Organic electroluminescence: materials and devices

Abstract

Electroluminescence (EL) inorganics is a phenomenon of both fundamental and practical interest. Defined as direct conversion of electricity into light incorporates many physical processes which are not yet fully understood. Considered originally as an organic analogue of the Destriau effect that is high-field or intrinsic EL found previously in inorganic phosphors, remains still obscure since narrow bands in organic solids rule out electron acceleration to energies allowing impact generation of electronic excited states. Therefore, the search for wide-band organic materials is a challenge to design and to fabricate organic intrinsic EL devices. The recombination radiation following the excess charge carrier injection into a luminescent material stands for injection EL. This type of EL is commonly accepted as a basis for the emission of light from organic materials sandwiched between metal electrodes. In this review, some consequences of the band width and energy level positions in low-molecular weight organic materials and polymers are discussed. Fundamental concepts are illustrated using the notions of recombination ((tau) rec ) and transit ((tau) T ) times of charge carriers. Injection-controlled ((tau) rec greater than (tau) T ) and volume-controlled ((tau) rec less than (tau) T ) injection EL modes are distinguished in organic light- emitting-diodes (LEDs). Two routes to fabricate spectrally tunable organic LEDs are discussed: (a) building multilayer devices with various materials as chromophores (emitters) and (b) employing single-layer conjugated polymer systems with different main-chain molecular structures and blending luminescent polymers into the host polymer matrix, or molecularly-doped non-conjugated polymers with appropriate selection of the optically active component. Charge carrier trapping and excitonic interactions are important factors affecting the recombination routes and production of emitting states as is demonstrated on EL from single organic crystals. Using photoinjecting contacts instead of dark-injecting electrodes allows us to design a unique light transducer combining an organic EL diode with the photosensitive film. Selected examples of organic LEDs and light transducers are presented. Some directions for further work are given.