p-Type Doping of Small-Molecule Organic Semiconductors using Organic Vapor Phase Deposition (OVPD)

ABSTRACTIn the past few years, organic vapor phase deposition (OVPD) has been demonstrated to be an effective deposition method for high-performance monochrome and white organic light emitting diodes (OLEDs) [1-4]. OVPD provides good material utilization efficiency and large achievable deposition rates.An application of p-type doping is the improvement of hole injection either from the anode contact or from a charge generation layer in stacked OLEDs [5]. Nevertheless, no reports on p-type doping using OVPD can be found in literature, in part due to the thermal instability and high chemical sensitivity of organic dopants.In this work, p-type doping using an AIXTRON Gen-1 OVPD tool with two different show-erhead designs is examined. NDP-2 (NOVALED) and N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (NPB) were used as p-type dopant (guest) and hole-conducting host, respectively. p-Type doped hole-only devices were fabricated and compared with undoped ones.Two different showerhead designs (made either of aluminum or stainless steel) were investi-gated with respect to OLED performance to determine possible side reactions.Highly efficient monochrome red OLEDs including a p-type doped hole transport layer were demonstrated exhibiting a current efficiency of 31 cd/A, a power efficiency of 26 lm/W and a driving voltage of 3.7 V without improved light outcoupling (all values at 1000 cd/m2).