Low Band Gap Copolymers Research Articles

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT.

Conjugated donor-acceptor copolymers hold great potential as materials for high-performance organic photovoltaics, organic transistors and organic thermoelectric devices. Their low optical bandgap is achieved by alternation of donor and acceptor moieties along the polymer chain, leading to a pronounced charge-transfer character of electronic excitations. However, the influence of appended side chains and of chemical defects of the backbone on their photophysical and conformational properties remains largely unexplored on the level of individual chains. Here, we employ room temperature single-molecule photoluminescence spectroscopy on four compounds based on the prototypical copolymer PCDTBT with systematically changed chemical structure. Our results show that an increasing density of statistically added hexyl chains to the TBT comonomer distorts the molecular conformation, likely through the increase of average dihedral angles along the backbone. We find that, although the conformation becomes more twisted with high hexyl density, the side chains appear to stabilize the backbone in this twisted conformation. In addition, we demonstrate that homocoupling defects along the backbone barely influence the PL spectra of single chains, and thus intra-chain electronic properties.

Direct observation of conformations of a high-mobility n-type low-bandgap copolymer in solutions and solid films.

The aggregation morphologies of conjugated polymers in solutions and solid films are important for their optoelectronic applications. Due to the amorphous state of the polymers, it remains a great challenge to determine their conformations in either liquids or solids. Herein, a ps/fs synchronized 2D IR technique is applied to investigate the molecular conformations of a high-mobility n-type low-bandgap copolymer, N2200, dissolved in CHCl3 and CCl4, and in solid films cast from both solutions by the vibrational cross-angle method. In CCl4, the polymer forms more aggregates and folds more and the backbone dihedral angle of C-C(NDI)/C-S(Thiophene) of its average conformation is about 10° more distorted than that in CHCl3 and the most stable conformation for a free molecule. Anti-intuitively, the solid films cast from both solutions have the same molecular conformation, and the conformation is similar to that of the polar CHCl3 rather than the conformation of the less polar CCl4. The results imply that the interaction between the polymer backbones is probably stronger than its interaction with CCl4, which can naturally guide the rearrangement of polymer chains during the evaporation of solvent molecules. This work also implies that the balance and competition between the polymer/polymer interaction and the polymer/solvent interaction seem to be the dominant factors responsible for what morphology can form in a solid film cast from solution. It is not always true that different molecular conformations must exist in solid films grown from different solutions with different polarity or different extents of aggregates with different conformations.

Stable low-bandgap isoindigo-bisEDOT copolymer with superior electrochromic performance in NIR window

High-performance electrochromic materials with high optical contrast, coloration efficiency, and excellent stability have always been critical for polymer electrochromics, and will boost diverse applications including flexible displays and wearable electronics. Herein, we develop a donor-acceptor-donor type copolymer from strong electron-withdrawing acceptor unit isoindigo and electron-donating moiety bis-EDOT by synthesizing and electropolymerizing its precursor (E)-1,1′-dihexyl-6,6′-bis(2,2′,3,3′-tetrahydro-[5,5′-bithieno[3,4-b][1,4]dioxin]-7-yl)-[3,3′-biindolinylidene]-2,2′‑dione (BisE−IDOH−BisE). Such polymer exhibits a low optical bandgap of 1.02 eV with stable and reversible electrochromism from kelly-green to sky blue. Further kinetic studies demonstrate that this polymer displays intriguing overall electrochromic performances throughout the NIR window with simultaneously high optical contrast up to 90%, high coloration efficiency of 1025 cm2 C−1, fast switching time of 1.1 s, and excellent reversible stability (< 4% loss upon 5000 cycling), which presents one of the best overall performances among the reported electrochromic polymers so far. With these amazing electrochromic properties, isoindigo-EDOT based polymers can be further developed towards device patterning and real applications like flexible indoor decoration displays and near infrared camouflage.

One-pot synthesis and thermal stability of thiophene-bridged thieno[3,2-b]thiophene donor-based copolymers

Three low band gap (below 1.6 eV) copolymers based on 2,5-bis(3-octylthiophen-2-yl)thieno[3,2-b]thiophene) donor unit and isoindigo acceptor units were designed and successfully synthesized by the direct arylation polycondensation method. The backbones of the copolymers were chosen to be similar while the alkyl side chain substituted on the isoindigo units were systematically tailored as 2-hexyldecyl in P1, 2-octyldecyl in P2 and 2-decyltetradecyl in P3. The thermal stabilities of the copolymers was found to be highly dependent on the lengths of alkyl side chains. The long alkyl side chains in P3 were found to be detrimental to its stability. P2 was found to have a higher thermal stability was found in due to the optimized side chains that reduced steric repulsion between the side chains in the donor and acceptor units. The thermogravimetric analysis also revealed that the backbone decomposition of P2 is at higher temperature than P3 and P1. The results confirmed the importance of alkyl side chain optimization for the stability of copolymers

