2,7-Carbazole and thieno[3,4-c]pyrrole-4,6-dione based copolymers with deep highest occupied molecular orbital for photovoltaic cells

Three kinds of donor-acceptor (D-A) type photovoltaic polymers were synthesized based on 2,7-carbazole and thieno[3,4-c]pyrrole-4,6-dione (TPD). The conjugation of weakly electron (e)-donating 2,7-carbazole and strongly e-accepting TPD moieties yielded a deep highest occupied molecular orbital (HOMO) and its energy level was fine-controlled to be -5.72, -5.67 and -5.57 eV through the incorporation of thiophene (T), thieno[3,2-b]thiophene (TT) and bithiophene (BT) as a ??-bridge. Polymer:[6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) based bulk heterojunction solar cells exhibited a high open-circuit voltage (VOC) in the range, 0.86-0.94 V, suggesting good agreement with the measured HOMO levels. Despite the high VOC, the thiophene (or thienothiophene)-containing PCTTPD (or PCTTTPD) showed poor power conversion efficiency (PCE, 1.14 and 1.25%) because of the very low short-circuit current density (JSC). The voltage-dependent photocurrent and photoluminescence quenching measurements suggested that hole transfer from PC71BM to polymer depends strongly on the HOMO level of the polymer. The PCTTPD and PCTTTPD devices suffered from electron-hole recombination at the polymer/PC71BM interfaces because of the insufficient energy offset between the HOMOs of the polymer and PC71BM. The PCBTTPD:PC71BM device showed the best PCE of 3.42% with a VOC and JSC of 0.86 V and 7.79 mA cm-2, respectively. These results show that photovoltaic polymers should be designed carefully to have a deep HOMO level for a high VOC and sufficient energy offset for ensuring efficient hole transfer from PC71BM to the polymer. © 2015 Elsevier B.V. All rights reservedclose0

Effect of alkyl chain topology on the structure, optoelectronic properties and solar cell performance of thienopyrroledione-cored oligothiophene chromophores

We have investigated a series of oligothiophenes containing a central thienopyrroledione group with rhodanine, dicyanovinyl and octylcyanoacrylate end-capping groups. For each end capping group, two alkyl chain configurations were explored by appending n-octyl chains to the oligothiophenes in both proximal and distal topologies. Substitution of different alkyl topologies and end-capping groups altered not only intramolecular conformations but also intermolecular interactions, thus affecting frontier molecular orbitals and bulk properties such as optical, thermal transitions, solid state packing, and device properties. The electronic properties of the materials were probed in field effect transistors (FETs), single carrier diodes and bulk heterojunction (BHJ) solar cell devices. FET devices revealed that all materials behaved as p-type semiconductors with mobilities in the range of 10(-5) to 10(-3) cm(2) V-1 s(-1). In solar cell devices, we observed that the optimal end capping group was rhodanine, while the optimal alkyl chain configuration was the proximal configuration. The rhodanine capped molecule with proximal alkyl chain isomerism led to a power conversion efficiency of similar to 2%. Grazing-incidence wide-angle X-ray scattering studies of this series of molecules revealed a strong tendency to undergo edge-on packing with no pi-pi stacking in the vertical direction, which may limit their performance in BHJ solar cells.clos

Semicrystalline D–A Copolymers with Different Chain Curvature for Applications in Polymer Optoelectronic Devices

Thiophene- and thienothiophene-based donor–acceptor (D–A) type semicrystalline copolymers with different backbone curvatures, <b>PTBT14</b> and <b>PTTBT14</b>, were designed and synthesized. Both the polymers exhibit a nearly planar structure via noncovalent S···O and C–H···N attractive interactions, etc., in the polymer chain. <b>PTTBT14</b> is linear, whereas <b>PTBT14</b> is curved owing to ∼160° bond angle of the thiophene linkage. <b>PTTBT14</b> showed the higher degree of interchain ordering with edge-on orientation, resulting in efficient charge transport (0.26 cm² V^–1 s^–1 for <b>PTTBT14</b> compared to 0.02 cm² V^–1 s^–1 for <b>PTBT14</b>) in PFETs with remarkable morphological stability and no deterioration in device properties at temperatures up to 250 °C. On the other hand, the curved shape of <b>PTBT14</b> attributed to its improved photovoltaic properties with a power conversion efficiency of 5.56%. The linear <b>PTTBT14</b> showed much stronger self-interactions with negligible morphological changes and little miscibility with PC₆₁BM, showing the poor photovoltaic characteristics

Benzotriazole-Containing Planar Conjugated Polymers with Noncovalent Conformational Locks for Thermally Stable and Efficient Polymer Field-Effect Transistors

We report a series of benzotriazole-based semicrystalline π-conjugated polymers with noncovalent conformational locks for applications in polymer field-effect transistors. The benzotriazole moiety is a versatile electron-deficient building block that offers two chemically functionalizable sites, 2­(N) and 5, 6­(C) positions, allowing easy modulation of the solution processability and electronic structures of the resulting polymers. Fluorine or alkoxy substituents were introduced to the benzotriazole unit to enhance the molecular ordering through intra- and intermolecular F···S, F···H–C, C–F···π_F, or S···O attractive interactions. The fluorinated polymer (<b>PTBTz-F</b>) showed remarkably enhanced hole mobility (μ_h = 1.9 cm²/(V·s), on/off ratio = 8 × 10⁷) upon thermal annealing at 305 °C, compared to the unsubstituted one (<b>PTBTz</b>) (μ_h = 7.0 × 10^–3 cm²/(V·s), on/off ratio = 3 × 10⁶). Alkoxy unit substitution (<b>PTBTz-OR</b>) also improved the carrier mobility up to 0.019 cm²/(V·s) with an on/off ratio of 4 × 10⁵. Fluorine or alkoxy substitution induced tight interchain ordering with edge-on orientation, as confirmed by X-ray diffraction measurements. In particular, fluorinated <b>PTBTz-F</b> showed high thermal stability (<i>T</i>_d 453 °C) and the remarkable device characteristics with deep frontier orbital levels

Semicrystalline D-A copolymers with different chain curvature for applications in polymer optoelectronic devices

Thiophene- and thienothiophene-based donor-acceptor (D-A) type semicrystalline copolymers with different backbone curvatures, PTBT14 and PTTBT14, were designed and synthesized. Both the polymers exhibit a nearly planar structure via noncovalent S??????O and C-H??????N attractive interactions, etc., in the polymer chain. PTTBT14 is linear, whereas PTBT14 is curved owing to ???160?? bond angle of the thiophene linkage. PTTBT14 showed the higher degree of interchain ordering with edge-on orientation, resulting in efficient charge transport (0.26 cm2 V-1 s-1 for PTTBT14 compared to 0.02 cm2 V-1 s-1 for PTBT14) in PFETs with remarkable morphological stability and no deterioration in device properties at temperatures up to 250 ??C. On the other hand, the curved shape of PTBT14 attributed to its improved photovoltaic properties with a power conversion efficiency of 5.56%. The linear PTTBT14 showed much stronger self-interactions with negligible morphological changes and little miscibility with PC 61BM, showing the poor photovoltaic characteristics.close2