The LUMO and HOMO energy levels (ELUMO/EHOMO) are key parameters for conjugated polymers, which can greatly affect their applications in organic opto-electronic devices. In this manuscript, with donor–acceptor (D–A) type conjugated polymers based on double B←N bridged bipyridine (BNBP) unit, we report fine-tuning of ELUMO of conjugated polymers in a wide range via substitutions on both D unit and A unit. We synthesize eight D–A type conjugated polymers with alternating electron-deficient BNBP unit and electron-rich bithiophene (BT) unit in the main chain. By changing the substitutes on BNBP or BT, the ELUMO of these polymers can be finely tuned in a wide range from −3.3 eV to −3.7 eV. We comprehensively investigate the electronic structures, photophysical properties, charge-transporting properties and polymer solar cell (PSC) device applications of these polymers. In PSC devices, these BNBP-based polymers can be used either as electron donors (with high-lying ELUMO/EHOMO) or as electron acceptors (with low-lying ELUMO/EHOMO). The PSC device with the BNBP-based polymer donor exhibits a PCE of 2.92% and the PSC device with the BNBP-based polymer acceptor exhibits a PCE of 5.16%. These results indicate a new approach to modulate the LUMO energy levels of D–A type conjugated polymers by modifications on both D unit and A unit.

A new polymer electron acceptor (P-BNBP-CDT) composed of an alternating double B←N bridged bipyridine (BNBP) unit and a cyclopenta-[2,1-b:3,4-b′]-dithiophene (CDT) unit has been developed. P-BNBP-CDT exhibits strong light absorption in the visible range of 500–650 nm and suitable LUMO/HOMO energy levels (ELUMO/HOMO) of −3.45 eV/−5.64 eV, which are very complementary to that (ELUMO/HOMO = −3.2 eV/−5.2 eV) of the widely-used polymer donor, poly(3-hexylthiophene) (P3HT). All-polymer solar cells (all-PSCs) with P3HT as an electron donor and P-BNBP-CDT as an electron acceptor exhibit power conversion efficiencies (PCEs) exceeding 1.0% with high donor : acceptor blend ratios (w : w, from 0.5 : 1 to 9 : 1). The highest PCE of these devices is 1.76% with a high donor : acceptor blend ratio of 5 : 1. These results not only indicate that BNBP-based polymers are promising for P3HT : polymer acceptor devices, but also suggest the potential for low cost and facile device processing of all-PSCs.

Polymer electron acceptors are the key materials in all-polymer solar cells (all-PSCs). In this review, we focused on introducing the principle of boron-nitrogen coordination bond (B←N), and summarizing our recent research on polymer electron acceptors containing B←N unit for efficient all-PSC devices. Two approaches have been reported to design polymer electron acceptors using B←N unit. The one is to replace a C–C unit by a B←N unit in conjugated polymers to transform a polymer electron donor to a polymer electron acceptor. The other approach is to construct novel electron-deficient building block based on B←N unit for polymer electron acceptors. The polymer electron acceptors containing B←N unit showed tunable lowest unoccupied molecular orbital (LUMO) energy levels and exhibited excellent all-PSC device performance with power conversion efficiency of exceeding 6%. These results indicate that organic boron chemistry is a new toolbox to develop functional polymer materials for optoelectronic device applications.

B←N coordination bond can be used to develop polymer electron acceptors for efficient all-polymer solar cells (all-PSCs). Here, we report a new alternating conjugated polymer containing two building blocks based on B←N unit. The polymer exhibits strong light absorption in the visible range, low-lying LUMO/HOMO energy levels and moderate electron mobility. The resulting all-PSC devices exhibit power conversion efficiencies of 1.50%–2.47%.

In this communication, we report a series of polymer semiconductors based on a novel electron-deficient building block, double B←N bridged bipyridine (BNBP). These polymers show ambipolar or unipolar n-channel charge-transporting characteristics with electron mobilities in the range of 0.02–0.32 cm2 V−1 s−1 in organic thin film transistors.

A key parameter for polymer electron acceptors is the lowest unoccupied molecular orbital (LUMO) energy level (ELUMO). For state-of-the-art polymer electron acceptors based on the naphthalene diimide (NDI) unit, their ELUMO are low-lying and cannot be tuned, leading to a low open-circuit voltage (Voc) of the resulting all-polymer solar cells (all-PSCs). We report that polymer electron acceptors based on the double B←N bridged bipyridine (BNBP) unit exhibit tunable ELUMO because of their delocalized LUMOs over polymer backbones. The ELUMO of the copolymer of the BNBP unit and selenophene unit (P-BNBP-Se) is lower by 0.16 eV than that of the copolymer of the BNBP unit and thiophene unit (P-BNBP-T). As a result, the energy levels of P-BNBP-Se match well with the widely-used polymer donor, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th). The electron mobility of P-BNBP-Se (μe = 2.07 × 10−4 cm2 V−1 s−1) is also higher than that of P-BNBP-T (μe = 7.16 × 10−5 cm2 V−1 s−1). While the all-PSC device based on the PTB7-Th:P-BNBP-T blend shows a moderate power conversion efficiency (PCE) of 2.27%, the corresponding device with P-BNBP-Se as the acceptor exhibits a PCE as high as 4.26%. Moreover, owing to the suitable ELUMO of P-BNBP-Se, the all-PSC device of P-BNBP-Se shows a Voc up to 1.03 V, which is higher by 0.22 V than that with the conventional NDI-based polymer acceptor. These results indicate that BNBP-based polymers can give all-PSCs with high PCEs, remarkably high Voc values and small photon energy losses.

