Influence of Systematic Incorporation of Conjugation-Break Spacers into Semi-Random Polymers on Mechanical and Electronic Properties

Control of Properties through Hydrogen Bonding Interactions in Conjugated Polymers

Molecular design is crucial for endowing conjugated polymers (CPs) with unique properties and enhanced electronic performance. Introducing Hydrogen-bonding (H-bonding) into CPs has been a broadly exploited, yet still emerging strategy capable of tuning a range of properties encompassing solubility, crystallinity, electronic properties, solid-state morphology, and stability, as well as mechanical properties and self-healing properties. Different H-bonding groups can be utilized to tailor CPs properties based on the applications of interest. This review provides an overview of classes of H-bonding CPs (assorted by the different H-bond functional groups), the synthetic methods to introduce the corresponding H-bond functional groups and the impact of H-bonding in CPs on corresponding electronic and materials properties. Recent advances in addressing the trade-off between electronic performance and mechanical durability are also highlighted. Furthermore, insights into future directions and prospects for H-bonded CPs are discussed.

A skeletal randomization strategy for high-performance quinoidal-aromatic polymers

Enhancing the solution-processability of conjugated polymers (CPs) without diminishing their thin-film crystallinity is crucial for optimizing charge transport in organic field-effect transistors (OFETs). However, this presents a classic "Goldilocks zone" dilemma, as conventional solubility-tuning methods for CPs typically yield an inverse correlation between solubility and crystallinity. To address this fundamental issue, a straightforward skeletal randomization strategy is implemented to construct a quinoid-donor conjugated polymer, PA4T-Ra, that contains para-azaquinodimethane (p-AQM) and oligothiophenes as repeat units. A systematic study is conducted to contrast its properties against polymer homologues constructed following conventional solubility-tuning strategies. An unusually concurrent improvement of solubility and crystallinity is realized in the random polymer PA4T-Ra, which shows moderate polymer chain aggregation, the highest crystallinity and the least lattice disorder. Consequently, PA4T-Ra-based OFETs, fabricated under ambient air conditions, deliver an excellent hole mobility of 3.11 cm2 V-1 s-1, which is about 30 times higher than that of the other homologues and ranks among the highest for quinoidal CPs. These findings debunk the prevalent assumption that a random polymer backbone sequence results in decreased crystallinity. The considerable advantages of the skeletal randomization strategy illuminate new possibilities for the control of polymer aggregation and future design of high-performance CPs, potentially accelerating the development and commercialization of organic electronics.

Unraveling the Effect of Stereoisomerism on Mobility-Stretchability Properties of n-Type Semiconducting Polymers with Biobased Epimers as Conjugation Break Spacers

The development of intrinsically stretchable n-type semiconducting polymers has garnered much interest in recent years. In this study, three biobased dianhydrohexitol epimers of isosorbide (ISB), isomannide (IMN), and isoidide (IID), derived from cellulose, were incorporated into the backbone of a naphthalenediimide (NDI)-based n-type semiconducting polymer as conjugation break spacers (CBSs). Accordingly, three polymers were synthesized through the Migita-Kosugi-Stille coupling polymerization with NDI, bithiophene, and CBSs, and the mobility-stretchability properties of these polymers were investigated and compared with those of their analogues with conventional alkyl-based CBSs. Experimental results showed that the different configurations of these epimers in CBSs sufficiently modulate the melt entropies, surface aggregation, crystallographic parameters, chain entanglements, and mobility-stretchability properties. Comparable ductility and edge-on preferred stacking were observed in polymers with endo- or exo-configurations in IMN- and IID-based polymers. By contrast, ISB with endo-/exo-configurations exhibits an excellent chain-realigning capability, a reduced crack density, and a proceeding bimodal orientation under tensile strain. Therefore, the ISB-based polymer exhibits high orthogonal electron mobility retention of (53 and 56)% at 100% strain. This study is one of the few examples where biobased moieties are incorporated into semiconducting polymers as stress-relaxation units. Additionally, this is the first study to report on the effect of stereoisomerism of epimers on the morphology and mobility-stretchability properties of semiconducting polymers.

