OLED continues to draw attention as a main display technology. OLED displays promises myriad merits compared to other displays such as CRT and LCD, and these include fast response time, ultra-thin panel structure, low-power consumption and wide viewing angle. However the manufacturing cost is high. The continued advance in and improvement of LCD technology are setting hurdles for the introduction of OLEDs products to market. In this regard, printing technology could offer a means to lower the production cost of OLEDs, and combining printing technology and conventional OLEDs leads to the evolution of PLEDs.

Electroluminescence from conjugated polymers was first discovered by Prof. Richard H. Friend’s group at Cambridge University during an investigation on the electrical properties of poly(p-phenylene vinylene) (PPV) in 1990(Nature 1990, 347, 539). Yellow-green light with emission maximum at 551 nm was observed from this bright yellow polymer upon excitation with a flow of electric current between two electrodes. To deal with the solubility problem, Heeger et al. subsequently fabricated a PLED using soluble PPV derivatives(Appl. Phys. Lett. 1991, 1982).

As PLEDs can be used for large area flat panel display and are relatively inexpensive, it has been receiving a growing attention in recent years. Blue PLEDs has been successfully fabricated by Uchida et al. using polyalkylfuorene (PF) as emitter (Synth. Met. 1992, 57, 4168). In the early stage of development, PLEDs were usually fabricated by spin coating.

However there are many disadvantages such as solution wastage and lack of lateral patterning capability associated with this fabrication technique, thus limiting the commercial applications of PLEDs. To overcome these drawbacks, inkjet printing has been introduced by Yang et al. (Appl. Phys. Lett. 1998, 2660) and nowadays PLEDs could be fabricated using commercially available inkjet printer.

Motivated by these findings and developments, display companies such as Seiko Epson Corporation, Philips and Samsung electronics have joined the efforts to develop inkjet printing PLEDs. In recent years, red-, green- and blue-light emitting polymers have been actively pursued for the fabrication of full color panels. 

Phosphorescent metal-organic complexes, which have been successfully used in the development of high performance OLEDs through vacuum deposition process, have also been used to make classes of light emitting polymers through attachment to polymer backbones. Some of the recent exapmles are: Skyblue-emitting device by Holdcroft et al. (Macromolecules 2006, 9157-9165) Thompson and co-workers reported high efficiency green light-emitting PLED with a maximum external quantum efficiency (EQE) of 10.5 % (Chem. Mater. 2006, 18, 386). A near white light-emitting (CIE: x = 0.30, y = 0.43) PLED has been fabricated by using a polymer with both blue and red emitting units attached on it (J. Am. Chem. Soc. 2004, 15389).

Besides attaching phosphorescent materials in polymer backbone, soluble phosphorescent materials are also new candidate for printable OLED. Recently, Merck announced that solution processable phosphorescent small molecules materials will be one of their main focus on OLED materials development.