ABSTRACT
Conduction ................................................................... 116 3.3 Effects of Doping ......................................................... 124 3.4 Effects of Discrete Traps ............................................ 129 3.5 Heterojunction Device................................................. 133 3.6 Summary...................................................................... 139 Acknowledgments................................................................. 139 Appendix: Theoretical Models ............................................. 139 References............................................................................. 142
3.1 INTRODUCTION
This chapter deals with a subset of carrier transport problems in organic light-emitting diodes (OLEDs): those problems within the space charge-limited (SCL) conduction regime. In particular, we emphasize the case in which trapped charges exist in high concentrations. Typical organic materials (e.g., Alq3) used in modern OLEDs have few intrinsic carriers. During normal operation, high concentrations of electrons and holes are injected from opposite electrodes and then recombine within a suitable light-emitting layer. These electrons and holes do not usually have a uniform distribution within the organic layers. Carrier concentrations toward the respective injecting electrodes are usually much higher than those within the body of the organic layer, and space charges are formed. A large body of theoretical and experimental work has been devoted to the study of carrier transport and recombination within the SCL conduction regime.1-7 The space charge can involve traps, namely, energy states that are within the energy gap and are away from the conducting bands by more than a few kT, where k is the Boltzmann constant and T is the absolute temperature. Carriers in these states (traps) have low escaping probability and tend to be stationary even when an electric field is applied.
