Donor-acceptor Type Copolymers Research Articles

High Performance Shortwave Infrared Organic Photodetectors Adopting Thiadiazole Quinoxaline‐Based Copolymers

AbstractDetection of light in the near‐infrared (NIR) and shortwave infrared (SWIR) spectral range is essential for the wide practical applications of biometrics, surveillance, machine vision, etc., in which organic molecules have gained growing interest as the potential sensing materials. In this study, the newly synthesized donor–acceptor type copolymers consisting of [1,2,5]thiadiazolo[3,4‐g]quinoxaline core unit with phenyl pendant groups as the acceptor and thiophene unit as the donor for achieving high‐efficiency organic SWIR photodetectors are presented. A further variation of the alkoxy chain lengths in the core unit allows the optimized device to exhibit a high external quantum efficiency of 19.5%–21.7% at 1200 nm under the low reverse bias of −1 V, leading to a high detectivity up to 2.96 × 1010 Jones for the spectral range beyond 1 µm. This further suggests the possible origin of the molecular structure‐dependent device performance in terms of the molecular stacking, morphological changes, and charge‐transporting properties in the bulk heterojunction blends.

Novel Dithienopyrrole-Based Conjugated Copolymers: Importance of Backbone Planarity in Achieving High Electrical Conductivity and Thermoelectric Performance.

The development of conjugated polymers with structures that are suitable for efficient molecular doping and charge transport is a key challenge in the construction of high-performance conjugated polymer-based thermoelectric devices. In this study, three novel conjugated polymers based on dithienopyrrole (DTP) are synthesized and their thermoelectric properties are compared. When doped with p-dopant, a donor-acceptor type copolymer, DPP-MeDTP, exhibits higher electrical conductivity and thermoelectric power factor compared to the other donor-donor type copolymers. The high electrical conductivity of DPP-MeDTP compared to the other polymers originates from the high degree of backbone planarity and molecular order, which contributes to its high charge carrier mobility. In addition, the highly crystalline structure of DPP-MeDTP is well maintained upon doping, while the crystalline order of the other polymers decreases significantly upon doping. The findings of this work not only provide insights into the design of DTP-based conjugated polymers for thermoelectric use but also demonstrate the significance of a high degree of molecular order and structural robustness upon doping to achieve high thermoelectric performance.

Optimistic exploitation of Poly(3-hexyl thiophene-benzothiadiazole) D-A copolymer for low Voc loss bulk heterojunction solar cells

A novel copolymer possessing greater potential to yield high open circuit voltage for photovoltaic devices is presented here. Donor-Acceptor (D-A) type copolymer, based on benzothiadiazole (BTZ) as acceptor unit and 3-hexylthiophene (3-HT) as donor unit, was designed, synthesize, characterized and its possibilities for solar cell applications were studied. Copolymers, synthesized using direct arylation, were undergone electrochemical, thermal, morphological studies along with the determination of its electronic structure. Potential of the copolymer to function as vital part of a photovoltaic device was analyzed by fabricating solar cells with basic device structure of ITO/ZnO/P(3-HT-BTZ):PCBM/Ag. Here the entire device fabrication except Ag deposition was undergone in an open atmosphere without using any glove box setup. Photovoltaic (PV) parameters of the device, especially Voc observed to have a strong dependence on copolymer-PCBM interfacial area, where the probability of electron-hole recombination is dominant. Device configuration having an optimum copolymer-PCBM interfacial area ensures efficient exciton dissociation and long charge carrier life time and yielded Voc of 1.85 V.

Versatile Solution-Processed Reductive Interface Layer for Contact Engineering of Staggered Organic Field-Effect Transistors

Efficient charge injection/extraction from/to contact electrodes is essential to realize organic electronic and optoelectronic devices with optimum characteristics for many applications. Herein, we studied a versatile reductive interlayer based on sodium borohydride (NaBH4) to control the contact properties of the staggered organic field-effect transistors (OFETs) either by doping and/or by regulating the contribution of charge carriers. The versatile functionalities of the NaBH4 layer are mainly determined by the alignment of frontier molecular orbitals of donor-acceptor (D-A) type copolymer semiconductors and the work function of the contact electrode. After incorporating the NaBH4 layer, the work function of the bottom-contact gold electrode can be decreased significantly by 1.0 eV, which makes it favorable to efficient electron injection. An Ohmic contact is achieved by the spontaneous injection of electrons to the n-type organic semiconductors with high electron affinity while converting the OFET operation mode to n-type characteristics by blocking the counter-charge carriers for the other types of ambipolar and p-type semiconductors. The solution-processed reducing agent can be a valuable approach to develop high-performance printed and flexible electronic devices through careful engineering to obtain proper contributions of charge carriers either as electrons or holes in various D-A copolymer semiconductors.

