Donor End-Capped Hexafluorinated Oligomers for Organic Solar Cells with 9.3% Efficiency by Engineering the Position of π-Bridge and Sequence of Two-Step Annealing

A pair of isomeric hexafluorinated oligomers (Th6FSe and Se6FTh), which are with the same aromatic compositions (difluorobenzothiadiazoles central core, IDT units, and donor end-capped groups), but differ in the π-bridge position (selenophene and thiophene), were designed and successfully synthesized. The potential of the resulted oligomers as donor materials for BHJ-OSCs was systematically investigated through optical absorption, AFM, TEM, GIXD, charge mobility measurement, and photovoltaic device fabrication. It was found that the π-bridge sequences in the resulted oligomers play a subtle but key role in device performances. Moreover, as a result of increase of crystalline content and desired phase separation after rapid SVA or combined TA and SVA treatment, the device performance of the resultant devices undergo significant enhancement. Notably, the Se6FTh devices showed a best PCE of ca. 9.3% with SVA+TA treatment, which is the highest PCE of BHJ-OSCs based on donor end-capped oligomers. These primary study demonstrated that the sequence of π-bridge and annealing treatments play critical roles for improving ordered and crystalline morphology and enhanced PCE, and hence can provide an useful strategy toward highly efficient oligomers for BHJ-OSCs.