Modified PEDOT-Based Materials for HILs

Ho et al. improved the photometric efficiency of polymeric OLEDs by increasing the work function of PEDOTiPSS with layer thickness to form a graded layer. The authors deposited PEDOTiPSS together with cationic poly(p-xylylene-a-tetrahydrothiophenium) (PXT) to dedope PEDOT. The concentration of PXT was increased from one to the next successively deposited layer to achieve a continuous increase of the work function and the electron blocking properties over the HIL. 

Lifetime improvements of a factor of two were reported by Lee when a PEDOTiPSS layer was cross-linked with glycidoxypropyltrimethoxysilane.  

This result was explained by improving the adhesion between PEDOTPSS and the light emitting polymer together with the assumption that released In ions from the ITO anode are trapped in the cross-linked layer. 

The influence of the ratio of PEDOT to PSS on device performance was studied on polymeric OLEDs. A significant increase in device efficiency was found when a layer of PEDOT PSS with a ratio of PEDOT to PSS of 1 20 was coated on a high conducting polymeric anode with a ratio of 1 2.5. The high PSS content is necessary to form a PSS-enriched surface to block electrons and to prevent excitons from being quenched, as discussed earlier. 

A correlation between PSS enrichment at the surface, work function, and device efficiency was found by Lee and Chung investigating various types of PEDOTPSS. The segregation of PSS to the surface was enhanced when low molecular weight PSS was employed. 

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