Energy level alignment at interfaces with pentacene: metals versus conducting polymers

Ultraviolet photoemission spectroscopy was used to determine the work function f and the hole injection barriers Dh of interfaces formed between pentacene and two fundamentally different electrode materials: (i) conductive polymers comprising poly(3,4-ethylenedioxythiophene) and sulfonate moieties, and (ii) Au with chemisorbed Cl. f of these substrates before pentacene deposition covers a wide range of 4.25–5.15 eV for the polymers, and extends to 5.8 eV for Au–Cl. Despite this huge variation in f,Dh after pentacene deposition is found to be almost identical for all cases ( 0.35 eV). The mechanism responsible for Dh being independent of initial surface f is proposed to be an interfacial charge transfer reaction leading to cationic pentacene species. These findings are compared to interfaces between pentacene and clean metal surfaces, where Dh was found to decrease with increasing substrate f.