Page - 69 - in Advanced Chemical Kinetics

decrease of the recombination rate. Theywere able to determine that the recombination rate constant isaconsequenceof theperovskitemorphological inhomogeneity. Recombination is an importantmechanismof depopulation of the excited state, fromwhich energy is generated.Controlling the exciton recombinationhasbeena strategy for enhancing thesolarcellefficiency,but itneedsanaccuratecharacterizationof thekineticsofallcompeting processes of deactivation and, sometimes, it can lead to a poorly effective control of the recombination. Other strategies have been developed, focusing on enhancing the exciton formation, other than avoiding recombination. Many studies have demonstrated that pro- cesses such as multiple exciton generation in quantum dots and singlet exciton fission in molecular chromophoreshavegreatlycontributed toenhance thepowerconversionefficiency of devices such as solar cells and fuels cells. To carefully characterize, both processes had proven to consist of an embracing strategy to promote higher efficiencies. Beard et al. [17] studied the characteristics of the mechanisms multiple exciton generation [18] and singlet exciton fission [19, 20], searching for their similarities, inorder togiveenough informationon howto improve theexciton formation insuchdevices, independentlyof thedevice configura- tion. They found that the twomechanisms are different, because inmultiple exciton genera- tion, twoexcitons are created in a single quantumdotwhereas in singlet exciton fission, two species are electronically coupled to give rise to an overall singlet excited state that allows a transition from the singlet excited state to two coupled triplet excited states. In the former, there is spin conservation, in the latter, two triplets are created, each one presenting half the energyof theprime singlet excited state.Alsodifferent are their dynamics. Excitonmultiplica- tion, in bothmechanisms, occurs very fast, nevertheless, the difference lies on lifetimes of the newly generated excitons. In exciton singlet fissionmechanism, the new excited triplet states present lifetimes of microseconds, originated from singlet states with lifetimes of nanosec- onds [19], whereas in multiple exciton generation, the excitons present lifetimes of picosec- onds [21].Despite thesedifferences, they concluded that in solar cells, the enhancement in the efficienciescalculatedconsideringbothmechanismsaresimilar.They informedthat there is still muchwork tobedoneregarding thesolar cell structures tominimizenon-radiative recombina- tionandprovidemoreefficiencytothem,butsolarcellswithpowerconversionefficiencyofover 30%canbeeasilyobtainedbymulti-excitongeneration.Also,Thompsonetal. [22]showedthatit ispossible to achievemoreefficient solar cells exploiting the singlet exciton fissionmechanism, andSemoninet al. [23] achievedan increase in theexternalphotocurrentefficiencyofquantum dotsolarcellsexploitingthemultipleexcitongenerationmechanism. Thephotophysicalprocesses thatare responsible for thepopulationofelectronicexcitedstates after the fast absorption of light by the absorber can be exploited for several imaginable applications. An example is thework ofWu et al. [24], where photolysis kinetics, quantum yield andbioavailability of a ketone (acetylacetone) duringUVirradiationwere investigated. They found that, after the absorption of UV light by the ketone, a series of photophysical processesovercame thephotochemical reactionsofdecomposition. Interestingly, theyobserved that the energy transfermechanisms that occur after the absorption of sunlight guarantee the highefficiencyof thephotochemical changes.Since thedegradationproductsof theketoneafter the photochemical reactions were similar to themetabolic products in biofermentation, they argue that the acetylacetonemay be used inwater treatment at the pre-treatment stage and New Materials to Solve Energy Issues through Photochemical and Photophysical Processes: The Kinetics Involved http://dx.doi.org/10.5772/intechopen.70467 69