Investigation of efficiency roll-off in FIrpic based phosphorescent

Transcription

1 Investigation of efficiency roll-off in FIrpic based phosphorescent organic light-emitting diodes Wenyu Ji 1, 2 and Furong Zhu 2, * 1 State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun , People s Republic of China 2 Department of Physics and Institute of Advanced Materials, Hong Kong Baptist University, Hong Kong Abstract This work reports our effort on understanding the efficiency roll-off in bilayer blue phosphorescent organic light emitting diodes (OLEDs), based on a blue phosphorescence from bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)-iridium (FIrpic) doped 4,4-N,N-dicarbazole-biphenyl (CBP) emissive layer. The luminous efficiency of a set of blue phosphorescent OLEDs with different FIrpic dopant concentrations was analyzed. It is found that the efficiency roll-off in FIrpic-based blue phosphorescent OLEDs is closely related to the exciton quenching induced by the radical cations of CBP + present in the CBP host. In addition, high current efficiency of 7.76 cd/a was achieved with a neat FIrpic emission layer, which is correlated to the introduction of fluorine atoms into the FIrpic. Theoretical calculation, based on the density functional theory, shows that the electron distribution in the lowest unoccupied molecular orbital of FIrpic is highly localized the pic ligand and strong Coulomb repulsion is occurred To whom correspondence should be addressed. Furong Zhu, frzhu@hkbu.edu.hk 1

2 between FIrpic molecules, which may decrease the effect of self-quenching of FIrpic on efficiency roll-off. Key words: phosphorescent OLEDs, phosphorescence, efficiency roll-off, exciton quenching 2

3 Ι. Introduction Phosphorescent organic light emitting diodes (OLEDs) have attracted significant attentions for application in active matrix OLED displays and solid-state lighting.[1-4] Cyclometalated Ir(III) complexes are commonly used phosphorescent dopants that can harvest both singlet and triplet excitons to achieve nearly 100% internal quantum efficiency in high efficiency OLED.[5] Bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium (FIrpic), a representative Ir(III)-based blue phosphorescent dopant, has been widely used as blue emission unit in OLED displays and white OLEDs.[6] However, the efficiency roll-off in phosphorescent OLEDs at high current density (brightness) is still one of the open challenges.[7] The mechanism of efficiency roll-off in FIrpic-based phosphorescent OLEDs has not fully understood yet. Generally, the sources of efficiency roll-off at high intensities are considered from two aspects: one is the imbalance between the numbers of electrons and holes in the emissive layer (EML) and the other is the nonradiative exciton quenching processes in the emissive layer (EML).[7] Much attention to date has focused on the role of quenching processes of triplet-triplet exciton annihilation (TTA) and triplet-polaron annihilation (TPA) as the source of efficiency roll-off. [8, 9] However it has been shown that TTA quenching process takes effect only at much higher current densities (>1000 ma/cm 2 ) for the short lifetime (<1 s) phosphorescent dopant based device. Thereby, the efficiency roll-off in FIrpic-based phosphorescent OLEDs cannot be fully explained by the TTA or TPA processes for an average exciton lifetime of ~2 s for the excitons forming in 3

4 FIrpic.[10] It is necessary to explore the origin of the roll-off to deeply understand the underlying physical mechanisms and improve the performance of FIrpic-based OLEDs. In this study, it is found that the efficiency roll-off in FIrpic-based phosphorescent OLEDs correlates closely with the exciton quenching induced by the leakage of exciton into the hole transporting layer and then quenching induced by radical cation CBP + that formed in the CBP due to imbalanced electron-hole current. Moreover, the self-quenching effect is very weak due to the localized electron distribution in the FIrpic lowest unoccupied molecular orbital (LUMO) and electrostatic interaction between FIrpic molecules. II. Experiment. The blue phosphorescent OLEDs with a structure of ITO/ poly(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS, ~25 nm)/cbp (10 nm)/cbp:firpic (x %, 30 nm, x = 5, 12, 25, 50, 80, and 100)/ 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 50 nm)/lif(1 nm)/al(100 nm) were built. Thin films of PEDOT:PSS, CBP, CBP:FIrpic, and TPBi in the OLEDs serve as the hole injection layer, hole transport layer, emission layer, and electron transport layer, respectively. The pre-patterned ITO/glass substrates, having a sheet resistance of 10 Ohms/sq, were cleaned in ultra-sonication using acetone, ethanol, deionized water, and isopropanol followed by an ex situ UV ozone treatment in air for 5 min. A 25 nm thick PEDOT:PSS film was spin-coated onto UV ozone treated ITO 4

