The small exciton binding energy of perovskite suggests that the long-lived photoluminescence and slow recovery of the ground state bleaching of the tetragonal phase at room temperature results primarily from the decay of free charges rather than the decay of the initially created excitons. Here we demonstrate the ground state bleaching recovery of the orthorhombic phase of methylammonium lead iodide (CH3NH3PbI3) is much faster than that of the tetragonal phase using temperature dependent transient absorption spectroscopy. The distribution in orientation of the methylammonium group which is disordered in the tetragonal phase and ordered in the orthorhombic phase results in smaller dielectric constant and larger exciton binding energy in the latter phase. We observe the recovery of the ground state bleaching in the orthorhombic phase to be comprised of decays of both excitons and free charges. Our findings suggest CH3NH3PbI3 behaves like a nonexcitonic semiconductor in the tetragonal phase and an excitonic semiconductor in the orthorhombic phase.

Photosystem II (PSII) initiates photosynthesis in plants through the absorption of light and subsequent conversion of excitation energy to chemical energy via charge separation. The pigment binding proteins associated with PSII assemble in the grana membrane into PSII supercomplexes and surrounding light harvesting complex II trimers. To understand the high efficiency of light harvesting in PSII requires quantitative insight into energy transfer and charge separation in PSII supercomplexes. We have constructed the first structure-based model of energy transfer in PSII supercomplexes. This model shows that the kinetics of light harvesting cannot be simplified to a single rate limiting step. Instead, substantial contributions arise from both excitation diffusion through the antenna pigments and transfer from the antenna to the reaction center (RC), where charge separation occurs. Because of the lack of a rate-limiting step, fitting kinetic models to fluorescence lifetime data cannot be used to derive mechanistic insight on light harvesting in PSII. This model will clarify the interpretation of chlorophyll fluorescence data from PSII supercomplexes, grana membranes, and leaves.