High brightness and efficiency of polymer-blend based light-emitting layers without the assistance of the charge-trapping effect.

Abstract

Polymeric light-emitting materials have been developed recently as an attractive solution-processable alternative to conventional vacuum-deposited small molecules in organic/polymeric light-emitting diodes, but they are still limited in terms of their performance, especially with low luminance and efficiency. We report on some noteworthy characteristics of a new type of single emitting layer (EML), composed of a blend of a host blue-emitting polyspirobifluorene-based copolymer and a guest yellow-emitting poly(p-phenylene vinylene) derivative copolymer. These host and guest polymers have nearly identical highest occupied molecular orbital levels of about 5.2 eV, and lowest unoccupied molecular orbital levels of about 2.4 eV and 2.9 eV, respectively, minimizing the prevailing charge-trapping properties of their blend. Even in the absence of the charge-trapping effect, it is shown that very bright green electroluminescent (EL) emission with a maximum luminance of ~142,000 cd/m2 can be realized for the blended host:guest EML at a moderate concentration (~5 wt%) of the guest polymer. Current efficiency is also observed to be up to ~14 cd/A, which is much higher than those (3.6~5.1 cd/A) of reference devices with pure host or pure guest polymeric EMLs. Moreover, there is a small change in green color emission, with CIE coordinates of (0.35, 0.60) even at high luminance, showing good color stability of the EL emission from the blended EML. These significant improvements in device performance are mainly attributed to efficient Förster resonance energy transfer between the host and guest polymers in the blended EML. Together with its simple structure and easy processability, the high brightness and efficiency of our blended polymeric EML provides a new platform for the development of solution-processable light-emitting devices and/or advanced emissive display devices.