Acceptor-acceptor type perylenediimide-based polymeric acceptors with large 2-decyltetradecyl flexible side chain: Synthesis and impact of fluorination

Acceptor-acceptor (A-A) type perylenediimide (PDI)-based polymeric acceptors (PAs) possessed a high air stability and an excellent charge mobility. However, few efforts devoted to A-A type PDI-based polymeric acceptors and the interplay between the structure and photovoltaic performance was still nebulous. Herein, three A-A type PDI-based PAs PPDI-DT-DTBT, PPDI-DT-DTFBT and PPDI-DT-DTFFBT with large flexible 2-decyltetradecyl (DT) side chains were developed to probe into the effect of different fluorination at the benzothiadiazole (BT) moiety. And the donor-acceptor (D-A) type polymer acceptor PPDI-DT-2T without BT unit was prepared to make the comparison. All of them exhibited the high thermo-stability and solution state photo-stability. Gradually increased fluorine substituents acquired the blue-shifted absorption, the increased bandgap, the raised E_LUMO, more planar molecular configuration and strengthened aggregation. Accordingly, with the widely developed donor polymer PTB7-Th as electron donor, bulk heterojunction device based on monfluorinated PPDI-DT-DTFBT obtained the middle V_OC of 0.70 V, the highest J_SC of 6.22 mA cm⁻² and FF of 37.56 %, contributing to the winning power conversion efficiency (PCE) of 1.63 %. By contrast, fluorine-free PPDI-DT-DTBT- and difluorinated PPDI-DT-DTFFBT-based devices acquired the inferior PCEs of 1.31 % and 1.24 %. The control polymer accepter PPDI-DT-2T got the PCE of 1.27 % regardless of the high V_OC of 0.80 V as the result of high E_LUMO. The raised PCE after monofluorination was mainly originated from the increased J_SC and FF, which was due to the improved absorption and the higher electron mobility. The observed low efficiency in these DT-side chain modified PDI polymeric acceptors was probably due to the large aggregation and the coarse surface morphology resulting from limited solubility. Interestingly, fluorine-free PPDI-DT-DTBT film exhibited the E_LUMO of −3.85 eV and afforded the highest OFET electron mobility of 2.2 × 10⁻² cm² V⁻¹ s⁻¹. This finding implied that it was cautious to prolong the flexible side chain during the A-A type PDI-containing PAs, however, monofluorinating the benzothiadiazole moiety was an effective scenario by the virtue of tuning the molecular configuration and lifting the charge mobility, with an aim at elevating the photovoltaic performance in all-polymer solar cells.