Reversible Switching in Capacitors and Organic Transistors Employing Dihydroazulene/Vinylheptafulvene Photochromic Switches

Organic field-effect transistors (OFETs) have been shown to rival or even outperform benchmark amorphous silicon-based devices. This has stimulated research efforts to further exploit organic transistors for more complex, multifunctional devices such as photodetectors or optical memories. One possibility to introduce additional functionality into OFETs is to employ molecular switches that undergo reversible isomerization under applied stimuli, such as irradiation with specific wavelengths in the UV-Vis regime. Both photoisomers exhibit different properties, such as frontier orbital energies, dipole moment or spatial extension resulting in a reversibly switchable electronic system once incorporated into an OFET device.<br/><br/>Here, we present that blends of semiconducting polymers and dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitches can be employed to fabricate OFETs with light/thermo-responsive device characteristics. Irradiation with UV light results in significant alteration of the transfer characteristics, especially of the subthreshold region of the gate voltage sweep. In contrast to previous work on spiropyrans or diarylethenes, the backreaction is induced thermally and not via visible light irradiation and reproducibly yields the characteristics of the pristine devices. Applying UV light of different doses shows that the magnitude of switching directly depends on the respective UV dose, hence enabling a multi-level electronic system.<br/><br/>We further demonstrate that DHA/VHF molecular switches can also be used to introduce stimuli-responsiveness into organic transistors by incorporating them into the gate dielectric of the device. To systematically explore this approach, we firstly focus on the evaluation of the dielectric properties of DHA/VHF blends with the dielectric polymer poly(methyl methacrylate) (PMMA) in metal-insulator-metal capacitors using impedance spectroscopy. Afterwards, these switchable dielectric blends are used as gate dielectrics in OFETs, allowing optical control of device characteristics and figures of merit such as field-effect mobility and threshold voltage.<br/><br/>Additionally, our results show that DHA/VHF molecular switches (unlike the established spiropyran (SP)/merocyanine (MC) photoswitch) exhibit excellent resistance to cycling fatigue when incorporated into both the semiconductor and the PMMA gate dielectric.<br/><br/>Bias stress experiments on transistors with DHA/VHF-PMMA and SP/MC-PMMA blend dielectrics reveal that the mechanism by which transistor performance is affected is fundamentally different for these two families of molecular switches.