Vertical organic light-emitting transistors with an evaporated Al gate inserted between hole-transporting layers

Similar to the configuration of static induction transistor (SIT), vertical organic light-emitting transistors (VOLETs) with an evaporated Al gate inserted between hole-transporting layers were fabricated and their static characteristics were characterized. By using Alq3 as the emissive layer, the basic transistor characteristics of the VOLETs were compared with two organic hole-transporting layers, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB). Our experimental data show that the intensity of emission from VOLETs can be tuned by adjusting gate voltages. The emission intensity of device ITO/TPD/Al/TPD/Alq3/Al decreases by increasing the applied gate voltages, showing an operation in depletion mode, however, it shows the operation in enhancement mode for VOLET device ITO/NPB/Al/NPB/Alq3/Al. Further, the reasons for these two different phenomena were discussed in terms of the dielectric constant of materials, carrier mobility and energy barriers at electrode/organic interface.