Novel Nitrogen-Containing Heterocyclic Non-Fullerene Acceptors for Organic PhotovoltaicCells: Different End-Capping Groups Leading to a Big Difference of Power Conversion Efficiencies

Novel cores for high performance nonfullerene acceptors (NFAs) remain to be developed. In this work, two new n-type nitrogen-containing organic heterocyclic NFAs, namely, BDTN-BF and BDTN-Th, were designed and synthesized based on a new seven fused-ring core (BDTN) with two different end-capping groups. As a result, BDTN-BF possessed similar absorption spectra in solution and solid state to BDTN-Th, but a slightly higher maximum molar extinction coefficient. Manufacturing the polymer solar cells with PM6 as the donor, the photovoltaic performance of BDTN-BF and BDTN-Th was investigated. The PM6:BDTN-BF-based device achieved the highest power conversion efficiency (PCE) of 11.54% with a high J_sc of 20.20 mA cm^–2, a fill factor (FF) of 61.46%, and a large V_oc of 0.93 V, and the energy loss (E_loss) was calculated to be 0.48 eV. Comparatively, the PM6:BDTN-Th-based device achieved the maximum PCE value of only 3.53% because of inadequate J_sc and FF. The higher J_sc and FF for the PM6:BDTN-BF-based device was mainly due to the effective electron transfer from PM6 to BDTN-BF, more balanced μ_h/μ_e, higher electron mobility of the neat film, better charge collection and dissociation efficiency, and more favorable morphology. These results demonstrate that the acceptors with nearly identical absorption spectra could result in a significant difference in photovoltaic performance, which stress the importance of end-capping units. Furthermore, few NFA-based devices achieve large V_oc and high J_sc simultaneously as one based on PM6:BDTN-BF, indicating that nitrogen hybridization of NFAs may be an efficient strategy to realize high and balanced V_oc and J_sc.