Cl alloying improves thermal stability and increases luminescence in iodine-rich inorganic perovskites.

The inorganic perovskite CsPbI3 shows promising photophysical properties for a range of potential optoelectronic applications but is metastable at room temperature. To address this, Br can be alloyed into the X-site to create compositions such as CsPbI2Br that are stable at room temperature but have bandgaps >1.9 eV - severely limiting solar applications. Herein, in an effort to achieve phase stable films with bandgaps <1.85 eV, we investigate alloying chlorine into iodine-rich triple-halide CsPb(I0.8Br0.2-x Cl x )3 with 0 < x < 0.1. We show that partial substitution of iodine with bromine and chlorine provides a path to maintain broadband terrestrial absorption while improving upon the perovskite phase stability due to chlorine's smaller size and larger ionization potential than bromine. At moderate Cl loading up to ≈5%, X-ray diffraction reveals an increasingly smaller orthorhombic unit cell, suggesting chlorine incorporation into the lattice. Most notably, this Cl incorporation is accompanied by a significant enhancement over Cl-free controls in the duration of black-phase stability of up to 7× at elevated temperatures. Additionally, we observe up to 5× increased steady state photoluminescence intensity (PL), along with a small blue-shift. In contrast, at high loading (≈10%), Cl accumulates in a second phase that is visible at grain boundaries via synchrotron fluorescence microscopy and negatively impacts the perovskite phase stability. Thus, replacing small fractions of bromine for chlorine in the iodine-rich inorganic perovskite lattice results in distinct improvement thermal stability and optoelectronic quality while minimally impacting the bandgap.

Strong-bonding hole-transport layers reduce ultraviolet degradation of perovskite solar cells.

The light-emitting diodes (LEDs) used in indoor testing of perovskite solar cells do not expose them to the levels of ultraviolet (UV) radiation that they would receive in actual outdoor use. We report degradation mechanisms of p-i-n-structured perovskite solar cells under unfiltered sunlight and with LEDs. Weak chemical bonding between perovskites and polymer hole-transporting materials (HTMs) and transparent conducting oxides (TCOs) dominate the accelerated A-site cation migration, rather than direct degradation of HTMs. An aromatic phosphonic acid, [2-(9-ethyl-9H-carbazol-3-yl)ethyl]phosphonic acid (EtCz3EPA), enhanced bonding at the perovskite/HTM/TCO region with a phosphonic acid group bonded to TCOs and a nitrogen group interacting with lead in perovskites. A hybrid HTM of EtCz3EPA with strong hole-extraction polymers retained high efficiency and improved the UV stability of perovskite devices, and a champion perovskite minimodule-independently measured by the Perovskite PV Accelerator for Commercializing Technologies (PACT) center-retained operational efficiency of >16% after 29 weeks of outdoor testing.

Two-dimensional perovskite templates for durable, efficient formamidinium perovskite solar cells.

We present a design strategy for fabricating ultrastable phase-pure films of formamidinium lead iodide (FAPbI3) by lattice templating using specific two-dimensional (2D) perovskites with FA as the cage cation. When a pure FAPbI3 precursor solution is brought in contact with the 2D perovskite, the black phase forms preferentially at 100°C, much lower than the standard FAPbI3 annealing temperature of 150°C. X-ray diffraction and optical spectroscopy suggest that the resulting FAPbI3 film compresses slightly to acquire the (011) interplanar distances of the 2D perovskite seed. The 2D-templated bulk FAPbI3 films exhibited an efficiency of 24.1% in a p-i-n architecture with 0.5-square centimeter active area and an exceptional durability, retaining 97% of their initial efficiency after 1000 hours under 85°C and maximum power point tracking.