PCPDTBT copolymer based high performance organic phototransistors utilizing improved chain alignment

Abstract In this article, we report single component thin film based organic phototransistors (OPTs) developed using the novel ribbon-floating film transfer method (ribbon-FTM). A low band-gap copolymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) has been employed to develop the photoactive thin films of the devices. The fabricated polymer films were highly anisotropic in nature. To investigate the effect of film anisotropy on the device performance, organic field effect transistors (OFETs) were fabricated in two configurations namely, parallel and perpendicular, wherein the polymeric chains were oriented in line and orthogonal to the transistor channel direction, respectively. Notable differences in charge carrier mobility and device on-off ratios were observed among the two sets of devices. An on-off ratio of 4.1 × 103 and an average mobility of 0.001 cm2/V were observed in case of the perpendicular oriented OFETs which were increased to 105 and 0.003 cm2/V for the parallel oriented OFETs. A probe into the contact resistance at the respective metal/polymer interfaces of the devices was carried out using the modified transmission line method (MTLM). 4 times lower contact resistance was observed in the parallel OFET which was anticipated to be responsible for its superior performance. Further, the fabricated devices were illuminated with monochromatic red light (Pin = 0.4 mW/cm2) to observe their performance as phototransistors. Appreciable values of photosensitivity (2 × 104) and photoresponsivity (17.6 A/W) were obtained for the parallel OPTs which were found to be much higher than those of the previously reported thin film based OPTs employing other materials fabricated through conventional film coating techniques. The results presented in this report are of key significance towards the development of highly sensitive and cost-effective phototransistors which can be used in opto-sensing and imaging applications.

Difluoroterthiophene as promising block to build highly planar conjugated polymer for polymer photovoltaic cells

Developing highly planar semiconducting polymer is essential for achieving high mobility and improving the photovoltaic performance of bulk-heterojunction polymer solar cells. In this contribution, two novel low-bandgap donor-acceptor copolymers, P1-3T and P2-3T2F, consisting of electron-accepting benzothiadiazole acceptor segment and electron-donating terthiophene donor segment with and without fluorine atoms were designed and synthesized. The density functional theory calculations and ultraviolet–visible absorption studies demonstrate that the fluorinated P2-3T2F polymer has more planar backbone conformation, deeper HOMO level, broader absorption spectrum with a vibronic shoulder peak, and higher absorption coefficient compared with the non-fluorinated analogue polymer, P1-3T. Furthermore, P2-3T2F exhibits good crystallinity and high hole mobility, presumably due to the well-ordered lamellar packing and the π-π stacking interaction between the backbones of the conjugated polymers. The optimized polymer solar cell based on P2-3T2F:PC61BM exhibits a short-circuit current density (Jsc) of 12.77 mA cm−2, a fill factor (FF) of 68.99%, an open-circuit voltage (Voc) of 0.80 V, and a maximum power conversion efficiency (PCE) of 7.14%, which is approximately 46% higher than that of the P1-3T:PC61BM device. The enhanced PCE is primarily due to increased light-harvesting ability and interconnected morphology with finely dispersed polymer-rich and PC61BM-rich domains, which improves the efficiency of exciton dissociation and rises the mobility of charge carriers, thus boosting the short-circuit current density and the FF value. Moreover, the relatively deep HOMO of P2-3T2F effectively increases the Voc.

Perylene diimide based low band gap copolymers: synthesis, characterization and their applications in perovskite solar cells

In this study, newly designed perylene diimide based polymers P1 and P2 were synthesized and were used as hole transporting materials in place of the more common spiro-OMeTAD in perovskite solar cells. The co-polymers P1-P2 were synthesized by Suzuki and direct arylation polymerizations of 2,9-bis(7-bromo-9,9-dioctyl-9H-fluoren-2-yl)anthra[2,1,9-def:6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H,9H)-tetrone M1 with 5,11-bis(2-hexyldecyl)-3,9-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5,11-dihydoindolo[3,2-b]carbazole M2 and 3-octylthiophene M3, respectively. P1 and P2 exhibit broad absorption spectra in thin film and solution with the absorption extending from the ultraviolet to visible and near infrared region of the solar spectrum. The water contact angles of P1-P2 were determined to be 100.8° and 122.9°, respectively, showing good hydrophobicity for the synthesized copolymers. Both polymers show steady state photoluminescence quenching when blended with perovskite. Perovskite solar cell devices fabricated using P1-P2 in place of spiro-OMeTAD show power conversion efficiencies of 13.02% and 10.74%, respectively and manifest the potential of this class of materials as promising alternatives to conventionally used spiro-OMeTAD.

