2.5. Charge generation kinetics
The charge-generation process is studied using femtosecond transient absorption (fs-TA) spectroscopy of the blend films. The contour plots of the time-resolved absorption difference spectra of D18:OC4-4Cl-Ph , D18:OC4-4Cl-Th and D18:OC4-4Cl-C8 blend films pumped at 800 nm are shown in Figure 5. The GSB signal of polymer donor can reflect the hole transfer process.[42] Specifically, the kinetics of 602 nm is chosen to represent the D18 GSB dynamics. After excitation, strong GSB peaks around short wavelength region (550–660 nm) appears inOC4-4Cl-Ph , OC4-4Cl-Th and OC4-4Cl-C8 based blend films, which represents the hole transport at the D/A interface. Biexponential function can be used to fit the hole transport kinetic process.[43] The hole transfer process consists of an ultrafast hole transfer process at the D/A contact, as defined byτ 1, and a diffusion-mediated mechanism that is significantly influenced by domain size and aggregation, as defined byτ 2[44]. Theτ 1/τ 2 are estimated to be 2.46/23.95, 2.81/22.22 and 1.73/18.95 ps for the OC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 blend films, respectively. Therefore, the D18:OC4-4Cl-C8 based device is more conducive to the dissociation and diffusion of excitons, which is more beneficial for improving the photovoltaic performance.