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

Enhancing Optoelectronic Anisotropy in Highly Oriented Thin Films by Fluorine Substitution in Novel Semiconducting Polymers

The recent past has witnessed remarkable progress in organic electronics, driven by the quest for flexible, lightweight, and cost-effective electronic devices. Semiconducting polymers (SCPs) have emerged as key materials in this field, offering unique electronic and optoelectronic properties along with mechanical flexibility. This study focuses on designing, synthesizing, and utilizing novel donor-acceptor (D-A) copolymer-based SCPs introducing a difluorothiophene moiety in the polymeric backbone. The importance of fluorine substitution for backbone planarity was verified by density functional theory calculations, comparing it with a nonfluorine substituted counterpart. Through the Unidirectional Floating Film Transfer Method (UFTM), we fabricated highly oriented thin films, resulting in increased optical anisotropy with dichroic ratios reaching 19.3 in PC20-FT thin films, one of the highest optical anisotropy observed for solution processable SCP thin films. X-ray diffraction and atomic force microscopy results validated the increase in the crystallinity and domain size with the increasing alkyl chain length. Finally, we elucidate these findings in the context of electrical applications by fabricating organic field-effect transistors revealing anisotropic charge transport achieving a promising mobility of 1.24 cm2V-1s-1 and mobility anisotropy of 39.5. This study offers insights into the design principles and performance optimization of SCP-based devices, paving the way for advancements in plastic electronics.

Donor-Acceptor Copolymer with a Linear Backbone Induced Ordered and Robust Doping Morphology for Efficient and Stable Organic Electrochemical Devices

Donor (D)-acceptor (A) copolymer-based organic mixed ionic-electronic conductors (OMIECs) exhibit intrinsic environmental stability for they have tailored energy levels. However, their figure-of-merit (μC*) is still falling behind the D-D polymers because of morphology deterioration during the electrochemical doping process. Herein, we developed two D-A copolymers with precisely regulated backbone curvature, namely PTBT-P and PTTBT-P. Compared to the curved PTBT-P and previously reported copolymers, PTTBT-P better keeps its backbone linear, leading to a long-range ordered doping morphology, which is revealed by the in operando X-ray technique. This optimized doping morphology enables a significantly improved operando charge mobility (μ) of 2.44 cm2 V-1 s-1 and a μC* value of 342 F cm-1 V-1 s-1, one of the highest values in D-A copolymer based on OECTs. Besides, we fabricated PTTBT-P-based electrochemical random-access memories and achieved ideal and robust conductance modulation. This study highlights the critical role of backbone curvature control in the optimization of doping morphology for efficient and robust organic electrochemical devices.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Design strategies for improving the crystallinity of covalent organic frameworks and conjugated polymers: a review

Highly crystalline covalent organic frameworks (COFs) or conjugated polymers (CPs) are very important and highly desirable because these materials would display better performance in diverse devices and provide more structure-property related information. However, how to achieve highly crystalline or single-crystal COFs and CPs is very challenging. Recently, many research studies have demonstrated the possibility of enhancing the crystallinity of COFs and CPs. Thus, it is timely to offer an overview of the important progress in improving the crystallinity of COFs and CPs from the viewpoint of design strategies. These strategies include polycondensation reaction optimization, improving the planarity, fluorine substitution, side chain engineering, and so on. Furthermore, the challenges and perspectives are also discussed to promote the realization of highly crystalline or single-crystal COFs and CPs.

Morphological Studies of Solution-Crystallized Thermoplastic Elastomers with Polyethylene Endblocks and a Random-Copolymer Midblock

Styrenic thermoplastic elastomers (TPEs) in the form of triblock copolymers possessing glassy endblocks and a rubbery midblock account for the largest global market of TPEs worldwide, and typically rely on microphase separation of the endblocks and the subsequent formation of rigid microdomains to ensure satisfactory network stabilization. In this study, the morphological characteristics of a relatively new family of crystallizable TPEs that instead consist of polyethylene endblocks and a random-copolymer midblock composed of styrene and (ethylene-co-butylene) moieties are investigated. Copolymer solutions prepared at logarithmic concentrations in a slightly endblock-selective solvent are subjected to crystallization under different time and temperature conditions to ascertain if copolymer self-assembly is directed by endblock crystallization or vice versa. According to transmission electron microscopy, semicrystalline aggregates develop at the lowest solution concentration examined (0.01 wt%), and the size and population of crystals, which dominate the copolymer morphologies, are observed to increase with increasing aging time. Real-space results are correlated with small- and wide-angle X-ray scattering to elucidate the concurrent roles of endblock crystallization and self-assembly of these unique TPEs in solution.

