Evolution of OLEDs

1 Evolution of OLEDs

Prior to the OLEDs, many display technologies such as cathode ray tube, inorganic light emitting diodes, liquid crystal displays, plasma displays were leading in the display market. All these displays have their own limitations including bulkiness,low viewing angle, color tunability, etc. The essential requirements of present generation displays are reproduction of good light quality,brightness, contrast, improved color variation, high resolution, low weight,reduction in thickness, reduction in cost, low power consumption. All these short comings are rectified in these OLED devices and a new flat panel display technology on these organic based devices commonly

known as OLEDs emerged.

Key advantages of these OLEDs are that the displays are flat, lighter in weight, emissive fast, operate at a very low voltage and offer the prospect of simple fabrication. They have excellent viewing angle and the potential of much low power consumption than backlit liquid crystal displays.

1.1 Structureof OLED

Structure of different OLEDs is shown in Fig.1. A single-layer OLED is made of a single organic layer sandwiched between the cathode and the anode. This layer must not only possess high quantum efficiency for photoluminescence, but the layer must also have good hole and electron transport properties. In a two-layer OLED, one organic layer is specifically chosen to transport holes and the other layer is chosen to transport electrons. Recombination of the hole–electron pair takes place at the interface between the two layers, which generates electroluminescence. In a three-layer OLED an additional layer is placed between the hole transporting layer and the electron-transporting layer. The emitting layer is primarily the site of hole–electron recombination and thus for electroluminescence. This cell structure is useful for emissive materials that do not possess high carrier transport properties. In a multi-layer OLED, an electron injection layer is also included. Introduction of multi-layer device structure eliminates the charge carrier leakage as well as exciton quenching,as excited states are generally quenched at the interface of the organic layer and the metal. Multilayer OLEDs consist of different layers namely ITO glass plate, hole injection layer (HIL), hole transport layer (HTL), emitting layer(EML), electron transporting layer (ETL) and anode.

Substrate:This is usually clear plastic, glass, or metal foil, which is a transparent andconductive substrate with high work function (ϕw ≈ 4.7–4.9 eV).

Anode:This is as a transparent electrode to inject holes into organic layers. Important requirement of this layer is that it must have low roughness and with high work function.

Hole injection layer (HIL): The materials with high mobility, electron blocking capacity and high glass transition temperature can be used as HIL.

Hole transport layer (HTL): Hole transporting layer plays an important role in transporting holes and blocking electrons, thus preventing electrons from reaching the opposite electrode without recombining with holes.

Emissive layer (EML): The layer in between HTL and ETL is a good emitter of visible photons, generally known as emissive layer (EML). This layer can be a material made of organic molecules or polymers with high efficiency, lifetime and colorpurity.

Electron transport layer (ETL): This layer should have good electron transporting and hole blocking properties.

Cathode:Cathode is typically a low work function metal alloy (ϕw ≈2.9–4.0 eV). The cathode injects electrons into emitting layers. It is transparent in top emitting devices. It must be stable to the organic layers under it.

Deposition of all these layers on ITO glass substrate itself is too critical because of the sensitivity of the material to different factors such as high temperature,incorporation of dust during fabrication. Various materials generally used in different layers of OLED are tabulated in Table 1.

1.2.Types of OLEDs

Different formulations of OLEDs namely, passive-matrix OLED (PMOLED), active-matrix OLED(AMOLED), transparent OLED, top-emitting OLED, bottom-emitting OLED, foldable OLED and white OLED along with their characteristics are described in Table 2.

1.3.Materials for OLEDs

Different classes of organic semiconductors used in OLEDs are shown in Fig. 2. OLED materials for displays can be made by the following materials, each of which has its own distinct fabrication process and a different set of advantages andlimitations. The color is decided by the band gap of the material. Small molecular material exists usually as crystal. A popular example is Alq3. Polymer material is usually presents in amorphous state in the device. Popular materials are PPV and MEH-PPV. Two key characteristics of OLEDs ased on organic materials are large size and moderate brightness.

1.4.Generation of Light from OLEDs

Meeting predicted that worldwide energy consumption needs over the next hundred years will require fundamental changes in how we generate and use energy. The systems that aim to produce white light for illumination purposes are collectively termed solid-state lighting (SSL). Solid-state devices have recently achieved electrical-to-optical power conversion efficiencies of 76% at infrared wavelengths. Unlike incandescent and fluorescent lightings, for which indirect processes (electricity to heat, electricity to gas discharges) limit efficiencies, there is no known fundamental physical barrier to SSL achieving similar (or even higher) efficiencies for bright white light. Even if “only” 50% efficient SSL were to be achieved and displaced current white-lighting technologies completely, the impact would be enormous. The elec-tricity used for lighting would be cut by 62%, and total electrical energy consumption would decrease by roughly 13%. The savings in energy production that could be enabledby SSL would also have an important impact on the environment. If we can simultaneously achieve long device lifetimes and high energy efficiency indevices by addressing the scientific challenges, SSL based on OLEDs could become a formidable economic force and the greatest technological achievement in the field of lighting.

Light generated by the organic emissive region of OLEDs can be emitted in (i)external modes, which can escape through the substrate in the forward viewingdirection, (ii) substrate-wave guiding modes, which extend from thesubstrate/air interface to the metal cathode; and (iii) organic-wave guidingmodes, which are confined within the high-refractive-index organic layers. The conventional planar-type OLEDs allow approximately 20% of all the light emission generated in an OLED to escape through the external modes, creating a very low light extraction efficiency of 20%. Various optical designs have been patented and/or reported,which may increase the light extraction efficiency to 30–40%, corresponding to a 50–100% increase. This technology incorporates a novel optical design to efficiently extract all wave guiding modes in an OLED, and can potentially lead to light extraction efficiencies up to 80%. The fabrication of this light extraction enhancement mechanism is compatible with existing high throughput,low-cost printing technologies, and will not noticeably change the electrical performance of the original OLED device. Unlike many existing methods, this technology will not change the emission spectrum, nor will it significantly alter the angular emission pattern. Therefore it can easily be incorporated into existing full-color displays or white-light-emitting devices without modifying the driving electronics. Resolution as good as 20 um can be achieved by making this technology suitable for monochromatic or full-color displays.

Qualityof light generated by OLEDs can be evaluated from three parameters namely Commission Internationale de L’Eclairage (CIE) coordinates, color rendering index (CRI), and correlated color temperature (CCT). The emitting light color of a lighting source can be characterized by the CIE coordinates, which describe how the human eyes perceive the emission color of any light source(with an arbitrary emission spectrum) with a pair of two numbers [x, y] in 1931 CIE chromaticity diagram.