Semiconducting Polymer Nanowires with Highly Aligned Molecules for Polymer Field Effect Transistors

Conjugated polymers have emerged as promising materials for next-generation electronics. However, in spite of having several advantages, such as a low cost, large area processability and flexibility, polymer-based electronics have their own limitations concerning low electrical performance. To achieve high-performance polymer electronic devices, various strategies have been suggested, including aligning polymer backbones in the desired orientation. In the present paper, we report a simple patterning technique using a polydimethylsiloxane (PDMS) mold that can fabricate highly aligned nanowires of a diketopyrrolopyrrole (DPP)-based donor–acceptor-type copolymer (poly (diketopyrrolopyrrole-alt-thieno [3,2-b] thiophene), DPP-DTT) for high-performance field effect transistors. The morphology of the patterns was controlled by changing the concentration of the DPP-based copolymer solution (1, 3, 5 mg mL−1). The molecular alignment properties of three different patterns were observed with a polarized optical microscope, polarized UV-vis spectroscopy and an X-ray diffractometer. DPP-DTT nanowires made with 1 mg mL−1 solution are highly aligned and the polymer field-effect transistors based on nanowires exhibit more than a five times higher charge carrier mobility as compared to spin-coated film-based devices.

Donor-acceptor conjugated copolymer with high thermoelectric performance: A case study of the oxidation process within chemical doping

The doping process and thermoelectric properties of donor-acceptor (D-A) type copolymers are investigated with the representative poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene]3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno[3,4-b]thiophenediyl)) (PTB7-Th). The PTB7-Th is doped by FeCl3 and only polarons are induced in its doped films. The results reveal that the electron-rich donor units within PTB7-Th lose electrons preferentially at the initial stage of the oxidation and then the acceptor units begin to be oxidized at a high doping concentration. The energy levels of polarons and the Fermi level of the doped PTB7-Th remain almost unchange with different doping levels. However, the morphology of the PTB7-Th films could be deteriorated as the doping levels are improved, which is one of the main reasons for the decrease of electrical conductivity at the later stage of doping. The best electrical conductivity and power factor areobtained to be 42.3 S⋅cm−1 and 33.9 μW⋅mK−2, respectively, in the doped PTB7-Th film at room temperature. The power factor is further improved to 38.3 μW⋅mK−2 at 75 °C. This work may provide meaningful experience for development of D-A type thermoelectric copolymers and may further improve the doping efficiency.

Mixed Conduction in an N‐Type Organic Semiconductor in the Absence of Hydrophilic Side‐Chains

AbstractOrganic electrochemical transistors (OECTs) are the building blocks of biosensors, neuromorphic devices, and complementary circuits. One rule in the materials design for OECTs is the inclusion of a hydrophilic component in the chemical structure to enable ion transport in the film. Here, it is shown that the ladder‐type, side‐chain free polymer poly(benzimidazobenzophenanthroline) (BBL) performs significantly better in OECTs than the donor–acceptor type copolymer bearing hydrophilic ethylene glycol side chains (P‐90). A combination of electrochemical techniques reveals that BBL exhibits a more efficient ion‐to‐electron coupling and higher OECT mobility than P‐90. In situ atomic force microscopy scans evidence that BBL, which swells negligibly in electrolytes, undergoes a drastic and permanent change in morphology upon electrochemical doping. In contrast, P‐90 substantially swells when immersed in electrolytes and shows moderate morphology changes induced by dopant ions. Ex situ grazing incidence wide‐angle X‐ray scattering suggests that the particular packing of BBL crystallites is minimally affected after doping, in contrast to P‐90. BBL's ability to show exceptional mixed transport is due to the crystallites’ connectivity, which resists water uptake. This side chain‐free route for the design of mixed conductors could bring the n‐type OECT performance closer to the bar set by their p‐type counterparts.

Fluorination strategy enables greatly improved performance for organic solar cells based on polythiophene derivatives

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have reached 18% recently, which have already met the demand of practical application. However, these outstanding results were generally achieved with donor-acceptor (D-A) type copolymer donors, which can hardly fulfill the low-cost large-scale production due to their complicated synthesis processes. Therefore, developing polymer donors with simple chemical structures is urgent for realizing low-cost OSCs. Polythiophene (PT) derivatives are currently regarded as promising candidates for such kind of donor materials, which has been illustrated in many works. In this work, two new alkylthio substituted PT derivatives, P301 and P302, were synthesized and tested as donors in the OSCs using Y5 as the acceptor. In comparison, the introduction of fluorine atoms on the backbone of P302 can not only downshift the energy levels, but also greatly improve the phase separation morphologies of the active layers, which is ascribed to the enhanced aggregation effect and the reduced miscibility with the non-fullerene acceptor. As a result, the P302:Y5-based OSC exhibits a significantly improved PCE of 9.65% than that of P301:Y5-based one, indicating the important role of fluorination in the construction of efficient PT derivative donors.