Enhancing the Photovoltaic Performance of a Benzo[c][1,2,5]thiadiazole-Based Polymer Donor via a Non-Fullerene Acceptor Pairing Strategy

As a well-known electron-withdrawing group, benzo[c][1,2,5]thiadiazole (BT) has been intensively studied and adopted to construct polymer donors with tunable band gaps. However, polymer solar cells (PSCs) with BT-based polymer donors, limited by the weak absorption and inflexible energy level of fullerene derivatives, usually suffer mediocre power conversion efficiencies (PCEs). Here, through subtly tailoring a BT unit with asymmetric fluoro and alkyloxy groups and judiciously pairing a BT-based polymer donor with three narrow band gap non-fullerene acceptors (e.g., IEICO-4F, ITOIC-2F, and IDTCN-O), active layers with complementary absorption spectra, small lowest unoccupied molecular orbital (LUMO) offsets, and preferred morphologies have been achieved. Consequently, PSCs with excellent J_sc values (over 20 mA/cm²) and high PCEs up to 12.33% have been obtained. To the best of our knowledge, the value of 12.33% is among the highest PCEs for BT-based polymers in binary PSCs so far. This work demonstrates that the cooperative effect of energy levels, absorption spectra, and morphologies between the donors and acceptors is crucial for governing the performance of organic photovoltaics.

π-Conjugated Polymers Incorporating a Novel Planar Quinoid Building Block with Extended Delocalization and High Charge Carrier Mobility

Two novel conjugated polymers incorporating quinoidal thiophene are successfully synthesized. By combining 1D nuclear magnetic resonance (NMR) and 2D nuclear Overhauser effect spectroscopy analyses, the isomeric form of the major quinoid monomer is clearly identified as the asymmetric Z, E-configuration. The quinoidal polymers are synthesized via Stille polymerization with thiophene or bithiophene. Both quinoidal polymers exhibit the low band gap of 1.45 eV and amphoteric redox behavior, indicating extended conjugation owing to the quinoidal backbone. These quinoidal polymers show ambipolar behaviors with high charge carrier mobilities when applied in organic field-effect transistors. In addition, the radial alignment of polymer chains achieved by off-center spin-coating leads to further improvement of device performance, with poly(quinoidal thiophene-bithiophene) exhibiting a high hole mobility of 8.09 cm² V^-1 s^-1 , which is the highest value among the quinoidal polymers up to now. Microstructural alteration via thermal annealing or off-center spin-coating is found to beneficially affect charge transport. The enhancement of crystallinity with strong π-π interactions and the nanofibrillar structure arising from planar well-delocalized quinoid units is considered to be responsible for the high charge carrier mobility.

Alkoxybenzothiadiazole-Based Fullerene and Nonfullerene Polymer Solar Cells with High Shunt Resistance for Indoor Photovoltaic Applications

We synthesized three semicrystalline polymers (PTTBT_BO, PDTBT_BO, and P2FDTBT_BO) by modulating the intra- and intermolecular noncovalent Coulombic interactions and investigated their photovoltaic characteristics under various light intensities. Low series (R_s) and high shunt (R_sh) resistances are essential prerequisites for good device properties under standard illumination (100 mW cm^-2). Considering these factors, among three polymers, PDTBT_BO polymer solar cells (PSCs) exhibited the most desirable characteristics, with peak power conversion efficiencies (PCE) of 7.52 and 9.60% by being blended with PC₇₁BM under standard and dim light (2.5 mW cm^-2), respectively. P2FDTBT_BO PSCs exhibited a low PCE of 3.69% under standard light due to significant charge recombination with high R_s (9.42 Ω cm²). However, the PCE was remarkably improved by 2.3 times (8.33% PCE) under dim light, showing negligible decrease in open-circuit voltage and remarkable increase in fill factor, which is due to an exceptionally high R_sh of over 1000 kΩ cm². R_s is less significant under dim light because the generated current is too small to cause noticeable R_s-induced voltage losses. Instead, high R_sh becomes more important to avoid leakage currents. This work provides important tips to further optimize PSCs for indoor applications with low-power electronic devices such as Internet of things sensors.

