Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

AgBiI4 powder, crystals, and polycrystalline films were synthesized by sealed tube solid state reactions, chemical vapor transport (CVT), and solution processing, respectively, and their structural, optical and electronic properties are reported. The structure of AgBiI4 is based unambiguously upon a cubic close packed iodide sublattice, but it presents an unusual crystallographic problem: we show that the reported structure, a cubic defect-spinel, cannot be distinguished from a metrically cubic layered structure analogous to CdCl2 using either powder or single crystal X-ray crystallography. In addition, we demonstrate the existence a noncubic CdCl2-type polymorph by isolation of nontwinned single crystals. The indirect optical band gap of AgBiI4 is measured to be 1.63(1) eV, comparable to the indirect band gap of 1.69(1) eV measured for BiI3 and smaller than that reported for other bismuth halides, suggesting that structures with a close-packed iodide sublattice may give narrower band gaps than those with perovskite structures. Band edge states closely resemble those of BiI3; however, the p-type nature of AgBiI4 with low carrier concentration is more similar to MAPbI3 than the n-type BiI3. AgBiI4 shows good stability toward the AM1.5 solar spectrum when kept in a sealed environment and is thermally stable below 90 °C.

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu 2 AgBiI 6 : a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 10 5 cm –1 near the absorption onset, several times that of CH 3 NH 3 PbI 3 . Solution-processed Cu 2 AgBiI 6 thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm 2 V –1 s –1 ), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI 3 . The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs 2 AgBiI 6 thin films have not been synthesized to date, Cu 2 AgBiI 6 is a valuable example of a stable Ag + /Bi 3+ octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.

By correlating photovoltaic and material properties with metal content, we identify compositional ranges of low and high optoelectronic quality in (FA0.83Cs0.17)(Pb1−ySny)I3 perovskites.