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).