Switching the Electronic Properties of ZnO Surfaces with Negative T‐Type Photochromic Pyridyl‐dihydropyrene Layers and Impact of Fermi Level Pinning

Abstract

Remote control of the electronic energy levels by external stimuli such as light will enable optoelectronic devices with improved or additional functionalities. Here, we demonstrate that the electronic properties of ZnO interfaced with molecular negative T-type photoswitches, i.e., 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.This process is found to be fully reversible and efficient in the solid state. Using photoemission spectroscopy and density functional theory (DFT) modelling, we show that the photochromic ring closure/opening process results in reversible shift of the frontier occupied molecular level by 0.7 eV with respect to the Fermi level. Notably, in both molecular configurations, the energy level alignment at the ZnO/Py-DHP interface is governed by a Fermi level pinning of the lowest unoccupied molecular level. Moreover, upon switching, we observe an increase in the ionization energy for Py-DHP multilayers compared to that of a monolayer. We attribute this to a different preferred molecular orientation in monolayer versus multilayer films. Our results show that dynamic tuning of the interface energy level alignment at inorganic/organic junctions by external stimuli is feasible and will aid the development of photoprogrammable opto-electronic devices.

Enlightening Materials with Photoswitches

“…This research endeavor is an important step toward multifunctional optoelectronic devices, in which several interfaces are brought together that can be addressed individually by external stimuli to gain control over the electrical output of an electric device. Photoswitchable SAMs have moreover been used to tune the energy levels of various (semi)conducting oxides, such as ZnO and indium tin oxide . The observed effect was attributed to the fact that the frontier molecular orbitals are energetically perfectly suited to allow for resonant tunneling through the closed but not the open isomer.…”

UV Photochromism in Transition Metal Oxides and Hybrid Materials

“…Pyrolidine‐dihydropyrene (Py‐DHP) is a T‐type organic compound evaporated to ZnO surface to exhibit unique photochromic properties ( Figure a). [ 224 ] The ring opening/closure of the PyDHP switching is triggered by visible light (ring opening) and UV light (ring closure). For example, ring opening is initiated with 30 min illumination at λ = 565 nm and is reversed by UVA exposure.…”

“…Reproduced with permission. [ 224 ] Copyright 2019, Wiley. b) Chemical structure of the PA‐DAE molecule in its open and closed form.…”

Отрицательный Фотохромизм: Последние Достижения

“…Оно обесцвечиваясь под действием видимого излучения и возвращаясь в исходное состояние при нагревании при 50 С. Показано, что это соединение может использоваться для фотоуправления уровнями электронной энергии в оптоэлектронных устройствах [56].…”