Low-bandgap D-A1-D-A2 type copolymers based on TPTI unit for efficient fullerene and nonfullerene polymer solar cells

To obtain low-bandgap polymers paired well with fullerene and nonfullerene acceptors, here we adopted a D-A1-D-A2 motif to develop two new low-bandgap copolymers PTPTI-T-BDD and PTPTI-T-FBT, where thiophene was used as the D unit, thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]-isoquinoline-5,11(4H,10H)-dione (TPTI) was used as the A1 unit, and benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (BDD) and 5,6-difluoro-2,1,3-benzothiadiazole (FBT) were employed as the A2 unit, respectively. Effects of the electron-withdrawing strength of A2 unit on optoelectronic and photovoltaic properties of the PTPTI-T-BDD- and PTPTI-T-FBT-based fullerene and nonfullerene polymer solar cells (PSCs) were systematically investigated. When blended with PC71BM and ITIC, PTPTI-T-FBT-based PSCs showed a power conversion efficiency (PCE) of 6.20% and 6.03%, respectively, both of which are higher than that of PTPTI-T-BDD-based PSCs. Furthermore, ternary PSCs based on PBDB-T:PTPTI-T-FBT:PC71BM exhibited an improved PCE of 7.92%. This work suggests that constructing D-A1-D-A2 copolymers is a promising strategy to develop low-bandgap copolymers for efficient fullerene and nonfullerene PSCs with a reduced energy loss.

Analysis of bulk heterojunction organic solar cell blends by solid-state NMR relaxometry and sensitive external quantum efficiency – Impact of polymer side chain variation on nanoscale morphology

Abstract A significant number of organic electronic devices rely on blends of electron-donating and electron-accepting molecules. In bulk heterojunction organic photovoltaics, the nanoscopic phase behavior of the two individual components within the photoactive layer has a major impact on the charge separation and charge transport properties. For polymer:fullerene solar cells, it has been hypothesized that an increased accessibility of the electron-deficient monomer unit in push-pull type low bandgap polymers allows for fullerene ‘docking’. The close proximity of electron donor and acceptor molecules enables more efficient charge transfer, which is beneficial for the device efficiency. With this in mind, we synthesized a series of PBDTTPD [poly(benzodithiophene-thienopyrroledione)] low bandgap copolymers with varying side chains. Solar cells were fabricated for all polymers and the device characteristics were compared. The combination of proton wideline solid-state NMR (ssNMR) relaxometry and sensitive external quantum efficiency (sEQE) measurements was shown to provide essential information on donor-acceptor interactions and phase separation in bulk heterojunction organic photovoltaics. The reduced charge transfer state absorption and the observed phase separation of crystalline PC71BM domains for the polymers containing the most accessible methyl-TPD unit indicate a diminished contact between donor and acceptor, leading to a loss in performance.

Fluoro-Modulated Molecular Geometry in Diketopyrrolopyrrole-Based Low-Bandgap Copolymers for Tuning the Photovoltaic Performance.

Fluorination of conjugated polymers is an effective strategy to tune the energy levels for obtaining high power conversion efficiency (PCE) in organic solar cells. In this work, we have developed fluoro-modulated molecular geometries in diketopyrrolopyrrole based low-bandgap copolymers. In these polymers, planar conformation can be locked by intramolecular non-covalent interaction (intramolecular supramolecular interaction) between the sulfur atoms and the introduced F atoms (F···S interaction). By varying the fluorinated moieties, such a planarity can be disturbed and the molecular geometry is tuned. As a result, the polymer' properties can be modulated, including the ultraviolet-visible absorption spectrum to become broaden, charge mobility to be enhanced, open-circuit voltage (Voc) and short-circuited current (Jsc) to be elevated, and thus photovoltaic performance to be improved. The photovoltaic device based on PCFB, one of the fluorinated terpolymers, exhibited a high PCE near 8.5% with simultaneously enhanced Voc and Jsc relative to the non-fluorinated one (PCB).

Eco-Friendly Push-Coated Polymer Solar Cells with No Active Material Wastes Yield Power Conversion Efficiencies over 5.5.

