Organic Photovoltaics

The field of polymer organic photovoltaics (OPV) has made impressive improvements over the last 10 years with power conversion efficiencies increasing from 8% to 18%. The design of new polymer donors and non-fullerene (NFA) acceptors have been the driving force for these large improvements. The Reynolds group designs new classes of polymers with minimal changes to explore how these small changes in the polymer can have larger impacts on the device performance, morphology and electronic properties. Understanding how to fine-tune a polymer donor can help design optimal structures for future devices. Below are some of the most recent published projects in the group.

A key factor for OPV materials implementation into industrial relevant devices is their active layer thickness tolerance as solar cell performances are typically reported with thicknesses on the order of 100–150 nm, but thicker films (ca. 300 nm) are needed for printing and roll-to-roll processing. With this in mind, we designed and synthesized two PM7 isomeric derivatives featuring a chlorinated benzodithiophene and ester functionalized terthiophene moieties for the incorporation into non-fullerene OSCs. The fundamental difference between the two isomeric polymers is the location of the ester side chains where the PM7 D1 esters are located on the outer thiophene units, whereas the esters on PM7 D2 are located on the central thiophene unit. This simple modification produced polymers with similar absorption profiles, electrochemical onsets, charge carrier mobilities when blended with ITIC-4F, and grazing-incidence wide-angle X-ray scattering patterns.

Thin-film (100 nm) OSCs were fabricated resulting in average PCEs of 11.6% for PM7, 12.1% for PM7 D1, and 9.9% for PM7 D2 when blended with ITIC-4F. In contrast, we observed significant differences in PCE when the active layer thickness was increased to 180 nm resulting in a decrease in average PCE for PM7 D2 (5.3%), whereas PM7 D1 was able to retain a 11.9% average PCE. In a follow-up study, PM7 D1-based all-polymer solar cells (all-PSCs) exhibit a high power conversion efficiency (PCE) of 9.13%, which outperforms that of the PM7-based all-PSC (PCE = 6.93%). Importantly, the ester structural modification has significant impact on the thin-film mechanical ductility and robustness. Specifically, the elongation properties of PM7 D1 and PM7 D2 pristine films are significantly improved by 2.5 times compared to that of PM7. The improved ductile properties of PM7 D1 and PM7 D2 also affect the mechanical ductility of the blend films, leading to 1.5-fold increase in crack onset strain compared with that of the PM7 blend film.

1. You, H.; Jones, A.L.; Ma, T.; Kim, G.; Lee, S.; Lee, J.; Kang, H.; Kim, T.; Reynolds, J.R.; Kim, B.J. Ester-functionalized, wide-bandgap derivatives of PM7 for simultaneous enhancement of photovoltaic performance and mechanical robustness of all-polymer solar cells. J. Mater. Chem. C., 2021, 9, 2775-2783.

2. Jones, A.L.; Ho, C.H.; Riley, P.R.; Angunawela, I.; Ade, H.; So, F.; Reynolds, J.R. Investigating the active layer thickness dependence of non-fullerene organic solar cells based on PM7 derivatives. J. Mater. Chem. C., 2021, 8, 15459-15469.