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

Abstract

Ultraviolet photoemission spectroscopy was used to determine the work function ϕ and the hole injection barriers Δ h 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. ϕ 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 ϕ, Δ h after pentacene deposition is found to be almost identical for all cases (≈0.35 eV). The mechanism responsible for Δ h being independent of initial surface ϕ 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 Δ h was found to decrease with increasing substrate ϕ.