Effects on Photovoltaic Performance of Dialkyloxy-benzothiadiazole Copolymers by Varying the Thienoacene Donor

A series of four donor-acceptor alternating copolymers based on dialkyloxy-benzothiadiazole (ROBT) as an acceptor and thienoacenes as donor units were synthesized and tested for polymer solar cells (PSCs). These new polymers had different donor units with varied electron-donating ability (thieno[3,2-b]thiophene (TT), dithieno[3,2-b:2',3'-d]thiophene (DTT), benzo[1,2-b:4,5-b']dithiophene (BDT), and naphtha[1,2-b:5,6-b']dithiophene (NDT)) in the polymer backbone. To understand the effect of these thienoacenes on the optoelectronic and photovoltaic properties of the copolymers, we systematically analyzed and compared the energy levels, crystallinity, morphology, charge recombination, and charge carrier mobility in the resulting polymers. In this series, optimized photovoltaic cells yielded power conversion efficiency (PCE) values of 6.25% (TT), 9.02% (DTT), 6.34% (BDT), and 2.29% (NDT) with different thienoacene donors. The introduction of DTT into the thienoacene-ROBT polymer enabled the generation of well-ordered molecular packings with a π-π stacking distance of 3.72 Å, high charge mobilities, and an interconnected nanofibrillar morphology in blend films. As a result, the PSC employing the polymer with DTT exhibited the highest PCE of 9.02%. Thus, our structure-property relationship studies of thienoacene-ROBT-based polymers emphasize that the molecular design of the polymers must be carefully optimized to develop high efficient PSCs. These findings will help us to understand the impact of the donor thienoacene on the optoelectronic and photovoltaic performance of polymers.

Charge delocalization characteristics of regioregular high mobility polymers

Controlling the regioregularity among the structural units of narrow bandgap conjugated polymer backbones has led to improvements in optoelectronic properties, for example in the mobilities observed in field effect transistor devices. To investigate how the regioregularity affects quantities relevant to hole transport, regioregular and regiorandom oligomers representative of polymeric structures were studied using density functional theory. Several structural and electronic characteristics of the oligomers were compared, including chain planarity, cation spin density, excess charges on molecular units and internal reorganizational energy. The main difference between the regioregular and regiorandom oligomers is found to be the conjugated backbone planarity, while the reorganizational energies calculated are quite similar across the molecular family. This work constitutes the first step on understanding the complex interplay of atomistic changes and an oligomer backbone structure toward modeling the charge transport properties.

Thiophene-benzothiadiazole based D–A1–D–A2 type alternating copolymers for polymer solar cells

A series of D–A₁–D–A₂ type regioregular copolymers based on difluorobenzothiadiazole (DFBT) and dialkoxybenzothiadiazole (ROBT) structures for high performance PSCs.

Simultaneous enhancement of performance and insensitivity to active layer thickness for OPVs by functionalizing π-spacer's side chain

A novel BTI based copolymer was designed by incorporating a decylthio modified π-spacer into the backbone for improving the photovoltaic performance.

Straight chain D–A copolymers based on thienothiophene and benzothiadiazole for efficient polymer field effect transistors and photovoltaic cells

Fine modulation of chain linearity suggests an effective way to control interchain ordering for specific application in polymer solar cells or field effect transistors.

