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In contrast with findings on the wild-type Rhodobacter sphaeroides reaction center, biexponential P H → PH charge recombination is shown to be weakly dependent on temperature between 78 and 298 K in three variants with single amino acids exchanged in the vicinity of primary electron acceptors. These mutated reaction centers have diverse overall kinetics of charge recombination, spanning an average lifetime from ~2 to ~20 ns. Despite these differences a protein relaxation model applied previously to wild-type reaction centers was successfully used to relate the observed kinetics to the temporal evolution of the free energy level of the state P H relative to P B. We conclude that the observed variety in the kinetics of charge recombination, together with their weak temperature dependence, is caused by a combination of factors that are each affected to a different extent by the point mutations in a particular mutant complex. These are as follows: (1) the initial free energy gap between the states P B and P H, (2) the intrinsic rate of P B → PB charge recombination, and (3) the rate of protein relaxation in response to the appearance of the charge separated states. In the case of a mutant which displays rapid P H recombination (ELL), most of this recombination occurs in an unrelaxed protein in which P B and P H are almost isoenergetic. In contrast, in a mutant in which P H recombination is relatively slow (GML), most of the recombination occurs in a relaxed protein in which P H is much lower in energy than P H. The weak temperature dependence in the ELL reaction center and a YLH mutant was modeled in two ways: (1) by assuming that the initial P B and P H states in an unrelaxed protein are isoenergetic, whereas the final free energy gap between these states following the protein relaxation is large (~250 meV or more), independent of temperature and (2) by assuming that the initial and final free energy gaps between P B and P H are moderate and temperature dependent. In the case of the GML mutant, it was concluded that the free energy gap between P B and P H is large at all times. [ABSTRACT FROM AUTHOR]

Perovskite solar cells, composed of a mixture of methylammonium (MA) and formamidinium (FA) cations [in the benchmark proportions of (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 ] and titania as an electron-accepting material, are prepared under different conditions, with the objective of finding correlations between the solar cell performance and several important stationary and dynamical parameters of the material. The effects of humidity, oxygen, the use of anti-solvent, and the presence and quality of a mesoporous titania layer are investigated. It is found that an increase in the photocurrent corresponds to a higher content of the desired cubic perovskite phase and to increased long-wavelength absorption of the sample. On the contrary, for poorer-quality cells, additional short-wavelength bands in both absorption and emission spectra are present. Furthermore, a higher photocurrent of the cells is correlated with faster interfacial charge-transfer dynamics. For the highest photocurrent of >20 mA cm -2 , the characteristic times of about 1 μs are observed by electrochemical impedance spectroscopy, and emission half-lifetimes of about 6 ns by time-resolved fluorescence spectroscopy (upon excitation with 420 nm pulses of ≈0.5 mW power). Both first- and second-order rate constants, extracted from the emission measurements, are greater for the cells showing higher photocurrents, probably owing to a more rapid charge injection., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)