Improved Efficiency in Organic Solar Cells Based on A2-D-A1-D-A2 Nonfullerene Acceptors with a Benzoselenadiazole Core Induced by Higher Dipole Moment and Dielectric Constant

Two A(2)-D-A(1)-D-A(2) nonfused small molecules (NC1 and NC2) have been synthesized. The compounds contain benzoselenadiazole as a core acceptor (A(1)), cyclopentadithiophene as a donor (D), and rhodanine and CN-rhodanine as terminal acceptors (A(2)), and their optical and electrochemical properties have been investigated. The dielectric constant and dipole moment of NC2 are higher than that of NC1, and this leads to a lower binding energy for the excitons generated after the absorption of photons. The NC1- and NC2-based bulk heterojunction organic solar cells, using a wide-band gap copolymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b ']dithiophene))-alt-(5,5-(1 ',3 '-di-2-thienyl-5 ',7 '-bis(2-ethylhexyl)benzo[1 ',2 '-c:4 ',5 '-c ']dithiophene-4,8-dione)] (PBDB-T) as a donor, attain overall power conversion efficiencies of 12.26% and 14.93%, respectively. The superior power conversion efficiency of the NC2-based organic solar cells is predominantly due to the higher fill factor and short circuit current, although the open circuit voltage is higher for the NC1 counterpart. The photoluminescence spectrum of PBDB-T overlaps the absorption spectra of NC1 and NC2, and this indicates energy transfer from PBDB-T to either NC1 or NC2. This overlap is more pronounced for NC2. This leads to more efficient energy transfer (excitons) from PDBB-T to NC2, and the excitons are more effectively dissociated into free charge carriers, thus leading to a higher short circuit current. The higher fill factor for the NC2-based devices is associated with reduced charge recombination and more balanced charge transport, as confirmed by the more marked phase separation, compact pi-pi stacking distance, and increased crystal coherence length for the PBDB-T:NC2 active layer. The energy loss for the NC2-based polymer solar cell is lower (0.46 eV) than that for the NC1-based counterpart (0.51 eV), and this may be linked to the higher values for both the dielectric constant and dipole moment, which in turn lead to a lower driving force for hole transfer from NC2 to PBDB-T when compared to NC1 to PBDB-T.