Electron-blocking mechanisms at the hole transport layer-emissive layer interface in polymer light-emitting diodes. Enhanced device performance with a novel electron-blocking interlayer

The interface between the hole transport layer (HTL) and emissive layer (EML) in polymer light-emitting diodes (PLEDs) has attracted intense research attentioin since the initial discovery of PLEDs in 1989. In this contribution, we analyze the electron-blocking properties of various HTL at this interface and their effect on PLED device performance. We find that poly(3,4- ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) - a conventional PLED HTL - does not possess optimum electron-blocking properties and that PLED device performance can be significantly enhanced by inserting a new type of electron-blocking layer (EBL) between the PEDOT-PSS HTL and EML. The new EBLs developed in this study consist of two major components: a siloxane-derivatized, crosslinkable, TPD-like triarylamine hole-transporting material, such as 4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi₂), and a hole-transporting polymer, such as poly(9,9-dioctyl-fluorene-co-N-(4- butylphenyl) diphenylamine) (TFB). TPDSi₂ undergoes crosslinking in air and rendering the TPDSi₂ + TFB blend insoluble. With the TPDSi₂ + TFB EBL inserted between PEDOT-PSS and BT layers, PLED device current density is reduced, device light output and current efficiency are dramatically increased (maximum current efficiency ∼ 17 cd/A). Our result shows: 1) insufficient electron-blocking by PEDOT-PSS is another reason for the poor performance of PEDOT-PSS/BT based devices; 2) PLED device performance can be dramatically enhanced with a triarylamine siloxane-based EBLs.