Two donor–acceptor (D–A) backbone copolymers with different fused ring acceptors, i.e., dithieno [2,3‐e:3′,2′‐g]isoindole‐7,9(8 H)‐dione (DTID) and dithieno[3′,2′:5,6;2″,3″:7,8]naphtho[2,3‐d]imidazol‐9(10 H)‐one (DTNID) and same benzo[1,2‐b:4,5‐b′]dithiophene with alkylated aromatic side as side chains (BDTT) donor, denoted as P133 and P135 are synthesized and used as donor along with narrow bandgap nonfullerene Y6 acceptor for the preparation of polymer solar cells (PSCs). The dielectric constant of P135 :Y6 is higher than that of P133 :Y6 due to the strong electron deficient ability of DTNID compared to DTID, which fostered exciton dissociation and charge transport, constrained charge recombination, and ultimately boosted the power conversion efficiency of P135 :Y6 to 15.11%, which is higher than P133 :Y6 (10.24%). Therefore, these investigations confirm the pronounced potential of fused ring DTNID as an acceptor unit for emerging D–A copolymers for PSCs with high efficiency.

The power conversion efficiency of the ternary PSCs (16.32%) is higher than that for binary counterparts, i.e., 13.16% and 12.62% for P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6, respectively.

A donor–acceptor copolymer based on thiazole substituted benzodithiophene (BDTTz) (donor unit) and thieno[3,4‐c]pyrrole‐4‐6‐dione (TPD) (acceptor unit) P(BDTTz‐TPD) denoted as P132 is investigated. P132 shows an absorption profile from 300 to 680 nm and the low‐lying highest occupied molecular orbital (HOMO) energy level of −5.54 eV. P132 as donor along with fullerene derivative (PC 71 BM) or nonfullerene denoted as BThIND‐Cl as acceptor are used for the fabrication of polymer solar cells. The P132:BThIND‐Cl‐based polymer solar cell shows high open‐circuit voltage as compared with P132:PC 71 BM which is attributed to the upshifted lowest unoccupied molecular orbital level of BThIND‐Cl and low energy loss. Moreover, P132:BThIND‐Cl attains higher short‐circuit current than that for P132:PC 71 BM which is attributed to the broader absorption spectra and low optical band gap of BThIND‐Cl, which leads to high light‐harvesting ability of the P132:BThIND‐Cl active layer. The polymer solar cells based on P132:PC 71 BM and P132:BThIND‐Cl attain power conversion efficiencies of 7.03% and 15.52%, respectively. The charge transport is more balanced in P132:BThIND‐Cl and results in a high value of fill factor. Small energy loss in P132:BThIND‐Cl‐based PSCs is attributed to the low HOMO offsets between P132 and BThIND‐Cl.

Two D-A copolymers consisting of fused ring pyrrolo-dithieno-quinoxaline acceptors are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2-ethylhexyloxy (P117) side chains. These copolymers are used as donors and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron-deficient fused ring pyrrolo-bithieno-quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells (PSCs) based on P134:Y6 attain the power conversion efficiency (PCE) of 15.42%, which is higher than the P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 can be associated with more well-adjusted charge transport, weak charge recombination, proficient exciton generation, and dissociation into free charge carriers and their subsequent charge collection owing to the dense π-π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor-weak donor tactic for developing efficient D-A copolymers consists of quinoxaline acceptor for PSCs.

