Optimizing the Crystallization Behavior and Film Morphology of Donor–Acceptor Conjugated Semiconducting Polymers by Side-Chain–Solvent Interaction in Nonpolar Solvents

Increasing the Content of Edge-On Orientation to Improve the Hole Mobility of IDTBT Film by the van der Waals Interaction between the Side Chain and Alkane Additives

The molecular orientation in the conjugated polymer film critically impacts the performance of organic electronic devices. As for organic field-effect transistors (OFETs), the edge-on orientation is beneficial for efficient interchain charge transport. However, for the near amorphous indacenodithiophene-co-benzothiadiazole (IDTBT) film, the face-on orientation was dominated in the drop cast thin film. Herein, we proposed a strategy to increase the edge-on orientation in IDTBT films by inserting n-hexadecane (16C) additives with a high boiling point between the side chains. Because the 16C additives had a similar structure to that of the side chain, the 16C additives were distributed around the side chain in the nucleation stage. Then, the backbone aggregated quickly with the nuclei formation speed (k) nearly twice as high as that without the 16C additives. In the growth stage, some face-on nuclei turned to edge-on nuclei due to the van der Waals interactions between the side chain and 16C additives. This was verified by the results of in situ GIWAXS, where the intensity of (010) decreased and the intensity of (100) increased in the out-of-plane direction. The edge-on orientation content of the film with 16C additives reached 57%, higher than that of the neat film (33%). Moreover, compared to the neat IDTBT film, the lamellar packing distance of the film with 16C additives increased the lamellar packing distance from 16.97 to 23.26 Å and the π-π stacking distance decreased from 4.36 to 4.19 Å. The hole mobility of the film with 16C additives (3.24 ± 0.19 cm2 V-1 s-1) was higher than that of the neat film (0.94 ± 0.07 cm2 V-1 s-1).

A High-Performance Organic Photovoltaic System with Versatile Solution Processability

Recently developed organic photovoltaic (OPV) materials have simultaneously closed the gaps in efficiency, stability, and cost for single-junction devices. Nonetheless, the developed OPV materials still pose big challenges in meeting the requirements for practical applications, especially regarding the prevalent issues of solution processability. Herein, a highly efficient polymer donor, named DP3, incorporating an electron-rich benzo[1,2-b:4,5-b']dithiophene unit as well as two similar and simple acceptor units is presented. Its primary objective is to enhance the interchain and/or intrachain interactions and ultimately fine-tune bulk-heterojunction microstructure. The DP3:L8-BO system demonstrates the highest power conversion efficiency (PCE) of 19.12%. This system also exhibits high-performance devices with over 18% efficiencies for five batches with various molecular weights (23.6-80.8 KDa), six different blend thicknesses (95-308 nm), differenced coating speeds (3.0-29.1 m min-1), with promising PCEs of 18.65% and 15.53% for toluene-processed small-area (0.029 cm2) cells and large-area (15.40 cm2) modules, thereby demonstrating versatile solution processability of the designed DP3:L8-BO system that is a strong candidate for commercial applications.

Alcohol-Processable All-Polymer n-Type Thermoelectrics

The general strategy for n-type organic thermoelectric is to blend n-type conjugated polymer hosts with small molecule dopants. In this work, all-polymer n-type thermoelectric is reported by dissolving a novel n-type conjugated polymer and a polymer dopant, poly(ethyleneimine) (PEI), in alcohol solution, followed by spin-coating to give polymer host/polymer dopant blend film. To this end, an alcohol-soluble n-type conjugated polymer is developed by attaching polar and branched oligo (ethylene glycol) (OEG) side chains to a cyano-substituted poly(thiophene-alt-co-thiazole) main chain. The main chain results in the n-type property and the OEG side chain leads to the solubility in hexafluorineisopropanol (HFIP). In the polymer host/polymer dopant blend film, the Coulombic interaction between the dopant counterions and the negatively charged polymer chains is reduced and the ordered stacking of the polymer host is preserved. As a result, the polymer host/polymer dopant blend exhibits the power factor of 36.9 µW m-1 K-1, which is one time higher than that of the control polymer host/small molecule dopant blend. Moreover, the polymer host/polymer dopant blend shows much better thermal stability than the control polymer host/small molecule dopant blend. This research demonstrates the high performance and excellent stability of all-polymer n-type thermoelectric.

