Enhanced Synthesis and Photovoltaic Performance of Donor-Acceptor Polymers With Ultralow HOMO Energy Levels Based on Pyrrolo[3,4-b]Dithieno[2,3-f:2′-h] Quinoxalindione Derivatives

An improved synthetic strategy for pyrrolo[3,4-b]dithieno[2,3-f:2′-h]quinoxalindione derivatives was developed as acceptor units in donor-acceptor (D-A) polymers for organic solar cells. The approach involves the efficient fusion of two pendant thienyl groups in 6-(2-octyldodecyl)-2,3-di(thiophen-3-yl)-5H-pyrrolo[3,4-b]pyrazine-5,7(6H)-dione into a dithienoquinoxaline skeleton via FeCl₃-mediated oxidative coupling, achieving excellent yields. Compared to previous methods, this route demonstrated higher yields, shorter reaction times, and simplified purification processes. Two polymers, PQB and PQDB, synthesized from the monomer and BDT-Cl, exhibited high thermal stability with decomposition temperatures of 400.52°C and 420.37°C, respectively. Their absorption maxima were 468 and 464 nm in solution, and 467 and 485 nm as films, with optical bandgaps of 1.90 and 1.85 eV. HOMO energy levels were calculated as −5.82 and −5.64 eV, and LUMO energy levels as −3.56 and −3.50 eV, respectively. The HOMO energy levels are among the lowest observed, presenting a new avenue for a distinct class of wide-band polymers. Optimized solar cells based on PQDB and PCBM, fabricated with 1 vol% DIO and without annealing, achieved a power conversion efficiency (PCE) of 3.18%.