5 substrates at 4000 rpm in air. They were then annealed at 120 C for 30 min in a nitrogen-purged glove box (MBRAUN) with oxygen and moisture levels less than 0.1 ppm. PEDOT:PSS-coated ITO/glass substrates were then quickly loaded into the vacuum chamber that is connected to glove box for device fabrication. All functional layers used in the phosphorescent OLEDs were deposited at pressure below Torr. Detailed processes of fabrication and measurement for OLEDs have been described in our previous paper [11, 12]. CBP, FIrpic, and TPBi were purchased from LumTec, and PEDOT:PSS was purchased from H.C. Starck. All the materials were used as received without purification. The OLEDs had an active area of 10 cm 2, defined by the overlap area between the front ITO anode and the rear Al cathode. Ш. Results and Discussion Figure 1a shows the current density-efficiency characteristics of all seven devices. We can see that the current efficiency of devices is increased with FIrpic concentration increased from 5% to 50%, then decreased with further increasing the concentration, which can be explained as follows. The flat energy level alignment of the device in this study is shown in Figure 1b and the triplet exciton energy is 2.53 ev, 2.62 ev, and 2.67 ev for CBP, FIrpic, and TPBi, respectively. [13, 14] From the energy level alignment we can deduce that the excitons are formed at the CBP:FIrpic/TPBi and CBP/FIrpic interface for doping and neat FIrpic (referred to the 100% concertration) devices, respectively. The charges, including holes and electrons, are primarily injected into CBP and only a small amount of the charges are trapped by 5

6 FIrpic when the FIrpic is at a low doping level as reported by Lu group.[14] Moreover, the energy transfer from CBP to FIrpic is inefficient due to the low triplet energy level of CBP. These two effects result in a low efficiency when the FIrpic concentration is too low. With the increase of doping concentration, the electrons are directly injected into the FIrpic molecules from TPBi and most of holes are trapped by FIrpic, which leads to the excitons are dominantly formed on FIrpic and increase its contribution to the electroluminescence emission. The mechanism concerned above is also confirmed by the electrical properties of the devices as shown in Figure 1c. We can see that the current density is increased at the same voltage for higher doping concentration, which is due to the increased electron transport by FIrpic in CBP:FIrpic layer. In addition, we found that the 30 nm neat FIrpic based device possesses an intermediate current density, which is ascribed to the lower electron mobility and higher hole mobility of CBP than those of FIrpic. It is worth noting that the efficiency roll-off is reduced with the FIrpic concentration increased and the 30 nm neat FIrpic based device possesses the lowest efficiency roll-off. The normalized current efficiency is shown in Figure 2. The J 80 (current density to 80% decay in current efficiency), is monotonically increasing with dopant concentration increased, 16.4, 17.6, 17.9, 32.5, 54.0, and 96.2 ma/cm 2 for 5%, 12%, 25%, 50%, 80%, and 100% Firpic based devices, respectively. This phenomenon is very different from the accepted concept that the efficiency roll-off or exciton quenching will be deteriorative due to TTA and TPA processes with a higher phosphorescent dopant concentration. Based on the above analysis, here, we attribute 6

