Physics of Organic Semiconductors, 2nd, Completely New Revised Edition

3

Electronic Structure of Molecular Solids: Bridge to the Electrical Conduction

Nobuo Ueno

3.1 Introduction

Organic semiconductors, discovered in the mid-twentieth century [1–3], have larger bandgap and smaller bandwidth than their inorganic counterparts. The number of thermally excited carriers in the organic films is not enough to give sufficient current. We thus need injection of carriers into the organic film from electrodes to realize sufficient current in organic devices. This chapter describes fundamental aspects of electronic structure of organic semiconductors and the method to bridge the electronic structure and electrical property using ultraviolet photoemission spectroscopy (UPS).

As it is well known, the electrical conductivity (σ) is given by

(3.1)

where n is the carrier concentration, q is the charge of the carrier concerned, and μ is the charge-carrier mobility. This relation simply indicates that we must increase n and μ, if we need larger conductivity and electrical current. To obtain sufficient current in organic films, we need to inject charge carriers effectively from electrodes to increase n. As the carrier injection is dominated by the charge injection barrier height that is the energy difference between the Fermi level (E_F) and the highest occupied molecular orbital (HOMO) state (for hole) or the lowest unoccupied molecular orbital (LUMO) state (for electron) ...