Synthesis DifferentSide-chains Of Twisted Benzodiperylenediimides For Non-fullerene Solar Cells

Bulk heterojunction?BHJ?solar cells become a hot research topic,because they are solution processable and flexible.Excellent acceptors play an important role in the development of BHJ solar cells.Traditional fullerene derivatives as widly used acceptors have disadvantages such as high cost,weak visible absorption,and the difficult modification.Nonfullerene acceptors can overcome above shortcomings.Herein,we synthesized five twisted benzodiperylenediimides?C5,C8,C11,C20,C23?with different side-chains at the nitrogen atom for the first time.Their solubility,photophysical properties,electrochemical characters,and applications in BHJ solar cells were investigated.Twisted benzodiperylenediimides with large rigid conjugated core and strong absorption is an excellent acceptor in non-fullerene solar cells.Since side chains of semiconductors play a crucial role in the solar cells,TBDPDI acceptors with different side chains?1-ethylpropyl;2-ethylhexyl;1-pentylhexyl;2-octyldodecyl;1-undecyldodecyl?were synthesized.In solution,TBDPDI compounds with alkyl chains branched at 1-position compounds?C5,C11,and C23?show significantly different absorption profiles and fluorescence intensity with those branched at 2-position compounds?C8 and C20?,due to stronger aggregation of the latter.Nevertheless,alkyl chains have little effect on the molecular orbital energy levels and optical band gaps,as verified by cyclic voltammetry and solid state absorption.Due to their complementary absorption and matchable energy levels with donor of PCE10,these acceptors and PCE10 were used together to fabricate bulk heterojunction?BHJ?solar cells.Because of inferior phase separation with large domain size around 100 nm and bulky insulated side chains,acceptors with long alkyl chains?C20 and C23?have the low electron mobility??e?around 10-8 cm2 V^-1 s^-1 and the low power conversion efficiency?PCE?of solar cells.TBDPDI with 1-pentylhexyl?C11?gives the highest PCE of 5.0% under the optimized condition,which is attributed to proper phase separation with domain size around 20 nm and highest ?e of 10-6 cm2 V^-1 s^-1.