Paper TC+EM+EN+TF-WeA10
Interfacial Layer Engineering of Transparent Conductive Oxides for Optoelectronic Device Application

Wednesday, October 30, 2013, 5:00 pm, Room 102 B

Transparent conductive oxides (TCOs) have widespread utility as electrical contacts in photovoltaic (PV) and other optoelectronic devices, such as display screens and organic light emitting diodes (OLEDs). The TCO surface chemistry can be tailored through the addition of interfacial layers (IFLs), such as polymers, covalently bonded organofunctional silanes, and chemisorbed small molecules. These IFLs can be used to optimize rates of charged carrier transfer, and increase the compatibility of the polar TCO with nonpolar materials used in OPVs and OLEDs. Understanding the interactions between these materials and the TCO interfaces is essential to controlling device performance. For example, PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)), a polymer commonly used in OLED and OPV devices, functions as an electron blocking layer for the TCO anode, improving device efficiency. However, the polymer is highly acidic and can be corrosive to the TCO layer; in the case of organic PV devices, this can limit their lifetime.

We used organofunctional silanes, including allyl triethoxy silane (ATES), octa-decyl-trichloro silane (OTS), 3-aminopropyl-triethoxy silane (APTES) and 3-aminopropyl-dimethyl-ethoxy silane ( APDMES ), to modify indium tin oxide (ITO), aluminum doped zinc oxide (AZO) and fluorine doped tin oxide (FTO). We characterized the electrical and optical properties and surface energies of the silanized TCOs, and compared the results to a standard OPV polymer, PEDOT:PSS. Results demonstrate that varying the functionality and deposition conditions of the silane is a simple method of tuning and customizing the surface energy of the hydrophilic TCO; water contact angles ranging from 57° (APDMES) to 94° (OTS) are achieved without affecting the TCO transparency or conductivity. Additionally, both bare and silanized ITO, AZO and FTO were exposed to damp heat (DH, 85 °C, 85% relative humidity) for up to 1000 hours. After each exposure a standard cleaning process was used and the TCOs’ electrical and optical properties and surface energies were determined. Using contact angle measurements with multiple fluids, the surface energies of the TCOs were tracked, and the largest change in total surface energy was found for AZO, then ITO, with FTO remaining essentially unchanged for the conditions studied. In preliminary degradation studies of TCO/silane stacks, ATES was found to delay and reduce the resistivity increase of ITO in damp heat. Further degradation data of TCO/silane stacks, with and without encapsulation will be presented.