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

Abstract

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 (C 8 O-BTBT-OC 8) 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 cm 2 V −1 s −1 , a threshold voltage close to 0 V, and an on/off current ratio above 10 7. In situ lattice phonon Raman microscopy is used to study the stability of the SIP polymorph. It is found that films based on only C 8 O-BTBT-OC 8 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.

Organic Semiconductor/Insulator Blends for Elastic Field‐Effect Transistors and Sensors

“…The 2,7-dioctyloxy [1]benzothieno [3,2-b]benzothiophene (C8O-BTBT-OC8) revealed in solution sheared PS blends greater device performance and significantly longer stability of the surface-induced herringbone structure than the neat semiconductor films. [77] In situ phonon Raman microscopy proved a slow structural transition of C8O-BTBT-OC8 into the unfavorable bulk cofacial phase of poor electrical performance. The stability time of the herringbone structure in blend films depended also on M w of PS.…”

Structures, Properties, and Device Applications for [1]Benzothieno[3,2‐b]Benzothiophene Derivatives

“…fabricated C8O‐BTBT‐OC8:PS thin film and the metastable surface‐induced herringbone phase was stabilized after adding PS into the solution. [ 151 ] Especially, the devices showed the long‐term stability over 1 year.…”

Organic Semiconductor–Insulator Blends for Organic Field‐Effect Transistors

“…Salzillo et al found that the introduction of PS into 2,7‐dioctyloxy[1]benzothieno[3,2‐b]benzothiophene (C8O‐BTBT‐OC8) could stabilize the metastable polymorphs during the crystallization. [ 39 ] As a result, the obtained device sustained a hole mobility of 0.3–0.6 cm 2 V −1 s −1 and a very slight V th shift of 1 V even after 1.5 years. In general, insulator blending can lead to more electrically and environmentally durable OSCs in practical applications through improved film quality or insulator encapsulation.…”