High efficiency organic light-emitting diodes with PEDOT-based conducting polymer anodes

We report current-voltage-luminance measurements on a series of organic light-emitting diodes (OLEDs) based on a Lumation Green 1300 Series (LG1300) light-emitting polymer (LEP) provided by the Sumitomo Chemical Company, Ltd. The devices used either indium tin oxide (ITO) or vapour-phase polymerised poly(3,4-ethylenedioxythiophene) (VPP-PEDOT) as the underlying anode material with or without a surface coating of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Four devices were tested with the structures: ITO/LG1300/Ca, VPP-PEDOT/LG1300/Ca, ITO/PEDOT:PSS/LG1300/Ca and VPP-PEDOT/PEDOT:PSS/LG1300/Ca. The two devices with uncoated anodes exhibited low luminosities and efficiencies due to poor hole injection at the anode: < 0.12 lm W-1, < 0.75 cd A-1 for ITO; and < 0.023 lm W-1, < 0.144 cd A-1 for VPP-PEDOT. The two devices with PEDOT:PSS-coated anodes exhibited much higher luminosities and efficiencies: 3.8lmW-1, 8.4 cd A-1 for ITO/PEDOT:PSS; and 2.6 lm W-1, 10.2 cd A-1 for VPP-PEDOT/PEDOT:PSS at ~10,000 cd m-2. Electromodulation measurements suggest the improved efficiencies are attributable to the poly(styrenesulfonate) component of PEDOT:PSS, which causes electrons to become trapped at the PEDOT:PSS/LG1300 interface and in so doing increases the rate of hole injection into the active layer. The results reported in this manuscript indicate that VPP-PEDOT is a viable alternative anode to ITO, capable of yielding superior efficiencies (cd A-1) in otherwise identical OLEDs.