A review on the light extraction techniques in organic electroluminescent devices
Introduction
Organic light emitting diodes (OLEDs) have rapidly developed over the last decade because of their potential applications in flat-panel displays, domestic solid state lighting etc. The underlying principle of light generation mechanism in organic light-emitting devices (OLEDs) is the radiative decay of molecular excited states, also known as excitons [1], [2], [3], [4], [5], [6]. The basic device structure of a conventional OLED consists of glass substrate coated with transparent conducting oxide, an organic and polymeric light emitting layer sandwiched between hole transport layer (HTL) and electron transport layer (ETL) and on the top of the device a metal cathode is deposited. In most of the conventional small molecule organic materials only the radiative decay of singlet excitons is responsible for generating light and the energy of triplet excitons is wasted because of the slow radiative decay and generates delayed fluorescence [7]. Thus the internal quantum efficiency ηint (the ratio of the total number of photons generated within the organic emitter to the number of injected electrons) of fluorescent OLEDs is only 25% [2], [7]. It has been demonstrated that the internal quantum efficiency of OLEDs can be achieved near 100% by means of harvesting both singlet and triplet molecular excitation states using electro-phosphorescent materials, which is nearly fourfold increase in efficiency as compared to singlet – harvesting fluorescent organic materials [8], [9], [10]. The external quantum efficiency ηext (ratio of the total number of photons emitted by the OLED into the viewing direction to the number of electrons injected into organic emitter) of an OLED device is related to the internal quantum efficiency ηint and the external coupling efficiency ηcoupling (the ratio of the total number of photons coupled out in the forward direction to the number of injected electrons) by the following relation [11], [12]In this expression γ is the electron–hole charge balance factor, ηexc is the fraction of total number of excitons formed which result in radiative decay (ηexc ∼1/4, and ∼1 for fluorescence, and electro-phosphorescence based OLED materials, respectively), and Φp is the intrinsic quantum efficiency for radiative decay (including both fluorescence and phosphorescence) [11], [12]. The internal quantum efficiency of OLEDs is mainly affected by the non-radiative electron–hole recombination loss and the singlet–triplet branching ratio [7], [11], [12]. Most of the electro-phosphorescent OLED materials have very small non-radiative loss. On the other hand, despite achieving near 100% internal quantum efficiency, the external coupling efficiency (ηext coupling) of the conventional OLED device remains very low. Assuming isotropic emission in the organic layer and a perfectly reflecting cathode, the fraction of generated light escaping from the substrate is [11], [12], [13], [14]where n is the refractive index of OLED material and ξ is a constant that depends on the dipole alignment and the geometry of the OLED device. For most of the organic materials n is about 1.7 and taking the value of 2 for ξ, the internal coupling efficiency is only about 20%. According to classical ray optics theory about 80% of generated light is lost in wave-guided modes due to glass substrate and ITO/organic material (Fig. 1) which means that the majority of the light is either trapped inside the glass substrate and device, or emitted out from the edges of an OLED device [13], [14], [15], [16]. The external quantum efficiency ηext of OLED is given by [11]:where q is electronic charge, λ is wavelength, Idet(λ) detected photocurrent, h Planck’s constant, c speed of light in vacuum, f is OLED-to-detector coupling factor, IOLED is current and R(λ) is the detector responsivity. For OLED applications in large area display and general illumination, it is the power efficiency also known as wall plug efficiency ηpower (W/W) which is generally used for quantifying the light amount and is defined as the ratio of total output light power to the input electrical power and can be determined from known external quantum efficiency [2], [11]where V is external driving voltage, hν/e is the average energy of emitted photon of frequency ν. Finally the luminous efficiency ηL = ηpower·l (lm/W) is calculated, where l is luminous efficacy.
On applying a DC-field light is generated from thin organic emitting layer spontaneously in all the directions and propagates via various modes, that is, external modes (escape from the substrate surface), substrate-, and ITO/organic wave-guided modes due to total internal reflection (TIR) [13], [14], [15], [16] as depicted in Fig. 1. For incidence angles larger than critical angle TIR occurs as can be seen from Fig. 1 and because of which ∼30% light is trapped inside the glass substrate and ∼50% light is propagated though ITO-organic wave-guided modes shown by dotted lines. For the purpose of applications in general illumination and flat-panel displays, light emitted from the substrate surface (external modes) is only 20% of the total emitted light from inside the OLED device [13], [14], [15], [16]. In this article we discuss novel approaches that have been implemented to improve the external coupling in OLEDs by means of various internal and external device modification techniques such as, substrate modification methods, use of scattering medium, micro-lens arrays, micro-cavity, photonic crystals and nano-cavity, nano-particles, nano-wires, nano-structures and surface plasmon-enhanced techniques. Some of the efficient techniques are reviewed and future prospects are presented.