Achieving High Performance Stretchable Conjugated Polymers via Donor Structure Engineering

A backbone engineering strategy is developed to tune the mechanical and electrical properties of conjugated polymer semiconductors. Four Donor-Acceptor (D-A) polymers, named PTDPPSe, PTDPPTT, PTDPPBT, and PTDPPTVT, are synthesized using selenophene (Se), thienothiophene (TT), bithiophene (BT), and thienylenevinylenethiophene (TVT) as the donors and siloxane side chain modified diketopyrrolopyrrole (DPP) as acceptor. The influences of the donor structure on the polymer energy level, film morphology, molecular stacking, carrier transport properties, and tensile properties are all examined. The films of PTDPPSe show the best stretchability with crack-onset-strain greater than 100%, but the worst electrical properties with a mobility of only 0.54 cm2  V-1  s-1 . The replacement of the Se donor with larger conjugated donors, that is, TT, BT, and TVT, significantly improves the mobility of conjugated polymers but also leads to reduced stretchability. Remarkably, PTDPPBT exhibits moderate stretchability with crack-onset-strain ≈50% and excellent electrical properties. At 50% strain, it has a mobility of 2.37 cm2 V-1  s-1 parallel to the stretched direction, which is higher than the mobility of most stretchable conjugated polymers in this stretching state.

High-Performance Intrinsically Stretchable Polymer Solar Cell with Record Efficiency and Stretchability Enabled by Thymine-Functionalized Terpolymer

Designing new polymer semiconductors for intrinsically stretchable polymer solar cells (IS-PSCs) with high power conversion efficiency (PCE) and durability is critical for wearable electronics applications. Nearly all high-performance PSCs are constructed using fully conjugated polymer donors (P_D) and small-molecule acceptors (SMA). However, a successful molecular design of P_Ds for high-performance and mechanically durable IS-PSCs without sacrificing conjugation has not been realized. In this study, we design a novel thymine side chain terminated 6,7-difluoro-quinoxaline (Q-Thy) monomer and synthesize a series of fully conjugated P_Ds (PM7-Thy5, PM7-Thy10, PM7-Thy20) featuring Q-Thy. The Q-Thy units capable of inducing dimerizable hydrogen bonding enable strong intermolecular P_D assembly and highly efficient and mechanically robust PSCs. The PM7-Thy10:SMA blend demonstrates a combination of high PCE (>17%) in rigid devices and excellent stretchability (crack-onset value >13.5%). More importantly, PM7-Thy10-based IS-PSCs show an unprecedented combination of PCE (13.7%) and ultrahigh mechanical durability (maintaining 80% of initial PCE after 43% strain), illustrating the promising potential for commercialization in wearable applications.

Enhancing the Elasticity of Conjugated Polymers through Precise Control of the Spacing between the Backbone and Siloxane Side-Chains

Intrinsically stretchable conjugated polymers (CPs) have extensively been studied for the development of novel flexible electronic devices. In this work, a method to control the elastic properties of CPs has been proposed via regulation of spacer length between the siloxane side-chain and the backbone. The target polymers were CP films with the structure P(mC-Si) for four different numbers of the spacer methylene groups, namely, m = 5, 6, 7, and 8. The effect of spacer length on the aggregation state as well as on electrical and elastic properties of the prepared films was then investigated. An adjustable lamellar spacing (d_L-L), in addition to improved elastic properties, was achieved as the spacer length was changed in the prepared polymer films. Moreover, P(7C-Si) has a sufficient d_L-L value of 35.77 Å, which provides enough space for inter-chain sliding to dissipate stress. This facilitated the dissipation of stress during the straining process. At a strain value of 100% in the vertical direction, the mobility of the P(7C-Si) film was 0.79 cm² V^-1 s^-1 and reduced to 84.0% of the initial value without any applied strain. The study provides clear evidence that tuning the spacer length between the silicone endgroup and backbone is an effective way to improve the intrinsic stretchability of CPs with siloxane side chains.

