Page - 68 - in Advanced Chemical Kinetics

better everyone’s lifeworldwide. Examples are the several uses of photochemistrykinetics in distinctprocessesanditsapplicationtonewmaterialsdevelopment, inspecial thoseforenergy conversionandenergyharvesting [6–11]. Recently, research into optoelectronic organic materials is being developed to describe newoptionswith potential for applications in emissive devices, sensors and solar cells [7]. Although these materials have been successfully tested as part of these devices, they are numerous and a serious difficulty has been to determine which characteristics are deter- minant for a material to present a specific property and how to replicate that in others. The answer invariably has been found in determining the kinetics of deactivation of the electronic excited states and, therefore, of the photophysical properties andphotochemical processes. The efficiency of a device containing organic electroluminescent compounds is strictly related to the efficiency of the exciton formation and, thus, it depends on the conjugation lengths [7], which determine the mechanisms of energy transfer among the material [12]. For instance, in their work, Arkan and Izadyar studied the mechanism of charge transfer and the rate of exciton formation and dissociation in dye-sensitized solar cells based on TiO2/Si/porphyrins. They observed the rate of exciton formation/dissocia- tion inmetal-porphyrins, revealing the occurrence of an efficient charge transport in these systems. Indeed, it isexpectedthatefficientsolarcellspresentgreatabilityofexcitonformation,efficient exciton transport and charge transport from the donor to the acceptor [13] tominimize the influence of the competitiveprocesses such as exciton recombination that reduces the energy conversionefficiency [14]. Exciton formation is a driving force of the solar cell efficiency, which causes the exciton recombination to be an event that needs to be controlled. In several devices, recombination must be understood to be avoided to guarantee the highest efficiency.Many solar cells have beenbasedonperovskite due to their ability of delivering efficiencies as high as 22% [15]. In theirwork,Dar et al. characterized the charge carrier recombination process that occurs in a bromide-basedperovskitebymeasuringthetransientabsorptionkineticsareseveralexcitation intensities (5–100 μJ cm�2). For that, they assumed that the carrier dynamics is mainly governedbybimolecular recombination,beingexpressedanddecaykinetics: dn=dt¼γ tð Þn2 (11) Where, indisorderedsystems, the time-dependent recombination isapproximately to [16]: γ tð Þ¼γ0t�α (12) That gives the carrier concentration kinetics: 1/n =�1/n0 =γ0 t1�α/(1�α), independent of the initial carrierdensityand, thus, independentof theexcitation intensity. Through this treatment, they identified the time-dependent recombinationasa functionof the morphology of the perovskite. They found that the polycrystalline perovskite structure pre- sents grain boundaries that are physical obstacles for the carriermotion, which results in a Advanced Chemical Kinetics68