FAPbI₃-based Thin Single Crystal Perovskite for Efficient, Stable Perovskite Solar Cell

Perovskite solar cells (PSCs) have been rising in the last decade with solvent, compositional, and interface engineering methods to improve film crystallinity, optimize light absorption wavelength, inhibit intrinsic defect and passivate surface charge accumulation sites, reaching device efficiency of 25.7%. However, the lackluster stability performance remains lingering due to a large number of perovskite defects existing on the surface and grain boundaries inducing ion migration and interface degradation from moisture, oxygen, and other constituent ions thus limiting the operational stability of PSCs. Countless works have reported enhancing PSCs stability by using stable transport layer and metal contact especially passivating perovskite defect and grains boundaries to stop the degradation process from its root. Nevertheless, the source of the degradation remains as the intrinsic defect and grains boundary are always present in the polycrystalline film.<br/><br/>Single crystal as free-grain perovskite is an appropriate candidate for stable perovskite solar cells with defect density a few magnitude orders lower than polycrystalline film. Recent works on inverse temperature crystallization (ITC) demonstrated the fabrication of thin single crystal perovskite that is proportional to the solar cell application. 22.8% PCE has been achieved by employing formamidinium (FA) cation to MAPbI₃-based perovskite solar cells. However, as the advantages of FAPbI₃-based perovskite is obvious with a narrower band gap, superior crystal quality, and higher thermal stability, the move to FAPbI₃-based perovskite is inevitable. Nevertheless, synthesizing FAPbI₃ pure alpha-phase is challenging with non-perovskite delta-phase as stable form at room temperature and limitation of precursor volume on space confined ITC method.<br/><br/>Here in this study, we demonstrated the growth of thin single crystal FAPbI₃-based perovskite with cesium incorporation for stable alpha-phase PSCs. We confirmed the presence of cesium in the crystal with EDX SEM following the stoichiometric reaction. We performed a double 85 test to observe the crystal under harsh conditions. The alpha-phase FAPbI₃-based single perovskite is stable more than two-fold than its polycrystalline counterpart and the current best single crystal PSC MA_xFA_yPbI₃ under relative humidity 85%, and temperature 85^oC test conditions. With a bandgap closing to ~1.47 eV, we show the possibility of an exceeding 23% efficiency device owing to the high current density reaching 27.8 mA/cm² enabling thin single crystal perovskite closer to the highest efficiency p-i-n polycrystalline perovskite devices to date. We also performed MPPT tracking to observe device operational stability under light irradiation in an inert atmosphere.