Theoretical Designing of Novel Donor Acceptor Type Copolymer Comprising of Thiophene

Using ab initio band structure results of three novel donor acceptor polymers (A)x PCDT, (B)x PMCT and (C)x PFTh as the input, the electronic structures and conduction properties of their periodic and aperiodic copolymer (AmBnCk)x have been investigated. The method involves using negative factor counting method based on Dean’s negative eigenvalue theorem. In this article, the quasi-one-dimensional Type II staggered copolymers comprising of thiophene units on the basis of the band alignments of the constituent homopolymers were studied. The trends in their electronic structures and conduction properties as a function of (i) block sizes (m, n, k) and (ii) arrangement of the blocks (periodic or aperiodic) in the various copolymer chains are discussed. These trends are important guidelines to the experimentalists for designing novel electrically conducting polymers with tailor made conduction properties.

Enhancement the photovoltaic performance of conjugated polymer based on simple head-to-head alkylthio side chains engineered bithiophene

In this article, three novel and simple molecular structure with donor-acceptor (D-A) type copolymers via only head-to-head alkoxy (OR) and/or alkylthio (SR) side chains onto the bithiophene (BT) as donor units and fluorinated benzotriazole (FBTA) as acceptor unit, namely, PBTOR-FBTA, PBTOSR-FBTA and PBTSR-FBTA, were successfully designed and synthesized. The impacts of sulfur-oxygen (S⋯O) or sulfur-sulfur (S⋯S) noncovalent interactions on their physicochemical properties, molecular stacking, carrier mobility, morphologies of blend films, as well as their photovoltaic performance were deeply and systematically studied. The introduction of SR side-chains suddenly lowered the highest occupied molecular orbital (HOMO) energy levels, blue-shifted absorption, enhanced π-π stacking, as well as improved morphology of the photoactive layer blends in comparison with the reference polymer without SR side-chain. Polymer solar cells (PSCs) were fabricated to estimate their photovoltaic performance of the polymers. Under an optimized blend ratio of PBTSR-FBTA:PC71BM (1:0.8, w/w), the PBTSR-FBTA-based device exhibits a higher power conversion efficiency (PCE) of 6.25%, which is about 3.34 and 1.87 folds than that of the PBTOR-FBTA and PBTOSR-FBTA-based devices, respectively. Our research results demonstrate that the modification of the simple and low-cost SR side chains is an effective strategy to improve the photovoltaic performance of the polymers.

The impact of [1,2,5]chalcogenazolo[3,4‐f]‐benzo[1,2,3]triazole structure on the optoelectronic properties of conjugated polymers

AbstractTwo novel conjugated near‐infrared (NIR) absorbing donor–acceptor type copolymers comprising benzodithiophene as the donor and [1,2,5]chalcogenazolo[3,4‐f]‐benzo[1,2,3]triazole derivatives as the acceptors, spaced with thiophene as the π‐bridge, were designed and synthesized via Stille polycondensation reaction. The effect of acceptor strength on optoelectronic properties was targeted and investigated. Branched alkyl chains (the extended 2‐octyl‐1‐dodecyl alkyl chain; C8C12) were introduced to 5H‐[1,2,3]triazolo[4′,5′:4,5]benzo[1,2‐c][1,2,5]thiadiazole and 5H‐[1,2,3]triazolo[4′,5′:4,5]benzo[1,2‐c][1,2,5]selenadiazole for enhanced solubility of polymers which ease the processability hence device constructions. The strongly electron‐withdrawing units lead to a substantial change in the absorption properties via promotion of the intramolecular charge transfer band alongside the π–π* transition. The resultant soluble polymers were characterized via cyclic voltammetry to determine highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels as −5.00 and −3.92 eV for PSBT and −4.86 and −4.04 eV for PSeBT, respectively. Electronic band gaps of the copolymers were calculated as 1.08 eV for PSBT and 0.82 eV for PSeBT, respectively. NIR absorbing copolymers were used to construct electrochromic devices.

Thienoquinolinone as a new building block for wide bandgap semiconducting polymer donors for organic solar cells

A new heterocyclic aromatic structure, thieno[3,2-c]quinolin-4(5H)-one (TQO), was designed and synthesized as an electron-accepting building block for donor–acceptor-type copolymers.

