Effect of PEDOT:PSS on the performance of solution-processed blue phosphorescent organic light-emitting diodes with an exciplex host

In this study, efficient solution-processed organic light-emitting diodes (OLEDs) based on a light-emitting layer composed of a blue emissive phosphorescent dendrimer and exciplex host were investigated. Employing poly(styrene sulfonic acid) doped poly(ethylenedioxythiophene):poly(styrenesulfonate) (m-PEDOT:PSS) as the hole injection layer resulted in devices with external quantum efficiencies (EQEs) over 20% for luminances of up to 100 cd m−2. A feature of these devices was a relatively slow electroluminescence turn-on for the initial voltage scan, which was absent when m-PEDOT:PSS was replaced with molybdenum oxide as the hole injection layer. The initial turn-on rate was found to decrease with increasing PSSH content, with the results suggesting that proton migration from (m-)PEDOT:PSS is the cause of the change in the device performance. The overall device performance was found to be scan dependent, with sequential voltage scans leading to a decrease in EQE, which is ascribed at least in part to electromer formation. We also demonstrate solution-processed OLEDs with the same exciplex host and bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as the emitter, that have maximum EQE and power efficiencies of 17.0% and 25.3 lm W−1, respectively

Light-emitting dendrimer:exciplex host-based solution-processed white organic light-emitting diodes

Solution-processed monochrome and white organic light-emitting diodes (OLEDs) that have a light-emitting layer composed of a 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and (5-terphenyl-1,3-phenylene)bis(diphenylphosphine oxide) (POPH) exciplex host and soluble blue, green and/or red phosphorescent dendrimers have been fabricated and characterised. The OLED performance characteristics were found to be scan dependent, with the first scans having large external quantum efficiencies (EQEs) at low luminance, with subsequent scans showing stable performance. The monochrome (blue, green and red) films were found to have high photoluminescence quantum yields, relatively balanced hole and electron mobilities, and the OLEDs had stable EQEs of between 9 and 12% at 100 cd m−2 over multiple scans. The blue, green and red emissive materials were blended with the exciplex host, with their ratio tuned to achieve white emission. The optimal blend ratio provided white OLEDs that had EQEs of 11.7% and 10.6% at 100 cd m−2 and 1000 cd m−2, respectively. The best balance of 1931 Commission Internationale d'Eclairage (CIE) coordinates (0.40,0.40), Colour Rendering Index (70), and Delta uv (0.003) was achieved for an OLED with the light-emitting layer containing the blue, green and red dendrimers in a ratio of 20.0:0.4:0.8 wt%. A feature of the white OLEDs was the stability of the CIE coordinates, with a change of only (0.013,0.005) between a luminance of 100 cd m−2 and 4000 cd m−2