Broadening the optical absorption of organic photovoltaic (OPV) materials by enhancing the intramolecular push-pull effect is a general and effective method to improve the power conversion efficiencies of OPV cells. However, in terms of the electron acceptors, the most common molecular design strategy of halogenation usually results in down-shifted molecular energy levels, thereby leading to decreased open-circuit voltages in the devices. Herein, we report a chlorinated non-fullerene acceptor, which exhibits an extended optical absorption and meanwhile displays a higher voltage than its fluorinated counterpart in the devices. This unexpected phenomenon can be ascribed to the reduced non-radiative energy loss (0.206 eV). Due to the simultaneously improved short-circuit current density and open-circuit voltage, a high efficiency of 16.5% is achieved. This study demonstrates that finely tuning the OPV materials to reduce the bandgap-voltage offset has great potential for boosting the efficiency.

We demonstrate a facile and environmentally friendly approach to prepare well-dispersed graphene sheets by g-ray induced reduction of a graphene oxide (GO) suspension in N,N-dimethyl formamide (DMF) at room temperature. GO is reduced by the electrons generated from the radiolysis of DMF under g-ray irradiation. The reduced GO by g-ray irradiation (G-RGO) can be re-dispersed in many organic solvents, and the resulting suspensions are stable for two weeks due to the stabilization of N(CH 3 ) 2 + groups on G-RGO. Additionally, G-RGO is efficient in improving the conductivity of polystyrene (PS). Its PS nanocomposites exhibit a sharp transition from electrically insulating to conducting with a low percolation threshold of 0.24 vol% and a high electrical conductivity of 45 S m À1 is obtained with only 2.3 vol% of G-RGO. The superior electrical conductivity is attributed to the uniform dispersion of the G-RGO sheets in the PS matrix.

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