Recently, halide double perovskites (HDPs), such as Cs₂AgBiBr₆, have been reported as promising nontoxic alternatives to lead halide perovskites. However, it remains unclear whether the charge-transport properties of these materials are as favorable as for lead-based perovskites. In this work, we study the mobilities of charges in Cs₂AgBiBr₆ and in mixed antimony-bismuth Cs₂AgBi_1-xSb_xBr₆, in which the band gap is tunable from 2.0 to 1.6 eV. Using temperature-dependent time-resolved microwave conductivity techniques, we find that the mobility is proportional to T^-p (with p ≈ 1.5). Importantly, this indicates that phonon scattering is the dominant scattering mechanism determining the charge carrier mobility in these HDPs similar to the state-of-the-art lead-based perovskites. Finally, we show that wet chemical processing of Cs₂AgBi_1-xSb_xBr₆ powders is a successful route to prepare thin films of these materials, which paves the way toward photovoltaic devices based on nontoxic HDPs with tunable band gaps.

Tailoring the physical properties of hybrid lead metal halide APbX₃ perovskites by means of compositional engineering is one of the key factors contributing to the development of highly efficient and stable perovskite solar cells. While the beneficial effects of partial ionic replacement at the A- and X-sites are largely demonstrated, partial replacement of Pb²⁺ is less explored. Here, we developed a solution-based procedure to prepare thin films of mixed-metal MAPb_1-aMn_aI₃ perovskites. Although Mn²⁺ ions have a size that can potentially fit in the B-sites of MAPbI₃, using a combination of structural and chemical analysis, we show that only less than 10% of Pb²⁺ can be replaced by Mn²⁺. A 3% replacement of Pb²⁺ by Mn²⁺ leads to an elongation of the charge carrier lifetimes as concluded from time-resolved PL measurements. However, by analysis of the time-resolved microwave conductance data, we show that the charge carrier mobilities are largely unbalanced, which is in accordance with density functional theory (DFT) calculations indicating that the effective mass of the hole is much higher than that of the electron. Increasing the concentration of Mn²⁺ in the precursor solution above 10% results in formation of amorphous Mn-rich domains in the film, while the perovskite lattice becomes depleted of Mn²⁺. These domains negatively affect the charge carrier mobilities and shorten the lifetime of photogenerated carriers. The resulting reduction in charge carrier diffusion lengths will severely limit the photovoltaic properties of solar cells prepared from these mixed metal halide perovskites.

Double perovskites, comprising two different cations, are potential nontoxic alternatives to lead halide perovskites. Here, we characterized thin films and crystals of Cs₂AgBiBr₆ by time-resolved microwave conductance (TRMC), which probes formation and decay of mobile charges upon pulsed irradiation. Optical excitation of films results in the formation of charges with a yield times mobility product, φΣμ > 1 cm²/Vs. On excitation of millimeter-sized crystals, the TRMC signals show, apart from a fast decay, a long-lived tail. Interestingly, this tail is dominant when exciting close to the bandgap, implying the presence of mobile charges with microsecond lifetimes. From the temperature and intensity dependence of the TRMC signals, we deduce a shallow trap state density of around 10¹⁶/cm³ in the bulk of the crystal. Despite this high concentration, trap-assisted recombination of charges in the bulk appears to be slow, which is promising for photovoltaic applications.

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, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in A^IB^IIX₃ perovskites, the band-gap transition in A^I ₂BB′X₆ double 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 for 1, which results in long (μs) microwave conductivity lifetimes. We further describe a facile self-doping reaction in 2 through Br₂ loss at ambient conditions.

In view of its band gap of 2.2 eV and its stability, methylammonium lead bromide (MAPbBr₃) is a possible candidate to serve as a light absorber in a subcell of a multijunction solar cell. Using complementary temperature-dependent time-resolved microwave conductance (TRMC) and photoluminescence (TRPL) measurements, we demonstrate that the exciton yield increases with lower temperature at the expense of the charge carrier generation yield. The low-energy emission at around 580 nm in the cubic phase and the second broad emission peak at 622 nm in the orthorhombic phase originate from radiative recombination of charges trapped in defects with mobile countercharges. We present a kinetic model describing both the decay in conductance as well as the slow ingrowth of the TRPL. Knowledge of defect states at the surface of various crystal phases is of interest to reach higher open-circuit voltages in MAPbBr₃-based cells.

Halide double perovskites have recently been developed as less toxic analogs of the lead perovskite solar-cell absorbers APbX₃ (A = monovalent cation; X = Br or I). However, all known halide double perovskites have large bandgaps that afford weak visible-light absorption. The first halide double perovskite evaluated as an absorber, Cs₂AgBiBr₆ (1), has a bandgap of 1.95 eV. Here, we show that dilute alloying decreases 1's bandgap by ca. 0.5 eV. Importantly, time-resolved photoconductivity measurements reveal long-lived carriers with microsecond lifetimes in the alloyed material, which is very promising for photovoltaic applications. The alloyed perovskite described herein is the first double perovskite to show comparable bandgap energy and carrier lifetime to those of (CH₃NH₃)PbI₃. By describing how energy- and symmetry-matched impurity orbitals, at low concentrations, dramatically alter 1's band edges, we open a potential pathway for the large and diverse family of halide double perovskites to compete with APbX₃ absorbers.