Molecularly Hybridized Conduction in DPP-Based Donor-Acceptor Copolymers toward High-Performance Iono-Electronics

Iono-electronics, that is, transducing devices able to translate ionic injection into electrical output, continue to demand a variety of mixed ionic-electronic conductors (MIECs). Though polar sidechains are widely used in designing novel polymer MIECs, it remains unclear to chemists how much balance is needed between the two antagonistic modes of transport (ion permeability and electronic charge transport) to yield high-performance materials. Here, the impact of molecularly hybridizing ion permeability and charge mobility in semiconducting polymers on their performance in electrochemical and synaptic transistors is investigated. A series of diketopyrrolopyrrole (DPP)-based copolymers are employed to demonstrate the multifunctionality attained by controlling the density of polar sidechains along the backbone. Notably, efficient electrochemical signal transduction and reliable synaptic plasticity are demonstrated via controlled ion insertion and retention. The newly designed DPP-based copolymers further demonstrate unprecedented thermal tolerance among organic mixed ionic-electronic conductors, a key property in the manufacturing of organic electronics.

Intrinsically Stretchable, Highly Efficient Organic Solar Cells Enabled by Polymer Donors Featuring Hydrogen-Bonding Spacers

Intrinsically stretchable organic solar cells (IS-OSCs), consisting of all stretchable layers, are attracting significant attention as a future power source for wearable electronics. However, most of the efficient active layers for OSCs are mechanically brittle due to their rigid molecular structures designed for high electrical and optical properties. Here, a series of new polymer donors (P_D s, PhAmX) featuring phenyl amide (N¹ ,N³ -bis((5-bromothiophen-2-yl)methyl)isophthalamide, PhAm)-based flexible spacer (FS) inducing hydrogen-bonding (H-bonding) interactions is developed. These P_D s enable IS-OSCs with a high power conversion efficiency (PCE) of 12.73% and excellent stretchability (PCE retention of >80% of the initial value at 32% strain), representing the best performances among the reported IS-OSCs to date. The incorporation of PhAm-based FS enhances the molecular ordering of P_D s as well as their interactions with a Y7 acceptor, enhancing the mechanical stretchability and electrical properties simultaneously. It is also found that in rigid OSCs, the PhAm5:Y7 blend achieves a much higher PCE of 17.5% compared to that of the reference PM6:Y7 blend. The impact of the PhAm-FS linker on the mechanical and photovoltaic properties of OSCs is thoroughly investigated.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Polyurethane-Based Stretchable Semiconductor Nanofilms with High Intrinsic Recovery Similar to Conventional Elastomers

Polymer semiconductors with large elastic recovery (ER) under high strain in thin film state are highly desirable for stretchable electronics. Here we report a type of stretchable semiconductor PU(DPP)x, by copolymerization of oligodiketopyrrolopyrrole-based conjugated block and hydrogenated polybutadiene flexible block via urethane linkage for intermolecular hydrogen bonding. By regulating block ratio, PU(DPP)35 with 35 wt % conjugated block exhibits high intrinsic ER > 80% under 175% strain (ε) in pseudo free-standing thin film state, comparable with commercial elastomers, and crack onset strain (COS) > 300% along with maximum hole mobility of 0.19 cm2 V-1 s-1 in organic thin film transistors to bring it to the best performing block copolymer-type stretchable semiconductors. Enhanced mobility is achieved using PU(DPP)35 as the binder for conjugated polymer PDPPT3. The 25 wt %-PDPPT3 blend displays mobility up to 1.28 cm2 V-1 s-1 along with COS ∼120%, and 10 wt %-PDPPT3 blend exhibits ER of 78% at ε = 150%, COS of ∼230%, modulus of 36.5 MPa, maximum mobility of 0.62 cm2 V-1 s-1 and no obvious degradation of mobility at ε = 150% after 100 cycles of strain. Moreover, the structural similarity enables the blend film uniform and stable microstructure against mechanical and thermal deformation. Notably, PU(DPP)35 and the blend are characterized by high mechanical performance similar to that of commercial elastomers in thin film state, and demonstrate their potential for high performance stretchable electronics.

