Figure 4.The decay dynamics of the singlet excitons in (a) OC4-4Cl-Ph , (b) OC4-4Cl-Th and (c)OC4-4Cl-C8 films.
2.4. Exciton diffusion lengths
According to previous reports, a longer exciton diffusion distance is help to suppress charge recombination, which is beneficial for facilitates higher current density.[40] Therefore, the exciton diffusion lengths (L Ds) of OC4-4Cl-Ph , OC4-4Cl-Thand OC4-4Cl-C8 neat films are estimated by using transient absorption spectroscopy (TAS) and the exciton-exciton annihilation (EEA) model. The ground-state-bleaching (GSB) signals (Figure S1) emerge after photoexcitation of OC4-4Cl-Ph , OC4-4Cl-Th andOC4-4Cl-C8 films at 800 nm. The decay dynamics of the singlet excitons are shown in Figure 4. The quenching of excitons is generally considered into two parts: trap-induced recombination and bimolecular EEA process, which corresponded to the rate decay parameters κand γ , respectively, which can be used to calculate the exciton diffusion coefficient (D). The calculated D values ofOC4-4Cl-Ph , OC4-4Cl-Th andOC4-4Cl-C8 are 4.72×10-2, 4.94 ×10-2 and 11.69×10-2cm2 s-1, respectively. TheL D values can be calculated by the equation:L D=(D τ)1/2, where τ is the exciton lifetime. The calculated L D values ofOC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 are 10.01, 9.41 and 11.94 nm (Table S1), respectively. The large L D values ofOC4-4Cl-C8 mean the longer exciton diffusion distances, which can effectively reduce the exciton recombination.[41]