Strong optical absorption by a semiconductor is a highly desirable property for many optoelectronic and photovoltaic applications. The optimal thickness of a semiconductor absorber is primarily ...determined by its absorption coefficient. To date, this parameter has been considered as a fundamental material property, and efforts to realize thinner photovoltaics have relied on light-trapping structures that add complexity and cost. Here we demonstrate that engineering cation disorder in a ternary chalcogenide semiconductor leads to considerable absorption increase due to enhancement of the optical transition matrix elements. We show that cation-disorder-engineered AgBiS2 colloidal nanocrystals offer an absorption coefficient that is higher than other photovoltaic materials, enabling highly efficient extremely thin absorber photovoltaic devices. We report solution-processed, environmentally friendly, 30-nm-thick solar cells with short-circuit current density of 27 mA cm−2, a power conversion efficiency of 9.17% (8.85% certified) and high stability under ambient conditions.AgBiS2 nanocrystals with enhanced optical absorption yield efficient ultrathin solar cells.
Vacancy-ordered double perovskites of the general formula A 2 BX 6 are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated BX 6 units with A-site cations ...occupying the cuboctahedral voids. Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs2SnI6 has recently been explored for applications in photovoltaic devices. To elucidate the structure–property relationships of these materials, we have synthesized solid-solution Cs2Sn1–x Te x I6. However, even though tellurium substitution increases electronic dispersion via closer I–I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility. Density functional calculations of native defects in Cs2SnI6 reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material tolerant to these defect states. The increased covalency of Te–I bonding renders the formation of iodine vacancy states unfavorable and is responsible for the reduction in conductivity upon Te substitution. Additionally, Cs2TeI6 is intolerant to the formation of these defects, because the defect level occurs deep within the band gap and thus localizes potential mobile charge carriers. In these vacancy-ordered double perovskites, the close-packed lattice of iodine provides significant electronic dispersion, while the interaction of the B- and X-site ions dictates the properties as they pertain to electronic structure and defect tolerance. This simplified perspective based on extensive experimental and theoretical analysis provides a platform from which to understand structure–property relationships in functional perovskite halides
Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform ...industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed.
Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) exhibit unusually low free‐carrier concentrations despite being processed at low‐temperatures from solution. We demonstrate, ...through quantum mechanical calculations, that an origin of this phenomenon is a prevalence of ionic over electronic disorder in stoichiometric materials. Schottky defect formation provides a mechanism to self‐regulate the concentration of charge carriers through ionic compensation of charged point defects. The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature. This behavior, which goes against established defect conventions for inorganic semiconductors, has implications for photovoltaic performance.
Missing ion action: Schottky defects are found to be a dominant mode of equilibrium stoichiometric disorder in the photovoltaic material CH3NH3PbI3. This behavior can explain the previous finding that “the compounds show intense color, but there is no significant conductivity”.
Bismuth-based solar absorbers have recently garnered attention due to their promise as cheap, nontoxic, and efficient photovoltaics. To date, however, most show poor efficiencies far below those seen ...in commercial technologies. In this work, we investigate two such promising materials, BiSI and BiSeI, using relativistic first-principles methods with the aim of identifying their suitability for photovoltaic applications. Both compounds show excellent optoelectronic properties with ideal band gaps and strong optical absorption, leading to high predicted device performance. Using defect analysis, we reveal the electronic and structural effects that can lead to the presence of deep trap states, which may help explain the prior poor performance of these materials. Crucially, detailed mapping of the range of experimentally accessible synthesis conditions allows us to provide strategies to avoid the formation of killer defects in the future.
Hybrid halide perovskites have recently emerged as a highly efficient class of light absorbers; however, there are increasing concerns over their long-term stability. Recently, incorporation of SCN– ...has been suggested as a novel route to improving stability without negatively impacting performance. Intriguingly, despite crystallizing in a 2D layered structure, (CH3NH3)2Pb(SCN)2I2 (MAPSI) possesses an ideal band gap of 1.53 eV, close to that of the 3D connected champion hybrid perovskite absorber, CH3NH3PbI3 (MAPI). Here, we identify, using hybrid density functional theory, the origin of the smaller than expected band gap of MAPSI through a detailed comparison with the electronic structure of MAPI. Furthermore, assessment of the MAPSI structure reveals that it is thermodynamically stable with respect to phase separation, a likely source of the increased stability reported in experiment.
The methylammonium lead halides have become champion photoactive semiconductors for solar cell applications; however, issues still remain with respect to chemical instability and potential toxicity. ...Recently, the Cs2AgBiX6 (X = Cl, Br) double perovskite family has been synthesized and investigated as stable nontoxic replacements. We probe the chemical bonding, physical properties, and cation anti-site disorder of Cs2AgBiX6 and related compounds from first-principles. We demonstrate that the combination of Ag(I) and Bi(III) leads to the wide indirect band gaps with large carrier effective masses owing to a mismatch in angular momentum of the frontier atomic orbitals. The spectroscopically limited photovoltaic conversion efficiency is less than 10% for X = Cl or Br. This limitation can be overcome by replacing Ag with In or Tl; however, the resulting compounds are predicted to be unstable thermodynamically. The search for nontoxic bismuth perovskites must expand beyond the Cs2AgBiX6 motif.
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