Enhancing Long-Term Device Stability Using Thin Film Blends of Small Molecule Semiconductors and Insulating Polymers to Trap Surface-Induced Polymorphs

The lack of long-term stability in thin films of organic semiconductors can often be caused by the low structural stability of metastable phases that are frequently formed upon deposition on a substrate surface. Here, thin films of 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene (C8O-BTBT-OC8) and blends of this material with polystyrene by solution shearing are fabricated. Both types of films exhibit the metastable surface-induced herringbone phase (SIP) in all the tested coating conditions. The blended films reveal a higher device performance with a field-effect mobility close to 1 cm2 V−1 s−1, a threshold voltage close to 0 V, and an on/off current ratio above 107. In situ lattice phonon Raman microscopy is used to study the stability of the SIP polymorph. It is found that films based on only C8O-BTBT-OC8 slowly evolve to the Bulk cofacial phase, significantly impacting device electrical performance. In contrast, the blended films stabilize the SIP phase, leading to devices that maintain a high performance over 1.5 years. This work demonstrates that blending small-molecule organic semiconductors with insulating binding polymers can trap metastable polymorphs, which can lead to devices with both improved performance and long-term stability.