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.

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr crystals does not change due the presence of Bi in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.

Cs 2 AgBiBr 6 thin film preparation for characterization. The double-perovskite thin films studied in this work were all prepared through sequential vapour deposition. In a vacuumsealed chamber, AgBr (99% Fluka), BiBr 3 (≥98% Sigma Aldrich) and CsBr (99.9% Sigma Aldrich) were placed in separate crucibles and sequentially thermally evaporated onto the substrates. In particular, the standard procedure we optimized evaporated 90 nm of AgBr, 120 nm of BiBr 3 and 150 nm of CsBr to obtain 300 nm of Cs 2 AgBiBr 6 . This basic stack was repeated the necessary number of times to achieve the desired total film thickness. To achieve thicknesses that are not multiples of 300 nm (like the 750 nm reported in the text), we ran the last evaporation cycle depositing half of each precursor thickness, keeping always the same precursors ratio (1:1.3:1.6 AgBr:BiBr 3 :CsBr). After the deposition of the desired thickness, we annealed the samples on a hotplate in air at 250 ºC for 30 minutes. The post-deposition annealing temperature and time were optimised to deliver maximum solar cell performance.Solar cell preparation. FTO or ITO coated glasses were cleaned by sequential sonication in soap, water, acetone and isopropanol. After being dried with a N 2 gun, the substrates were further cleaned by O 2 plasma for 10 minutes. Titanium isopropoxide (140 µl in 1 ml of EtOH) was added to 1 ml of acidic EtOH (10 µl of HCl 2M in 1 ml EtOH), and deposited on the FTO substrates by spincoating at 2000 rpm for 45 sec with 2000 rpm/sec acceleration. Following this, the films were annealed at 150°C for 15 min and 500°C for 30 min. SnO 2 layers were prepared by spincoating at 3000 rpm (200 rpm/sec) for 30 sec of a solution of SnCl 4 ⋅5H 2 O in isopropanol (17.5 mg/ml) on top of the FTO or ITO coated glasses. The so-prepared films were annealed at 100°C for 10 min followed by an annealing at 180°C for 30 min. The SnO 2 and TiO 2 films were placed in the vacuum chamber, and the Cs 2 AgBiBr 6 film was deposited as previously presented. The hole transport material (Spiro-OMeTAD, Lumtec) was dissolved in chlorobenzene (85 mg/ml) and doped with 20 µl of LiTFSI (500 mg/ml in BuOH) and 30 µl of tert-butylpyridine. The solution was then deposited on the active layer by spincoating in air at 2000 rpm (2000 rpm/sec) for 45 sec. The devices were then left overnight in a desiccator in air atmosphere, and then completed by the evaporation of 100 nm silver contacts. All the

Lead halide perovskites are materials with excellent optoelectronic and photovoltaic properties. However, some hurdles remain prior to commercialization of these materials, such as chemical stability, phase stability, sensitivity to moisture, and potential issues due to the toxicity of lead. Here, we report a new type of lead-free perovskite related compound, CsPdBr. This compound is solution processable, exhibits long-lived photoluminescence, and an optical band gap of 1.6 eV. Density functional theory calculations indicate that this compound has dispersive electronic bands, with electron and hole effective masses of 0.53 and 0.85 m, respectively. In addition, CsPdBr is resistant to water, in contrast to lead-halide perovskites, indicating excellent prospects for long-term stability. These combined properties demonstrate that CsPdBr is a promising novel compound for optoelectronic applications.

Impressive power conversion efficiencies coupled with the relative ease of fabrication have made perovskite solar cells a front runner for next-generation photovoltaics. Although perovskite films and optoelectronic devices have been widely studied, relatively little is known about the chemistry of the precursor solutions. Here, we present a study on the hydrolysis of N,N-dimethylformamide, correlating how pH changes related to its degradation affect the crystallization of MAPbI3 xClx perovskite films. By careful manipulation of the pH, and the resulting colloid distribution in precursor solutions, we fabricate perovskite films with greatly improved crystallinity, which when incorporated into photovoltaic devices reproducibly yield efficiencies of over 18%. Extending this method to the mixed cation, mixed halide perovskite FA0.83MA0.17Pb(I0.83Br0.17)3, we obtain power conversion efficiencies of up to 19.9% and open-circuit voltages of 1.21 V for a material with a bandgap of 1.57 eV, achieving the lowest yet reported loss in potential from bandgap to a VOC of only 360 mV