Influence of para-alkyl chain length of the bay-phenyl unit on properties and photovoltaic performance of asymmetrical perylenediimide derivatives

In this report, three asymmetrical perylenediimide derivatives (PDI) substituted on the bay-position with para-alkylphenyl groups were synthesized, on which the substituted alkyl side chain was n-propyl (4-PP-PDI), n-hexyl (4-HP-PDI), or n-nonyl (4-NP-PDI) group. The effect of alkyl chain length on the optical and electrochemical properties, thermal behavior, as well as the photovoltaic performance of these materials in solution processed polymer solar cells were systematically studied. Results indicated that the para-alkyl side chain length showed negligible influence on the spectroscopy and redox behaviors of the materials, but significant influence on the photovoltaic performance. The propyl substituted compound 4-PP-PDI showed the best photovoltaic performance of V_OC = 0.63 V, J_SC = 1.93 mA cm^-2, FF = 0.63 and a power conversion efficiency of 0.77%, which was attributed to the balanced intermolecular interaction of PDI molecules and the donor-acceptor phase separation of the blend films.