BACKGROUND:  Two factors negatively impact the efficiency of polymer LEDs. These are (1) the balance of electrons and holes and (2) their confinement. Most conjugated polymers have unbalanced charge transport properties, since the mobility of holes is often greater than that of electrons. Generally in PLEDs, the balance of electrons and holes is attained by modifying the charge injection contact and LiF (for example) is used as the cathode contact for efficient electron injection. The charge confinement can then be attained by inserting a charge blocking layer between the electron transport layer and hole transport layer. To facilitate fabrication, it would be better to have a charge injection layer for one type of charge function as the charge blocking layer for the other type of charge.

INNOVATION:  This technology is an enhancement of the PLED by material engineering (adopting polymer blends which traps one type of charge) and interface engineering by inserting an electron injection and hole blocking layer. A blend of high bandgap and low bandgap polymers is used to introduce charge traps in the light-emitting polymer. The innovation lies in materials, blend ratios, thicknesses, fabrication techniques and the use of dopants. The devices yield a peak efficiency of 16 lumens/watt - an improvement resulting from the combination of self-balanced efficient charge injection and charge confinement.

DEVELOPMENT-TO-DATE:  Multiple devices have been created and tested. Results show:

• The leakage current before light injection is low, on the order of 10**-5 mA/cm2;
• Light emission is observed at low applied external voltage, about 2.3V;
• Low operating voltages are recorded for white PLEDs;
• The method has been demonstrated to be successful in improving the efficiency of red, green, and white PLEDs; and
• The devices exhibit high quantum efficiencies. More detailed results are available upon request.

ABOUT THE LAB:  This innovation was generated by Prof. Yang Yang's Labs at UCLA, which is involved in a wide range of research involving conjugated polymers and their applications in (opto)electronic devices, such as light-emitting diodes, photodiodes, and field effect transistors. The lab emphasizes the study of the metal/polymer interface, newer fabrication technologies and fabrication of flexible optoelectronic devices. The web site for the lab is http://www.seas.ucla.edu/yylabs.

Reference: UCLA Case No. 2005-602

US Patent Number: WO 2006/116054