Push-coating is a simple process that can be employed for extremely low-cost polymer electronic device production. Here, we demonstrate its application to the fabrication of poly(2,7-carbazole- alt-dithienylbenzothiadiazole) (PCDTBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) active layers processed in air, yielding similar photovoltaic performances as thermally annealed spin-coated thin films when used in inverted polymer solar cells (PSCs). During push-coating, the polydimethylsiloxane layer temporarily traps the deposition solvent, resulting in simultaneous film formation and solvent annealing effect. This removes the necessity for a postdeposition thermal annealing step which is required for spin-coated PSCs to produce high photovoltaic performances. Optimized PSC active layers are produced with a push-coating time of 5 min at room temperature with 20 times less hazardous solvent and 40 times less active material than spin-coating. Annealed spin-coated active layers and active layers push-coated for 5 min both produce average power conversion efficiencies (PCEs) of 5.77%, while those push-coated for a shorter time of 1 min yield a slightly lower value of 5.59%. We demonstrate that, despite differences in their donor:acceptor vertical concentration gradients, unencapsulated PCDTBT:PC71BM active layers push-coated for 1 min produce PSCs with similar operational stability and upscaling capacity as thermally annealed spin-coated ones. As fast device fabrication can be achieved with short-time push-coating, we further demonstrate the potential of this deposition technique by manufacturing push-coated PSC-based semitransparent photovoltaic devices with a PCE of 4.23%, relatively neutral colors and an average visible transparency of 40.2%. Our work thus confirms that push-coating is not limited to the widely employed poly(3-hexylthiophene-2,5-diyl) but can also be used with low band gap copolymers and opens the path to low-cost and eco-friendly, yet efficient and stable PSCs.

In-Operando Study of the Effects of Solvent Additives on the Stability of Organic Solar Cells Based on PTB7-Th:PC71BM

Degradation of organic solar cells is among the fundamental problems that hinder their successful breakthrough. We probe in-operando the degradation behavior of organic solar cells based on the high-efficiency low-bandgap benzodithiophene copolymer PTB7-Th and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The influence of the solvent additives 1,8-diiodooctane and o-chlorobenzaldehyde on the degradation mechanism is studied with in situ grazing incidence small-angle X-ray scattering during operation. Changes in the IV characteristics are correlated for the first time in situ with changes in the morphology showing that the use of a solvent additive causes a change in device degradation.

Green solvent-processed efficient non-fullerene organic solar cells enabled by low-bandgap copolymer donors with EDOT side chains

By a combination of side chain and ternary blend strategies, novel BDT-TT-based copolymer donors were developed to improve the green solvent solubility, crystallinity, energy level, carrier mobility and blend morphology. Non-fullerene binary and ternary blend devices based on PTB-EDOTS exhibited high PCEs of 10.18% and 12.26%, respectively.

Ultrafast excited state dynamics of a bithiophene‐isoindigo copolymer obtained by direct arylation polycondensation and its application in indium tin oxide‐free solar cells

ABSTRACTA low band gap copolymer, P2TI (Eg = 1.6.eV), with bithiophene as donor and isoindigo as acceptor units is designed and synthesized by the direct arylation polycondensation (DAP) method. Absorbance of the polymer spans from 300 to 780 nm. It exhibits an absorption coefficient (ε) of 96 L/g cm in solution at its maxima. A HOMO level of −5.42 eV and LUMO level of −3.72 eV is measured by square wave voltammetry. ITO‐free solar cells fabricated using a P2TI:PCBM71 bulk heterojunction shows a moderate efficiency of 1.02% with a high open circuit voltage of 0.81 V. An intramolecular charge transfer state is found in the relaxation of P2TI in solution, which is generated with a time constant of 2 ps as measured by femtosecond‐transient absorption spectroscopy. Charge carriers were generated in &lt;250 fs in P2TI:PCBM71 films. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1475–1483

New copolymers with fluorinated and non-fluorinated benzothiadiazole units for efficient single layer near infra-red photodiodes with fast time response

We describe the synthesis and photodetecting properties of new, low band-gap copolymers: P1 with 5,6-difluoro-2,1,3-benzothiadiazole and P2 with 2,1,3-benzothiadiazole, both containing diketopyrrolopyrrole acceptor units. These compounds have been used for fabrication of single layer, solution processed photodetectors. For this purpose, two blends with fullerene derivative (6,6)-phenyl-C61-butyric acid methyl ester ([60]PCBM): P1:[60]PCBM and P2:[60]PCBM, were prepared and applied as active layers in bulk-heterojunction photodiodes. For near infrared light (810 nm), these photodetectors, in spite of single layer structure, display competitive performance, showing specific detectivity up to 5·1011 Jones, responsivity up to 0.3 A/W and rise and fall times of the transient signals below 10 μs.

