New D-A Type Conjugated Polymers Incorporated Cyano Groups For Polymer Solar Cells

As a potential and promising renewable energy source, bulk heterojunction (BHJ)polymer solar cell (PSC) has attracted significant attention recently. However, up tonow the device power conversion efficiency (PCE) still remains one of the key factorshindering its practical application. It is well–known that the PCE is proportional to theopen circuit voltage (Voc), short circuit current density (Jsc) and fill factor (FF) of PSCs.An ideal polymer should possess broad absorption to harvest as much sunlight aspossible for high Jsc, relatively deep highest occupied molecular orbital (HOMO) forhigh Voc, and high hole mobility with better planarity for high FF. In addition,molecular weight of the polymer and morphology of the active layers should beoptimized as well. In the strategy of LBG copolymer design, D A type copolymershave been mostly developed and investigated recently due to their tunableopotoelectronic properties by adjusting electron rich or electron deficient units tofacilitate polymer’s intramolecular charge transfer (ICT). To obtain high Voc, Jsc, andFF, many efforts have been devoted to modify the chemical structure of conjugatedpolymers to tune their optoelectronic properties, and consequently improve theperformance of PSCs.1) Four LBG polymers, POP, POT, PTP and PTT, consisting of BDT donorswith different side chains and BCNV acceptors with different π bridges, weresynthesized by Pd catalyzed Stille cross coupling reaction. Broader absorption andnarrower bandgaps were achieved (from POP and PTP to POT and PTT) when thephenyl bridges were replaced with thienyl bridges. Due to the introduction of CN groups and choice of BDT donor unit, all polymers have relative deep HOMOsbetween5.48eV and5.37eV, and resulted in prospective high Vocs in PSCs, although the PCEs of the PSC devices were not sufficient. Systematicallyinvestigations suggested that the disadvantages, such as poor backbone planarity,defects/impurities of polymer and low polaron photogeneration efficiency resultedfrom BCNV, should be account for these low devices performances.2) We investigated for the first time the effects of different CN group numbers onthe optoelectronic, structural, morphology and photovoltaic properties of threeconjugated polymers POT DH, POT HCN and POT DCN. With increasedCN group number, broader absorption, smaller optical bandgap and lower HOMOlevels can be obtained. The planarity of polymers also decreases with increasedCN group, leading to different inter molecular packing and morphology. Theincorporation of two CN groups results in poor morphology, inefficient charge transferand very low device performance. The best PCE of4.21%was demonstrated byPOT HCN with one CN group. Thus we believe that, by controlling the number ofintroduced CN groups, we can generally fine tune the planarity and LUMO/HOMOlevels of this class of polymers to achieve desired optoelectronic properties andmorphology for high photovoltaic performance. This also provides a feasible way foroptimizing other photovoltaic semiconducting polymers by adjusting the number ofelectron–withdrawing units.