The energy level of higher occupied molecular orbital (HOMO) and lower unoccupied molecular orbital (LUMO) alignment plays a very important role in the study of the charge transport and injection mechanism. This is because organic semiconductors have negligible doping concentration and have very less charge density [
3]. Due to this reason, electrodes are responsible for injecting all charges in OLED devices. In this paper, the theoretical model for recombination is presented and discussed. Also, the effect of applied voltage on recombination is investigated in stacked nano-structured organic light emitting diode [
5]. To enhance the current density, distribution can be solved by taking in to consideration of drift diffusion equations [
6]. These equations are based on Langevin recombination rate. Also, the Poole Frenkel model depends on electric field for the calculation of carrier mobility of the device to be simulated [
6]. To enhance the device performance, vast understanding and knowledge of the physical phenomena and methodology is needed regarding the transport and injection characteristics of charge carriers. Lee et al. reported in Ref. [
17] that the properties and thickness of the electron transport layer (ETL) can be crucial for the achievement of improved charge balance. Also, ETL is also responsible for the recombination zone detention in phosphorescent OLED to enhance device performances. Also the refractive index of ETL is a factor to enhance the performance. It has been observed that lower the refractive index; higher will be the performance of the device [
18]. Position and orientation of the molecules of emitter strongly influence the efficiency roll-off in OLEDs. This can be investigated by changing the distance between the emitter and metal cathode. Molecular doping of organic semiconductors is also an important step in enhancing the performance of an OLED. A lot of work has been done on the doped transport layer applications. This is due to the fact that the conductivity of organic materials can be enhanced by doping with acceptor or donor atoms. In phosphorescent organic materials, the emitting layer (EML) is doped with another material such as Bis[2-(4,6-difluorophenyl)pyridinato-C2,
N](picolinato)iridium(III) (FIrpic), fac-Tris(2-phenylpyridine)iridium (Ir(ppy)
3) etc. This is done to form a host-guest system to improve the quantum efficiency. In the present work, Ir(ppy)
3 is used as the guest material for doping of CBP. This depends on the selection of doping material as already discussed about CBP-Ir(ppy)
3 host-guest materials. In the present work, electron acceptor dopants like HAT-CN are used to the doped hole transporting layers [
2–
4,
13,
19].