Synthesis and characterization of soluble donor-acceptor type copolymers based on benzotriazole, quinoxaline and benzene units with multicolor electrochromism

Abstract Three soluble donor-acceptor (D-A) type copolymers employing benzene (B) as donor, 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo [d][1,2,3]triazole (Z) and 2,3-bis((4-(2-octyldodecyl)oxy)phenyl)-quinoxaline (Q) as acceptors were synthesized through chemical polymerization. A variety of characterization methods such as cyclic voltammetry, UV–vis spectroscopy, colorimetry and thermogravimetric analysis were executed to detect the electrochromic properties of polymers. All the polymers displayed multicolor in the redox process with medium band gaps, and different molar ratio of B/Z/Q allowed them to cover diverse color changes, containing orange-red/brown-yellow/cyan/green (PBZQ-1), orange-red/yellow/light grass green (PBZQ-2), and red/black/grey-blue (PBZQ-3). The multichromism of the polymers involved RGB and black colors. Meanwhile, with the increase of Z unit ratio and the decrease of Q unit ratio, the polymer demonstrated the reduced onset oxidation potential and optical band gap, as well as the different kinetic parameters. Moreover, the three polymers exhibited good solubility, desirable thermal stability, relatively large optical contrast and high coloration efficiency. The above positive results implied that these copolymers were expected to be the credible candidates for the electrochromic devices with commercial values.

Use of surface photo-reactive nanometal printing for polymer thin-film transistors: contact resistance and short-channel effects

Abstract

Synthesis of a new wide-bandgap conjugated copolymer with 3-trifluoromethylthiophene monomer and Its application to non-fullerene polymer solar cells

In this study, a novel wide-bandgap conjugate donor-acceptor type copolymer, PBDT-TFMTh, containing benzodithiophene (BDT) as a donor unit and trifluoromethyl (TFM)-substituted thiophene (Th) as an acceptor unit was designed and synthesized. The physical, optical, and electrochemical properties of PBDT-TFMTh were studied in detail and it was found to have lower-lying highest occupied and lowest unoccupied molecular orbitals. The polymer solar cell with an active layer consisting of PBDT-TFMTh and acceptor IDT(TCV)2 showed a relatively high power conversion efficiency of 2.25% after thermal annealing at 120 °C.

Photophysical and Photovoltaic Characterization of Flourene-Anthracene-Benzothiadiazole Based Donor–Acceptor Type Copolymers for Bulk Heterojunction Polymer Solar Cells

Photophysical and Photovoltaic Characterization of Flourene-Anthracene-Benzothiadiazole Based Donor–Acceptor Type Copolymers for Bulk Heterojunction Polymer Solar Cells

Acceptor Unit Effects for Ambipolar Organic Field-Effect Transistors Based on TIPS-Benzodithiophene Copolymers

Two narrow band gap triisopropylsilyl substituted benzo[1,2-b:4,5-b] dithiophene (TIPS-BDT) derivatives, P1 (1.65 eV) and P2 (1.46 eV) are synthesized for ambipolar organic field-effect transistors and complementary inverters. Two electron acceptor units, heptadecanyl substituted thieno[3,4-c]pyrrole-4,6-dione (TPD) and ethylhexyl substituted diketopyrrolo[3,4-c]pyrrole (DPP) are incorporated to tune the structure and resulting properties of the donor-acceptor type copolymers. Structural modification based on the acceptor unit variation, resulted in comparable electrochemical, optical, microstructural, and charge transporting properties, as well as environmental and operational stability. TIPS-BDT copolymers with TPD acceptor units show comparatively superior performance, with field effect mobility ∼10-3 cm2V-1s-1 for both holes and electrons and inverter gain ∼18 with poly(methyl methacrylate) gate dielectric.

Synthesis and characterization of a donor-acceptor type copolymer containing ester-functionalized benzo[c][1,2,5]thiadiazole as an accepting building block for organic photovoltaics

The benzo[c][1,2,5]thiadiazole (BT) based copolymer, poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-alt-(diethyl 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6-dicarboxylate)] (PBDTBT-COOR), was synthesized by Stille polycondensation. The BT unit with carboxylate ester groups was designed and synthesized as an electron-withdrawing building block for donor-acceptor type copolymers with the aim of achieving good solubility and low-lying highest occupied molecular orbital energy levels of the synthesized polymer. Consequently, the synthesized PBDTBT-COOR copolymer showed excellent solubility in common organic solvents and a relatively deep HOMO energy level. An inverted organic photovoltaic device was fabricated using PBDTBT-COOR and [6,6]-phenyl C71 butyric acid methyl ester as the electron donor and the acceptor, respectively, with the device structure of indium tin oxide/ZnO/active layer/MoO3/Ag. Under the illumination condition of AM 1.5 G, i.e., 100 mW/cm2, the maximum power conversion efficiency of the device was 3.15% with an open-circuit voltage of 0.87 V, short circuit current of 9.68 mA/cm2, and a fill factor of 37.6%.

