Simulation of Mixed-Host Emitting Layer based Organic Light Emitting Diodes C. RIKU a,, Y. Y. KEE a, T. S. ONG a, S. S. YAP b and T. Y. TOU a* a Faculty of Engineering, Multimedia University, 631000 Cyberjaya, Malaysia b Faculty of Engineering, University of Malaya, 50603 Kuala Lampur, Malaysia *Email: tytou@mmu.edu.my Abstract. SimOLED simulator is used in this work to investigate the efficiency of the mixed-host organic light emitting devices (MH-OLEDs). Tris-(8-hydroxyquinoline) aluminum(3) (Alq3) and N,N-diphenyl-N,N-Bis(3-methylphenyl)-1,1- diphenyl-4,4-diamine (TPD) are used as the electron transport layer (ETL) material and hole transport layer ( HTL) material respectively, and the indium-doped tin oxide and aluminum (Al) as anode and cathode. Three MH-OLEDs, A, B and C with the same structure of ITO / HTM (15 nm) / Mixed host (70 nm) / ETM (10 nm) /Al, are stimulated with ratios TPD:Alq3 of 3:5, 5:5, and 5:3 respectively. The Poole-Frenkel model for electron and hole mobilities is employed to compute the current density-applied voltage-luminance characteristics, distribution of the electric field, carrier concentrations and recombination rate. Keywords: efficiency, Poole-Frenkel model, recombination zone, uniformly mixed OLEDs PACS: 73.50.Gr, 73.61.Ph, 78.66.Qn INTRODUCTION The organic light-emitting device (OLED) is known to have advantages such as low cost, large area display, easy processing and flexible substrate as compared to inorganic counterparts [1-3]. But challenges like degradation and life time require OLEDs need to be further improved in order to match with the inorganic LED performance [4]. In recent years, there were intensive efforts for improving the device performance such as the operational lifetime, thus the mixed host (MH) architecture for the emissive layer (EML) was studied by some research groups [5]. For example, a barrier builds up to enhance carrier confinement due to the HTL/ETL interface in conventional heterojunction (HJ) OLEDs, but it forms chemically unstable cations into the interface by which the formation of non-radiative trapping centers accelerates, therefore increase the driving voltage and resulting in luminance decay of an OLED. In contrast, for MH-OLEDs, the HTL/EML interface become blurry which reduces carrier buildup and widens the EML, hence, extending the operation lifetime [6]. Some experimental studies have been done to explain the device performance of MH-OLEDs in term of charge transportation and recombination qualitatively [7-8]. In this work, however, we attempt to simulate both electrical and optical behavior of an OLED by using Poole-Frenkel model for the charge carrier. DEVICE STRUCTURES AND PARAMETERS VALUES Figure 1 shows the mixed host (MH) and conventional HJ-OLEDs. For OLED A, B and C, the ratio of hole transport material (HTM) to the electron transport material (ETM), which are uniformly mixed, are 3:5, 5:5 and 5:3, respectively. The device structures of all uniformly-mixed (UM) OLEDs are as follow: ITO / TPD (15 nm) / TPD: Alq 3 (70 nm) / Alq3 (10 nm) /Al. For comparison, the HJ-OLED with structure of ITO / TPD (45 nm) / Alq 3 (50 nm) /Al is also simulated. The total organic thickness of all OLEDs is fixed at 95 nm. The Poole- Frenkel mobility (PFM) model for charge carrier distribution is employed in the simulation to compute the current density-applied voltage-luminance (J-V-L) characteristics, distribution of the electric field, carrier concentrations and recombination rate.
FIGURE 1. Schematic structure of (a) HJ-OLED and (b) UM-OLEDs. For OLED simulations, parameters which have to be defined are the energy level and mobilities of the materials [9]. Due to the mixed transport materials [10], the lowest unoccupied molecular orbit (LUMO) of the mixed host is assumed to be the LUMO of the electron transport material (ETM), whereas the highest occupied molecular orbit (HOMO) of the mixed host is assumed to be the HOMO of the hole transport material (HTM). Since there is no unified model to describe the bipolar mobilities in the mixed host, hereafter it is proposed to assume that the mobilities lies between the mobilities of each material [11]. An electric field dependent carrier mobility E of the mixed host layer (MHL) is based on the Poole-Frenkel [12-14] equation as following where, E exp E E is the charge carrier mobility under electric field E, 0 0 (1) is the mobility at zero electric field and is the Poole-Frenkel field dependent factor. Since and are both constants, the mobility of the organic 0 material under an electric field can be calculated using the equation (1). The experimental works [15], shows that the mixed host mobilities is power dependence of the concentration of the components, thus equations (2) and (3) are used to calculate the mobilities for the mixed host layer- CHTM 1 C HTM (2) mix, P HTM, P ETM, P CHTM 1 C HTM (3) mix, n HTM, n ETM, n where, mix,n p is the electron (hole) mobility of the mixed host layer, HTM,n p is the electron (hole) mobility of the hole transport material and ETM,n p is the electron (hole) mobility of the electron transport material. The device performance is simulated and the numerical results are analyzed after defining the energy level and mobilities of the mixed host layer in simulator. The energy level and mobilities of the electron transport material (ETM) and the hole transport material (HTM) are given in Table 1. TABLE 1. Input parameters used in the simulation of HJ-OLED and MH-OLEDs. Parameters TPD Alq3 OLED A OLED B OLED C HOMO (ev) -5.4-5.7-5.4-5.4-5.4 LUMO (ev) -2.5-3.1-3.1-3.1-3.1 (cm 2 / Vs) 7E-05 1.8E-08 3.99E-07 1.12E-06 3.15E-06 p 0 (cm 1/2 / V 1/2 ) p 0 2E-03 2E-03 2E-03 2E-03 2E-03 (cm 2 / Vs) 1.8E-08 7E-05 3.15E-06 1.12E-06 3.99E-07 n 0 (cm 1/2 / V 1/2 ) n 0 2E-03 2E-03 2E-03 2E-03 2E-03