The LUMO/HOMO energy levels of conjugated polymers are key parameters for their applications as polymer electron donors for polymer solar cells (PSCs). The widely-used strategy to tune the LUMO/HOMO levels of polymer donors is to develop D–A type polymers based on an alternating electron-donating unit (D) and an electron-accepting unit (A). In this paper, we report a novel approach to tune the LUMO/HOMO levels of polymer donors via replacing a C–C unit by a B ← N unit for enhanced PSC device performance. The control polymer PCPDT shows the LUMO/HOMO levels of −2.71 eV/−4.98 eV, which are both much higher than those required for an ideal polymer donor. By replacing a C–C unit with a B ← N unit, the resulting polymer PBNCPDT exhibits much lower LUMO/HOMO levels of −3.23 eV/−5.20 eV. PBNCPDT also shows a narrower optical bandgap (Eg = 1.73 eV) than that (Eg = 1.85 eV) of PCPDT, which is helpful for harvesting of sunlight. Moreover, PBNCPDT with the B ← N unit is not a typical D–A type conjugated polymer because its LUMO and HOMO are both delocalized over the whole conjugated framework. As the control PSC device based on PCPDT exhibits an open-circuit voltage (Voc) of 0.45 V and power conversion efficiency (PCE) of 0.63%, the device of PBNCPDT shows much improved Voc of 0.82 V and PCE of 3.74%. These results indicate that a B ← N unit can be used to develop polymer donors for high-performance PSC devices.

A key parameter for polymer electron acceptors is the lowest unoccupied molecular orbital (LUMO) energy level (ELUMO). For state-of-the-art polymer electron acceptors based on the naphthalene diimide (NDI) unit, their ELUMO are low-lying and cannot be tuned, leading to a low open-circuit voltage (Voc) of the resulting all-polymer solar cells (all-PSCs). We report that polymer electron acceptors based on the double B←N bridged bipyridine (BNBP) unit exhibit tunable ELUMO because of their delocalized LUMOs over polymer backbones. The ELUMO of the copolymer of the BNBP unit and selenophene unit (P-BNBP-Se) is lower by 0.16 eV than that of the copolymer of the BNBP unit and thiophene unit (P-BNBP-T). As a result, the energy levels of P-BNBP-Se match well with the widely-used polymer donor, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th). The electron mobility of P-BNBP-Se (μe = 2.07 × 10−4 cm2 V−1 s−1) is also higher than that of P-BNBP-T (μe = 7.16 × 10−5 cm2 V−1 s−1). While the all-PSC device based on the PTB7-Th:P-BNBP-T blend shows a moderate power conversion efficiency (PCE) of 2.27%, the corresponding device with P-BNBP-Se as the acceptor exhibits a PCE as high as 4.26%. Moreover, owing to the suitable ELUMO of P-BNBP-Se, the all-PSC device of P-BNBP-Se shows a Voc up to 1.03 V, which is higher by 0.22 V than that with the conventional NDI-based polymer acceptor. These results indicate that BNBP-based polymers can give all-PSCs with high PCEs, remarkably high Voc values and small photon energy losses.

A new polymer electron acceptor (P-BNBP-CDT) composed of an alternating double B←N bridged bipyridine (BNBP) unit and a cyclopenta-[2,1-b:3,4-b′]-dithiophene (CDT) unit has been developed. P-BNBP-CDT exhibits strong light absorption in the visible range of 500–650 nm and suitable LUMO/HOMO energy levels (ELUMO/HOMO) of −3.45 eV/−5.64 eV, which are very complementary to that (ELUMO/HOMO = −3.2 eV/−5.2 eV) of the widely-used polymer donor, poly(3-hexylthiophene) (P3HT). All-polymer solar cells (all-PSCs) with P3HT as an electron donor and P-BNBP-CDT as an electron acceptor exhibit power conversion efficiencies (PCEs) exceeding 1.0% with high donor:acceptor blend ratios (w:w, from 0.5:1 to 9:1). The highest PCE of these devices is 1.76% with a high donor:acceptor blend ratio of 5:1. These results not only indicate that BNBP-based polymers are promising for P3HT:polymer acceptor devices, but also suggest the potential for low cost and facile device processing of all-PSCs.

In this communication, we report a series of polymer semiconductors based on a novel electron-deficient building block, double B←N bridged bipyridine (BNBP). These polymers show ambipolar or unipolar n-channel charge-transporting characteristics with electron mobilities in the range of 0.02–0.32 cm2 V−1 s−1 in organic thin film transistors.