Charge injection barrier heights across multilayer organic thin films

Organic multilayer structures having a tetrathiafulvalene (TTF) or tetracyanoquinodimethane (TCNQ) interlayer were grown by physical vapor deposition on gold, indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) PEDOT-PSS, substrates chosen to be representative of the anode materials typically used in the manufacture of organic electronic devices. The top layer was either p-quarterphenyl (Qp) or the hole transport material N,N'-bis(3-methylphenyl)N,N'-diphenylbenzidine (TPD). The valence electronic structure and vacuum level of the compound interface were monitored as a function of film thickness by ultraviolet photoelectron spectroscopy (UPS). Here we report the dependence of the energy level alignment on the substrate work function and discuss how an organic interlayer having a strong electron accepting or donating characteristic might be used to control the charge injection from a conducting electrode to a semiconducting organic hole transport material.