A series of nine alternating donor–acceptor copolymer semiconductors based on naphthalene diimide (NDI) acceptor and seven different thiophene moieties with varied electron-donating strength and conformations has been synthesized, characterized, and used in n-channel and ambipolar organic field-effect transistors (OFETs). The NDI copolymers had moderate to high molecular weights, and most of them exhibited moderate crystallinity in thin films and fibers. The LUMO energy levels of the NDI copolymers, at −3.9 to −3.8 eV, were constant as the donor moiety was varied. However, the HOMO energy levels could be tuned over a wide range from −5.3 eV in P8 to −5.9 eV in P1 and P3. As semiconductors in n-channel OFETs with gold source/drain electrodes, the NDI copolymers exhibited good electron transport with maximum electron mobility of 0.07 cm2/(V s) in P5. Although head-to-head (HH) linkage induced backbone torsion, polymer P4 showed substantial electron mobility of 0.012 cm2/(V s) in bottom-gate/top-contact device geometry. Some of the copolymers with high-lying HOMO levels (P7 and P8) exhibited ambipolar charge transport in OFETs with high electron mobilities (0.006–0.02 cm2/(V s)) and significant hole mobilities (>10–3 cm2/(V s)). Varying the device geometry from top-contact to bottom-contact leads to the appearance or enhancement of hole transport in P4, P6, P7, and P8. Copolymers with smaller alkyl side chains on the imide group of NDI have enhanced carrier mobilities than those with bulkier alkyl side chains. These results show underlying structure–property relationships in NDI-based copolymer semiconductors while demonstrating their promise in n-channel and ambipolar transistors.Keywords: ambipolar charge transport; donor−acceptor conjugated copolymer; electron transport; n-channel organic transistor; n-type polymer semiconductor; naphthalene diimide copolymer;

Three new donor−acceptor conjugated polymers incorporating thieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor and dialkoxybithiophene or cyclopentadithiophene units as donor have been synthesized and explored in bulk heterojunction (BHJ) solar cells and organic field-effect transistors (OFETs). The TPD-containing polymers had optical band gaps of 1.50−1.70 eV and HOMO levels of −4.85 to −5.26 eV. The highly electron-rich character of dialkoxybithiophene in P1 and P2 destabilizes their HOMOs which significantly affects the photovoltaic efficiency. However, polymer P3 containing cyclopentadithiophene donor units results in a deeper HOMO level of −5.26 eV. The field-effect mobility of holes varied from 2 × 10−4 cm2/(V s) in P2 to ∼1 × 10−2 cm2/(V s) in P3-based transistors. BHJ solar cells using polymer P1 or P2 as the electron donor and (6,6)-phenyl-C71-butyric acid ester (PC71BM) as the electron acceptor exhibit low open circuit voltages (Voc = 0.40−0.60 V) and power conversion efficiencies below 1.5%. However, BHJ solar cells based on the TPD−cyclopentadithiophene copolymer (P3) achieved a high Voc of ∼0.8 V and power conversation efficiency greater than 3%. These results demonstrate the tuning of the open circuit voltage and thus the photovoltaic efficiency of BHJ solar cells based on copolymers containing thieno[3,4-c]pyrrole-4,6-dione acceptor.

Phenyleneethynylene-based conjugated copolymers using benzo[1,2-d:4,5-d′]bis[1,3]dioxole (BDO) in the repeating unit are reported. The electronic structure of the BDO unit imparts a localized HOMO topology while the LUMO is delocalized over the polymer backbone, so that the lowest optical absorption band of the polymer has considerable intramolecular charge transfer character. This contrasts with published donor−acceptor polymers with localized LUMO and delocalized HOMO. The very large Stokes shifts of the monomers, which are due to the small oscillator strength of the lowest optical transition, are largely retained in the polymers as a result of covalently constrained dihedral angles in the substituents (not the backbone), as predicted/explained by calculations.

The synthesis and characterization of two new thiophene copolymers with backbone phthalimide units is reported. Thin-film optical and wide-angle X-ray diffraction measurements indicate extended electronic conjugation and close intermolecular π-stacking for both polymers. Ambient carrier mobility of thin-film transistors prepared from these polymers is as high as 0.28 cm2/(V s) with an on/off ratio greater than 105.

Bulk heterojunction solar cells based on blends of the new low band gap donor–acceptor copolymer, poly(N-(dodecyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide) (PhBT12), and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were systematically investigated. The PhBT12/fullerene blend films were found to exhibit a crystalline nanoscale morphology with space-charge-limited mobility of holes as high as 4.0 × 10−4 cm2/Vs without thermal annealing, leading to moderately efficient devices. The performance of the solar cells varied significantly with PhBT12/fullerene composition, reaching a power conversion efficiency of 2.0% with a current density of 6.43 mA/cm2 and a fill factor of 0.55 for the 1:1 PhBT12/PC71BM blend devices. However, thermally annealed (120 °C) PhBT12/fullerene blend devices had negligible photovoltaic properties due to micrometer scale phase separation of the blends which is attributed to the long side chains. We expect that better photovoltaic performance can be achieved by modifying the polymer side chain length and the device processing as well. These results show that phthalimide-based donor–acceptor copolymer semiconductors, exemplified by PhBT12, are promising low band gap materials for developing efficient bulk heterojunction solar cells.

A series of copolymers of 3,3′-dialkylbithiophenes and tetrafluorobenzene units are reported along with non-fluorinated controls. All are characterized by NMR, gel permeation chromatography, differential scanning calorimetry, optical spectroscopy, and wide-angle X-ray diffraction (WAXD). The fluorine substituents enhance backbone planarity, order, and π-stacking in the solid-state as revealed by optical spectroscopy and WAXD. Peak oxidation potential is also increased by 0.55 V relative to a benchmark polymeric semiconductor, poly(3-hexylthiophene). Head-to-head 3-alkylthiophene linkages do not preclude backbone planarity provided the side chains are not too bulky.

Bulk heterojunction solar cells based on blends of the new low band gap donor–acceptor copolymer, poly(N-(dodecyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide) (PhBT12), and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were systematically investigated. The PhBT12/fullerene blend films were found to exhibit a crystalline nanoscale morphology with space-charge-limited mobility of holes as high as 4.0 × 10−4 cm2/Vs without thermal annealing, leading to moderately efficient devices. The performance of the solar cells varied significantly with PhBT12/fullerene composition, reaching a power conversion efficiency of 2.0% with a current density of 6.43 mA/cm2 and a fill factor of 0.55 for the 1:1 PhBT12/PC71BM blend devices. However, thermally annealed (120 °C) PhBT12/fullerene blend devices had negligible photovoltaic properties due to micrometer scale phase separation of the blends which is attributed to the long side chains. We expect that better photovoltaic performance can be achieved by modifying the polymer side chain length and the device processing as well. These results show that phthalimide-based donor–acceptor copolymer semiconductors, exemplified by PhBT12, are promising low band gap materials for developing efficient bulk heterojunction solar cells.

A series of donor–acceptor poly(phenylene ethynylene)s (PPEs) with N,N′-dialkylpyromellitic diimide (PMDI) as electron acceptor and various donor units are reported. Optoelectronic properties were investigated by UV/vis absorption and electrochemical measurements, revealing constant LUMO energies (∼−3.6 eV) across the series with HOMO energy levels governed by the donor monomers and optical band gaps from 2.30 to 1.50 eV. With a high volume fraction of solubilizing side chains, the polymers are soluble in common organic solvents, but solution NMR measurements and thermochromism in solution indicate aggregation due to additive intermolecular interactions between donor and acceptor units.