A wide variety of charge carrier dynamics, such as transport, separation, and extraction, occur at the interfaces of planar heterojunction solar cells. Such factors can affect the overall device performance. Therefore, understanding the buried interfacial molecular structure in various devices and the correlation between interfacial structure and function has become increasingly important. Current characterization techniques for thin films such as X-ray diffraction, cross section scanning electronmicroscopy, and UV-visible absorption spectroscopy are unable to provide the needed molecular structural information at buried interfaces. In this study, by controlling the structure of the hole transport layer (HTL) in a perovskite solar cell and applying a surface/interface-sensitive nonlinear vibrational spectroscopic technique (sum frequency generation vibrational spectroscopy (SFG)), we successfully probed the molecular structure at the buried interface and correlated its structural characteristics to solar cell performance. Here, an edge-on (normal to the interface) polythiophene (PT) interfacial molecular orientation at the buried perovskite (photoactive layer)/PT (HTL) interface showed more than two times the power conversion efficiency (PCE) of a lying down (tangential) PT interfacial orientation. The difference in interfacial molecular structure was achieved by altering the alkyl side chain length of the PT derivatives, where PT with a shorter alkyl side chain showed an edge-on interfacial orientation with a higher PCE than that of PT with a longer alkyl side chain. With similar band gap alignment and bulk structure within the PT layer, it is believed that the interfacial molecular structural variation (i.e., the orientation difference) of the various PT derivatives is the underlying cause of the difference in perovskite solar cell PCE.
Organic−inorganic hybrid perovskites have been intensively studied for their use in optoelectronic devices due to their utilization of lowcost, earth-abundant precursors that are solution-processed at low-temperatures into high-quality devices. Despite this progress, interdevice variability and long-term stability have prevented the widespread commercial adoption of perovskite devices, especially for high-energy photon detectors. Using methylammonium lead iodide perovskite single crystals grown via inversetemperature crystallization, we demonstrate a facile solution-based technique to coat the single-crystalline bulk with a micrometer-scale thick surface layer comprised of a wider band gap two-dimensional Ruddlesden−Popper (RP) hybrid perovskite. The resulting perovskite room-temperature γ-ray detector devices exhibit greatly improved device yield and repeatability from run-to-run and device-to-device within a given processing run. With an energy resolution of under 15% (12.0 keV) for incident 81 keV photons, this solution-based technique resolves interdevice variability concerns and could pave the way for low-cost, scalable manufacturing of optoelectronic devices based on RP hybrid perovskite films.
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