The technique that will cause less damage

The organic light emitting diodes (OLEDs) has been proven for their commercialization in terms of performance, colour accuracy and lifetime.

However, it will be great challenge to OLED to compete with the liquid crystal display (LCD) in applications. While low contrast and viewing angle dependency in the LCD and plasma displays are can’t compare with the OLED displays, high power efficiency makes OLED as a beneficial display technique. However, OLEDs can achieve considerable display applications; notable achievement in high transmittance of organic electronic devices. The organic layers in the OLED can be tuned to absorb only in the ultraviolet region but there will be large shift in the emission zone.

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So, we can integrate OLED as a see-through display, for a moment in the automobile windshield to guide the driver or may be for visual information in the medical surgeries. For transparent OLEDs, transparent electrodes required on the top and bottom sides of the device. Currently, due to high electrical conductivity (80 ohm/sq) and transparency, we choose indium tin oxide (ITO) as a bottom electrode in the inverted structure of OLED. A layer thickness of 80 nm has transparency above 80% over the wavelength of the visible region (400-700 nm). Commonly, the high-quality deposition of ITO is used with plasma-assisted technique. Sputtering, which is well-known technique for transparent conducting oxides (TCOs) deposition. Moreover, high-energy particles are emitted during the process of the sputter deposition which will significantly damage the organic layers.

As an effect, increase in the leakage current, lifetime of OLED, and low efficiency have been reported. Therefore, multi-layered is needed to protect the organic materials against the metal deposition on the top electrode. There are several minor issues to overcome to achieve efficient buffer layer to maintain quality of transparent OLEDs: efficient protection against impinging particles to the organic layers, a high transparency in the wide spectrum of visible region and the efficient electrical conductivity are expectable.

There should be facilitated charge injection to the adjacent organic layer from the buffer layer. Additionally, high- protective against deposition technique that will cause less damage to the underlying organic films, a nonreactive material needed as a buffer material which in terms to form high dense thin film for good stability or ability. Materials with all these properties are very rare. Thus, no concept for fully describing the transparent buffer layers are key factors for the high efficiency transparent OLEDs has been published. Metal films offer protective properties and allows for the ohmic charge injection into the adjacent organic layers, but transmittance is limited. Even though the layer thickness (for example, Ag thin film) is reduced to the 8 nm, the transmittance is cannot exceed more than 50% . Moreover, in the transparent displays, windshield of the automobile is required high transparency of 75% for safety.

In comparison, organic buffer layers can exhibit high transmittance such as, Mg: Ag, pentacene or copper phthalocyanine (CuPc).  Parthasarathy et al. reported the sputter deposition of CuPc on the organic molecule effectively, which distribute the energy of impinging particles over the several bonds. The planar molecules like pentacene grow on the polycrystalline substrate which grain sizes are greater than 5nm.

Therefore, surface morphology or surface roughness of the films will change the quality of deposited ITO as a top electrode which significantly drops the conductivity of the top electrode


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