FIGURE 3.9 Archetype organic light-emitting diode structure. SOURCE: Willner et al. (2012). ©IEEE (2012). Reprinted, with permission from Proceedings of the IEEE.
transport layer that moves electrons from the cathode metal contact to the light emissive layer, or “EML.” This layer is typically composed of two different molecules, a charge conductive “host” molecule into which is doped a molecule at very small concentration (~1 to 8 percent by weight) that gives off light of the desired color (or wavelength) under excitation from electrons and holes in the device. This dopant is called the light emissive “guest.” The “HTL” is the hole transport layer whose purpose is to transport positively charged “holes” from the anode contact to the EML. The transparent conducting anode through which the light is viewed is invariably composed of ITO, and the cathode is a metal (such as aluminum doped with lithium) capable of forming an ohmic contact with the ETL for the efficient injection of electrons. Typical OLED structures used in high-efficiency and high-reliability applications are considerably more complex than the structure shown in Figure 3.9. However, in all cases, the total thickness of organic layers rarely exceeds 100 nanometers (1 nanometer = 10–7 centimeter) (Willner et al., 2012). The committee also notes that in contrast to LEDs, OLEDs can be made integral to the luminaire rather than being added to it, in contrast to all alternative lighting solutions. This structural adaptability provides new design possibilities for SSL.
The mechanism for light emission in organic, thinfilm OLEDs (Box 3.2) is fundamentally different than in inorganic semiconductor LEDs described earlier in this chapter. When an electron and its oppositely charged counterpart, the hole, are conducted to the same molecule within the EML, they put the molecule into an excited state. This excitation is maintained for a brief period of time (from nanoseconds to microseconds). While it exists, the excitation can hop from molecule to molecule, which are very densely packed within the EML. This mobile excitation (called an “exciton”) eventually decays by the recombination of the electron and the hole (i.e., the electron “falls into” the hole that is located on the same molecule as the electron). This decay process often emits light whose energy is equal to that of the difference in energies between the electron and hole. By changing the composition or structure of the molecule, the wavelength (color) of light emission can be varied. In fact, small chemical modifications can change the color emission from the ultraviolet, through the blue and green, to the red. In all cases, the light emission can be extremely efficient, with 100 percent conversion of electrons to photons having been reported across the visible light spectrum (Willner et al., 2012).
In a manner similar to the calculation of the EQE of an inorganic LED, the EQE of an OLED depends on both an intrinsic efficiency, for the material and device, and an outcoupling or extraction efficiency, where
where ɸ is the absolute efficiency of a molecule to emit light once excited, Y is the probability that every injected electron and a hole can simultaneously exist on a light-emissive molecule, ƞout is the outcoupling efficiency to be discussed below, and X is the ratio of emissive molecular excited states that an electron and hole can reside on in a single molecule to the total number of possible excited states. X is also known as the excited state ratio. For the best emissive molecules, ɸ = 1, which is often the case with state-of-the-art materials. Furthermore, Y = 1 in properly engineered device structures.
The power efficiency (ƞP) of the light source is its most important operational parameter. Here the optical power out per the input electrical power is related to the quantum efficiency following the formula:
Here, ɵ is the overlap of the light source with the spectral sensitivity of the eye, and Vλ is related to the energy of the emitted photon. The operating voltage of the OLED is V— clearly the power efficiency decreases as V increases. For a given device geometry, the operating voltage is related to the device drive current and thus also has an important influence on the device lifetime.
In conventional OLEDs fabricated on glass substrates, through mechanisms similar to those in inorganic LEDs, much of the emitted light is trapped within the glass substrate or absorbed in the layers that comprise the device (see Figure 3.10), resulting in an extraction or outcoupling efficiency of only ~20 percent. However, low-cost schemes have been reported that can increase this efficiency to 40 to 60 percent (see below). Nevertheless, one of the grand challenges facing OLEDs is how to extract more of the emitted light in a cost-effective and highly efficient manner. This will