7 the efficiency roll-off to two aspects: one is the lack of charge balance and hence exciton quenching by the CBP +. Holes are already the dominant carrier in these structure devices, which will result in the formation of CBP + and quench the exciton formed on FIrpic. To test this hypothesis, we fabricated a device with a structure of ITO/hexaazatriphenylene-hexacarbonitrile (HAT-CN, 2 nm)/cbp (33 nm)/cbp:firpic (20% and 100%, 30 nm)/tpbi (50 nm)/lif (1 nm)/al (100 nm). For the HAT-CN based devices, the UV ozone treated ITO substrates were directly loaded into the same vacuum chamber to complete the device fabrication. Here the 20% doping concentration is an optimized parameter for HAT-CN based device and 100% means a neat FIrpic layer. This device shows a lower current density at the same voltage than that of PEDOT:PSS based devices as shown in Figure 3a, which is due to the lower hole injection efficiency of HAT-CN based device. The current density efficiency properties is shown in Figure 3b and inset is the normalized efficiency at different current densities. HAT-CN based device shows a lower efficiency roll-off, implying a weak effect of CBP + on the device performance. These results further confirm our evaluation that CBP + is formed and quencher the excitons in FIrpic. The other origin of the efficiency roll-off is attributed to the back energy transfer from FIrpic to the host of CBP. This interaction between FIrpic and CBP has been discussed in a previous publication that describes endothermic energy transfer as a mechanism in a FIrpic:CBP device.[13] Figure 4a shows the electroluminescence spectra of all seven devices at current density of 5 ma/cm 2. As can be seen, a contribution from CBP host near 393 nm is observed and the ratio of emission intensity from CBP to that of FIrpic 7

8 is decreased with increasing FIrpic concentration, which indicates more excitons is formed in FIrpic with the amount of FIrpic molecules increasing, consequently enhancing the emission from FIrpic. The electroluminescence spectra exhibit a broadening and shift toward the longer wavelength, which is accompanied by the broadening and growth of the long wavelength tail band. These are characteristic features of extensive aggregation of FIrpic molecules and similar phenomena have been reported in fac-tris(2-phenylpyridinato-n, C2')iridium (III) [Ir(ppy) 3 ] based devices.[15] It is known that there are two emission bands associated with two different triplet states on FIrpic, having an emission peak around 470 nm with a sub-peak around 500 nm. [14] This may be due to the back energy transfer from the higher triplet of FIrpic to CBP and a favored energy transfer from TPBi to the lower triplet state of FIrpic, [14] resulting in a relative high intensity near 500 nm for the low doping concentration devices. To better describe the rationality of the above discussion, we define the efficiency enhancing factor: (E) = E(c)/E(0) - 1 E(c) is the normalized current efficiency of PEDOT:PSS based devices for different dopant concentration, from 12% to 100%. E(0) is the normalized efficiency of 5% doping concentration device. The enhancement factors of (E) at different current density are shown in Figure 4b. We can see that the (E) is increased gradually with the increase of current density for the devices with higher FIrpic concentration, 8

9 which further confirms that higher concentration of FIrpic within the recombination region is in favor of the exciton formation on FIrpic and consequently reduce the efficiency roll-off. Figures 5(a-d) show the spectra of 5%, 50%, 80%, and 100% FIrpic based devices with PEDOT:PSS as the HIL at current density of 1, 2.5, 5, 10, 20, 40, and 100 ma/cm 2. We can see that the intensity of main emission peak around 475 nm is enhanced with the current density increased, which further confirms that more charges (or excitons) are directly injected into (or formed on) FIrpic molecules at higher current density. The intensity distribution for each device is almost current independent when the doping concentration is over 25%, which is attributed to the direct formation of excitons by charge injection into FIrpic. Generally, the doped OLEDs are more difficult to adapt for mass production processes than those based on non-doped ones considering the reproducibility of the optimum doping level. Achieving all non-doped white OLEDs is still a big challenge, especially for the blue emission component. In our study, it is surprising that the device with a neat 30 nm FIrpic can still maintain high efficiency, 4.06 cd/a for PEDOT:PSS based device. Moreover, the current efficiency reaches 7.76 cd/a with HAT-CN as the HIL as shown in Figure 6. The efficiency roll-off with neat FIrpic layer in HAT-CN device is much reduced. For example, the current efficiency is only decreased by 19% from the peak value of 7.76 cd/a to 6.28 cd/a at 40 ma/cm 2, and to 4.90 cd/a (37%) at 100 ma/cm 2. It is distinctly different from that Ir(ppy) 3 based devices as reported by Cao et al.,[15] in which the device efficiency is very low and 9

