Interfacial charge injection barriers in organic light-emitting diodes : The effect of thin interlayers of organic donor-acceptor molecules TTF and TCNQ

摘要

Energy level alignment at interfaces between tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) films on representative anode materials Au, ITO and PEDOT-PSS are investigated by ultraviolet photoelectron spectroscopy (UPS). Both materials have broadly uniform behavior independent of the chemical composition of the substrate. The position of the vacuum level of the deposited films is fixed with respect to the substrate Fermi energy, and the hole injection barrier is likewise constant. The magnitude of the vacuum level shift is a simple linear function of the substrate work function. A 4 nm thick interlayer in TTF/TCNQ/Au and TCNQ/TTF/Au compound interfaces does not alter the alignment of the outer material with the substrate. Charge transfer in conjunction with subsequent structural relaxation of the ionic donor or acceptor molecule is proposed to explain the results. Lastly, we discuss how donor-acceptor molecules might be used to mediate charge injection into organic light emitting diodes (OLEDs) and similar organic electronic devices.