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Herein, we demonstrate a facile technique for transforming a low‐energy‐converting quinoxaline‐based polymer into an efficient polymeric donor for non‐fullerene acceptor‐based organic solar cells (NFA‐OSCs). Alternating copolymers, namely P(BDTSi‐DTfQ), incorporating electron‐rich 4,8‐bis(triisopropylsilylethynyl)‐benzo[1,2‐b:4,5‐′]dithiophene (BDTSi) and electron‐deficient 2,3‐didodecyl‐6‐fluoro‐5,8‐di(thiophen‐2‐yl)quinoxaline (DTfQ) units were synthesized. The properties of P(BDTSi‐DTfQ) were thoroughly studied and briefly compared to those of the reported polymers, namely P(BDTSi‐DTffQ), made up of BDTSi and 2,3‐didodecyl‐6,7‐difluoro‐5,8‐di(thiophen‐2‐yl)quinoxaline (DTffQ) units. Polymer P(BDTSi‐DTfQ) exhibited a lower bandgap (Eg) and higher highest occupied and lowest unoccupied energy levels (HOMO and LUMO) than P(BDTSi‐DTffQ). The estimated Eg and HOMO/LUMO for P(BDTSi‐DTfQ) were 1.90 eV and −5.46 eV/−3.56 eV, respectively, and for P(BDTSi‐DTffQ) the same were 1.94 eV and −5.58 eV/−3.64 eV, respectively. Interestingly, the NFA‐OSCs made from P(BDTSi‐DTfQ) as the donor and NFA, namely ITIC, as the acceptor, gave a power conversion efficiency (PCE) of 3.68%, which is much higher than the PCE obtained (⁓0.75%) for the OSCs prepared by using the P(BDTSi‐DTffQ):ITIC blend. Noticeably, the energy levels of P(BDTSi‐DTfQ) were found to be favorable for efficient charge separation when it was blended with ITIC. This blend not only allowed a better charge separation at the donor/acceptor interfaces but also significantly lowered bimolecular recombination. The overall effect was to provide a higher PCE. However, P(BDTSi‐DTffQ) showed mismatched energy levels with ITIC resulting in a higher bimolecular recombination and lower PCE. [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects of polymer backbone curvature on the performance of non-fullerene organic solar cells (NFA-OSCs) fabricated using an electron-donating π-conjugated polymer and a non-fullerene electron-acceptor (NFA), 3,9-bis[2-methylene(3-(1,1-dicyanomethylene)-indanone)]-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d :2ʹ,3ʹ- dʹ ]-s-indaceno[1,2 -b :5,6- bʹ ]dithiophene (ITIC). The NFA-OSCs are fabricated using structurally similar bis(pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (BPPD)-based π–conjugated polymers, i.e., P1 and P2, and they indicate almost identical absorption and energy levels but dissimilar backbone curvatures. P1 comprises two-dimensional conjugated benzodithiophene (BDTT) and thiophene π-bridged BPPD derivatives, and P2 comprises BDTT and thieno[3,2-b]thiophene π-bridged BPPD derivatives. The absorption maximum and highest occupied/lowest unoccupied molecular orbital levels of P1 and P2 are ~505 nm and ~˗5.4/˗3.4 eV, respectively. However, P1 shows a twisted backbone, whereas P2 exhibits a well-controlled wavy backbone owing to the presence of slightly different-structured π-bridges, such as thiophene and thieno[3,2-b]thiophene, on their backbones. The NFA-OSCs are fabricated with the structures of glass-ITO/PEDOT:PSS/P1 and P2:ITIC+0.5 vol% 1,8-diiodoctane (DIO)/PFN-Br/Al provide quite different device performances with power conversion efficiency of 0.88% and 6.25%, respectively. In general, the curvature of the polymer backbone significantly affects the nanoscale network formation between the polymer and ITIC, thereby resulting in a significant difference in the carrier transport and photovoltaic performances of the NFA-OSCs. Image 1 • Polymer backbone curvature effects on NFA-OSCs were studied. • Twisted backbone polymer offered poor PCE of 0.9%. • Controlled wavy backbone polymer provided relatively higher PCE of 6.2%. • Polymer backbone curvature affects the morphology, carrier transport, and PCE. • Controlled wavy backbone polymers are favorable to NFA-OSCs. [ABSTRACT FROM AUTHOR]

Three new benzodithiophene- alt -di(thiophen-2-yl)quinoxaline polymers were prepared with the aim of exploring 5,8-bis(thiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (TFQ)-based polymers on fullerene-free organic solar cells (OSCs). The electron-rich benzo[1,2-b:4,5-bʹ]dithiophene unit contains three different substituents, namely 2,6-bis(trimethyltin)-4,8-bis(2-butyloctyloxy)benzo[1,2-b:4,5-bʹ]dithiophene (Distannyl BDTO), 2,6-bis(trimethyltin)-4,8-bis(5-(2-butyloctyl)thiophen-2-yl)benzo[1,2-b:4,5-bʹ]dithiophene (Distannyl BDTT), and 2,6-bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-ʹ]dithiophene (Distannyl BDTSi). These were polymerized with electron deficient 5,8-bis(5-bromothiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (Dibromo TFQ) to afford the polymers P(BDTO-TFQ) , P(BDTT-TFQ) , and P(BDTSi-TFQ). The estimated band-gaps (E g s) and the highest occupied molecular orbital levels (HOMOs) were 1.88, 1.86, and 1.94 eV, and −5.35, −5.44, and −5.58 eV, respectively. The OSCs made with a blend of polymer and ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2ʹ,3′-dʹ]-s-indaceno[1,2-b:5,6-bʹ]dithiophene) gave the maximum power-conversion efficiencies (PCE s) of 3.49, 7.06, and 0.75%, respectively. Overall, the substituents attached at the 4- and 8-positions of the BDT unit of the TFQ-based polymers were found to exhibit greatly altered properties, such as their absorption, energy levels, and crystallinity. This resulted in dissimilar photovoltaic performance for the resulting polymers. Image 1021 • Quinoxaline-based three new polymers with different substituents were prepared. • The calculated optical bands were 1.88 eV, 1.86 eV, and 1.94 eV, respectively. • The determined HOMO levels were −5.35 eV, −5.44 eV, and −5.58 eV, respectively. • The maximum PCEs of NFA OSCs were 3.49%, 7.06%, and 0.75%, respectively. • The substituents on the polymer backbone were greatly alter their properties. [ABSTRACT FROM AUTHOR]