteristics

We have theoretically examined the geometries, electronic density of states and band structures of cubic and hexagonal A₂AgCrCl₆ (A = Cs, Rb, K, Na, Li) using meta-GGA SCAN-rVV10. The optimized lattice density was found to vary between 2.68 and 4.08 g cm^-3 for cubic-A₂AgCrCl₆, with the fundamental electronic bandgap (direct) in the range of 0.66-0.69 eV. The cell density of hexagonal A₂AgCrCl₆ was between 2.97 and 3.93 g cm^-3, but with an indirect bandgap of 0.93-1.02 eV. The valence band maximum and the conduction band minimum of A₂AgCrCl₆ were confirmed to be essentially of Cr(3d) character, but the contributions from the orbital states of Cl(3p) to the VBM were also appreciable. Cubic A₂AgCrCl₆ (A = Cs, Rb, K) was identified to possess genuine perovskite stoichiometry, evaluated using various geometry-based indices (viz. octahedral factor, tolerance factor, and global instability index). This was not so for A₂AgCrCl₆ (A = Na, Li), and was due to the small size of Na and Li cations that caused the critical strain of CrCl₆ octahedra and a significant decrease in the cell volume. However, all the five A₂AgCrCl₆ displayed nearly similar optical properties, including the nature of the oscillator peaks in the dielectric function, absorption coefficient, photoconductivity, reflectivity, and Tauc spectra. The zero-limit of the refractive index was calculated around 2.25 and 2.00 for cubic and hexagonal A₂AgCrCl₆, respectively, and the extinction coefficient was very small for all cases. The nature of the optical bandgap and transition peaks discussed in this study of cubic and hexagonal Cs₂AgCrCl₆ agreed well with the experiment. The examination of phonon band dispersion led to the conclusion that cubic-A₂AgCrCl₆ (A = Cs, Rb) are the only halide double perovskites of the entire series that are dynamically stable.

Colloidal Bi-Doped Cs2Ag1-x Na x InCl6 Nanocrystals: Undercoordinated Surface Cl Ions Limit their Light Emission Efficiency

Understanding and tuning the ligand shell composition in colloidal halide perovskite nanocrystals (NCs) has been done systematically only for Pb-based perovskites, while much less is known on the surface of Pb-free perovskite systems. Here, we reveal the ligand shell architecture of Bi-doped Cs₂Ag_1-x Na _x InCl₆NCs via nuclear magnetic resonance analysis. This material, in its bulk form, was found to have a photoluminescence quantum yield (PLQY) as high as 86%, a record value for halide double perovskites. Our results show that both amines and carboxylic acids are present and homogeneously distributed over the surface of the NCs. Notably, even for an optimized surface ligand coating, achieved by combining dodecanoic acid and decylamine, a maximum PLQY value of only 37% is reached, with no further improvements observed when exploiting post-synthesis ligand exchange procedures (involving Cs-oleate, different ammonium halides, thiocyanates and sulfonic acids). Our density functional theory calculations indicate that, even with the best ligands combination, a small fraction of unpassivated surface sites, namely undercoordinated Cl ions, is sufficient to create deep trap states, opposite to the case of Pb-based perovskites that exhibit much higher defect tolerance. This was corroborated by our transient absorption measurements, which showed that an ultrafast trapping of holes (most likely mediated by surface Cl-trap states) competes with their localization at the AgCl₆ octahedra, from where, instead, they can undergo an optically active recombination yielding the observed PL emission. Our results highlight that alternative surface passivation strategies should be devised to further optimize the PLQY of double perovskite NCs, which might include their incorporation inside inorganic shells.

The Cs2AgRhCl6 Halide Double Perovskite: A Dynamically Stable Lead-Free Transition-Metal Driven Semiconducting Material for Optoelectronics

A-Site doping with alkali ions, and/or metal substitution at the B and B'-sites, are among the key strategies in the innovative development of A 2BB'X6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A 2AgRhCl6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A 2AgRhCl6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs2AgBiX6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A 2AgRhCl6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57-0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0-4.5 eV, enabled us to demonstrate that A 2AgRhCl6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs2AgRhCl6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable.

Why choosing the right partner is important: stabilization of ternary CsyGUAxFA(1-y-x)PbI3 perovskites

Lead halide perovskites with mixtures of monovalent cations have attracted wide attention due to the possibility of preferentially stabilizing the perovskite phase with respect to photovoltaically less suitable competing phases. Here, we present a theoretical analysis and interpretation of the phase stability of binary (CH6N3)x[HC(NH2)2](1-x)PbI3 = GUAxFA(1-x)PbI3 and ternary CsyGUAxFA(1-y-x)PbI3 mixtures. We first estimate if such mixtures are stable and if they lead to a stabilization of the perovskite phase based on static Density Functional Theory (DFT) calculations. In order to investigate the finite temperature stability of the phases, we also employ first-principles molecular dynamics (MD) simulations. It turns out that in contrast to the FA+-rich case of FA/Cs mixtures, although mixing of FA/GUA is possible, it is not sufficient to stabilize the perovskite phase at room temperature. In contrast, stable ternary mixtures that contain 17% of Cs+ can be formed that lead to a preferential stabilization of the perovskite phase. In such a way, the enthalpic destabilization due to the introduction of a too large/too small cation that lies outside the Goldschmidt tolerance range can be (partially) compensated through the introduction of a third cation with complementary size. This allows to suggest a new design principle for the preparation of stable perovskite structures at room temperature with cations that lie outside the Goldschmidt range through mixtures with size-complementary cations in such a way that the effective average cation radius of the mixture lies within the stability range.

Lead-Free Small-Bandgap Cs2CuSbCl6 Double Perovskite Nanocrystals

Lead-free halide double perovskites (DPs) have attracted great attention due to their stability, nontoxicity and good photophysical property. In this work, we report a new, small-bandgap Cs₂CuSbCl₆ DP nanocrystals (NCs) with a bandgap of 1.66 eV, which is the smallest bandgap in reported lead-free three-dimensional DP NCs. Density functional theory calculations confirm that Cs₂CuSbCl₆ DP has an indirect bandgap of 1.70 eV, in good agreement with the experimental result. The photophysical property of Cs₂CuSbCl₆ NCs is studied by the combination of femtosecond transient absorption (TA) and nanosecond TA techniques. In addition, the Cs₂CuSbCl₆ NCs exhibit good stability exposed in the air. These results show that these NCs will have great potential to be used as a light-absorbing material for photovoltaic applications.

Designing Two-Dimensional Properties in Three-Dimensional Halide Perovskites via Orbital Engineering

Manipulating the orbital hybridization between the metal cation and the halide anion to achieve novel properties is highly desired. Here, we present an orbital engineering strategy to construct two-dimensional (2D) electronic structures in three-dimensional (3D) halide perovskites by rationally controlling the hybridization between the d orbitals of the metal cations and the halide p orbitals. Taking Cs₂Au(I)Au(III)I₆ as an example, we demonstrate that the flat conduction band and valence band at the band edges can be achieved simultaneously by combining two metal cations with different d orbital configurations using first-principles calculations. The band structure and predicted carrier mobilities show huge anisotropy along in-plane and out-of-plane directions, confirming the 2D electronic properties. In addition, the strong anisotropic optical and mechanical properties (e.g., 2D-like properties) are also presented. Our work provides orbital engineering guidance for achieving low-dimensional properties with strong anisotropy in 3D halide perovskites for novel electronic and photonic applications.