Spin-cast thin semiconducting polymer interlayer for improving device efficiency of polymer light-emitting diodes

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Abstract
We report that adding a thin ( ∼ 10 nm ) semiconducting polymer interlayer between apoly(styrenesulphonate)-doped poly(3,4-ethylenedioxythiophene) (PEDT:PSS) hole transporter and an emissive semiconductor significantly improves the device efficiency of polymer light-emitting diodes(LEDs). With the interlayer, the external quantum efficiency (EQE) increases from 0.7%( 0.4 cd ∕ A at 3.7 V ) to 1.9% ( 1.0 cd ∕ A at 3.3 V ) at 100 cd ∕ m 2 for red LEDs and from 1.9%( 6.2 cd ∕ A at 3.4 V ) to 3.0% ( 10.1 cd ∕ A at 3.0 V ) at 1000 cd ∕ m 2 for green LEDs. An EQE of 4.0% is also observed in blue LEDs (35% increase). The interlayer is spin-coated directly on top of the PEDT:PSS layer from a poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) (TFB) solution. This interlayer prevents significant quenching of radiative excitons at the PEDT:PSS interface by acting as an efficient exciton blocking layer.