RESULTS AND DISCUSSIONS From the current density-voltage (J-V) characteristic curve in Figure 2(a), the uniformly MH-OLEDs have a current density higher than the HJ-OLED due to its bipolar characteristics. At same voltage, the current density of uniformly MH-OLEDs is increased with increasing mixing ratio of TPD. When the mixing ratio of TPD is smaller (OLED A), the hole mobility in EML is lower than the electron mobility. For equal mixture ratio (OLED B), the current density is increased because of balanced electron-hole mobility in EML. When TPD ratio (OLED C) is increased, then EML is more hole transport dominated, therefore the current density becomes higher. In Figure 2(b), the MH-OLED gives higher brightness than HJ-OLED before breakdown. Thus, it is reasonable to expect that the recombination processes and transport of charge carriers in UM-OLEDs are different from that in the conventional HJ-OLED, as shown in Figure 3. FIGURE 2. Simulated (a) J-V characteristics of OLEDs and (b) Lv-J characteristics of OLED. FIGURE 3. The charge carrier density: (a) Hole density, (b) Electron density, and (c) recombination zone distribution in HJ- OLED and UM-OLEDs.
In OLED A, the electrons are pushed to the HTM/Mixed host interface, where most of the recombination took place. In contrast, OLED C, the holes and recombination zone are pushed to the mixed host/etm interface. The recombination zones in OLEDs A and C are broader than in HJ-OLED, nevertheless, most of the recombination still took place near the interface, which might be quenched by the accumulated charge carriers. The OLED B has a balanced hole and electron mobility in the mixed layer. Therefore, for mixed layers, electron and hole are widely distributed. An expanded recombination zone is also obtained which improve the lifetime of UM-OLEDs over HJ-OLED [16]. In the case of high Alq3 concentrations in MH layer, the average hopping distance for electrons would be smaller which increase electron mobility and reverse incident occurs for higher TPD concentration in TPD/Alq3 mixed host layer. As a result, the recombination zone in Figure 3(c) shifts (from anode to cathode) with increasing TPD concentration. Since, the HJ-OLED has a narrow recombination zone that is close to the hole accumulation region, it is observed in Figure 4, that both the power efficiency and current efficiency of HJ-OLED are lower than all the UM-OLEDs. OLED C has the highest efficiency (power and current efficiency) among the three uniformly MH OLEDs because the recombination occurred near the electron accumulation zone. It is noted that all efficiency curves sharply decline as the current density increased due to an increased charge carrier density under higher current densities and henceforth the charge carrier quenching increase with increasing of TPD ratio. In Table 2, the maximum brightness, maximum power efficiency ( ) and electroluminescence (EL) efficiency ( ) of UM- OLEDs are compared with conventional HJ-OLED. The electroluminescence efficiency ( ) and power efficiency ( ) of device A, B and C are improved by a factor of 1.78, 2.06 and 2.62 and 1.31, 1.46 and 1.75 correspondingly, as compared to HJ-OLED which agree with the experimental work [18-19]. FIGURE 4. Simulated Results: (a) power efficiency - current density curve and (b) Current Efficiency - current density curve for HJ-OLED and UM-OLEDs. TABLE 2. Performance comparison of heterojunction OLED and UM-OLEDs. Improvement Over Max. Brightness, Max. EL Efficiency, Max. Power Efficiency, Devices Lv (cd/m 2 HJ-OLED ) γ (cd/a) η (lm/w) γ η HJ-OLED 648.14 0.76 0.24 - - OLED A 1380.32 1.36 0.31 1.78 1.31 OLED B 1927.13 1.57 0.35 2.06 1.46 OLED C 3856.37 1.99 0.42 2.62 1.75 CONCLUSION A quantitative electrical model, known as SimOLED, based on the mobility of carriers with different mixed ratios of TPD/Alq3 and the recombination zone distribution in MH layer has been used. By utilizing the field dependent model of this simulator, it is observed that the hole carrier hopping distance decreased with increased of TPD molecules in mixed layer and hence it prompted the hole current conduction, whereas the mobility of the electron carriers was reduced with the increase of electrons hopping distance. Additionally, the holes injected from HTL/mixed layer interface dispersed for a longer distance before recombination with electrons than the electrons which were injected from ETL/mixed layer interface. As a result, the recombination zone was shifted from anode to cathode as TPD ratio was increased from OLED A to OLED C.
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