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.