We have designed and synthesized two wide bandgap new donor-acceptor (D-A) copolymers consisting of the same alkylthiazole-substituted benzo[1,2-b;4,5-b ']dithiophene (BDTTz) donor unit and but different acceptor units, i.e., thiazolo[5,4-d]thiazole (TTZ) (P122) and 1,3,-4 thiadiazole (TDz) (P123) and investigated their optical and electrochemical properties. We have employed these copolymers as donor and fullerene (PC71BM) and narrow bandgap non-fullerene (Y6) as acceptor, to fabricate binary and ternary bulk heterojunction polymer solar cells (PSCs). The overall power conversion efficiency (PCE) of optimized binary bulk heterojunction PSCs based on P122:Y6 and P123:Y6 is 12.60% and 13.16%, respectively. The higher PCE for PSCs based on P123 than P122 counterparts may be associated with the broader absorption profile of the P123 and more charge carrier mobilities than that for the P122 active layer. With the incorporation of small amount of PC71BM into either P122:Y6 or P123:Y6 binary blend, the corresponding ternary PSCs showed an overall PCE of 14.89% and 15.52%, respectively, which is higher than the binary counterparts using either Y6 or PC71BM as acceptor. Incorporating the PC71BM in the binary host blend increases the absorption in the 300-500 nm wavelength region, generating more excitons in the active ternary layer and helping to dissociate the excitons into free charge carriers more effectively. The more appropriate nanoscale phase separation in the active ternary layer than the binary counterpart may be one of the reasons for higher PCE.

Two donor-acceptor (D-A) copolymers, P128 and P129, containing benzodithiophene (BDT) and naphthiodithiophene (NDT) donor unit, respectively, and the alike anthra[1,2-b:4,3,bMODIFIER LETTER PRIME:6,7-c"] trithiophene-8-12-dione(A3T) as an acceptor unit, are synthesized. The P129 (NDT donor) exhibits enhanced chain planarity and linearity of backbone compared to P128 (BDT donor), which leads to a considerable increase in both the light-harvesting ability and hole mobility. In addition to it, the incorporation of the NDT unit creates a strong modification in the morphology with a well-mixed interpenetrating nanofibrillar network with an increased donor-acceptor interface in the P129:Y6 bulk heterojunction active layer that leads to a high short-circuit current density and fill factor. The power conversion efficiency (PCE) of the improved polymer solar cells based on P128:Y6 and P129:Y6 is 6.25% and 14.55%, respectively. The negative HOMO offset (-0.02 eV) at P128/Y6 interface restricts the hole transfer from Y6 to P128 and results in low value of short-circuit current and thereby poor PCE. Finally, the ternary PSCs are constructed with a weight ratio of 0.5:0.5 between P128 and P129 and the weight component of Y6 is kept constant and the PSC attains a PCE of 16.19%. This increase in the PCE can be attributed to the formation of semiconducting alloy based on P128 and P129 donors and the transformation of HOMO offset from negative (-0.02 eV for Y6/P128) to positive (0.04 eV for Y6/P128:P129).

Two donor–acceptor (D-A) copolymers, P128 and P129 , containing benzodithiophene (BDT) and naphthiodithiophene (NDT) donor unit, respectively, and the alike anthra[1,2-b:4,3,bʹ:6,7-cʺ] trithiophene-8-12-dione(A3T) as an acceptor unit, are synthesized. The P129 ( NDT donor) exhibits enhanced chain planarity and linearity of backbone compared to P128 (BDT donor), which leads to a considerable increase in both the light-harvesting ability and hole mobility. In addition to it, the incorporation of the NDT unit creates a strong modification in the morphology with a well-mixed interpenetrating nanofibrillar network with an increased donor–acceptor interface in the P129 :Y6 bulk heterojunction active layer that leads to a high short-circuit current density and fill factor. The power conversion efficiency (PCE) of the improved polymer solar cells based on P128 :Y6 and P129:Y6 is 6.25% and 14.55%, respectively. The negative HOMO offset (−0.02 eV) at P128 /Y6 interface restricts the hole transfer from Y6 to P128 and results in low value of short-circuit current and thereby poor PCE. Finally, the ternary PSCs are constructed with a weight ratio of 0.5:0.5 between P128 and P129 and the weight component of Y6 is kept constant and the PSC attains a PCE of 16.19%. This increase in the PCE can be attributed to the formation of semiconducting alloy based on P128 and P129 donors and the transformation of HOMO offset from negative (−0.02 eV for Y6/ P128 ) to positive (0.04 eV for Y6/ P128 : P129 ).

