Charge transfer dynamics in fully operational dye sensitised solar cells consisting of an electrolyte or organic spiroOMeTAD in contact with a highly porous electrodeposited ZnO film sensitised with a monolayer of the indoline dye DN216 were observed using ultrafast transient absorption spectroscopy. From the temporal evolution of spectral signatures assigned with the help of spectroelectrochemical experiments to the population and depopulation of initial, transient and final states, a model was completed for the multistep injection of photoexcited electrons from the molecular absorber to the ZnO acceptor. Injection was found to occur via three different paths with three characteristic rates: directly from the dye's lowest unoccupied molecular orbital into the ZnO conduction band (200 fs) and via intermediate molecular dominated and surface dominated hybrid states (2 ps and 10 ps, respectively).
Perovskite based photovoltaic devices are popularised by the rapid increase in their efficiencies. Understanding the fundamental physics and chemistry processes occurring upon excitation is key. We monitored the temporal evolution of the population and depopulation dynamics of various electronic states in FA MA PbI Br by means of ultrafast transient absorption spectroscopy in the visible and near infrared spectral regions in order to build a fully consistent charge dynamics model of the initial photoprocesses. Upon photoexcitation with 3.2 eV photon energy, hot electrons and holes are generated in the lowest conduction and highest valence bands, away from the bandgap, and cool to the band edges with a time constant of 500 fs. Geminate recombination of excitons occurs with a time constant of 66 ps, which increases to approximately 130 ps at the optical bandgap. From a systematic study of the excited state population dynamics and its dependence on charge carrier density, we determined the nonlinear recombination rate constants characteristic to FA MA PbI Br . The coefficient describing the non-geminate recombination of free electrons and holes is independent of the k vector as well as the charge carrier density and equal to 1×10 s cm , while the Auger recombination coefficient decreases with increasing charge carrier density in the range of (2-50)×10 s cm .
The Cover Feature shows the ultrafast charge dynamics which occur upon photoexcitation of a mixed‐halide – mixed‐cation lead‐based perovskite thin film depicted together with the spectroscopic signatures obtained from femtosecond transient absorption spectroscopy. A diagram of the perovskite structure and its chemical composition is shown alongside. More information can be found in the Article by I. Minda et al. on page 3010 in Issue 22, 2018 (DOI: 10.1002/cphc.201800547).
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