Asymmetric Hybrid Siloxane Side Chains for Enhanced Mobility and Mechanical Properties of Diketopyrrolopyrrole-Based Polymers

High performance organic field effect transistor devices based on intrinsically scalable materials are of great significance in wearable electronics. In this work, an exclusive approach is reported to rationale the carrier mobility and stretchability of the conjugate polymers (CPs) by modifying the symmetry of the side chains species. Semiconductor CPs with symmetrical alkyl side chains (P(C-C)), symmetrical siloxane side chains (P(Si-Si)), and asymmetrical silicon-carbon side chains (P(C-Si)) are synthesized to investigate the influence of these side chains on the carrier mobility and mechanical behavior. The result shows that silicon-carbon asymmetric side chains can modulate the aggregation degree of polymer chains with a coherence length of 134 Å and maintain the mobility at 0.90 cm² V^-1 s^-1 . P(C-Si) exhibits superior tensile properties that even elongation up to 100% the value of mobility retains a majority properties. The main reason is that the lowest coherence length of P(C-Si) polymer leads to an increased proportion of amorphous zones in its polymer film, which efficiently dissipates mechanical stresses. This study provides an efficient strategy for the design and synthesis of the CPs with high carrier transport properties-mechanical stability.

Pushing the Limits of Flexibility and Stretchability of Solar Cells: A Review

Emerging forms of soft, flexible, and stretchable electronics promise to revolutionize the electronics industries of the future offering radically new products that combine multiple functionalities, including power generation, with arbitrary form factor. For example, skin-like electronics promise to transform the human-machine-interface, but the softness of the skin is incompatible with traditional electronic components. To address this issue, new strategies toward soft and wearable electronic systems are currently being pursued, which also include stretchable photovoltaics as self-powering systems for use in autonomous and stretchable electronics of the future. Here recent developments in the field of stretchable photovoltaics are reviewed and their potential for various emerging applications are examined. Emphasis is placed on the different strategies to induce stretchability including extrinsic and intrinsic approaches. In the former case, engineering and patterning of the materials and devices are key elements while intrinsically stretchable systems rely on mechanically compliant materials such as elastomers and organic conjugated polymers. The result is a review article that provides a comprehensive summary of the progress to date in the field of stretchable solar cells from the nanoscale to macroscopic functional devices. The article is concluded by discussing the emerging trends and future developments.

Materials Design and Optimization for Next-Generation Solar Cell and Light-Emitting Technologies

We review some of the most potent directions in the design of materials for next-generation solar cell and light-emitting technologies that go beyond traditional solid-state inorganic semiconductor-based devices, from both the experimental and computational standpoints. We focus on selected recent conceptual advances in tackling issues which are expected to significantly impact applied literature in the coming years. Specifically, we consider solution processability, design of dopant-free charge transport materials, two-dimensional conjugated polymeric semiconductors, and colloidal quantum dot assemblies in the fields of experimental synthesis, characterization, and device fabrication. Key modeling issues that we consider are calculations of optical properties and of effects of aggregation, including recent advances in methods beyond linear-response time-dependent density functional theory and recent insights into the effects of correlation when going beyond the single-particle ansatz as well as in the context of modeling of thermally activated fluorescence.