A Highly Efficient Near-Infrared-Emissive Copolymer with a N=N Double-Bond π-Conjugated System Based on a Fused Azobenzene-Boron Complex.

Fused azobenzene-boron complexes (BAzs) show highly efficient near-infrared (NIR) emission from the nitrogen-nitrogen double bond (N=N) containing π-conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron-nitrogen (B-N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor-acceptor (D-A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λPL =751 nm, ΦPL =0.25) and in the film (λPL =821 nm, ΦPL =0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation-induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π-conjugated materials.

(Invited) Thermoelectric Properties of π-Conjugated Polymers in Ionic-Liquid-Gated Transistors

Thermal energy havesting from low-temperature heat sources, such as human body, is a promissing method to feed energy to vast number of micro-devices and sensors in Internet of Things (IoT) technologies. Thermoelectric generator, used for this purpose, can directly convert heat into electric power through Seebeck effect. In recent years, organic semiconductor materials, such as π-conjugated polymers, have been attracting much attention as a potential candidate for light-weight, flexible, and low-toxic thermoelectric generators. However, thermoelectric performance of polymer-based devices are still low compared to that of inorganic materials, although various novel polymers exhibitting high mobilities have been synthesized so far. The possible reason for this is the absence of both the suitable doping method and the fundamental understanding of thermoelectric properties in these polymer materials. In general, the power factor (P) of a thermoelectric device is described as a product of the electrical conductivity (σ) and Seebeck coefficient (S) as P=S2σ, whereas these two quantities tend to show opposite behavior upon carrier doping. Thus, it is indispensable to clarify the accurate relationship between σ and S in doped films in order to obtain a guideline to improve P. In semicrystalline polymers such as PBTTT (Fig. 1(a)), an empirical relation of S∝σ -1/4 (P∝σ 1/2) has been reported in a wide range of σ by using chemical doping, although no physical background for this relation has been clarified [1]. However, different σ-S relation has also been pointed out recently based on the data analyses of various reports including the same data set in ref. 1 [2]. Such difference seems to arise from the different doping conditions in chemical doping. In this presentation, we report our recent challenge to determine the accurate σ-S relation in various high-mobility polymer materials by using an electrolyte-gated thin-film transistor (TFT) structure, which enables a continuous electrochemical doping on the same device up to high carrier densities. Fig. 1(b) shows a schematic illustration of the TFT device and measurement setup [3]. We adopted an ionic liquid as the electrolyte, which was drop-casted on the polymer thin film fabricated on the glass substrate. A temperature gradient was given by a couple of Peltier devices and the induced voltage (ΔV) and temperature difference (ΔT) between source (S) and drain (D) electrodes were measured to obtain seebeck coefficient (S=ΔV/ΔT) under various gate voltages (V g). The electrical conductivity was simultaneously measured at each V g. Fig. 1(c) shows an example of σ-S and σ-P relations upon electrochemical doping obtained for two PBTTT devices. We observe highly reproducible data for these devices. In the low conductivity region (σ<100 S/cm), the empirical relation of S∝σ -1/4 is clearly confirmed, whereas the deviation from this relation becomes evident in the high conductivity region (σ>100 S/cm). The maximum of P appears around the boundary of low and high conductivity regions. In the high conductivity region, the σ-S relation seems to approach S∝σ -1, typical to the Mott’s relation expected for metals. This behavior is consistent with the semiconductor-metal transition of PBTTT occuring at highly-doped regions, which is observed from the temperature dependence of the conductivity as well as the electron spin resonance (ESR) measurements. Similar σ-S relation was commonly observed in a wide range of polymers such as donor-acceptor (DA) type copolymers. The maximum value of P can be enhanced by the surface treatment of the substrate by self-assembled monolayers or by aligning the polymer main chain, presumably due to the increase of the mobility, although the S∝σ -1/4 relation is unchanged in the low conductivity regions. The above σ-S relation insensitive to the mobility is indicative of the contribution of domain boundaries in thermoelectric properties. [1] A. M. Glaudell et al., Adv. Energy Mater. 5, 1401072 (2015). [2] S. D. Kang et al., Nat. Mater. 16, 252 (2017). [3] J. Pu et al., Phys. Rev. B. 94, 014312 (2016). Figure 1