The Basics of Organic Light Emitting Diodes
This tutorial gives a short introduction to the field of Organic Light Emitting Diodes (OLED's).
The tutorial is divided in 4 sections:
What are OLED's ?
Organic light emitting diodes are a relatively new technology for solid state light sources. A typical OLED consists of two organic layers (electron and hole
transport layers), embedded between two electrodes. The top electrode is usually a metallic mirror with high reflectivity and the bottom electrode a transparent ITO layer on top of the glass substrate.
Construction of an OLED
Working principle
When a voltage is applied to the electrodes the charges start moving in the device under the influence of the electric field. Electrons leave the cathode and holes move from the anode in opposite direction. The recombination of this charges leads to the creation of a photon with a frequency given by the energy gap (E = hν) between the LUMO and HOMO levels of the emitting molecules. Therefore, the electrical power applied to the electrodes is transformed into light.
Charge transport and light generation in OLED's
Different materials and dopants can be used to generate different colors and the combination of them allows building up a white light source.
Materials used in OLED's
Efficiency
The ratio between the electrical power supplied to the device and the optical power that comes out through the glass substrate determines the efficiency of our device. It can be divided into the internal efficiency, that is the number of generated photons per injected electron, and the optical outcoupling efficiency which is the percentage of the generated light that is able to escape from the device through the glass substrate.
Limitation of the efficiency by total internal reflection
The latter depends strongly on the refractive index of the emitting material (n = 1.7), the refractive index of the substrate (n = 1.5) and the reflectivity of the cathode. In a thick device (relative to the wavelength of light), and in absence of scattering effects, the outcoupling efficiency (η) is given by the following analytical formula following from Snell's law:
But in actual OLED's the thickness of every layer is in the order of the wavelength of the generated light, yielding an angular distribution very different from isotropic. Interference effects like Wide-Angle or Multiple Beam interferences are presents in the actual OLED. Therefore, the thickness of every layer and the location of the emitter play an important role on the angular distribution of the generated photons.
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| Wide angle interference | Multiple beam interference |
The emission depends strongly on several factors:
- The wavelength
- The location of the emitter
- The thickness of the emitting layer
But, in general the most restrictive parameters for the outcoupling efficiency are the reflectivity of the metallic mirror and the refractive index of the organic layers.
The amount of light that is emitted into air can be enhanced by optimizing the thickness of the different layers.
Maximizing the efficiency by varying the thikness of the different layers
The presence of scattering centers in the substrate or especially in the active medium, where most of the light is trapped, could result in an increment of the outcoupling efficiency. Photonic crystals or nanostructured surfaces can also be applied to avoid lateral propagation of the light inside the OLED, bringing more light into the escape cone.
Scattering centers in OLED's can modify the direction of the trapped light
Applications and advantages
OLED's can be used for general lighting as well as for displays, backlight sources in LCD displays, signaling (emergency lighting, traffic signals) or automotive applications (dashboard).
Applications using OLED's
OLED technology has some major advantages for these applications:
- High efficiency and large area sources.
- High brightness and wide viewing angle.
- Thin, flat and lightweight.
- Low voltage and fast switching technology.
- Form freedom and tunable emission.
- Flexible displays.
- Low cost production.
But there are still a few breakthroughs to be made, especially in lifetime and efficiency issues.
More about OLED's: http://www.extra.research.philips.com/euprojects/olla/