Suppressing the non-radiative energy loss of organic solar cells by embedding nitroxide radical blocks in wide bandgap conjugated polymer donors

Abstract

The large non-radiative recombination losses (ΔV^non-rad_oc) of non-fullerene acceptor-based organic solar cells (NFA-OSCs) were recognized as the main limitation to further boosting their efficiency to a new regime. In this report, three nitroxide radical modified PM6 analogous polymers labeled as PM6-BTMP1, PM6-BTMP2 and PM6-BTMP5 were built from 2,6-bis(trimethylstannyl)-4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-FSn) and 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethyl-hexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDDBr) through embedding the third nitroxide radical component 2,5-dibromo-1,4-bis(4-oxy-2,2,6,6-tetramethylpiperidine-1-oxyl)phthalate (BTMP) with molar feed ratios of 50 : 49 : 1, 50 : 48 : 2 and 50 : 45 : 5. With the incorporation of a small amount of BTMP blocks containing nitroxide radicals in terpolymers, the absorption and electrochemical characteristics were almost not changed as compared to the notable PM6, but the power conversion efficiencies (PCEs) of the NFA-OSCs from the blends of BTMP-modified PM6 polymers paired with Y6 were decreased to 12.65–13.38% in contrast to that of 15.87% for the devices from PM6:Y6, alongside the decrease of ΔV^non-rad_oc and a remarkable increase in open circuit voltages (V_OCs), as well as a decline in short current densities (J_SCs) and fill factors (FFs). Interestingly, upon the utilization of the BTMP-modified PM6 polymer as a solid additive, the PCEs of the devices from PM6:Y6 were improved by 1.64–12.67% relative to that of 15.78% for the NFA-OSCs from the pristine PM6:Y6 blends (PCEs, 15.87% vs. 16.13–17.88%). Particularly interesting NFA-OSCs, which presented a PCE of 17.88%, were realized in the devices with 2.0 wt% PM6-BTMP2 additives. Besides these, the reasons behind the observed decrease in the performance and mitigated ΔV^non-rad_oc of the devices from the BTMP-modified PM6 as donors, as well as the improvement of the PCEs in the NFA-OSCs with BTMP-modified PM6 as solid additives, were investigated by a set of physical measurements. The results not only point to a potential avenue to develop novel polymer donors to mitigate the ΔV^non-rad_oc of the NFA-OSCs, but also provide a supplement to further optimize the nitroxide radical polymer additives to boost the efficiency of organic solar cells.