Polymer Fiber Rigid Network with High Glass Transition Temperature Reinforces Stability of Organic Photovoltaics

Organic photovoltaics (OPVs) need to overcome limitations such as insufficient thermal stability to be commercialized. The reported approaches to improve stability either rely on the development of new materials or on tailoring the donor/acceptor morphology, however, exhibiting limited applicability. Therefore, it is timely to develop an easy method to enhance thermal stability without having to develop new donor/acceptor materials or donor-acceptor compatibilizers, or by introducing another third component. Herein, a unique approach is presented, based on constructing a polymer fiber rigid network with a high glass transition temperature (Tg) to impede the movement of acceptor and donor molecules, to immobilize the active layer morphology, and thereby to improve thermal stability. A high-Tg one-dimensional aramid nanofiber (ANF) is utilized for network construction. Inverted OPVs with ANF network yield superior thermal stability compared to the ANF-free counterpart. The ANF network-incorporated active layer demonstrates significantly more stable morphology than the ANF-free counterpart, thereby leaving fundamental processes such as charge separation, transport, and collection, determining the device efficiency, largely unaltered. This strategy is also successfully applied to other photovoltaic systems. The strategy of incorporating a polymer fiber rigid network with high Tg offers a distinct perspective addressing the challenge of thermal instability with simplicity and universality.

Unraveling the Molecular Weight Dependence of High Magnetic Field to Manipulate the Semiconducting Polymer Molecular Orientation

The magnetic alignment of molecules, which exploits the anisotropy of diamagnetic susceptibility, provides a clean and versatile approach to the structural manipulation of semiconducting polymers. Here, the magnetic-alignment dynamics of two molecular-weight (MW) batches of a diketopyrrolopyrrole (DPP)-based copolymer (PDVT-8) were investigated. Microstructural characterizations revealed that the magnetically aligned, high-MW (Mn = 53.7 kDa) PDVT-8 film exhibited a higher degree of backbone chain alignment and film crystallinity compared with the low-MW (Mn = 17.6 kDa) PDVT-8 film grown via the same magnetic alignment method. We found that as the MW increases, the degree of preaggregation of the polymer molecules in solution significantly increases and the aggregation mode changes from H-aggregation to J-aggregation through a cooperative assembly mechanism. These events improved the responsiveness of high-MW polymer molecules to magnetic fields. Field-effect transistors based on the magnetic aligned high-MW PDVT-8 films exhibited a 6.8-fold increase in hole mobility compared to the spin-coated films, along with a mobility anisotropy ratio of 12.6. This work establishes a significant correlation among chain aggregation behavior in solution, polymer film microstructures, magnetic responsiveness, and carrier transport performance in donor-acceptor polymer systems.

Three-in-One Strategy Enables Single-Component Organic Solar Cells with Record Efficiency and High Stability

Single-component organic solar cells (SCOSCs) with covalently bonding donor and acceptor are becoming increasingly attractive because of their superior stability over traditional multicomponent blend organic solar cells (OSCs). Nevertheless, the efficiency of SCOSCs is far behind the state-of-the-art multicomponent OSCs. Herein, by combination of the advantages of three-component and single-component devices, this work reports an innovative three-in-one strategy to boost the performance of SCOSCs. In this three-in-one strategy, three independent components (PM6, D18, and PYIT) are covalently linked together to create a new single-component active layer based on ternary conjugated block copolymer (TCBC) PM6-D18-b-PYIT by a facile polymerization. Precisely manipulating the component ratios in the polymer chains of PM6-D18-b-PYIT is able to broaden light utilization, promote charge dynamics, optimize, and stabilize film morphology, contributing to the simultaneously enhanced efficiency and stability of the SCOSCs. Ultimately, the PM6-D18-b-PYIT-based device exhibits a power conversion efficiency (PCE) of 14.89%, which is the highest efficiency of the reported SCOSCs. Thanks to the aggregation restriction of each component and chain entanglement in the three-in-one system, the PM6-D18-b-PYIT-based SCOSC displays significantly higher stability than the corresponding two-component (PM6-D18:PYIT) and three-component (PM6:D18:PYIT). These results demonstrate that the three-in-one strategy is facile and promising for developing SCOSCs with superior efficiency and stability.

