Dynamic Photoswitching of Electron Energy Levels at Hybrid ZnO/Organic Photochromic Molecule Junctions

Abstract

The functionality of interfaces in hybrid inorganic/organic (opto)electronic devices is determined by the alignment of the respective frontier energy levels at both sides of the heterojunctions. Controlling the interface electronic landscape is a key element for achieving favourable level alignment for energy and charge transfer processes. Here, it is shown that the electronic properties of polar ZnO surfaces can be reversibly modified using organic photochromic switches. By employing a range of surface characterization techniques combined with density functional theory calculations, it is demonstrated that self-assembled monolayers (SAMs) of photochromic phosphonic acid diarylethenes (PA-DAEs) can be employed to reversibly change the electronic properties of polar ZnO/ SAM structures by light stimuli. The highest occupied molecular orbital level of PA-DAE is raised by 0.7 eV and the lowest unoccupied one lowered by 0.9 eV, respectively, upon illumination by ultraviolet light and the levels shift back to their original position upon illumination by green light. The results thus provide a pathway to tailor hybrid interface electronic properties in a dynamic manner upon simple light illumination, which can be exploited to reversibly tune the electrical properties of photoswitchable (opto)electronic devices.

Dynamically Switching the Electronic and Electrostatic Properties of Indium–Tin Oxide Electrodes with Photochromic Monolayers: Toward Photoswitchable Optoelectronic Devices

“…The synthesis of the PA-DAE molecules is reported in Ref 14 . The deposition of PA-DAE on ITO (roughness < 1 nm, purchased from Thin Film Devices Inc.) is based on a procedure that has been optimized for phosphonic acids and extensively reported previously, 3,14 and is as follows: the SAM was fabricated by immersion of the ITO coated glass into a 1 mM solution of the PA-DAE switch (in the open form) in anhydrous THF for two hours, and subsequently annealed in air for 40 min on a hot plate at 90 °C.…”

“…The results are consistent with the observation for PA-DAE SAM on ZnO (where the water contact angle increases from 46° to 101°). 14…”

Electronic and Optical Excitations at the Pyridine/ZnO(101¯0) Hybrid Interface

“…[1][2][3] These properties together with a type-I level alignment at the hybrid interface would be ideally suited for light-emitting applications. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The adsorption of a molecular layer on inorganic surfaces has also been exploited to tune the work function of the inorganic component. [6][7][8][9][10] Prerequisite for that is the lightinduced creation of hybrid or charge-transfer excitons, which exhibit the hole to a large extent on one side of the interface while the excited electron would reside on the other side.…”

Simultaneous Effect of Ultraviolet Radiation and Surface Modification on the Work Function and Hole Injection Properties of ZnO Thin Films

“…Motivated by this finding, we investigated the effect of the UV radiation on the WF of SAM‐treated ZnO. Benzyl phosphonic acid (BPA), 24‐fluorobenzyl phosphonic acid, (pyrimidin‐2‐yl) methylphosphonic acid (PyPA), and BPA:FBPA mixtures were bound to the ZnO surface following a procedure reported previously . The open squares in Figure show the WFs of such treated ZnO substrates, which were prepared and measured in the dark.…”

“…Among those, zinc oxide (ZnO) is widely used as a hole blocking electrode in electron only devices or as transparent electron‐extracting cathode in organic and hybrid solar cells Work function (WF) measurements with ultraviolet photoelectron spectroscopy (UPS) typically reveal values of ≈4 eV for untreated ZnO, rendering it an appropriate electrode for n‐type organic materials. Self‐assembled monolayers (SAMs) based on phosphonic acids (PAs) are frequently employed as interface modifiers to tune the WF of ZnO, enabling efficient injection of electrons and even holes into a wide range of semiconducting molecules and polymers . In contrast, ultraviolet (UV) illumination (light‐soaking) was shown to lead to transient and persistent changes of the WF of metal oxides, and particularly of ZnO .…”