Effect of low-dose doping of small-molecules on the surface potential and carrier balance of TADF OLEDs

Abstract

Solution-Processed organic light emitting diodes (OLEDs) have been intensively studied due to the simple preparation process. However, the devices made of small molecular luminescent materials always suffer from the reduce of the carrier transport capacity by the influence of molecular agglomeration. In this work, small-molecules material 4,4',4''-tris(N-carbazolyl)-triphenylamine (TCTA) is successfully introduced into the molecular aggregation site formed by traditional host material 1,3-Di-9-carbazolylbenzene (mCP), improving the charge mobility of the emitting layer. Doped mCP film has more compact molecular packing, which makes it has better charge transfer performance as a solid solvent of 2,3,4,6-tetrakis(9H-carbazole-9-yl)−5-fluorobenzonitrile (4CzFCN). Besides, 3 wt% of TCTA is beneficial to optimized electrochemical doping, lowering the surface potential of emitting layer and promoting holes injection. Doped host-guest system device exhibits a maximum current efficiency (CEmax = 29.6 cd/A) which has improved about 60% compared to the device with host of pure mCP (CEmax = 18.5 cd/A). The findings may provide a strategy for the future research on the film structure of solution-processed small molecule OLEDs.