Precise Modulation of Circularly Polarized Luminescence via Polymer Chiral Co-assembly and Contactless Dynamic Chiral Communication

Circularly polarized luminescence (CPL) plays a pivotal role in cutting-edge display and information technologies. Currently achieving precise color control and dynamic signal regulation in CPL still remains challenging due to the elusory relationship between fluorescence and chirality. Inspired by the natural mechanisms governing color formation and chiral interaction, we proposed an addition-subtraction principle theory to address this issue. Three fluorene-based polymers synthesized by Suzuki polycondensation with different electron-deficient monomers exhibit similar structures and UV/Vis absorption, but distinct fluorescence emissions due to intramolecular charge transfer. Based on this, precise-color CPL-active films are obtained through quantitative supramolecular co-assembly directed by addition principle. Particularly, an ideal white-emitting CPL film (CIE coordinates: (0.33, 0.33)) is facilely fabricated with a high quantum yield of 80.8 % and a dissymmetry factor (glum) of 1.4×10-2. Structural analysis reveals that the ordered stacking orientation favors higher glum. Furthermore, to address the dynamically regulated challenge, the comparable subtraction principle is proposed, involving a contactless chiral communication between excited and ground states. The representative system consisting of as-prepared fluorene-based polymers and chirality-selective absorption azobenzene (Azo)-containing polymers is constructed, achieving CPL weakening, reversal, and enhancement. Finally, a switchable quick response code is realized based on trans-cis isomerization of Azo moiety.

Meticulous Molecular Engineering of Crystal Orientation and Morphology in Conjugated Polymer Thin Films for Field-Effect Transistors

The ability to precisely tailor molecular packing and film morphology in conjugated polymers offers a robust means to control their optoelectronic properties. This, however, remains a grand challenge. Herein, we report the dependency of molecular packing of an important conjugated polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), on a set of intrinsic parameters and unveil the correlation between their crystalline structures and charge transport characteristics. Specifically, a family of PBTTT with varying side chains (i.e., hexyl, octyl, decyl, dodecyl, tetradecyl, and hexadecyl referred to as C6, C8, C10, C12, C14, and C16, respectively) and molecular weights (MWs) with a focus on C14 are judiciously designed and synthesized. Various crystalline structures are yielded by tuning the alkyl chain and MW of PBTTT together with thermal annealing. It reveals that extending the alkyl chain length of PBTTT to C14, along with a larger MW and heating at 180 °C, promotes the formation of edge-on crystallites with significantly improved orientation and ordering. Furthermore, these distinct crystalline structures greatly impact their charge mobilities. This study sheds light on the tailored design of crystalline structures in PBTTT through a synergetic approach, which paves the way for potential applications of PBTTT and other conjugated polymers in optoelectronic devices with enhanced performance.

Alkoxythiophene-Directed Fibrillization of Polymer Donor for Efficient Organic Solar Cells

Realizing fibrillar molecular framework is highly encouraged in organic solar cells (OSCs) due to the merit of efficient charge carrier transport. This is however mainly achieved via the chemical structural design of photovoltaic semiconductors. In this work, through the utilization of three alkoxythiophene additives, T-2OMe, T-OEH, and T-2OEH, the intermolecular interactions among a series of BDT-type polymer donors, i.e., PM6, D18, PBDB-T, and PTB7-Th, are tuned to self-assemble into nanofibrils during solution casting. X-ray technique and molecular dynamics simulation reveal that the alkoxythiophene with (2-ethylhexyl)oxy (─OEH) chains can attach on the 2-ethylhexyl (EH) chains of these polymer donors and promote their self-assembly into 1D nanofibrils, in their neat films as well as photovoltaic blends with L8-BO. By adapting these fibrillar polymer donors to construct pseudo-bulk heterojunction (P-BHJ) OSCs via layer-by-layer deposition, generally improved device performance is seen, with power conversion efficiencies enhanced from 18.2% to 19.2% (certified 18.96%) and from 17.9% to 18.7% for the PM6/L8-BO and D18/L8-BO devices, respectively. This work provides a physical approach to promote the fibrillar charge transport channels for efficient photovoltaics.

