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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
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book Advanced Chemical Kinetics"
Advanced Chemical Kinetics
- Title
- Advanced Chemical Kinetics
- Author
- Muhammad Akhyar Farrukh
- Editor
- InTech
- Location
- Rijeka
- Date
- 2018
- Language
- English
- License
- CC BY 4.0
- ISBN
- 978-953-51-3816-7
- Size
- 18.0 x 26.0 cm
- Pages
- 226
- Keywords
- Engineering and Technology, Chemistry, Physical Chemistry, Chemical Kinetics
- Categories
- Naturwissenschaften Chemie