The branching point of the side-chain of naphthalenediimide (NDI)-based conjugated polymers is systematically controlled by incorporating four different side-chains, i.e., 2-hexyloctyl (P(NDI1-T)), 3-hexylnonyl (P(NDI2-T)), 4-hexyldecyl (P(NDI3-T)), and 5-hexylundecyl (P(NDI4-T)). When the branching point is located farther away from the conjugated backbones, steric hindrance around the backbone is relaxed and the intermolecular interactions between the polymer chains become stronger, which promotes the formation of crystalline structures in thin film state. In particular, thermally annealed films of P(NDI3-T) and P(NDI4-T), which have branching points far away from the backbone, possess more-developed bimodal structure along both the face-on and edge-on orientations. Consequently, the field-effect electron mobilities of P(NDIm-T) polymers are monotonically increased from 0.03 cm 2 V −1 s −1 in P(NDI1-T) to 0.22 cm 2 V −1 s −1 in P(NDI4-T), accompanied by reduced activation energy and contact resistance of the thin films. In addition, when the series of P(NDIm-T) polymers is applied in all-polymer solar cells (all-PSCs) as electron acceptor, remarkably high-power conversion efficiency of 7.1% is achieved along with enhanced current density in P(NDI3-T)based all-PSCs, which is mainly attributed to red-shifted light absorption and enhanced electron-transporting ability.

We report a series of new n-type random copolymers (P­(NDI2OD-Se-Th x), where x = 0, 0.5, 0.7, 0.8, 0.9, 1.0) consisting of naphthalene diimide (NDI), selenophene-2,2′-thiophene (Se-Th), and seleno­[3,2-b]­thiophene (SeTh) to demonstrate their use in producing efficient all-polymer solar cells (all-PSCs) and organic field-effect transistors (OFETs). To investigate the effect of polymer crystallinity on the performance of all-PSCs and OFETs, we tuned the composition of the Se-Th and SeTh moieties in the P­(NDI2OD-Se-Th x) polymers, resulting in enhanced crystalline properties with a higher Se-Th ratio. Thus, the OFET electron mobility was increased with a higher Se-Th ratio, exhibiting the highest value of 1.38 × 10–1 cm2 V–1 s–1 with P­(NDI2OD-Se-Th 1.0). However, the performance of all-PSCs based on PBDB-T:P­(NDI2OD-Se-Th x) showed a nonlinear trend relative to the Se-Th ratio and the performance was optimized with P­(NDI2OD-Se-Th 0.8) exhibiting the highest power conversion efficiency of 8.30%. This is attributed to the stronger crystallization-driven phase separation in all-polymer blends for higher Se-Th ratio. At the optimal crystallinity of P­(NDI2OD-Se-Th 0.8) in all-PSCs, the degree of phase separation, domain purity, and electron mobility were optimized, resulting in enhanced charge generation and transport. Our works describe the structure–property–performance relationships to design effective n-type polymers in terms of crystalline and electrical properties suitable for both efficient OFETs and all-PSCs.

Quinoxaline (Qx) derivatives are promising building units for efficient photovoltaic polymers owing to their strong light absorption and high charge-transport abilities, but they have been used exclusively in the construction of polymer donors. Herein, for the first time, Qx-based polymer acceptors (P A s) were developed by introducing electron-withdrawing cyano (CN) groups into the Qx moiety (QxCN). A series of QxCN-based P A s, P(QxCN-T2), P(QxCN-TVT), and P(QxCN-T3), were synthesized by copolymerizing the QxCN unit with bithiophene, (E)-1,2-di-(thiophene-2-yl)ethene, and terthiophene, respectively. All of the P A s exhibited unipolar n-type characteristics with organic field-effect transistor (OFET) mobilities of around 10 À 2 cm 2 V À 1 s À 1 . In space-charge-limited current devices, P-(QxCN-T2) and P(QxCN-TVT) exhibited electron mobilities greater than 1.0 × 10 À 4 cm 2 V À 1 s À 1 , due to the well-ordered structure with tight π-π stacking. When the P A s were applied in allpolymer solar cells (all-PSCs), the highest performance of 5.32 % was achieved in the P(QxCN-T2)-based device. These results demonstrate the significant potential of Qx-based P A s for highperformance all-PSCs and OFETs.

Substitution of C atoms in a polymer backbone by N atoms allows for the facile tuning of the energy levels as well as the backbone conformation and packing structures of conjugated polymers. Herein, we report a series of three polymer acceptors (P As) with N atoms introduced at different positions of the backbone and investigate how these N atoms affect the device performances of all-polymer solar cells (all-PSCs). The three P As, namely, P­(NDI2DT-BTT), P­(NDI2DT-PTT), and P­(NDI2DT-BTTz), are composed of naphthalenediimide (NDI)-based and benzothiadiazole (BT)-based derivatives (dithiophene-BT (BTT), dithiophene-thiadiazolepyridine (PTT), and dithiazole-BT (BTTz)). The PTT and BTTz units are synthesized by replacing the C atoms in BT and thiophene, respectively, with N atoms, which effectively tune the optical, electrochemical, and charge-transporting properties of the corresponding P As. The all-PSCs using poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl))­benzo­[1,2-b:4,5-b′]­dithiophene)-co-(1,3-di­(5-thiophene-2-yl)-5,7-bis­(2-ethylhexyl)­benzo­[1,2-c:4,5-c′]­dithiophene-4,8-dione)] (PBDB-T) as a polymer donor and P­(NDI2DT-PTT) as P A exhibit a significantly enhanced power conversion efficiency (PCE) of 6.95%, whereas the all-PSCs based on the other P As show relatively lower PCEs (6.02% for PBDB-T:P­(NDI2DT-BTT) and 1.43% for PBDB-T:P­(NDI2DT-BTTz)). The high PCE of the PBDB-T:P­(NDI2DT-PTT) device is due to the superior charge transfer and charge dissociation, resulting from the closely matched energy levels between PBDB-T and P­(NDI2DT-PTT), as well as a more favorable bulk heterojunction morphology with improved miscibility. Importantly, the P­(NDI2DT-PTT)-based all-PSC device shows improved air stability compared to the P­(NDI2DT-BTT)-based device, which is most likely due to a decreased lowest unoccupied molecular orbital level of the P A. Our findings suggest that the incorporation of N atoms into the P As is an effective strategy for improving the efficiency and stability of all-PSCs.

Photovoltaic performance and mechanical robustness of all-polymer solar cells are simultaneously enhanced by developing ester-functionalized, wide-bandgap PM7 derivatives.