Insights into the Local Bulk-Heterojunction Packing Interactions and Donor–Acceptor Energy Level Offsets in Scalable Photovoltaic Polymers

An energy level offset in organic solar cells (OSCs) is necessary for efficient charge generation and separation. To date, there are several polymer donor–non-fullerene acceptor (NFA) bulk-heterojunction (BHJ) systems with a negligible ionization energy (IE) level offset achieving high power conversion efficiencies (PCEs) over 15%. Although these donor–acceptor pairs perform well in solar cells, there is little understanding on why some systems can achieve this phenomenon, and therefore, many of these BHJs are discovered through a trial-and-error process. Here, we investigate how OSC efficiencies can be modulated by adjusting the IE level offset in a series of PTQ10 n:m random terpolymer donors by means of solar cell performance (open circuit voltage (VOC) and short-circuit current (JSC)) when paired with Y6 and IDIC acceptors. PTQ10’s IE level was adjusted through a copolymerization of thiophene (n), bithiophene (m), and quinoxaline monomer units in different ratios, whereby 10% bithiophene leads to a 0.05 eV decrease in the polymer’s IE. The incorporation of 10% bithiophene (PTQ10 90:10) led to a 1.3 ± 0.5 mA/cm2 increase in JSC when paired with Y6 (PCE = 13.8 ± 0.4%) in conjunction with an incremental decrease in VOC and fill factor (FF) when compared to PTQ10 (PCE = 14.7 ± 0.1%). Increasing the bithiophene content to 20% (PTQ10 80:20) exacerbated the decrease in VOC and FF further without the benefit of increased JSC. The drop in FF with increasing bithiophene incorporation correlated with increasing edge-on orientation in the neat polymer and polymer:Y6 BHJ blend films, shown by grazing-incidence wide-angle X-ray scattering measurements. High-field solid-state (ss)NMR spectroscopy analysis of single component PTQ10, Y6, and PTQ10:Y6 BHJ blends provides a complementary insight into how a low IE level offset system (PTQ10:Y6) imparts high performance. By resolving inter- and intramolecular packing interactions at sub-nanometer distances, ssNMR results offer key insights into the changes in local structures and conformations in the vicinity of the alkoxy PTQ10 side chains and in the Y6 end group in BHJ blends when compared to the neat compounds. Despite the changes in local structures, the BHJ morphology maintains pure D-A domains and preserves the microstructure, which correlates with the high-performing solar cells. A synergic combination of chemical design, multiscale morphology characterization, and device physics shown in this study provides an excellent strategy to investigate the BHJ and its role in organic solar cell performance.