Self-Aligned Bilayers for Flexible Free-Standing Organic Field-Effect Transistors.

Hanna Zajaczkowska,Piotr Sleczkowski,Lothar Veith,Wojciech Pisula,Jacek Ulanski,Witold Waliszewski,Michal Borkowski,Bartlomiej Graczykowski,Paul W M Blom,Malgorzata A Bartkiewicz,Tomasz Marszalek

doi:10.1021/acsami.1c15208

Abstract

Free-standing and flexible field-effect transistors based on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/polystyrene bilayers are obtained by well-controlled phase separation of both components. The phase separation is induced by solvent vapor annealing of initially amorphous blend films, leading to crystallization of TIPS-pentacene as the top layer. The crystallinity and blend morphology strongly depend on the molecular weight of polystyrene, and under optimized conditions, distinct phase separation with a well-defined and trap-free interface between both fractions is achieved. Due to the distinct bilayer morphology, the resulting flexible field-effect transistors reveal similar charge carrier mobilities as rigid devices and additionally pronounced environmental and bias stress stabilities. The performance of the flexible transistors remains stable up to a strain of 1.8%, while above this deformation, a close relation between current and strain is observed that is required for applications in strain sensors.

Highlights

Solution-processable organic semiconductors offer great advantages for thin-film processing of mechanically flexible electronic devices.[1−5] The processing conditions determine to a great extent the molecular organization that is important for the charge carrier transport.[6−8] A high molecular order in large domains ensures an unhindered transport of charge carriers, especially in organic field-effect transistors (OFETs).[8−12] As a great advantage, the relatively low elastic modulus of organic semiconductors compared to their inorganic counterparts permits certain stretching and bending of the devices without serious degradation of the electrical performance within the mechanical properties of the active material.[13−16] Flexible OFETs require plastic substrates, which, show disadvantages
Toluene solution with a TIPS-pentacene/PS ratio of 1:3.3 was first spin cast on rigid auxiliary glass substrates to control the blend morphology during processing
Free-standing, self-aligned, and flexible TIPS-pentacene/PS bilayer films were fabricated for OFETs applications

Summary

■ INTRODUCTION

Solution-processable organic semiconductors offer great advantages for thin-film processing of mechanically flexible electronic devices.[1−5] The processing conditions determine to a great extent the molecular organization that is important for the charge carrier transport.[6−8] A high molecular order in large domains ensures an unhindered transport of charge carriers, especially in organic field-effect transistors (OFETs).[8−12] As a great advantage, the relatively low elastic modulus of organic semiconductors compared to their inorganic counterparts permits certain stretching and bending of the devices without serious degradation of the electrical performance within the mechanical properties of the active material.[13−16] Flexible OFETs require plastic substrates, which, show disadvantages. Calculations for blends of TIPS-pentacene and poly[bisphenol A carbonate-co4,40-(3,3,5-trimethylcyclohexy-lidene)diphenol carbonate] indicated an optimum ratio of 1:4 that was experimentally confirmed for phase-separated blend films with high charge carrier mobilities.[31] Besides the impact of ΔGm on the phase separation, the resulting blend morphology in thin films is governed by solute−solvent, solute−substrate, and solute− solute interactions that are determined by the blend composition, processing conditions, and surface energy of the substrate.[32] Blending of crystalline small-molecule semiconductors with an insulating polymer improves their surface wettability during processing and crystallinity in the final film, typically combined with vertical phase separation as required for an unhindered charge carrier transport in the in-plane direction of OFETs.[33−35] One reason for the higher crystallinity of the semiconducting phase is an extended drying time during solvent evaporation in the presence of the insulating polymer. The close relation between source−drain current and strain applied to the flexible OFETs make these devices attractive for applications in low-cost strain sensors

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES