An innovative possibility to introduce additional functionality to organic field-effect transistors (OFETs) is to employ photochromic molecules, which undergo reversible isomerization under applied stimuli such as irradiation with specific wavelengths. As a result, the transistors not only can be switched on/off by the applied voltages, they can also be programmed by alternate triggers, such as light. Here, reversible switching of OFETs is presented by blending various dihydroazulene/vinylheptafulvene photoswitches into polythiophene-based conjugated polymers. In result, the transfer characteristics of the transistors are altered significantly through UV irradiation. In contrast to current literature on different photoswitches such as spiropyrans or diarylethenes, the backreaction is induced thermally and not via visible light irradiation and reproducibly yields the pristine transistor characteristics. This reversible switching upon alternating UV irradiation and thermal annealing is quantified by figures of merit such as the magnitude of drain current, threshold voltage, and subthreshold swing. Irradiating the devices with different doses of UV light shows that the magnitude of switching directly depends on the respective UV dose, hence enabling a multi-level electronic system. Furthermore, long-term cyclability over 100 steps of repeated UV light exposure and thermal annealing is demonstrated.

Derivatives of the hexacyano-[3]-radialene anion radical (CN6-CP•−) emerge as a promising new family of p-dopants having a doping strength comparable to that of archetypical dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). Here, mixed solution (MxS) and sequential processing (SqP) doping methods are compared by using a model semiconductor poly(3-hexylthiophene) (P3HT) and the dopant CN6-CP•−NBu4+ (NBu4+ = tetrabutylammonium). MxS films show a moderate yet thickness-independent conductivity of ≈0.1 S cm−1. For the SqP case, the highest conductivity value of ≈6 S cm−1 is achieved for the thinnest (1.5–3 nm) films whereas conductivity drops two orders of magnitudes for 100 times thicker films. These results are explained in terms of an interfacial doping mechanism realized in the SqP films, where only layers close to the P3HT/dopant interface are doped efficiently, whereas internal P3HT layers remain essentially undoped. This structure is in agreement with transmission electron microscopy, atomic force microscopy, and Kelvin probe force microscopy results. The temperature-dependent conductivity measurements reveal a lower activation energy for charge carriers in SqP samples than in MxS films (79 meV vs 110 meV), which could be a reason for their superior conductivity.