Investigation of excited state characteristics in polymer donor and non-fullerene acceptor molecules

The high exciton binding energy of organic semiconductor materials limits charge separation efficiency. Investigating the excited state characteristics and modulation mechanisms of polymer donor and non-fullerene acceptor molecules is crucial for optimizing material design and enhancing the performance of organic photovoltaic devices. Therefore, this study focuses on investigating the excited state characteristics in polymer and non-fullerene organic materials. The tightbinding quantum mechanical approach is employed to systematically compare the excited state characteristics (including lattice geometry, band structure, and binding energy) between polymer donor and non-fullerene acceptor molecules, with particular emphasis on the role of electron-phonon coupling in modulating these excited state characteristics. The results indicate that non-fullerene acceptor molecules exhibit smaller lattice distortion, narrower bandgap, and lower binding energy compared to polymer donor molecules. It is precisely due to the distinct excited state characteristics of the polymer donor and non-fullerene acceptor molecules that the exciton binding energy in the organic photovoltaic systems they constitute can be effectively reduced, while also providing a favorable energy-level offset for exciton dissociation. This significantly enhances the efficiency of charge transfer and separation. Furthermore, reduction of the electron-lattice coupling strength can further diminish these parameters in both polymer donor and nonfullerene acceptor molecules. By enhancing the electron-donating capability of central groups or the electron-withdrawing capacity of end groups in non-fullerene acceptor molecules, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels can be shifted upward or downward, respectively. The upshifted HOMO and LUMO energy levels are accompanied by an increase in molecular polarizability and a decrease in reorganization energy, while the downshifted HOMO and LUMO energy levels lead to an enhanced molecular dipole moment and improved electron affinity. This optimized energy-level structure further reduces the binding energy and enables efficient charge separation. These findings demonstrate that the efficient charge transfer and separation in polymer/non-fullerene organic photovoltaic systems originate from their distinct molecular excited state characteristics. This fundamental understanding enables the rational design of high-performance organic optoelectronic materials and the development of novel organic photovoltaic devices through strategic modulation of the electron-phonon coupling strength and push-pull electronic structures of non-fullerene acceptors.