Absorption spectra of polymer FBT-Th4 (1,4) (M n = 46.4 Kg/mol, E g = 1.62 eV, and HOMO = -5.36 eV) indicate strong interchain aggregation ability. High hole mobilities up to 1.92 cm(2) (V s)(-1) are demonstrated in OFETs fabricated under mild conditions. Inverted solar cells with active layer thicknesses ranging from 100 to 440 nm display PCEs exceeding 6.5%, with the highest efficiency of 7.64% achieved with a 230 nm thick active layer.

No abstract

Quinoidal structures incorporating expanded paraquinodimethane (p-QM) units have garnered great interest as functional organic electronic, optical, and magnetic materials. The direct use of the compact p-QM unit as an electronic building block, however, has been inhibited by the high reactivity conveyed by its biradical character. Herein, we introduce a stable p-QM variant, namely p-azaquinodimethane (p-AQM), that incorporates nitrogen atoms in the central ring and alkoxy substituents on the periphery to increase the stability of the quinoidal structure. The succinct synthesis from readily available precursors leads to regio-and stereospecific p-AQMs that can be readily integrated into the backbone of conjugated polymers. The quinoidal character of the p-AQM unit endows the resulting polymers with narrow band gaps and high carrier transport mobilities. The study of a series of copolymers employing different numbers of thiophene units revealed an unconventional trend in band gaps, which is distinct from the widely adopted donor−acceptor approach to tuning the band gaps of conjugated polymers. Theoretical calculations have shed light on the nature of this trend, which may provide a unique class of conjugated polymers with promising optical and electronic properties.

Side-chain random copolymers show high 3-D hole transport and offer excellent active layer thickness tolerance.

Three benzotriazole (BTA)-based conjugated polymers PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, with electron-donating segments of fluorene, carbazole, and dialkoxybenzene, respectively, were successfully synthesized as new polymeric donors in bulk-heterojunction (BHJ) photovoltaic cells (PVCs). All the copolymers exhibited good solubility in common organic solvents and good thermal stability. The optical band gaps for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are 2.24, 2.18, and 1.87 eV, respectively. The HOMO levels for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are -5.67, -5.54, and -5.20 eV, respectively, which were determined by the electron-donating segments. The LUMO levels for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are -3.43, -3.36, and -3.33 eV, respectively, which were mainly dominated by the BTA unit. For a blend ratio of polymer:PCBM = 1:2, BHJ PVCs with Al cathode displayed power conversion efficiencies (PCE) of 0.9%, 1.51%, and 1.16% for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, respectively. Alcohol-soluble poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3 0 -(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) was selected for cathode modification. For BHJ PVCs with PF-DTBTA and PCz-DTBTA as the donors, open-circuit voltages (V oc ) of the PVCs could all be elevated with the PFN/Al bilayer cathode, whereas no such improvements were found for PVCs with PPh-DTBTA as the donor. The behaviors could be attributed to the different V oc losses when using Al cathode. Using PFN/ Al bilayer cathode could also improve short-curcuit current and fill factor for the three BTA-based copolymers. The N-N interactions between the BTA-based polymers and the PFN may modify interfacial contact, resulting in enhancement of the electron extraction from the acceptor phase to the cathode and decreasing hole-electron recombination in the active layer. Consequently, PFN/Al bilayer cathode elevated PCE values to 1.3%, 2.75%, and 1.39% for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, respectively. The most significant increasing of PCE with a calculated value of 80% was found for PCz-DTBTA as the donor, and this might be related to the additional N-N interactions between the carbazole segments and the PFN. The results would supply useful information to understand the contribution of an interfacial layer on the photovoltaic performance. Our results also suggest that the bilayer cathode would have great potential to elevate PCE of BHJ PVCs.