Donor–acceptor (D–A) conjugated copolymers are one of known classes of organic optoelectronic materials and have been well developed. However, less attention has been paid on acceptor–acceptor (A–A) conjugated analogs. In this work, two types of A–A conjugated copolymers, namely P1-Cn and P2-Cn (n is the carbon number of their alkyl side chains), were designed and synthesized based on perylenediimide (PDI) and 2,1,3-benzothiadiazole (BT). Different from P1-Cn, P2-Cn polymers have additional acetylene π-spacers between PDI and BT and thus hold a more planar backbone configuration. Property studies revealed that P2-Cn polymers possess a much red-extended UV–vis absorption spectrum, stronger π–π interchain interactions, and one-order larger electron mobility in their neat film state than P1-Cn. However, all-polymer solar cells using P1-Cn as acceptor component and poly(3-hexyl thiophene) or poly(2,7-(9,9-didodecyl-fluoene)-alt−5,5′-(4,7-dithienyl-2-yl-2,1,3-benzothiadiazole) as donor component exhibited much better performance than those based on P2-Cn. Apart from their backbone chemical structure, the side chains were found to have little influence on the photophysical, electrochemical, and photovoltaic properties for both P1-Cn and P2-Cn polymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1200–1215

Besides the donor–acceptor (D–A) type, acceptor–acceptor (A–A) polymers are another class of important alternative conjugated copolymers, but have been less studied in the past. In this study, two kinds of A–A polymers, P1 and P2, have been designed and synthesized based on diketopyrrolopyrrole in combination with the second electron-deficient unit, perylenediimide or thieno[3,4-c]pyrrole-4,6-dione. UV–vis absorption spectroscopy revealed that these two kinds of polymers have a band gap of 1.28–1.33 eV. Their highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are around −5.6 and −4.0 eV for P1 polymers, whereas −5.4 and −3.7 eV for P2 polymers, respectively. Density functional theory study disclosed that P1 backbone is in a vastly twisting state, whereas that of P2 is completely planar. Furthermore, organic field-effect transistor devices were fabricated using these two kinds of polymers as the active material. Of interest, the devices based on P1 polymers displayed n-channel behaviors with an electron mobility in the order of 10⁻⁴ cm² V⁻¹ s⁻¹. In contrast, the P2-based devices exhibited only p-channel charge transportation characteristics with a hole mobility in the order of 10⁻³ cm² V⁻¹ s⁻¹. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2356–2366

A series of donor-acceptor oligomer OBTTh_n (n=1–7) and polymer PBTTh₁ and PBTTh₂ composed of alternative 2,1,3-benzothiadiazole and 3-hexylthiophene have been designed and synthesized for the purpose of investigation on the effect of chain length and side-chain regioregularity on their basic properties and photovoltaic performance. In the OBTTh_n oligomers and PBTTh₁ polymer, all the hexyl side chains on thienyl units orient toward the same direction. Upon elongation of the chain length, the intramolecular charge transfer (ICT) absorption band in solution gradually redshifts from 398 nm for OBTTh₁ to 505 nm for OBTTh₇, then to 512 nm for PBTTh₁ polymer. Meanwhile, the HOMO energy level increases from −5.45 eV (OBTTh₁) to −5.08 eV (OBTTh₇) and −5.09 eV (PBTTh₁), and the LUMO energy level decreases from −3.11 eV (OBTTh₁) to −3.30 eV (OBTTh₇) and −3.33 eV (PBTTh₁), thus giving a smaller and smaller energy bandgap for higher oligomers and polymers. Theoretical calculation suggests straight line-like backbone geometry for this series of oligomers and polymer. On the other hand, polymer PBTTh₂ possesses a different side-chain regioregularity, in which every two neighbor hexyl side chains are arranged in different orienting direction. It is theoretically suggested to have curved line-like backbone geometry. In solution, it shows similar photophysical and electrochemical properties as PBTTh₁. However in film state, it displays a less redshift in the ICT band as refer to that in solution than PBTTh₁. In combination with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), these oligomers and polymers were used as donor material to fabricate organic bulk heterojunction solar cells. Again, chain length-dependent device photovoltaic performance was observed. The device based on OBTTh₄ showed a power conversion efficiency of 0.16%, while it increased to 0.36% and 0.49% for the devices based on OBTTh₆ and PBTTh₁, respectively. However, the side-chain regioregularity has less influence on the device photovoltaic output since the device based on PBTTh₂ displayed an efficiency of 0.52%, comparable to that of PBTTh₁.

To tailor organic p/n heterojunctions with molecular-level precision, a rational design strategy using side-chain incompatibility of a covalently connected donor–acceptor (D–A) dyad has been successfully carried out. An oligothiophene–perylenediimide dyad, when modified with triethylene glycol side chains at one terminus and dodecyl side chains at the other (2 Amphi), self-assembles into nanofibers with a long-range D/A heterojunction. In contrast, when the dyad is modified with dodecyl side chains at both termini (2 Lipo), ill-defined microfibers result. In steady-state measurements using microgap electrodes, a cast film of the nanofiber of 2 Amphi displays far better photoconducting properties than that of the microfiber of 2 Lipo. Flash-photolysis time-resolved microwave conductivity measurements, in conjunction with transient absorption spectroscopy, clearly indicate that the nanofiber of 2 Amphi intrinsically allows for better carrier generation and transport properties than the microfibrous assembly of 2 Lipo.