Organic light emitting devices ͑OLEDs͒ have been the subject of intense research because of their potential for flat panel display and solid state lighting applications. While small molecule OLEDs with very high efficiencies have been demonstrated, solution processable devices are more desirable for large size flat panel display and solid state applications because they are compatible with low cost, large area roll-to-roll manufacturing process. In this review paper, we will present the recent progress made in solution processable OLEDs. The paper will be divided into three parts. In the first part of the paper, we will focus on the recent development of fluorescent polymer OLEDs based on conjugated polyfluorene copolymers. Specifically, we will present results of carrier transport and injection measurements, and discuss how the charge transport and injection properties affect the device performance. In the second part of the paper, we will focus on the recent progress on phosphorescent dye-dispersed nonconjugated polymer OLEDs. Specifically, we will present our recent results on high efficiency green and blue emitting devices based on the dye-dispersed polymer approach. Similar to fluorescent conjugated polymer OLEDs, charge transport and injection properties in dye-dispersed polymer OLEDs also play an important role in the device performance. In the third part of this paper, we will present our results on white emitting phosphorescent OLEDs. Two approaches have been used to demonstrate white emitting OLEDs. First, white emitting OLEDs were made using blue emitting OLEDs with downconversion phosphors. Second, white emitting OLEDs were made by dispersing red, green, and blue phosphorescent dyes into the light emitting layer. High efficiency devices have been demonstrated with both approaches.

We present a highly efficient white electroluminescence device by the combination of a solution processed blue organic phosphorescence light-emitting diode with appropriate down-conversion phosphor system. The use of this down-conversion system produced an extraordinary enhancement on device performance, resulting in a white electroluminescence device with luminance efficacy of 25lm∕W at luminance efficiency reaching 39cd∕A. The extraordinary enhancement on device performance is attributed to isotropic radiation pattern of the excited phosphor particles, leading to high light extraction properties.

We demonstrate high efficiency organic light-emitting devices by incorporation of a nanoscale interfacial layer between a hole-conducting layer (PEDOT:PSS) and a light-emitting polymer layer (LEP) to improve hole injection. The interfacial layer has appropriate highest occupied molecular orbital level in order to act as a bridge for efficient hole injection from the PEDOT:PSS into the LEP. As an example we have incorporated a suitable interfacial layer into a green-emitting single-layer electrophosphorescent light-emitting diode. Devices with the interfacial layer show a peak efficiency of 41lm∕W, an improvement of more than 25% in their performance over comparable devices without the interfacial layer. The results presented here introduce a novel method to improve hole injection and thus efficiency in organic electroluminescent devices.

We report highly efficient solution processed blue electrophosphorescent organic light emitting diodes (PHOLEDs) utilizing a phosphorescent dye and a nonconjugated polymer host, molecularly doped with electron transporting molecules. Based on a bilayer device architecture blue PHOLEDs with luminous efficacy of 14lm∕W at luminous efficiency reaching 22cd∕A are demonstrated. Analysis of device performance indicates that this high efficiency is achieved by a combination of improved charge balance and light outcoupling efficiency. Our results demonstrate that simple solution processed devices can have efficiencies similar to those published to date for small molecule multilayer PHOLEDs based on the same emitter.