Spectroelectrochemical characterization of charge injection processes in thin films on ITO

Authors : Zeynep Ozkan Araci, R. Clayton Shallcross, Neal R. Armstrong, and S. Scott Saavedra

The efficiency of charge transfer across interfaces between layers of dissimilar materials is a crucial factor in the performance of several types of organic electronic devices, such as organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). We are studying the kinetics of charge transfer in different types of conductive films deposited on indium tin oxide (ITO) electrodes using a novel spectroelectrochemical technique, potential modulated attenuated total reflectance (PM-ATR). Poly(3,4-ethylenedioxythiophene/)/poly(styrenesulfonate) (PEDOT/PSS) is frequently used to facilitate hole injection between ITO and the light emitting layer in an OLED. Charge transfer between PEDOT/PSS and ITO cannot be measured electrochemically due to high non-Faradaic background, which is eliminated using the optical method. The charge transfer rate measured using PM-ATR is in the range of 13 – 22 s-1; we are
currently evaluating organic monolayers as promoters to enhance this rate. We are also studying semiconductor nanoparticles tethered to ITO. Our recent experiments with CdSe nanoparticle films show that we can easily measure reversible charge injection at submonolayer surface coverage. Our data represent the first reported measurements of charge transfer rates in these types of films on ITO.

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