10 the external quantum efficiency with neat Ir(ppy) 3 layer is only about 1% due to the self-quenching of Ir(ppy) 3. In contrary, the self-quenching interactions between FIrpic molecules is much weaker than that of Ir(ppy) 3. It is believed that the fluorination on the ppy ligand hinders self-quenching interactions.[16] In order to explore the underlying mechanism of FIrpic based devices, we calculate the electron density distributions of the highest occupied molecular orbital (HOMO), LUMO, and electrostatic surface potentials with density functional theory and the results are shown in Figure 7 (a-c). The electron density distributions of the FIrpic HOMO are mainly located on the Ir d-orbital and the phenyl rings of the ppy ligands as shown in Figure 7a. In contrast to Ir(ppy) 3, of which the LUMO is almost entirely ligand-centered as reported by Himmetoglu et al.,[17] the LUMO of FIrpic is entirely contributed from the pic ligand due to the introduction of fluorine atoms, which will decrease the overlap of LUMOs, and hence the quenching process due to energy transfer, between different FIrpic molecules. In addition, the fluorine and oxygen atoms exhibit high electronegativity as shown in Figure 7c, which will prevent the aggregation of FIrpic molecules at high doping concentration by Coulomb repulsion interaction and reduce the efficiency roll-off to some extent. All the results indicate that it is needful to design phosphorescent dye molecules with localized electron distributions for HOMO or/and LUMO energy level, hence reducing the overlap of HOMOs (LUMOs) between different molecules, and high surface electric potential (electronegativity or electropositive) to increase the Coulomb repulsion interaction to reduce the self-quenching occurred at high doping concentration. 10

11 Conclusions In conclusion, we fabricated phosphorescent dye based OLEDs with different doping concentration of FIrpic, as well as a neat FIrpic layer, as the emission layer. The exciton quenching mechanism in these devices is studied and we demonstrate that the radical cations CBP + and back energy transfer are the main quenching source, which result in the device efficiency roll-off. In addition, the fluorination on the ppy ligand in FIrpic molecule plays an important role in hindering the self-quenching interaction due to localized electron distributions of FIrpic LUMO energy level and strong Coulomb repulsion interaction between FIrpic molecules. Our results shed light on that the origins of efficiency roll-off is correlates closely with quenching process due to radical cations CBP + in CBP:FIrpic based OLEDs under the current density less than 100 ma/cm 2 and provide new guides for designing efficient and doping concentration independent (or weak dependent) phosphorescent small molecule materials. 11

12 Acknowledgements This research was supported by the National Natural Science Foundation of China (Nos ). 12

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15 Figure captions Figure 1 (a) The current density-efficiency curves of devices; (b) The molecular structure of FIrpic and energy level of materials used in this study; (c) The voltage-current density characteristics of devices with different FIrpic doping concentration. Figure 2 The normalized current efficiency-current density curves for all seven PEDOT:PSS based devices. Figure 3 (a) The current density-voltage characteristics of PEDOT:PSS based device with 5% FIrpic doping concentration, as well as that of HAT-CN based device with optimum FIrpic doping concentration of 20%; (b) The current efficiency-current density curves for PEDOT:PSS (5%) and HAT-CN (20%) based devices. Figure 4 (a) The EL spectra of PEDOT:PSS based devices at 5 ma/cm 2 and (b) The enhancement factors of (E) for normalized efficiency at different current density. Figure 5 (a) The normalized EL spectra of all seven PEDOT:PSS based devices and (b) the efficiency enhancing factor for the PEDOT:PSS device relative to that of 5% doping concentration. These enhancing factor are obtained from the normalized efficiencies, not the practical efficiency enhancement. Figure 6 (a) The normalized EL spectra for PEDOT:PSS based device (5%, 50%, 80%, and 100% FIrpic layer) at different current densities from 1 to 100 ma/cm 2. Figure 7 The calculated HOMO (a) and LUMO (b) energy level for FIrpic molecule; (c) The ESP of FIrpic. The red zone indicates electrically negative and blue zones indicate electrically positive. 15

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