Revealing photoinduced charge transfer mechanism across $\pi $-conjugated heterojunctions

The adapted Su-Schrieffer-Heeger (aSSH) model is extended to the $\pi $-conjugated bulk heterojunction system. The New aSSH Hamiltonian incorporated interchain $\pi $-$\pi $ stacking and dynamic electron-phonon coupling effects. Excellent agreements are found between the computed photoadsorption and photoinduced adsorption spectra and their corresponding experimental measurements. It is found that excitons generated in the bulk poly-(p-phenylene vinylene) (PPV) phase must overcome an energy barrier of 0.23 eV to reach heterojunction interface. These interfacial excitons show clear charge separations, with their electron states leaning towards the interface. Therefore, electron transfers from the D$_1^{\mathrm{\ast }}$ state of PPV to the t$_{\mathrm{1u}}^{\mathrm{\ast }}$ state of C$_{60}$ follow non-adiabatic mechanisms, which are accelerated by the 0.97 eV energy drop, close vicinity of the D$_1^{\mathrm{\ast}}$ state to the C$_{60}$ phase, and suppressed inversion symmetry that doubles the number of electron-accepting states. After non-adiabatic electron transfers, the hole D$_1$ states are screened by the optical phonons in PPV, forming self-localized hole polarons and moving further away from heterojunction interface.