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A study of the optical properties of poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2) is reported, identifying this polymer as one that possesses a desirable combination of charge transport and light emission properties. The optical and morphological properties of a series of polymer blends with F8T2 dispersed in poly(9,9‐dioctylfluorene) (PFO) are described and almost pure‐green emission from light emitting diodes (LEDs) based thereon is demonstrated. High luminance green electroluminescence from LEDs using only a thin film of F8T2 for emission is also reported. The latter demonstration for a polymer previously primarily of interest for effective charge transport constitutes an important step in the development of emissive materials for applications where a union of efficient light emission and effective charge transport is required.

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 vapourphase 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 À1 , < 0.75 cd A À1 for ITO; and < 0.023 lm W À1 , < 0.144 cd A À1 for VPP-PEDOT. The two devices with PEDOT:PSS-coated anodes exhibited much higher luminosities and efficiencies: 3.8 lm W À1 , 8.4 cd A À1 for ITO/PEDOT:PSS; and 2.6 lm W À1 , 10.2 cd A À1 for VPP-PEDOT/ PEDOT:PSS at $10 000 cd m À2 . 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 À1 ) in otherwise identical OLEDs.

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We report current-voltage-luminance and electromodulation measurements on a series of polymer light-emitting diodes, using indium tin oxide (ITO) coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the anode, poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine) (TFB) as an optional anodic interlayer material, poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) as the emissive layer, and either aluminium or (aluminium-capped) calcium as the cathode. Four device structures were investigated: ITO/PEDOT:PSS/F8T2/Al, ITO/PEDOT:PSS/F8T2/Ca, ITO/PEDOT:PSS/TFB/F8T2/Al, and ITO/PEDOT:PSS/TFB/F8T2/Ca. The devices with interlayers had substantially higher luminance and power efficiencies than their interlayer-free counterparts--a fact we attribute to the energy and mobility barriers that exist at the TFB-F8T2 interface. These barriers play two crucial roles in enhancing device efficiency: firstly, they cause the most easily injected charge carrier to accumulate at the TFB-F8T2 interface until efficient injection of the opposite carrier type becomes favourable; and, secondly, they inhibit electron and hole 'seepage' across the interface, thereby reducing leakage currents. The beneficial influence of these two effects is most marked for the interlayer-containing Al device which, in spite of a sizeable 0.9 eV barrier to electron injection at the cathode, exhibited surprisingly high luminous and power efficiencies of 2.4 cd A(-1) and 1.1 lm W(-1) at an arbitrary reference luminance of 2500 cd m(-2). This compares with peak values of just 0.11 cd A(-1) and 0.07 lm W(-1) at 25 cd m(-2) for the equivalent interlayer-free device (falling to 0.058 cd A(-1) and 0.025 lm W(-1) at 100 cd m(-2)). The interlayer-containing Ca device had luminous and power efficiencies of 3.5 cd A(-1) and 2.9 lm W(-1) at 2500 cd m(-2) compared to 1.1 cd A(-1) and 0.7 lm W(-1) for the equivalent interlayer-free device at 2500 cd m(-2).