Developing efficient wide-bandgap copolymer donor materials to match with narrow bandgap non-fullerene acceptors is continuously ongoing for polymer solar cells. Herein, two new D-A copolymers are designed and synthesized by embedding the same anthra[1,2-b:4,3-b':6,7-c"] trithiophene-8,12-dione (A3T) acceptor unit and different donor units, i.e., BDTTZ (P126) and BDTTh (P127). These copolymers showed broad absorption from 350 to 680 nm and deeper HOMO energy level. We have used these two copolymers as donors and a narrow bandgap non-fullerene acceptor Y6 to prepare bulk heterojunction polymer solar cells (PSCs). After the optimization, P126:Y6 and P127:Y6 attained overall power conversion efficiency of 15.07% and 12.27%, respectively. The higher PCE for the P126 than P127 is associated with the more efficient photon harvesting and photogenerated excitons, balanced charge transport, and low energy loss. Our results may help to design new polymers with a deeper highest occupied molecular orbital level that will be well-matched with non-fullerene acceptors.

To examine the effects of pendent alkyl chain on the benzodithiophenedione (BDD) acceptor in the donor-acceptor (D-A) conjugated copolymers, two D-A copolymers with same thiazole ring substituted benzo[1,2-b:4,5-b ']dithiophene (BDTTz) and different acceptors BDD with two pendent alkyl units PBDTTz-BDD and one pendent alkyl unit in benzo[1,2-b:4,5-c ']dithiophene-4,8-dione (BDTD) PBDTTz-BDTD are synthesized. There is small effect on the optical properties but significant effect on the intermolecular pi-pi stacking, charge transport, and photovoltaic performance. PBTTz-BDTD (with one pendent alkyl chain) on the BDD exhibits smaller pi-pi stacking distance as compared to PBDTTz-BDD (with two pendent alkyl chains). The balanced charge carrier mobilities for the PBDTTZ-BDTD based blends than those for PBDTTz-BDD based blends are beneficial for suppressing the recombination processes, resulting in higher short circuit current and fill factor for the polymer solar cells (PSCs) based on PBDTTz-BDTD. Combined with BThIND or Y6 as acceptor, PBDTTZ-BDTD based PSCs show power conversion efficiency of 12.85% and 14.08%, respectively, which are larger than that for PBDTTz-BDD counterparts (10.46% and 10.90% for BThIND and Y6, respectively). Thus, the photovoltaic performance of the D-A copolymer donors can be regulated through the slight variations of the pendent alkyl chains in the BDD acceptor unit.

Herein, the synthesis of a new medium bandgap nonfullerene acceptor DBTBT‐IC consisting of di‐benzothieno [3,2‐b][1]‐benzothiophene DBTBT as the central donor unit and IC as terminal acceptor units and its use as the acceptor for the fabrication of single binary and ternary polymer solar cells is reported. DBTBT‐IC exhibits a medium optical bandgap of about 1.65 eV. When paired with the wide bandgap‐conjugated polymer PDTNIT as donor, the polymer solar cells based on the optimized PDTNIT:DBTBT‐IC active layer realized a power conversion efficiency of 12.34% (short‐circuit current = 8.06 mA cm−2, open‐circuit voltage = 1.12 V, and fill factor (FF) = 0.62) which is higher than that for the PDTNIT:PC71BM counterpart, i.e., 9.08 % (short‐circuit current=14.58 mA cm−2, open‐circuit voltage = 0.93 V, and FF = 0.67). When a small amount of PC71BM was introduced into the host PDTNIT:DBTBT‐IC binary layer, the polymer solar cell based on the optimized PDTNIT:PC71BM:DBTBT‐IC (1:0.3:0.9) ternary active layer attained an excellent power conversion efficiency of about 15.92 %, mainly due to the increase in short‐circuit current and FF. The increase in the short‐circuit current may be associated with the broader absorption profile of the ternary active layer as compared with the binary counterparts and more efficient exciton utilization, due the partial energy transfer from PC71BM to DBTBT‐IC.

