Integrating charge mobility, stability and stretchability within conjugated polymer films for stretchable multifunctional sensors.

Conjugated polymers (CPs) are promising semiconductors for intrinsically stretchable electronic devices. Ideally, such CPs should exhibit high charge mobility, excellent stability, and high stretchability. However, converging all these desirable properties in CPs has not been achieved via molecular design and/or device engineering. This work details the design, synthesis and characterization of a random polythiophene (RP-T50) containing ~50 mol% of thiophene units with a thermocleavable tertiary ester side chain and ~50 mol% of unsubstituted thiophene units, which, upon thermocleavage of alkyl chains, shows significant improvement of charge mobility and stability. Thermal annealing a RP-T50 film coated on a stretchable polydimethylsiloxane substrate spontaneously generates wrinkling in the polymer film, which effectively enhances the stretchability of the polymer film. The wrinkled RP-T50-based stretchable sensors can effectively detect humidity, ethanol, temperature and light even under 50% uniaxial and 30% biaxial strains. Our discoveries offer new design rationale of strategically applying CPs to intrinsically stretchable electronic systems.

Charge Trapping in a Low-Crystalline High-Mobility Conjugated Polymer and Its Effects on the Operational Stability of Organic Field-Effect Transistors.

The effects of the microstructure of conjugated polymer thin films on charge trapping and operational stability of organic field-effect transistors (OFETs) are investigated. Device characteristics of OFETs based on two model conjugated polymers, poly(3-hexylthiophene) (P3HT) and a random 3-hexylthiophene-thiophene copolymer (RP33), are compared. P3HT films have high crystallinity and long-range molecular order, whereas RP33 films have low crystallinity and short-range molecular order as well as enhanced polymer backbone planarity. Experimental evidence shows that although the microstructure of the RP33 film provides efficient charge transport pathways, its high degree of structural disorder causes severe shallow trapping of charge carriers, which results in its inferior stability under bias stress. This study demonstrates that low-crystalline conjugated polymers with short-range order can provide a high charge-carrier mobility but at the same time be inappropriate for practical OFETs because of their poor intrinsic operational stability.

In-depth optical characterization of poly(3-hexylthiophene) after formation of nanosecond laser-induced periodic surface structures.

Herein, poly(3-hexylthiophene) films with periodic wavy surface structures are generated upon laser irradiation at a wavelength of 530 nm using a pulse duration of 5 ns and a repetition frequency of 10 Hz. The optical properties of the films irradiated with 1200, 3000, and 6000 pulses, respectively, are studied using various techniques.

Control of Crystallite Orientation in Diketopyrrolopyrrole-Based Semiconducting Polymers via Tuning of Intermolecular Interactions.

Numerous previous studies have focused on the notion that semiconducting polymers with an edge-on dominant orientation are advantageous for horizontal charge transport, whereas polymers with a face-on dominant orientation are advantageous for vertical charge transport, since the crystallite orientation determines the π-π stacking direction, which in turn affects the interchain charge transport direction. Here, we report that the crystallite orientation is dependent on the intermolecular interactions in the semiconducting polymer. In this study, we control the intermolecular interactions in a donor-acceptor (D-A) semiconducting polymer via side chain engineering. To perform side chain engineering, we use two different polymers: one with side chains on only A units (PDPP-B) and the other with side chains on both D and A units (PDPP-C8). We observe that PDPP-C8 is characterized by weaker intermolecular interactions due to the additional side chains on D units. A morphological analysis reveals that PDPP-B and PDPP-C8 films have microstructures that are characterized by edge-on and face-on dominant orientations, respectively. Therefore, we demonstrate that our strategies effectively control intermolecular interactions and, consequently, the crystallite orientation. Finally, we compare the vertical and horizontal mobilities of both polymer films. These results show that the crystallite orientation has significant influence on charge transport behaviors.

Role of Disorder in the Extent of Interchain Delocalization and Polaron Generation in Polythiophene Crystalline Domains.

To understand how disorder within conjugated polymer aggregates influences the polaron generation process, we investigated poly(3-hexylthiophene) (P3HT) and a congeneric random copolymer incorporating 33 mol % substituent-free thiophene units (RP33). Steady-state absorption and fluorescence spectra showed that increasing the intrachain torsional disorder in aggregates increases the energy and breadth of the density of states (DOS). By extracting polaron dynamics in the transient absorption spectra, we found that an activation energy barrier of 0.05 eV is imposed on the charge separation process in P3HT, whereas that in RP33 is essentially barrierless. We also found that a significant amount of excitons in P3HT are deactivated by traps, while no trapped excitons are generated in RP33. This efficient polaron generation in RP33 was attributed to the excess energy and enhanced interchain delocalization of precursor states provided by the intrachain torsional disorder and the close-packing structure in the absence of hexyl substituents.

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.