On the use and influence of electron-blocking interlayers in polymer light-emitting diodes

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.