Two new wide bandgaps D-A copolymers compromising of same benzo [1,2-b:4, 5-b ']dithiophene functionalized with thiazole side-chain donor unit and different acceptors units, i. e., fluorinated benzotriazole (FBTA) (PBDTTZ-FBTA) and bis-thiophene-thieno-benzothiazole (BTZ) (PBDTTZ-BTZ) were synthesized and their optical and electrochemical properties were investigated. These copolymers exhibit appropriate frontier energy levels and complementary absorption to most middle and narrow bandgap non-fullerene small molecule non-fullerene acceptors. We have explored using these two copolymers as donors combined with a narrow bandgap non-fullerene acceptor, namely BThIND-Cl, for the construction of polymer solar cells. The optimized polymer solar cells employing PBDTTZ-FBTA: BThIND-Cl showed overall power conversion efficiency of 14.96 % (with energy loss of 0.51 eV), which is higher than that for PBDTTZ-BTZ: BThIND-Cl (11.68 % with energy loss of 0.56 eV). Although the PBDTTZ-BTZ exhibits a broader absorption profile than that of PBDTTZ-FBTA, the higher power conversion efficiency of the later copolymer may be correlated with the high charge carrier mobility, suppression of the bimolecular and trap-assisted recombination due to appropriate phase separation and compact pi-pi stacking distance in the active layer and low energy loss.

Three D–A conjugated copolymers based on the same 8,10‐dihydro‐9H‐bisthieno[2′,3′:7.8;3″,2″:5.6]naphtho[2,3‐d]imidazol‐9‐one (DTNIA) acceptor unit and different donor units, i.e., 2‐dodecylbenzo[1,2‐b:3,4‐b′:6,5‐b″]trithiophene (3TB) (P1), 5,6‐dioctylnaphtho[2,1‐b:3,4‐b′]dithiophene (DTN) (P2), and 4,5‐diundecylbenzo[2,1‐b:3,4‐b′]dithiophene (DTB) (P3), are formulated and synthesized. All the copolymers exhibit deep highest occupied molecular energy levels of −5.43, −5.50, and −5.51 eV for P1, P2, and P3, respectively, and show an optical bandgap of 2.18, 2.12, and 2.11 eV, for P1, P2, and P3, respectively. These copolymers are used as donors for the construction of polymer solar cells combining ITIC‐m as an electron acceptor. The optimized polymer solar cells based on P1:ITIC‐m, P2:ITIC‐m, and P3:ITIC‐m realize overall power conversion efficiency of ≈9.62%, 12.84%, and 11.80%, respectively. The greater value of open circuit voltage for P2 and P3 relative to P1 may be due to the deeper highest occupied molecular orbital energy level of P2 and P3 as compared to P1. The highest power conversion efficiency for the P2‐based polymer solar cells may be originated from the denser π–π stacking distance and relatively improved crystallinity, which are advantageous for balanced charge transport, resulting in a comparatively high fill factor and short circuit current.

ABSTRACTA series of novel donor–acceptor (D–A) random conjugated terpolymers P2‐P4 along with the homopolymers P1 (BDT‐DPP) and P5 (BDT‐BTDQ) were designed and synthesized by copolymerizing a benzo[1,2‐b:4,5‐b]dithiophene (BDT) donor with an electron‐deficient diketopyrrolo[3,4‐c]pyrrole (DPP) unit and a benzothiadiazolo[3,4‐e]quinoxaline (BTDQ) moieties of different electron‐withdrawing strengths, and the resultant terpolymers showed broad absorption profile ranging from 300 to 1200 nm. The HOMO levels of the polymers were adjusted from −5.23 to −5.11 eV, and the optical bandgaps were controlled from 1.32 to 1.13 eV by changing the molar ratio of DPP and BTDQ acceptors. These terpolymers were used as a donor along with PC71BM as an acceptor for the creation of polymer solar cells, and the performance was optimized via variable the donor to acceptor ratio and solvent vapor annealing. The polymer solar cells made from the random terpolymer P3 showed the highest overall power conversion efficiency of (9.27%), which is higher than that for the corresponding homo‐polymers counterparts, that is, P1 (7.27%) and P5 (7.68%). The results demonstrate that the designing of random D‐A1‐D‐A2 terpolymers may be the best approach for efficient polymer solar cells. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1478–1485