Effects of Regioregularity and Molecular Weight on the Growth of Polythiophene Nanofibrils and Mixes of Short and Long Nanofibrils To Enhance the Hole Transport

Morphological control over polythiophenes has been widely studied; however the impacts of regioregularity (RR) and molecular weight (MW) on their structural development have not been investigated systematically. This study examined a representative polythiophene, poly(3-hexylthiophene) (P3HT), to reveal that small differences in the RR can produce a large difference in the growth of nanofibrils. Low-RR P3HTs generated neat long nanofibrils (LNFs), whereas high-RR P3HTs formed short nanofibrils (SNFs). This study identified a critical RR (96-98%) depending on their MW, below which P3HT grew into LNFs and above which P3HT grew into SNFs. This study also found that the mixing ratio between high-RR P3HT and a low-RR P3HT in the solution phase is strongly correlated with the relative populations of SNF and LNF in the coated film. This study suggested that mixing high-RR and low-RR polymers may be a good strategy to optimize the electrical properties of polythiophenes for target applications. As an example, a mixture of high-RR (75%) P3HT and low-RR P3HT (25%) improved considerably the power conversion efficiency of bulk heterojunction polymer solar cells compared with the values obtained from the pure high-RR P3HT and the pure low-RR P3HT.

High-performance all-polymer solar cells via side-chain engineering of the polymer acceptor: the importance of the polymer packing structure and the nanoscale blend morphology

The effectiveness of side-chain engineering is demonstrated to produce highly efficient all-polymer solar cells (efficiency of 5.96%) using a series of naphthalene diimide-based polymer acceptors with controlled side chains. The dramatic changes in the polymer packing, blend morphology, and electron mobility of all-polymer solar cells elucidate clear trends in the photovoltaic performances.

Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules

Small molecules with alkyl side chains of different lengths were prepared with 2,2′-bithiophene, terthiophene and thiobarbituric acid as the central core, spacer and end-cap. Uniform, shorter chain lengths gave stronger intermolecular interactions, favoring crystallization.

Alkylidenefluorene–isoindigo copolymers with an optimized molecular conformation for spacer manipulation, π–π stacking and their application in efficient photovoltaic devices

D–(π)_n-A-type copolymers with different thienyl spacers (n = 0–2) between alkylidenefluorene and isoindigo (P1, P2, P3) were synthesized via a Suzuki coupling reaction.

Effect of chain curvature on the performance of diketopyrrolopyrrole-based polymer solar cells

Two polymer semiconductors with different degrees of chain curvature are designed and synthesized. The curved polymer blended with PC₇₁BM exhibits a higher PCE of 5.3% than that of a linear polymer.

Low band-gap weak donor–strong acceptor conjugated polymer for organic solar cell

With an additional weak acceptor, the low band-gap donor–acceptor conjugated polymer displayed a remarkable power conversion efficiency of 5.36%.

Amorphous thieno[3,2-b]thiophene and benzothiadiazole based copolymers for organic photovoltaics

Three types of amorphous thienothiophene (TT)-benzothiadiazole (BT) based copolymers (PFTTBT) were synthesized by incorporating alkyl-substituted fluorene moieties as a third component in the polymer backbone. Their optical, electrochemical, morphological, and photovoltaic properties were examined by a comparison with those of a crystalline TT-BT derivative (PTTBT14). PTTBT14 was reported to have a high hole mobility (0.26 cm(2)/(V s)) due to the pronounced interchain ordering but poor photovoltaic power conversion efficiency (PCE) of 2.4-2.6% was reported due to excessively strong self-interactions with poor miscibility with fullerene structures. By incorporating fluorene units, the UV-vis spectra showed an increased bandgap (∼1.9 eV) with the disappearance of the packing-originated shoulder peak, and the valence band decreased compared to crystalline PTTBT14. The amorphous PFTTBT polymers showed substantially improved photovoltaic properties compared to PTTBT14, even though they showed poor hole mobility (∼10(-6) cm2/(V s)) and fill factor. The optimal devices were achieved by blending with excess PC71BM (polymer:PC71BM=1:4 by weight), showing little improvement in the thermal and additive treatments. Under simulated solar illumination of AM 1.5 G, the best PCE of 6.6% was achieved for a PFehTTBT:PC71BM device with an open-circuit voltage of 0.92 V, a short-circuit current of 15.1 mA/cm2, and a fill factor of 0.48. These results suggest that it is useful to disrupt partially the interchain organizations of excessively crystalline polymers, enabling fine-control of intermolecular ordering and the morphological properties (i.e., miscibility with fullerene derivatives, etc.) to utilize the advantages of both crystalline and amorphous materials for further improving PCE of polymer solar cells.