Intrinsically stretchable naphthalenediimide–bithiophene conjugated statistical terpolymers using branched conjugation break spacers for field–effect transistors

A series of naphthalene−diimide based conjugated polymers was synthesized through statistical terpolymerization with branched conjugation break spacers to enhance their mobility−stretchability properties in field-effect transistors.

A "Rigid-Flexible" Approach for Processable Perylene Diimide-Based Polymers: Influence of the Specific Architecture on the Morphological, Dielectric, Optical, and Electronic Properties

Conjugation-break flexible spacers in-between π-conjugated segments were utilized herein toward processable perylene diimide (PDI)-based polymers. Aromatic-aliphatic PDI-based polymers were developed via the two-phase polyetherification of a phenol-difunctional PDI monomer and aliphatic dibromides. These polyethers showed excellent solubility and film-forming ability and deep lowest unoccupied molecular orbital (LUMO) levels (-4.0 to -3.85 eV), indicating the preservation of good electron-accepting character or characteristics, despite the non-conjugated segments. Their thermodynamic properties, local dynamics, and ionic conductivity demonstrate the suppression of PDI's inherent tendency for aggregation and crystallization, suggesting PDI-polyethers as versatile candidates for organic electronic applications. Their dynamics investigation using dielectric spectroscopy revealed weak dipole moments arising from the distortion of the planar perylene cores. Blends of the PDI-polyethers (as electron acceptors) with P3HT (as a potential electron donor component) showed UV-vis absorbances from 350 to 650 nm and a tendency of the PDI-polyethers to intertwine with rr-P3HT and restrain its high crystallization tendency.

Allylamine PECVD Modification of PDMS as Simple Method to Obtain Conductive Flexible Polypyrrole Thin Films

In this paper, we report a one-step method to obtain conductive polypyrrole thin films on flexible substrates. To do this, substrates were modified through allylamine plasma grafting to create a high amount of reactive amine groups on PDMS surface. These groups are used during polypyrrole particle synthesis as anchoring points to immobilize the polymeric chains on the substrate during polymerization. Surface morphology of polypyrrole thin films are modified, tailoring the polyelectrolyte used in the polypyrrole synthesis obtaining different shapes of nanoparticles that conform to the film. Depending on the polyelectrolyte molecular weight, the shape of polypyrrole particles go from globular (500 nm diameter) to a more constructed and elongated shape. The films obtained with this methodology reflected great stability under simple bending as well as good conductivity values (between 2.2 ± 0.7 S/m to 5.6 ± 0.2 S/cm).

Asymmetric Side-Chain Engineering of Isoindigo-Based Polymers for Improved Stretchability and Applications in Field-Effect Transistors

Thus far, there is still no study systematically investigating the influence of asymmetric side-chain design on a polymer's stretchability and its associated stretchable device applications. Herein, three kinds of asymmetric side chains consisting of carbosilane side chain (Si-C8), siloxane-terminated side chain (SiO-C8), and decyltetradecane side chain (DT) are engineered in isoindigo-bithiophene (PII2T, P1-P3) and isoindigo-difluorobithiophene (PII2TF, P4-P6) conjugated polymers, and their structure-stretchability correlation is explored in field-effect transistor characterization. It is revealed that owing to the geometric difference between the side chains, different asymmetric side-chain combinations impose distinct influences on the molecular stacking and orientation of the derived polymers. Surprisingly, the combination of asymmetric side chains and backbone fluorination is shown to deliver the best stretchability and mechanical durability of the derived polymer. Consequently, P6 consisting of asymmetric Si-C8/DT side chains and fluorinated backbone possesses the best mobility preservation of 81% at 100% strain with the stretching force perpendicular to the charge-transporting direction. Moreover, it presents 90% mobility retention after 400 stretching-releasing cycles with 60% strain, greatly exceeding the value (36%) of the non-fluorinated counterpart (P3). Our results suggest that the rational design of asymmetric side chains and backbone fluorination provides an efficient way to enhance the intrinsic stretchability of conjugated polymers.