Systematic Modification of Indium Tin Oxide to Enhance Diode Device Behavior

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Systematic Modification of Indium Tin Oxide to Enhance Diode Device Behavior

Jing Guo,† Norbert Koch,‡ Jeffrey Schwartz,† and Steven L. Bernasek† † Department of Chemistry, Princeton University, Princeton, NJ 08544-1009 ‡Institut für Physik, Humboldt-Universität zu Berlin, D-12489 Berlin, Germany ABSTRACT

Monolayers of tin complexes of phenoxide ligands spanning a range of dipole moments were prepared on the surface of ITO via simple metathesis reactions. They were characterized by quartz crystal microgravimetry (QCM) and a Kelvin probe. A nearly linear relationship was found between the measured ITO work functions and dipoles of the surface complexes. Measurements of current densities of diode devices built on surface modified ITO anodes were made, and a correlation was found between the total surface dipole per unit area and these current densities. Simple OLED devices were also constructed using these modified anodes. INTRODUCTION

Surface modification of ITO has received wide attention as a means to control its anode properties, particularly to increase its work function in order to lower the barrier to hole injection in novel organic-based optoelectronic devices [1-6]. A simple electrostatic model has been proposed [7] within this context as a guide to enable this increase in work function. This model is based on an introduced dipole layer on the surface of the ITO [8,9]: The effected work function change (∆φ) should be directly proportional to the normal component of molecular dipole moments of surface-attached species (µz) and to the number of such species, per unit area, on the surface. Using organics to effect work function changes for ITO according to the "dipole model" has been described qualitatively, but no quantitative conformity to the "dipole model" had been established. We described surface modification of ITO in ultrahigh vacuum using a series of tin phenoxides [10,11], and we showed that a correlation existed between dipole moments of the parent phenols and changes in the ITO work function that occurred on formation of the surface complexes [11]. Surface loadings were not known. We have reported that the surface deposition and ligand metathetical reactions for tin alkoxides on ITO recorded in UHV [10,11] can be accomplished under normal laboratory conditions (see scheme 1). We found that performing these deposition and metathesis reactions on an ITO electrode-equipped quartz crystal microbalance (QCM) gives us surface complex loadings which, together with Kelvin probe vibrating capacitor measurements of the ITO work function, enabled calculation of surface complex dipole moments to be made and to be related to the “dipole model” [12]. The current density results of ITO treated with such modifications further reveal the quantitative

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control of device performance based on electrostatic theory. OLEDs based on these modified electrodes have now also been prepared and studied. R2

R2 O O O Sn

Sn O OH

ITO

OH

4

O

1

- HO

2

R2

R1 O

2 R3

R1 HO 3

- 2 HO

R1

R3

R3

O

O Sn