Optimization of the side-chain density to improve the charge transport and photovoltaic performances of a low band gap copolymer (original) (raw)
Related papers
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
Journal of Polymer Science Part A: Polymer Chemistry, 2011
Two new low-bandgap alternating copolymers (CEHTF and CEHTP) consisting of 4,6-bis(3 0-(2-ethylhexyl)thien-2 0-yl)thieno[3,4-c][1,2,5] thiadiazole and 9,9-bis(2-ethylhexyl)fluorene or 2,5-bis(isopentyloxy)benzene were synthesized by Suzuki coupling reaction of corresponding comonomers. Their optical, electrochemical, and photovoltaic (PV) properties were studied and are reported. Both the copolymers exhibited longwavelength absorption covering the whole visible spectral region, which is in CEHTP thin films extended up to near infrared region, ambipolar redox properties, and electrochromism. High-electron affinities and low-optical bandgap values, 1.37 and 1.15 eV, were determined for CEHTF and CEHTP, respectively. PV devices with bulk heterojunction made of blends of copolymers and fullerene derivative [6,6]-phenyl-C 61-butyric acid methyl ester ([60]PCBM) were prepared and characterized. Effects of intramolecular charge transfer strength and sidechain nature and length on photophysical properties are discussed. V
2012
Conjugated polymers with donor−acceptor architectures have been successfully applied in bulk heterojunction solar cell devices. Tuning the electron-withdrawing capability in donor−acceptor (D−A) conjugated polymers allows for design of new polymers with enhanced electrical and optical properties. In this paper, a series of D−A copolymers, PBDFDTBT (P1a), PBDTDTBT (P2a), PNDTDTBT (P3a), and PQDTDTBT (P4a), were selected and theoretically investigated using PBE0/6-311G** and TD-PBE0/6-311G**//PBE0/6-311G** methods. The calculated results agree well with the available experimental data of HOMO energy levels and band gaps. We further designed and studied four novel copolymers, P1b, P2b, P3b, and P4b, by substituting the 2,1,3-benzothiadiazole (BT) unit in P1a−P4a with a stronger unit of naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT), respectively. Compared with P1a−P4a, the newly designed polymers of P1b−P4b show better performance with the smaller band gaps and lower HOMO energy levels. The PCEs of ∼5%, ∼7%, ∼7%, and ∼7% for P1b−P4b, predicted by Scharber diagrams, are much higher than those of P1a−P4a when used in combination with PCBM. These results clearly reveal that tuning the electron-withdrawing capability in D−A conjugated polymers is an effective way to improve the electrical and optical properties and the efficiency of the photovoltaic device.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 2017
This paper studies donor-acceptor systems which incorporate benzodithiophene (BDT), benzodifuran (BDF) and benzodipyrrole (BDP) units as the electron-rich monomer with TT unit representing the electron-deficient monomer. This research is based on employing density functional theory (DFT) and time-dependent DFT (TD-DFT). The highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO), HOMO-LUMO gaps and dihedral-angles of these copolymers were calculated using oligomer extrapolation technique and periodic boundary condition (PBC) method. The optical band gaps and UV-vis absorption spectra of aforementioned copolymers were obtained by TD-DFT at the same level of theory. Based on the fair agreement between PBC-DFT calculated results and experimental data, the substituent effects of Cl, Br, CCH, COH, NO2, OH, SH and NH2 groups were investigated by PBC-DFT method. The difference between the ground and excited-states dipole moment (Δμge) of all derivativ...
RSC Adv., 2014
We report a comparative study on four donor-acceptor benzothiadiazole-based copolymers containing dithienyl or thienothiophene moieties for application in organic photovoltaic (OPV) devices. Bulkheterojunction OPV devices are fabricated having power conversion efficiencies ranging between 4 and 6%. Morphological, spectroscopic and charge-transport measurements are used to investigate the influence of either the dithienyl or thienothiophene moieties on the structure and photophysical properties of the copolymer and copolymer:PC 71 BM blend films and rationalise the solar cell characteristics. Although all copolymer:PC 71 BM blends exhibit comparable hole polaron yields, solar cell devices with the highest power conversion efficiencies are correlated with increased charge-carrier mobility of the copolymer and enhanced aggregation of PC 71 BM in the blend. † Electronic supplementary information (ESI) available: OPV device optimisation data, GIWAXS measurements of pure polymer lms, AFM scans of blend lm surface, PL spectra of neat and blend thin-lms and OFET transfer and output characteristics. See
Current Applied Physics, 2015
Three kinds of donor-acceptor (D-A) type photovoltaic polymers were synthesized based on 2,7carbazole 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 π-bridge. Polymer:[6,6]-phenyl-C 71 butyric acid methyl ester (PC 71 BM) based bulk heterojunction solar cells exhibited a high open-circuit voltage (V OC) in the range, 0.86~0.94 V, suggesting good agreement with the measured HOMO levels. Despite the high V OC , 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 (J SC). The voltage-dependent photocurrent and photoluminescence quenching measurements suggested that hole transfer from PC 71 BM to polymer depends strongly on the HOMO level of the polymer. The PCTTPD and PCTTTPD devices suffered from electron-hole recombination at the polymer/PC 71 BM interfaces because of the insufficient energy offset between the HOMOs of the polymer and PC 71 BM. The PCBTTPD:PC 71 BM device showed the best PCE of 3.42% with a V OC and J SC 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 V OC and sufficient energy offset for ensuring efficient hole transfer from PC 71 BM to the polymer.
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
International Journal of Quantum Chemistry, 2019
We report a density functional theory study of the effect of electron-withdrawing groups such as-F,-CN,-NO 2 on the geometrical, optoelectronic, intramolecular charge transfer (ICT), and photovoltaic properties of (E)-1,2-bis(5-alkyl-[2,3 0bithiophene]-2 0-yl)ethene (TVT-T) based donor-acceptor (D-A) copolymers with different acceptor units, that is, benzo[c][1,2,5]thiadiazole, benzo[c][1,2,5]oxadiazole, and benzo[c][1,2,5]selenadiazole. The computed optical absorption spectra of the designed compounds lie in the visible and near-infrared regions. Of all the studied copolymers,-CN substituted and Se-based compound displays the lowest HOMO-LUMO (E H-L) gap and optical band gap (E opt). The exciton binding energy (E b) is found to be smaller for O-incorporated compounds and-CN substituted copolymer as well, inferring more ICT. The electron-hole coherence concentrated over the D-A units is nearly the same for-CN and-NO 2 substituted compounds, but larger in-F derivatives, indicating weak electronhole coupling in the formers. Comparatively larger dipole moment (6.421 Debye-9.829 Debye) and charge transfer length (D CT) (1.976 Å-3.122 Å) for-CN derivatives lead to enhanced ICT properties. The designed donors yield good hole mobilities (0.127-6.61 cm 2 V −1 s −1) and the predicted power conversion efficiencies are calculated to be as high as~6%-7% for-CN and-NO 2 substituted compounds.