Unfolding the Correlation between Solution Aggregation and Solid-State Crystal Orientation in Donor-Acceptor Copolymers via Solvent Additive Processing

Tailoring the crystal orientation of donor-acceptor (D-A) copolymers is vital for boosting the performance of optoelectronic devices. Despite recent advances in controlling the crystal orientation of D-A copolymers in films, the investigation into their aggregates in solution and the correlation between the solution aggregates and solid-state crystal orientation has been limited. Herein, an effective solvent additive strategy is reported for tuning solution aggregates and the consequent solid-state structures of poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)). Specifically, the addition of 1-decanethiol (10-thiol) to the P(NDI2OD-T2) chloroform solution promoted the aggregation of P(NDI2OD-T2) chains because of the improved planarization of the backbones, which changed their crystal orientation in the film from coexisting edge-on and face-on to dominant edge-on when produced by drop-casting. The mechanism of this crystal orientation transformation is elucidated based on the interaction between 10-thiol and the side chains of P(NDI2OD-T2). The optical properties of P(NDI2OD-T2) films with different crystalline structures are closely correlated. Notably, the 10-thiol-enabled facile tailoring of the crystal orientation in P(NDI2OD-T2) can be readily applied to other D-A copolymers of interest. The findings of this study highlight a robust solvent additive strategy for regulating solution aggregates and crystal orientation in D-A copolymer films, which have applications in many optoelectronic devices.

Visualizing the multi-level assembly structures of conjugated molecular systems with chain-length dependent behavior

It remains challenging to understand the structural evolution of conjugated polymers from single chains to solvated aggregates and film microstructures, although it underpins the performance of optoelectrical devices fabricated via the mainstream solution processing method. With several ensemble visual measurements, here we unravel the morphological evolution process of a model system of isoindigo-based conjugated molecules, including the hidden molecular assembly pathways, the mesoscale network formation, and their unorthodox chain dependence. Short chains show rigid chain conformations forming discrete aggregates in solution, which further grow to form a highly ordered film that exhibits poor electrical performance. In contrast, long chains exhibit flexible chain conformations, creating interlinked aggregates networks in solution, which are directly imprinted into films, forming interconnective solid-state microstructure with excellent electrical performance. Visualizing multi-level assembly structures of conjugated molecules provides a deep understanding of the inheritance of assemblies from solution to solid-state, accelerating the optimization of device fabrication.

Density of States Engineering of n-Doped Conjugated Polymers for High Charge Transport Performances

Charge transport of conjugated polymers in functional devices closely relates to their density of states (DOS) distributions. However, systemic DOS engineering for conjugated polymers is challenging due to the lack of modulated methods and the unclear relationship between DOS and electrical properties. Here, the DOS distribution of conjugated polymers is engineered to enhance their electrical performances. The DOS distributions of polymer films are tailored using three processing solvents with different Hansen solubility parameters. The highest n-type electrical conductivity (39 ± 3 S cm^-1 ), the highest power factor (63 ± 11 µW m^-1 K^-2 ), and the highest Hall mobility (0.14 ± 0.02 cm² V^-1 s^-1 ) of the polymer (FBDPPV-OEG) are obtained in three films with three various DOS distributions, respectively. Through theoretical and experimental exploration, it is revealed that the carrier concentration and transport property of conjugated polymers can be efficiently controlled by DOS engineering, paving the way for rationally fabricating organic semiconductors.

Crystallization of D-A Conjugated Polymers: A Review of Recent Research

D-A conjugated polymers are key materials for organic solar cells and organic thin-film transistors, and their film structure is one of the most important factors in determining device performance. The formation of film structure largely depends on the crystallization process, but the crystallization of D-A conjugated polymers is not well understood. In this review, we attempted to achieve a clearer understanding of the crystallization of D-A conjugated polymers. We first summarized the features of D-A conjugated polymers, which can affect their crystallization process. Then, the crystallization process of D-A conjugated polymers was discussed, including the possible chain conformations in the solution as well as the nucleation and growth processes. After that, the crystal structure of D-A conjugated polymers, including the molecular orientation and polymorphism, was reviewed. We proposed that the nucleation process and the orientation of the nuclei on the substrate are critical for the crystal structure. Finally, we summarized the possible crystal morphologies of D-A conjugated polymers and explained their formation process in terms of nucleation and growth processes. This review provides fundamental knowledge on how to manipulate the crystallization process of D-A conjugated polymers to regulate their film structure.

