Modifications of the exciton lifetime and internal quantum efficiency for organic light-emitting devices with a weak/strong microcavity

Abstract

A comprehensive analysis is given on the modifications of the exciton lifetime and internal quantum efficiency ͑ int ͒ for organic light-emitting devices ͑OLEDs͒. A linear relation is derived between the exciton lifetime and int , which is difficult to measure directly. The internal quantum efficiency can thus be estimated easily through the measurement of the exciton lifetime. The exciton lifetimes for OLEDs with weak or strong microcavity are studied experimentally and theoretically. The modification of the exciton lifetime is well explained through the microcavity effect and surface plasmon resonance. An excellent agreement between the experimental and theoretical results is achieved. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2819610͔Organic light-emitting devices ͑OLEDs͒ with various device configurations have been the subject of intensive research due to their applications in display and lighting. [1][2][3][4] Considering the microcavity effect, OLEDs can be roughly categorized into two types, i.e., weak microcavity OLEDs and strong microcavity OLEDs. Conventional bottom emitting OLEDs are weak microcavity devices, while OLEDs with distributed Bragg reflectors or two metallic electrodes are considered as strong microcavity devices. Light emission properties, including the internal quantum efficiency ͑ int ͒, external quantum efficiency, exciton lifetime, and angular dependence, are distinct in the two types of OLEDs due to the Purcell effect. [5][6][7][8] Exciton lifetimes of emitters in planar dielectric microcavity structures, 9 near a partially reflecting surface, 10,11 in weak microcavity OLEDs, 12,13 and above metallic gratings 14 have been investigated either theoretically or experimentally. In this letter, a comprehensive analysis is given on the modifications of exciton lifetime for OLEDs with various device structures. A linear relation between the exciton lifetime and int will be derived, which means that int can be obtained indirectly through the measurement of exciton lifetime. In addition, we investigate both experimentally and theoretically the exciton lifetime for OLEDs with a weak or a strong microcavity.The theoretical analysis of exciton lifetime is based on a classical approach where the emitter is considered as an electric dipole running at a fixed current and with a random orientation. 6,15,16 As a consequence of Fermi's golden rule, the radiative decay rate ⌫ r ͑ ͒ at a wavelength in an OLED device is modified to 15,16 where ⌫ 0 ͑ ͒ is the radiative decay rate in the infinite medium. Here, F͑ ͒, the so-called Purcell factor, is the normalized total emission power of the electric dipole with random orientation ͑normalized by the total power of the dipole in the infinite medium͒. It should be noted that here, the emission power coupled to the metal electrode is also included in F. We consider such decay channel as radiative decay since this part of emission can potentially be coupled out from the device through, for example, patterning the substrate. 17 In addition, we typica...

High-efficiency fluorescent organic light-emitting diodes enabled by triplet-triplet annihilation and horizontal emitter orientation

“…q eff is the effective radiative quantum efficiency, derived from the (intrinsic) radiative quantum efficiency q by a modification induced by the OLED cavity due to the Purcell effect. [9][10][11][12] Classical limits for g EQE of 5% for fluorescent and 20% for phosphorescent emitters, respectively, have been stated following the assumptions made above, however, recent reports about efficiencies exceeding these limitations have been published: Delayed fluorescence, originating from triplet-triplet annihilation (TTA) [13][14][15][16][17][18] or thermally activated delayed fluorescence (TADF), [19][20][21][22][23][24] increases the radiative exciton fraction beyond 25% and has been identified as one reason for these high efficiencies. By using TADF emitters, comparable efficiencies to phosphorescent OLEDs can be realized.…”

Recent Advances in Transition Metal Complexes and Light‐Management Engineering in Organic Optoelectronic Devices

“…A microcavity is an optical resonator with coplanar reflectors separated at a distance based on the magnitude of the wavelength . They have been previously applied to inorganic light‐emitting diodes for enhancing emission intensity .…”

Electronic and Optical Processes of Organic Semiconductors

“…A good overview on how this effect impacts on organic semiconductor films, in particular in OLED structures, and how this can be described by classical electromagnetic treatment can be found in Furno and coworkers [333]. The modeling and engineering of such optical effects plays an increasing role for the optimization of OSCs and light-emitting diodes [232,[334][335][336][337]. The fluorescence resulting from the spontaneous emission after excitation is referred to as prompt fluorescence.…”