The Effect of Trap Density on the Trapping and De-trapping Processes in Determining the Turn-On Voltage of Double-Carrier Organic Light-Emitting Devices (OLEDs)

A double-carrier organic light-emitting device consisting of tris(8-hydroxyquinoline) aluminum (III) (Alq₃) as the emissive layer (OLED A) is fabricated. The trap density value obtained at voltage of 14 V when the device luminesces is 2.40 × 10¹⁵ cm⁻³. The rates of the capturing process (trapping) R_n and de-trapping, \(R_{n}^{\prime }\) of the charge carriers on the localized state of the double-carrier device are estimated. There are two significant regions observed in the plot, R_n > \(R_{n}^{\prime }\) and R_n < \(R_{n}^{\prime }\), and the intersection point of R_n with that of \(R_{n}^{\prime }\) indicates the voltage at which the device luminesces, known as the turn-on voltage of the device. Simulations are then performed by directly increasing the device trap density by various ratios to that of OLED A. The simulation has indicated that trap density has a strong influence on the R_n and \(R_{n}^{\prime }\) parameters. In order to verify the finding of the simulation, another double-carrier device (OLED B) with a hole transport layer (HTL) of poly(N,N′-bis-4 butyl phenyl-N,N′-bisphenyl) benzidine (poly-TPD) of 40-nm thickness is fabricated by adding the HTL layer to the existing structure of OLED A. The photoelectric characteristics analysis is performed on both types of OLED A and B. In this work, OLED B proved to have a higher value of trap density luminescence at a much lower turn-on voltage, as has been predicted by the simulation results due to the influence of trap density on the parameters R_n and \(R_{n}^{\prime }\) of the devices.