Role of Molecular Weight in the Mechanical Properties and Charge Transport of Conjugated Polymers Containing Siloxane Side Chains

The molecular weight is a key factor affecting the properties of conjugated polymers. To determine the critical molecular weights of conjugated polymers modified with siloxane side chains, poly-diketo-pyrrolopyrrole-selenophene (PTDPPSe-5Si) samples with molecular weights ranging from 20 to 350 kDa are synthesized. The critical molecular weight of the polymer is determined in the range of 60-100 kDa by testing the viscosity of the solution. When the molecular weight of the 27-60 kDa polymers is below the critical molecular weight, they exhibit a high crystallinity and low ductility. When the molecular weight of the 100 kDa polymer reaches the critical molecular weight, the crystallinity decreases, and the ductility increases. As the molecular weight increases, the polymer film also gradually changes from brittle to ductile. Furthermore, when the molecular weight of the 315 kDa polymer is much higher than the critical molecular weight, the film exhibits a significant ductility, which results in the polymer films showing no pronounced cracks after high-percentage stretching. Additionally, due to the oriented alignment of the molecular chains caused by stretching, the carrier mobility in the parallel direction becomes 2.14-fold of the initial film.

Controlling Solution-State Aggregation and Solid-State Microstructures of Conjugated Polymers by Tuning Backbone Conformation

Molecular ordering of conjugated polymers both in solution-state aggregates and in solid-state microstructures is a determining factor of the charge transport properties in optoelectronic devices. However, the effect of backbone conformation in conjugated polymers on assembly structures is still unclear. Herein, to understand such backbone conformation effect, three novel chlorinated benzodifurandionge-based oligo(p-phenylene vinylene) (BDOPV) polymers are systematically developed. These BDOPV-based polymers exhibit significantly twisted backbone conformation (near 90° interunit torsion angle) between conjugated units, which can prevent polymer chains from forming ordered assembly structures by increasing conformational energy penalty in closely packed chains. A higher rotational barrier of the torsion angle would further prevent polymer chains from assembling, finally resulting in non-aggregated chains in solution and highly disordered solid-state packing structures. This work will deepen the understanding of the relationship between polymer backbone conformation and assembly structures, contributing to the exploration of the structure-property relationship of polymers. This article is protected by copyright. All rights reserved.

Optimizing the Intercrystallite Connection of Donor-Acceptor Conjugated Semiconductor Polymer by Controlling the Crystallization Rate via Temperature

The charge carrier transport of conjugated polymer thin film is mainly decided by the crystalline domain and intercrystallite connection. High density tie-chain can provide an effective bridge between crystalline domains. Herein, the tie-chain connection behavior is optimized by decreasing the crystal region length (l_c ) and increasing the crystallization rate. Poly[4-(4,4-bis(2-octyldodecyl)-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT-ODD) is dissolved in nonpolar solvent isooctane and high ordered rod-like aggregations are formed. As the temperature increases, the changes of solution state and crystallization behavior lead to three different chain arrangement morphologies in the films: (1) at 25°C, large and separated crystal regions are formed; (2) at 55°C, small and well-connected crystal regions are formed due to faster crystallization rate and smaller nucleus size; (3) at 90°C, the amorphous film is formed. Further results show that the film prepared at 55°C has a smaller crystal region length (l_c , 7.6 nm) and higher tie-chains content. Thus, the film exhibits the best device mobility of 2.3 × 10^-3 cm² V^-1 s^-1 . This result shows the great influence of crystallization kinetics on the microstructure of conjugated polymer films and provides an effective way for the optimization of the intercrystallite tie-chain. This article is protected by copyright. All rights reserved.