Small band gap copolymers based on furan and diketopyrrolopyrrole for field-effect transistors and photovoltaic cells (original) (raw)
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Journal of Materials Chemistry, 2011
Four small band gap semiconducting copolymers based on electron deficient diketopyrrolopyrrole alternating with electron rich trimers containing furan and benzene or thiophene have been synthesized via Suzuki polymerization. The polymers have optical band gaps between 1.4 and 1.6 eV, optimized for solar energy conversion, and exhibit ambipolar charge transport in field-effect transistors with hole and electron mobilities higher than 10 À2 cm 2 V À1 s À1 . In solar cells the polymers are used as electron donors and provide power conversion efficiencies up to 3.7% in simulated solar light when mixed with [70]PCBM as acceptor. † Electronic supplementary information (ESI) available: Temperature dependent UV-vis spectra and additional transistor characterization. See
Journal of Polymer Science Part A: Polymer Chemistry, 2012
It has been shown recently, that the presence of alkyl side chains at the 3-positions on the thiophene rings placed next to 2,1,3-benzothiadiazole core in the backbone of several conjugated polymers results in severe steric hindrance and prevents efficient planarity of the thiophene-2,1,3-benzothiadiazole-thiophene (TBzT) segment. Both properties have a strong influence on the optoelectronic properties of the polymer and need to be considered when the polymer is to be used for organic electronics applications. In this work, we modified a previously synthesized oligothiophene copolymer, consisting of two 3,4 0-dialkyl-2,2 0bithiophene units attached to a 2,1,3-benzothiadiazole unit (TBzT segment) and a thieno[3,2-b]thiophene unit, by optimizing the lateral alkyl side chains following a density functional theory investigation. It is demonstrated that eliminating the alkyl side chains from the 3-positions of the TBzT segment and anchoring them onto the thieno[3,2-b]thiophene, using an efficient synthesis of the 3,6-dihexylthieno[3,2-b]thiophene unit, allows us to reduce the energy band gap. In addition, the chemical modification leads to a better charge transport and to an enhanced photovoltaic efficiency of polymer/fullerene blends. V
Low band gap polymers for organic photovoltaics
Solar Energy Materials and Solar Cells, 2007
Low band gap polymer materials and their application in organic photovoltaics (OPV) are reviewed. We detail the synthetic approaches to low band gap polymer materials starting from the early methodologies employing quinoid homopolymer structures to the current state of the art that relies on alternating copolymers of donor and acceptor groups where strategies for band gap design are possible. Current challenges for OPV such as chemical stability and energy level alignment are discussed. We finally provide a compilation of the most studied classes of low band gap materials and the results obtained in photovoltaic applications and give a tabular overview of rarely applied materials.
Journal of Materials Chemistry, 2012
We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, fieldeffect charge mobility, as well as organic photovoltaic characteristics between thiophene-and selenophene-bridged donor-acceptor low-band-gap copolymers. In these copolymers, a significant reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase of the short circuit current density (J SC ) but a slight decrease of the open circuit voltage (V OC ) of their bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a selenophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6bis(dodecyloxy)-4,7-di(selenophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pCzSe) or poly{4,8-bis(2ethylhexyloxy)benzo[1,2-b;4,5-b 0 ]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo [c] [1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor-acceptor copolymer: poly{9-dodecyl-9Hcarbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly {4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b 0 ]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Judging from our device data, the potential Se-Se interactions of the selenophene-bridged donor-acceptor copolymers, which is presumably beneficial for the fill factor (FF) of BHJ OPVs, is rather susceptible to the device fabrication conditions. † Electronic supplementary information (ESI) available: The drain current-voltage plots and transistor transfer characteristics of copolymers, details of computational study, 1 H NMR spectra of polymers. See
Organic Electronics, 2012
Poor processability and low molecular weights are often hindering the efficient utilization of novel conjugated polymers in optoelectronic devices. Increasing the alkyl side-chain density generally enhances the polymer solubility but may affect as well its optoelectronic properties. In this work, we use density functional theory to identify ways to increase the side-chain density of donor-acceptor alternate copolymers based on 2,1,3-benzothiadiazole, thiophene and thieno[3,2-b]thiophene units, without modifying there otherwise promising frontier orbital energy levels. Following the theoretical results, a new polymer could be synthesized, exhibiting good processability and improved charge transport. As a consequence, the photovoltaic device performances of this polymer family could be enhanced, reaching a 3.7% power conversion efficiency in a standard device configuration and without any post-deposition treatment.