Enhanced performance of organic light-emitting diodes with an air-stable n-type hole-injection layer

An air-stable n-type organic semiconductor, N,N-bis(4-trifluoromethoxybenzyl)-1,4,5,8-naphthalene-tetracarboxylic di-imide (NTCDI-OCF3), can function as an excellent hole-injection layer to improve the hole injection from an indium tin oxide (ITO) anode to a hole-transporting layer (HTL). Significantly improved hole injection was achieved by introducing an ultrathin layer of NTCDI-OCF3 between ITO and HTL, leading to a lower operational voltage and relatively less power consumption. The results can be attributed to the reduced hole-injection energy barrier from ITO to HTL, as shown by x-ray photoelectron spectroscopy measurements, due to the surface dipole that is formed by the NTCDI-OCF3. The thickness dependence of NTCDI-OCF3 was also examined.