Dependence of film morphology on deposition rate in ITO/TPD/Alq ₃/Al organic luminescent diodes

Organic multilayers of N,N′-diphenyl-N,N′-bis(3-methylphenyl)- 1,1′-biphenyl-4,4′-diamine(TPD)/aluminum tris(8-hydroxyquinoline) (Alq₃) were evaporated on the indium tin oxide (ITO) coated glass substrates under different conditions. The effect of deposition rate on the surface morphology of top Alq₃ films and I-V curves of organic luminescent diodes (OLED)s is studied, in order to optimize the Alq₃ film growth conditions. SEM and AFM images show dense, uniform surface for high deposition rate (>0.4 nm/s) devices, and local pinholes on the surface for device of low (<0.15 nm) deposition rate. I-V characteristic measurement shows resistive characteristics with no luminescence for low deposition rate devices, and diode characteristics with characteristic green luminescence for the high deposition rate. The deposition rate is identified as one of the key factors in the performance of the TPD/Alq₃ device due to its great influence on the surface morphology of top Alq₃ films.