All-inorganic CsPbI₃ Perovskite Materials And Solar Cells

All-inorganic lead halide perovskites solar cells have attracted enormous research interest in the recent three years owing to their significantly improved stability under thermal stress over the organic-inorganic hybrid perovskite counterparts.However,the cubic?-CsPbI₃ with the most appropriate bandgap for solar energy conversion among all inorganic perovskites is not structurally stable at room temperature and spontaneously transforms into unfavorable nonperovskite?-CsPbI₃.In this dissertation,we focus on the development of new black-phase CsPbI₃ perovskite,the orthorhombic?-CsPbI₃,with highly desired intrinsic thermodynamic stability and ideal electronic structure for high-performance solar cells.Without employing organic ligands or alloying with mixed cations/anions in the perovskite lattice,we present here for the first time a room-temperature-stable black-phase?-CsPbI₃ thin film by introducing a small amount of protic solvent with a high dielectric constant such as water（H₂O）into the perovskite precursor solution and depositing the film via a one-step spin-coating method followed by low-temperature annealing.The H₂O molecules can delicately tune the equilibrium of the proton transfer reaction in the precursor solution and hence manipulate the thermodynamic phase stability of CsPbI₃ polymorphs through tailoring the dimensions of perovskite grains.The exact crystal structure of?-CsPbI₃ is determined by Rietveld refinement of the XRD profile.The density functional theory（DFT）calculations reveal that?-CsPbI₃ has markedly lower surface free energies than?-CsPbI₃,thus thermodynamically preferred at remarkably enlarged surface areas beyond 8,600 m²/mol,as further evidenced by the phase stability test at room temperature in open air over a month.Moreover,the?-phase CsPbI₃ perovskite demonstrates ideal electronic structure and excellent optical properties,as demonstrated by the DFT calculations and photoluminescence（PL）and UV-vis absorption measurement.Inspired by the unique features of?-phase CsPbI₃ for photovoltaic applications,planar heterojunction solar cells are fabricated based on the prepared black-phase CsPbI₃ thin films.The?-CsPbI₃ devices show significantly enhanced photovoltaic performance with the best power conversion efficiency（PCE）of 11.3%,which is the highest PCE of pure CsPbI₃ perovskite solar cells reported to date.Furthermore,the?-CsPbI₃ devices exhibit substantially improved stability for months in ambient conditions and outstanding operational stability for hours under continuous solar illumination and constant forward bias at maximum power point.