A-A′-A structured methoxy-modified perylenediimide dimeric acceptors at the outside-bay position: Synthesis, spectroscopic property and photovoltaic application

Abstract

Acceptor-acceptor′-acceptor (A-A′-A) structured methoxy-modified perylene diimide (PDI-OMe) electron acceptors weren′t fully investigated so far. Herein, two A-A′-A type dimeric acceptors, denoted as DTBT-(PDI-OMe)2 and difluorinated DTFFBT-(PDI-OMe)2, where two methoxy-modified PDI-OMe subunits as biswings and electron-deficient units 4,7-di(thien-2- yl)benzothiadiazole (DTBT) and difluorinated 4,7-di(thien-2-yl)-5,6-difluorobenzothiadiazole (DTFFBT) as central linkers, were developed. And the methoxy-free DTBT-(PDI-HD)2 was utilized as reference material. Owing to the electron-donating characteristic of incorporating methoxy group at the outside bay position of PDI, DTBT-(PDI-OMe)2 possessed an enlarged optical bandgap but improved light-capturing ability, weakened aggregation and up-shifted ELUMO. Moreover, the decreased the thermal- and photo-stability, slightly red-shifted maximum absorption, the reduced the bandgap, weakened aggregation and the deepened ELUMO were seen after further difluorinating the central benzothiadiazole linker. Thereupon, compared with DTBT-(PDI-HD)2, PTB7-Th:DTBT-(PDI-OMe)2-based device achieved a 0.07V increased VOC of 0.86V, an 5.91% enhanced JSC of 3.94mAcm2 and a 20.69% elevated FF of 42.23% and thus a 38.83% raised PCE of 1.43%. Moreover, the further difluorinating the central benzothiadiazole linker led to a 20.98% decreased PCE of 1.13%, which was mainly limited by the 0.07V decreased VOC and 15.65% dropped FF regardless of slightly increased JSC in the DTFFBT-(PDI-OMe)2-based device. The dropped PCE was mainly due to the enlarged charge recombination, reduced electron mobility resulted from the weakened aggregation and insufficient microstructural phase separation. The work suggested that introducing the methoxy substituent at the outside bay position was an effective strategy for enhancing the voltage and efficiency, but it should be cautious to further incorporate fluorine atoms into the central electron-deficient linking cores during the A-A′-A structured methoxy-modified PDI dimers for high performance photovoltaic device.