Photoexcitations dynamics in pristine low bandgap π-conjugated copolymers

We present a comprehensive review on the optical properties in pristine low bandgap copolymers, namely PTB7 and PDTP–DFBT, which are used as electron donors in copolymer/phenyl C71 butyric acid methyl ester blends for high efficiency solar cell devices. The copolymer backbone chain comprises of donor (D) and acceptor (A) moieties, which lower the band gap to the near-IR spectral region. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in D–A copolymers we find at short times coexistence of two primary photoexcitation species, namely SE and triplet–triplet (TT) pairs, which are directly photogenerated upon photon absorption from the ground state within 300 fs, our transient time resolution. Using the transient magnetophotoinduced absorption (t-MPA) spectroscopy, we reveal the spin coupling between the SE and TT spin states from their correlated t-MPA responses. In addition, we show that the TT species dissociates into two individual triplet excitons (TE) in the picosecond time scale; however, the two-geminate TEs are entangled and maintain their spin coherence into the microsecond time domain.

Influence of side chains on low optical bandgap copolymers based on 2,1,3-benzoxadiazole for polymer solar cells

Abstract Two copolymers based on 2,1,3-benzoxadiazole (BO) as the acceptor unit and quaterthiophene as the donor unit, which possess different steric hindrance in the backbone, were designed and synthesized. The backbone conformations and electronic structures as well as the theoretically optimal structures of both copolymers were calculated. The influences of steric hindrance on photophysical and electrochemical properties of the polymers were investigated. With the steric hindrance of the polymers decreased, the optical bandgaps were decreased, and the HOMO levels upshifted while the LUMO levels downshifted. The weight ratio and annealing temperature on the performances of the polymer-based devices were evaluated in detail. The best device based on the polymer exhibited a moderate power conversion efficiency of 4.44%.

Low bandgap diketopyrrolopyrrole-based polymers with an asymmetric unit of fluoridated phenylene-thiophene for efficient polymer solar cells

Two low bandgap copolymers (PDPP-PT and PDPP-PTF2) based on asymmetrical phenylene-thiophene as donor unit and diketopyrrolopyrrole as acceptor unit were developed for polymer solar cells. Their thermal, optical, electronic properties and photovoltaic performances were systematically investigated. Both polymers exhibited narrow bandgaps (below 1.5 eV) with a wide absorption range (350–900 nm). The introduction of the fluorine endows the polymer PDPP-PTF2 with better planarity, higher hole mobility and preferable morphology compared with its non-fluorinated analogue PDPP-PT. The polymer PDPP-PTF2 with asymmetric donor unit of fluoridated phenylene-thiophene displayed the power conversion efficiency of 3.0% with an open-circuit voltage of 0.68 V and a short circuit current of 7.24 mA cm–2. This study demonstrates that asymmetric unit of fluoridated phenylene-thiophene is promising building blocks for the development of conjugated polymers for high-performance optoelectronic devices.

Low band gap conductive copolymer of thiophene with p‐phenylenediamine and its magnetic nanocomposite: Synthesis, characterization and biosensing activity

Thiophene and p‐phenylenediamine were utilized as monomers to prepare low band gap copolymer and nanocomposites with core–shell construction by a simple in situ emulsion polymerization. Different devices such as Fourier transform infrared, X‐ray diffraction, scanning electron microscopy, thermal gravimetric analysis, dynamic light scattering and vibrating sample magnetometer (VSM) were used for characterizing the copolymer and nanocomposites. Band gaps were estimated from Cyclic voltammetry to be 2.1 − 1.7 eV for copolymers and nanocomposite. The results that obtained by measuring the conductivity with Four‐probe method illustrated that the electrical conductivity of the nanocomposites is considerably higher than the copolymer which is dependent on the Fe content and doping degree. The nanocomposites showed superparamagnetic behavior with high‐saturated magnetization (Ms = 10–16 emu/g). The direct electrochemistry of myoglobin (Mb) entrapped in the Nafion film on a glassy carbon electrode modified with synthesized nanocomposite had been investigated in this paper. Cyclic voltammetric results showed that a pair of well‐defined quasi‐reversible redox peaks appeared in the pH 7.0 phosphate buffer solution, which was ascribed to the direct electron transfer of Mb heme Fe(III)/Fe(II) redox couples. Also, the antioxidant activity of new nanocomposites was measured by using 2, 2‐diphenyl‐1‐picrylhydrazyl assay. POLYM. COMPOS., 40:1034–1042, 2019. © 2018 Society of Plastics Engineers

Polymer solar cells based on D–A low bandgap copolymers containing fluorinated side chains of thiadiazoloquinoxaline acceptor and benzodithiophene donor units

The increase in the fluorine atoms in the copolymer improves the power conversion efficiency of the polymer solar cell.