Indium‐Tin‐Oxide (ITO) Work Function Tailoring by Covalently Bound Carboxylic Acid Self‐Assembled Monolayers

Abstract

Tuning the work function of electrodes is crucial to improve the carrier injection into organic optoelectronic devices in order to minimize energy losses. We have analyzed and explored the potential of self‐assembled monolayers (SAMs) covalently bound to an electrode to tune its work function for optimized organic optoelectronic devices. A systematic experimental photoelectron spectroscopy study of several carboxylic acid SAMs, deposited onto indium‐tin‐oxide (ITO), reveals work function changes up to 1.8 eV. Subsequently, the effects of different SAMs on the injection barrier in organic thin film transistors and thus their efficiency are studied. The data obtained can be described qualitatively, regarding the key factors for work function tuning, i.e., the intrinsic molecular dipole, the energetic position of the conducting molecular orbital as calculated by DFT and the contribution of the chemical bond at the electrode–SAM interface. Using the insights gained from the applied theoretical descriptions of work function tuning combined with the experimental findings, work function tailoring can be made possible and enables the design of an energetic matching interface.

The role of self-assembled monolayers in electronic devices

“…A number of reports related to this work are available in the literature. [110][111][112][113] In TFT devices, another important interface is the semiconductor/gate dielectric interface. This interface is responsible for the accumulation of charge carriers on the surface of the semiconductor.…”

“…Another issue with SAMs is the presence of defects, such as missing molecules on particular sites, that occur during SAM formation. These defects are found even when single crystalline surfaces such as Au (111) and optimal preparation conditions are used. 133,139,140 The presence of these defects limits the further application of SAMs in electronic devices.…”

The Impact of the Surface Modification on Tin-Doped Indium Oxide Nanocomposite Properties

“…Numerous computational and experimental methods have been implemented to investigate surface defects, surface interactions, binding between organic molecules and the surface, and structural properties of nanomaterials such as gold nanoparticles [ 18 ], CoBr 2 nanolayers [ 19 ], CdO and ZnO [ 20 ] metal oxides and SnO 2 /CdO, ITO and Zn 2 SiO 4 /ZnO nanocomposites [ 21 , 22 , 23 , 24 ]. Due to experimental limitations, it is difficult to analyze the interaction of organic molecules and surfaces at the molecular level.…”

Surface Dissociation Effect on Phosphonic Acid Self-Assembled Monolayer Formation on ZnO Nanowires

“… 5 , 8 − 10 For example, SAMs were applied onto ITO substrates to modulate the work function. 11 , 12 Among various head groups of SAMs, a phosphonic acid has particularly attracted the significant attention of many researchers 13 − 18 because phosphonic acids provide a more robust modification layer when compared to those formed from carboxylic acids, especially after annealing to maximize the number of P–O bonds. 19 − 22 For example, interesting applications using phosphonic acid SAMs on ZnO surfaces have been successfully demonstrated 8 − 10 , 13 − 15 , 21 − 23 since ZnO is one of the most popular metal oxide materials.…”