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Physics of Organic Semiconductors, 2nd, Completely New Revised Edition by Russell J. Holmes, Chihaya Adachi, Wolfgang Brutting

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4

Interfacial Doping for Efficient Charge Injection in Organic Semiconductors

Jae-Hyun Lee and Jang-Joo Kim

4.1 Introduction

The discovery of charge conduction in polymers and organic thin films has led to the application of new materials in electronic devices such as light-emitting diodes (OLED), lasers, field-effect transistors (OFETs), photovoltaics (OPVs), sensors, memory chips, and integrated circuits. The most fundamental characteristic of electronic devices is the electrical one represented by the current density–voltage (J–V) relationship.

The J–V characteristics of an organic semiconductor (OS) device are controlled by the charge injection at the electrode/organic and organic/organic interfaces and by the transport in bulk organic semiconductors. The energy level alignment at the interfaces dominantly controls the charge injection at the junctions. Lowering the energy barrier at the interfaces is important to facilitate efficient charge injection through the interfaces. In general, organic materials have low electrical conductivities due to the low intrinsic charge-carrier density and low mobility coming from the hopping transport of charges. The increasing charge density in organics is beneficial by increasing the conductivity of the organics.

Lowering of the injection barriers can be achieved through the proper selection of a metal electrode and an organic material so that the work function of the metal and the highest occupied molecular orbital (HOMO) energy level ...

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