Approaching Highly Stable Optoelectronic Device Operation at Elevated Temperature by Locking Backbone Torsion of Conjugated Polymers

The ability of organic electronics to maintain stability during prolonged operation at elevated temperatures is crucial for the durability and longevity of optoelectronic devices, thereby ensuring their commercial success. Achieving stable optoelectronic properties for conjugated polymers at high temperatures remains fundamentally challenging, and understanding the causes of their instability is even more complex. In this study, we identify backbone twisting motion as the primary reason for the unstable electrical and optical properties of donor-acceptor (D-A) conjugated polymers, using diketopyrrolopyrrole (DPP)-based polymers as a model system. For DPP polymers, the backbone thiophene-ring twist transition is responsible for shifts in the bandgap and alterations in charge transport properties, which ultimately results in the failure of thin film transistor devices. Holistically, we integrated multimodal tools to gain a deep understanding of the thermodynamic and structural properties of DPP D-A polymers, including dynamic mechanical analysis (DMA), quasi-elastic neutron scattering (QENS), and temperature-dependent Fourier transform infrared spectroscopy (FTIR). A significant change in segmental dynamics occurs around 400-420 K, attributed to the backbone thiophene-ring twist motion. Above this transition temperature, the twisting motion of the thiophene induces localization of the intrachain excitons, resulting in reduced charge carrier mobility, as well as a significant blue shift in optical absorption. Additionally, we demonstrated that intramolecular hydrogen bonding interaction within a newly synthesized DPP polymer can induce co-planarity in the backbone and suppress undesired backbone twisting at elevated temperatures, thereby ensuring a more stable charge transport property. Our work offers fundamental insights into the decline in device stability at elevated temperatures and outlines a pathway to overcome the bottlenecks that limit the performance of organic electronic devices at elevated operation temperatures.