Section snippets
Substrate modification techniques
The structure of electroluminescence emission pattern of OLEDs through surface and edge of the glass was characterized by Kim et al. [16]. It was found that in the absence of micro-cavity the surface emission is nearly Lambertian while the edge emission comprises discrete substrate reflection and leaky waveguide modes. A combined classical and quantum mechanical model was used to calculate the distribution of light emission from a planar OLED [17], [18]. It was found that the ITO/organic modes
Light extraction by scattering medium
For the application in general lighting the light extraction from OLEDs through light scattering is one of the effective choices because it offers inherent advantages, like constant color over all observation angles, symmetric illumination and uniform and Lambertian distribution [32], [33], [34], [35], [36]. An ordered monolayer of silica micro-spheres with a diameter 550 nm as a scattering medium was incorporated into the conventional two-layer OLED structure by Yamasaki et al. [32]. The
Light extraction by micro-lenses
Use of micro-lenses on the backside of the glass substrate is one of the most effective technique for extracting out substrate wave-guided modes. Significant improvement in the light out-coupling efficiency of OLEDs has been achieved using ordered micro-lens arrays [41], [42], [43], [44]. Moller and Forrest [41] fabricated the ordered micro-lens arrays (Fig. 4a) on the backside of OLED glass substrate with 10 μm diameter of PDMS with refractive index n = 1.4. Fig. 4b depicts the schematic diagram
Light out-coupling enhancement by micro-cavity structure
An OLED can be regarded as one-dimensional micro-cavity because the total thickness of organic films in the device is of the order of wavelength of light. There are two types of micro-cavities in OLED, that is, weak and strong micro-cavities [48]. Weak micro-cavity is formed with the conventional OLED structure due to the metal cathode and high refractive index anode (ITO) [49] while a strong micro-cavity OLED structure usually consists of a metal mirror on one side and a highly reflective
Light extraction by photonic crystals and nano-cavities
Photonic crystals (PCs), also known as photonic band gap (PBG) materials [75], [76] are periodic dielectric micro- (nano-) structures that have an energy band gap which forbids the propagation of a certain frequency range of light. PCs are new class of materials that provide novel capabilities for the control and manipulation of the flow of light [77], [78]. Recently photonic crystals PCs are being extensively exploited for light extraction in inorganic [79], [80] and organic LEDs [81], [82],
Surface plasmon enhanced OLEDs
Surface plasmons (SPs) are playing great role in merging photonics and electronics at nano-scale dimensions and opened up a new field of research called plasmonics [90]. Recently, the role of surface plasmons in the enhancement of light intensity in LEDs and OLEDs [91], [92], [93], [94], [95] has been studied extensively. Hobson et al. [93] have studied that the coupling between organic light-emitting materials and SP modes associated with the metallic cathode result in a loss of efficiency. In
Light-out-coupling by nano-structured films, nano-wires and nano-particles
Effect of nano-structured films, and nano-particles both metallic and magnetic nano-particles on the enhancement of extraction efficiency of OLEDs has been demonstrated recently [108], [109], [110]. In the case of nano-structured OLEDs both periodic and aperiodic nano-patterns have been used for extracting the light. Significant enhancement in light extraction efficiency was obtained by Lee et al. [109] by means of periodic nano-patterned OLEDs (Fig. 8a). It was shown that the periodic
Enhanced light out-coupling using aperiodic dielectric mirrors
One-dimensional and two-dimensional periodic structures lead to strong variations in brightness and color with viewing angle and the emission is enhanced only over a narrow range of wavelength and angle. Agarwal et al. [121] recently demonstrated that an aperiodic dielectric stack between the substrate and transparent anode in organic LED can enhance out-coupling efficiency up to 80%. A nine layer SiO2/SiNx were fabricated between the glass substrate and ITO layer and with increase in the
Summary and future prospects
A comparative study of the various light extraction techniques that have been implemented in OLEDs is presented in Table 1. It can be seen from Table 1 that various techniques have their own merits and demerits. Modifications in the device structure can be done according to the applications of the OLED. For example for small area displays a photonic crystal structure inside the OLED and micro-lenses outside the device may be good choice. On the other hand, for large area displays and general
Acknowledgement
The authors are grateful to DRDO, New Delhi for funding this project (No. ERIP/ER/073669).
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