Photophysics of organic/inorganic lead halide perovskites

In keeping with their nature of hybrid materials, halide perovskites have been found to possess a particularly attractive blend of optoelectronic properties. A wealth of literature has been produced, that on some occasions raised questions just as readily as it answered them. In particular, my thesis addresses the following four key issues. First, the nature of the band gap. Long assumed to be the direct, according to recent reports the band gap supposedly has a dual nature. This fundamental matter is addressed through a study of the radiative recombination rates as a function of temperature. Data show that in perovskites radiative recombination becomes faster with decreasing temperature, as indeed in all direct-band gap materials. Second, exciton formation. Countless reports agree that free carriers are the majority photoexcited species under typical operating conditions of a solar cell, while evidence of exciton formation and dissociation processes is still elusive. A differential photoluminescence technique is employed to access extremely low injection levels at which emission from primary (geminate) excitons outshines that of free carriers. Third, efficiency loss. Understanding what processes limit the power conversion efficiency of perovskite solar cells is essential for the technology to mature. Shockley-Read-Hall and interface recombination are unambiguously identified as the main recombination mechanisms in perovskites and perovskite heterostructures, respectively. Fourth, solution processed nanostructures. A solution method developed by co-worker Dr Daniela Marongiu is shown to be able to form stable self-assembled MAPbI3-xBrx nanocrystals in a MAPbBr3 matrix. A thorough investigation on the photophysics of this material reveals that photoexcited charge carriers are funneled from the matrix to the nanocrystals, lifting quantum yield by one order of magnitude.