Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

Attempts to improve the efficiency of kesterite solar cells by changing the intrinsic stoichiometry have not helped to boost the device efficiency beyond the current record of 12.6%. In this light, the addition of extrinsic elements to the Cu 2 ZnSn(S,Se) 4 matrix in various quantities has emerged as a popular topic aiming to ameliorate electronic properties of the solar cell absorbers. This article reviews extrinsic doping and alloying concepts for kesterite absorbers with the focus on those that do not alter the parent zinc-blende derived kesterite structure. The latest state-of-the-art of possible extrinsic elements is presented in the order of groups of the periodic table. The highest reported solar cell efficiencies for each extrinsic dopant are tabulated at the end. Several dopants like alkali elements and substitutional alloying with Ag, Cd or Ge have been shown to improve the device performance of kesterite solar cells as compared to the nominally undoped references, although it is often difficult to differentiate between pure electronic effects and other possible influences such as changes in the crystallization path, deviations in matrix composition and presence of alkali dopants coming from the substrates. The review is concluded with a suggestion to intensify efforts for identifying intrinsic defects that negatively affect electronic properties of the kesterite absorbers, and, if identified, to test extrinsic strategies that may compensate these defects. Characterization techniques must be developed and widely used to reliably access semiconductor absorber metrics such as the quasi-Fermi level splitting, defect concentration and their energetic position, and carrier lifetime in order to assist in search for effective doping/alloying strategies.

The performance of many emerging compound semiconductors for thin‐film solar cells is considerably lower than the Shockley–Queisser limit, and one of the main reasons for this is the presence of various deleterious defects. A partial or complete substitution of the cations presents a viable strategy to alter the characteristics of the detrimental defects and defect clusters. Particularly, it is hypothesized that double cation substitution could be a feasible strategy to mitigate the negative effects of different types of defects. In this study, the effects of double cation substitution on pure‐sulfide Cu2ZnSnS4 (CZTS) by partially substituting Cu with Ag, and Zn with Cd are explored. A 10.1% total‐area power conversion efficiency (10.8% active‐area efficiency) is achieved. The role of Cd, Ag, and Cd + Ag substitution is probed using temperature‐dependent photoluminescence, time‐resolved photoluminescence, current–voltage (IV), and external quantum efficiency (EQE) measurements. It is found that Cd improves the photovoltaic performance by altering the defect characteristics of acceptor states near the valence band, and Ag reduces nonradiative bulk recombination. It is believed that the double cation substitution approach can also be extended to other emerging photovoltaic materials, where defects are the main culprits for low performance.

Recently, considerable attention in the development of Cu 2 ZnSnS 4 (CZTS)-based thin-film solar cells has been given to the reduction of antisite defects via cation substitution. In this Letter, we report the substitution of copper atoms by silver, incorporated into the crystal lattice through a solution processable method. We observe an increase in open-circuit voltage (V OC ) by 50 mV and an accompanying rise in device efficiency from 4.9% to 7.2%. The incorporation of Ag is found to improve the grain size, enhance the depletion width of the pn-junction, and reduce the concentration of antisite defect states. This work demonstrates the promising role of Ag in reducing the V OC deficit of Cu-kesterite thin-film solar cells.

CZTS is a promising photoabsorber as a photocathode for water reduction to produce hydrogen but its performance is often limited by the presence of a high concentration of defects. Here, substitution of Cd improves the bulk quality of CZTS, resulting in a photocurrent enhancement of 4 mA cm À2 to 17 mA cm À2 at 0 V RHE . This improvement is attributed to the better charge collection, which may be due to the reduction in band tailing or band-edge defects and improved carrier transport properties.