Low Voltage Operation of Polymer Light-Emitting Device with Conducting Polymer Distributed Bragg Reflector

Abstract

ABSTRACTPolymer light-emitting devices with a conducting polymer distributed Bragg reflector (DBR) are reported. Polymer DBRs were formed from alternate spin-coating of poly (3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulphonate) (PSS) as the low-index layer and a hole-transporting polyfluorene derivative as the high-index layer onto ITO deposited glass substrates. The polymer DBR with 5 periods showed reflectivity of 40% at the wavelength of 560nm. Onto this polymer DBR, a green-light-emitting polyfluorene was spin-coated to give the emissive layer and Ca/Al was deposited to form the cathode served another reflector. These LEDs operated successfully at low voltages and emitted light having a narrower spectrum (FWHM=47nm) than structures without the DBR (FWHM=102nm). The peak wavelengths varied corresponding to the thickness of the emissive layer, that is, the cavity length as expected from calculations. The luminance turn-on voltage of the LED with the polymer DBR is 2V, which is the same as LEDs without a DBR. The current was found to be limited by injection in the low field region and limited by space charge density at high fields. The current does not depend on the number of layers in the DBR in the low field region, and this implies that there are no effective energy barriers in the polymer DBR.