Modification of indium tin oxide surfaces: Enhancement of solution electron transfer rates and efficiencies of organic thin-layer devices.

摘要

This dissertation has focused on the study of the ITO/organic heterojunction and the chemistries therein, and proposes appropriate strategies that enhance the interfacial physical and electronic properties for more efficient charge injection with application to organic thin-layer devices. We focused on four major aspects of this work: (i) To characterize the ITO surface and understand the chemistries that may be pertinent to interaction with adjacent organic layers in a device configuration. This developed into a working model of the ITO surface chemistry and provided a foundation for modification strategies. Characterization of the electronic properties of the surface indicated that less than 5% of the geometrical surface area is responsible for the bulk of current flow while the rest is electrically inactive. Likely, this is due to the build up of an In(OH)X type species which is isolating in nature. (ii) To determine the extent to which these chemistries are variable and propose circumstances where compositional changes can occur and characterize the resulting change in surface chemistry. It is shown that the surface chemistry of ITO is heterogeneous and possible very dynamic with respect to the surrounding environment. Solution pretreatments and plasma based etching had substantial effects on the surface chemistry. In particular, the amount of In(OH) X material present. (iii) To propose a strategy for modification of the interface which leads to enhanced physical and/or electrical properties. Modification of ITO surfaces by small molecules containing carboxylic acid functionalities is investigated. To accomplish this, a protocol for solution electrochemical probing was developed utilizing ferrocene/ferrocenium as the probe. Enhancements in the electron transfer rate coefficient were realized after modification of the ITO electrode. The enhancements are found to stem from a light etching mechanism which removes a portion of In(OH)X material and exposes a larger electrically active area. Additionally, an elecro-catalytic effect was observed with some of the modifiers used increasing k eff further. (iv) Apply these modifications to the development of model organic light emitting diodes (OLEDs) and organic photovoltaic devices (OPVs). Selected modification strategies including small molecule chemisorption and conductive polymer overlayers were utilized in OLED and OPV devices. Enhancements seen in solution electrochemical experiments are indicative of the enhancements seen for solid state devices. Modifications resulted in substantially lower leakage currents (3 orders of magnitude in some cases) as well as nearly doubling the efficiency. Also noted, the best devices utilized a combination of small molecule chemisorption and polymer overlayers.; An additional chapter describes the creation and characterization of electrochemically grown polymer nano-structures based on blazed angle diffraction gratings. The discussion details the micro-contact printing process and the electro-catalytic growth of the conductive polymers PANI and PEDOT to form diffraction grating structures in their own right. The resulting diffraction efficiency of these structures is shown to be sensitive to environmental conditions outlining possible uses as chemical sensors. This is demonstrated by utilizing these structures as working pH and potentio metric sensors based on the changing diffraction efficiency.