Ternary Organic Solar Cells Based on S, N‐Heteroacene Non‐Fullerene Acceptors with Unfused Architecture A‐D‐D‐A‐Type

In this study, two distinct unfused non‐fullerene acceptors (NFAs) are synthesized by arranging them in an A‐D‐D‐A pattern, both containing same D‐D central S, N‐heteroacene but different terminal acceptors, namely BTA (NFA‐2) and IC (NFA‐3). Their optical and electrochemical properties are investigated. Both NFA‐2 and NFA‐3 display the high lowest unoccupied molecular orbital energy level, leading to an increased open circuit voltage in the organic solar cells. PBDB‐T is chosen as polymer donor, showing spectral absorption that complements both NFAs. The optimized organic solar cells, based on PBDB‐T:NFA‐2 and PBDB‐T:NFA‐3 attained power conversion efficiency of 9.24% and 13.50%, respectively. Since the absorption characteristics of NFA‐2 and NFA‐3 are complementary, a small amount of NFA‐2 is added into PBDB‐T:NFA‐3 binary blend, the ternary organic solar cells attained a power conversion efficiency of 15.24%. The rise in power conversion efficiency is linked to the higher values of both short circuit current and fill factor. The increased short circuit current value in ternary organic solar cells is linked to the efficient use of excitons produced in NFA‐2 by transferring energy from NFA‐2 to NFA‐3 and effective exciton dissociation, faster charge extraction, decreased bimolecular and trap‐assisted recombination. The enhanced value of FF is also linked to the processes mentioned earlier. This investigation shows that it is advantageous to use separate non‐fused NFAs with absorption spectra that complement each other and have overlapped PL spectra of a medium bandgap acceptor along with the absorption spectra of a narrow bandgap NFA in ternary organic solar cells.