No abstract

Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long-term thermal and mechanical stability of associated devices, there is a need to determine the origin of the polymer ductility and toughness. In this work, we investigate a variety of high-performance conjugated polymers and relate their thermomechanical behavior to film toughness. Dynamic mechanical analysis (DMA) is used to probe thermomechanical relaxations of the conjugated polymers. Film ductility is measured as a function of temperature to determine the temperature that corresponds to a significant loss in film toughness. We systematically study polymers with changes to the side-chain structure, backbone structure, and crystallinity. We also compare polymers that have a clear glass transition (T g ) to those that do not. It is found that secondary thermal relaxations (sub-T g ) play a critical role in film toughness. This sub-T g is found to be a local molecular relaxation that appears to relate to side-chain and backbone mobility. We also find that many of the polymers considered continue to show moderate ductility below their sub-T g , which is attributed to crystallites or aggregates that have active slip systems. These results provide new insights into how conjugated polymer structure and related thermal relaxations influence film toughness that will assist in realizing mechanically robust devices.

The success of stretchable electronics based on conjugated polymers relies on having a thorough understanding of the polymer’s mechanical behavior over conditions likely encountered during operation. To meet this need, a novel approach to capture the stress–strain response of thin conjugated polymer films is introduced. This is achieved by laminating the polymer film of interest on a thin elastomer substrate and testing the composite specimen in a dynamic mechanical analyzer in a tensile test configuration. We term this approach as film laminated on thin elastomer (FLOTE) method. The benefits of this method include the ability to (1) determine the viscoelastic behavior of the conjugated polymer by testing over a broad range of temperatures and strain rates, (2) measure the film behavior over large cyclic strains, including under in-plane compression, and (3) capture the impact of the neighboring elastomer on the behavior of the polymer film. The focus is on the widely studied poly­(3-hexylthiophene) (P3HT) as a model system. We find that the viscoelastic characteristics of P3HT, varied by changing the specimen temperature, significantly impact film stability under cyclic strain. This includes showing that the hysteresis behavior of the films under cyclic strain changes significantly with sample temperature. In addition, it is found that, under cyclic loading, the composite has features consistent with Mullins’ effect. Based on these results, insights into polymer viscoelastic characteristics necessary to achieve high-performance stretchable electronics are gained.

The intrinsic degradation of conjugated polymer (CP) based solar cells (PSCs) due to morphological change by heat is an outstanding challenge. Increasing the glass transition temperature (T g ) of the materials used in PSCs can largely mitigate the thermal instability, yet most CPs used in high-efficiency PSCs only show low T g values, mainly due to the long and bulky side chains needed for solution processing of such polymers. Thermally removing cleavable side chains is an effective approach to regain the high T g of CPs after the film formation, thereby achieving higher stability; however, previous results using polythiophenes only achieved moderate efficiency (0.8% with PC 61 BM) after a high temperature (300 °C) treatment to remove all side chains. To better understand and utilize thermocleavable side chains (TCSs), we explore a series of regioregular polythiophenes having TCSs and hexyl side chains by varying the ratio of different side chains, from 0 mol % TCSs to 100 mol % TCSs at an increment of 20 mol %. Through a systematic investigation, we find that the polymers with more TCSs than hexyl side chains exhibit sufficient stability under a rather harsh condition (100 °C, in air and under continuous ambient light). While a complete removal of alkyl chains might offer a higher stability, the device efficiency was very low (∼0.14%); by contrast, the polymer having ∼70 mol % of TCSs achieved the highest efficiency (∼1.5%) after alkyl chain cleavage at 200 °C and significant morphological stability. Under our stability test (150 °C, 24 h and ambient light), these specific polymer:PC 61 BM based solar cells were able to retain 90% of the original efficiency. These key findings, together with mechanistic understanding of the thermocleavage process, provide valuable insight into the impact of TCS and present a new design rationale to achieve PSCs with both high efficiency and improved stability.

The thermomechanical behavior of polymer semiconductors plays an important role in the processing, morphology, and stability of organic electronic devices. However, donor-acceptor-based copolymers exhibit complex thermal relaxation behavior that is not well understood. This study uses dynamic mechanical analysis (DMA) to probe thermal relaxations of a systematic set of polymers based around the benzodithiophene (BDT) moiety. The loss tangent curves are resolved by fitting Gaussian functions to assign and distinguish different relaxations. Three prominent transitions are observed that correspond to: i) localized relaxations driven primarily by the side chains (γ ), ii) relaxations along the polymer backbone (β ), and iii) relaxations associated with aggregates (α ). The side chains are found to play a clear role in dictating T γ , and that mixing the side chain chemistry of the monomer to include alkyl and oligo(ethylene glycol) moieties results in splitting the γ -relaxation. The β relaxations are shown to be associated with backbone elements along with the monomer. In addition, through processing, it is shown that the α-relaxation is due to aggregate formation. Finally, it is demonstrated that the thermal relaxation behavior correlates well with the stress-strain behavior of the polymers, including hysteresis and permanent set in cyclically stretched films.