2

Recombination Mechanisms .............................. 49 2.4 The Nature of Emissive States .................................... 58

2.5 Injection Currents ......................................................... 66 2.5.1 Injection-Limited Currents (ILC)...................... 66 2.5.2 Diffusion-Controlled Current (DCC) ................. 70 2.5.3 Space-Charge-Limited Current (SCLC)............ 71 2.5.4 Double Injection.................................................. 72

2.6 EL Quantum Efficiency ................................................ 74 2.6.1 Basic Definitions................................................. 74 2.6.2 Steady-State EL.................................................. 76 2.6.3 Field Dependence of the EL QE........................ 83

2.7 Final Remarks ............................................................. 101 Acknowledgments................................................................. 103 References............................................................................. 103

2.1 INTRODUCTION

The various ways light is generated by applying an electric field to organic materials, without involving any intermediate energy forms — the phenomenon known as organic electroluminescence (EL) — render our present understanding of organic EL phenomenological at best. In dealing here with its microscopic mechanisms we wish to strike a note of challenge rather than achievement. The experimental material available reminds us in many respects of the organic crystal field in the pre-thin film era and the theory has been often adequate only to formulate the problems rather than to solve them. The laws of nature seem to be well known, the difficulties of interpretation come from the complexities of the problems arising from the diversity of emitting states, the structure of organic light-emitting diodes (LEDs), and from the great number of variables. We do not try to review all the aspects of the field of organic EL as they have been summarized in recent reviews,1-9 which contain references to original papers. Instead, a review of the general features of the optical properties of organic LEDs is given without attempting to achieve completeness by quoting all the important papers in the field. The main subject of the present chapter is correlating the optical properties of charge recombination underlying organic LEDs with their electrical characteristics. The ability

to convert the supplied electrical power into light in an organic LED is of primary interest, and this fundamental aspect is considered in some detail. Apart from the purely scientific interest, such considerations may be important also for modern LED technology.