LAUSR

DOI: 10.1002/admi.201900211 applications,[1–5] such as light-emitting/ sensing structures[6,7] and organic fieldeffect transistors (OFET).[2] Substantial efforts have been devoted to engineering the interfaces with regard to their energy level alignment in order to tune interfacial energy barriers, hence tailoring the charge carrier injection as well as extraction, and enhancing and expanding the functionality of electronic and optoelectronic devices.[6,8–10] For example, the use of interfacial layers (as thin as mono layers) composed of strong electron acceptor and donor molecules has proven feasible to substantially tune hole and electron injection/extraction properties, respectively.[6,9] However, this approach is limited to a static modification of the energy levels at the interface. More recently, the use of molecular switches has been shown to enable a dynamic energy level tuning,[11–13] which gives the interface multifunctional properties since the energy level alignment can be controlled and programmed via external stimuli, even in fully processed and encapsulated device stacks. These molecular compounds can be reversibly switched between two isomers by external stimuli, such as light, heat, and pH changes.[14,15] The switching between the two isomers fundamentally changes the electronic properties of the molecular compound, for example, the energy gap, the ionization Remote control of the electronic energy levels by external stimuli such as light will enable optoelectronic devices with improved or additional functionalities. Here, it is demonstrated that the electronic properties of ZnO interfaced with negative T-type photoswitches, that is, pyridyl-dihydropyrene (Py-DHP), can indeed be photomodulated. The process of forward switching of Py-DHP with green light from an isomer with a low energy gap to an isomer with a wider one is followed by a thermally activated backward transfer. Using photoemission spectroscopy and density functional theory modeling, it is shown that Py-DHP ring closure/opening reactions result in a reversible shift of frontier occupied molecular levels by 0.7 eV with respect to the Fermi level. Notably, in both molecular configurations, the energy level alignment at ZnO/Py-DHP interfaces is governed by a Fermi level pinning at the lowest unoccupied molecular level. Moreover, upon switching, an increase in the ionization energy for Py-DHP multilayers compared to that of a monolayer is observed. This is attributed to a different preferred molecular orientation in monolayer versus multilayers. The results show that a dynamic tuning of the energy level alignment at inorganic/organic interfaces by external stimuli is feasible and will aid the development of photoprogrammable optoelectronic devices.