Open Access
Add to BibSonomy
Abstract
Micro-cavity top-emitting organic light emitting diodes (TEOLEDs) are now receiving prominence as a technology for the active matrix display applications. The semi-transparent metal cathode plays the crucial role in realizing TEOLEDs structure. Here, we report the optimization results on Mg:Ag ratio as the semitransparent cathode deposited by vacuum thermal evaporation. The optimized Mg:Ag cathode with 1:10 ratio (wt %) shows a sheet resistance value as low as 5.2 Ω/□, an average transmittance of 49.7%, reflectance of 41.4%, and absorbance of 8.9% over the visible spectral region (400~700 nm). The fabricated red TEOLEDs device implemented using LiF (1nm)/Mg:Ag (1:10) cathode shows the voltage value of 4.17 V at a current density of 10.00 mA/cm2, and current efficiencies variation from 55.3 to 50.1 cd/A over the brightness range 2,000 – 12,000 cd/m2. The electroluminescence (EL) spectrum displays the light emission at 608 nm wavelength with a half width of 29.5 nm. The narrow half-width of red light emission is attributed to the micro-cavity effects due to the semitransparent cathode.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Micro-cavity top-emitting organic light-emitting diodes (TEOLEDs) technology is becoming increasingly important for active-matrix displays. TEOLEDs have merits of high aperture ratio as the active matrixed-circuitry is located at the bottom of the device. The top-emitting architecture comprising a reflective anode and a semitransparent cathode causes the micro-cavity effect. As a consequence, the device efficiency enhances at the forward direction [1–4]. As well, it narrows the broad emission spectra of organic emitters with its wavelength selective property, enhancing the color purity. Obviously, the top emissive structure with micro-cavity effect is required to be investigated extensively for the active matrix display applications owing to these various advantages [5–8].
One of the key technologies for high-quality OLED displays is to improve the optical and electrical characteristics of the semi-transparent cathode in micro-cavity TEOLEDs [9]. To inhibit the increase of driving voltage at display applications, the low sheet resistance is vitally required even when used as thin metal film. Such semi-transparent cathode is also desired to have the high reflectance and low absorption properties to enhance the micro-cavity effect [10]. In addition, the semi-transparent cathode ought to have good electron injection property without any injection barrier with electron transport layer. Hence, a very thin silver alloy layer due to its high conductivity was usually employed in top emitting devices to achieve strong micro-cavity effect such as a co-deposited Mg:Ag alloy or a multi-layered CsF/Yb/Ag or Al/Ag film as semi-transparent cathodes [11–15]. Although, magnesium, aluminum, and ytterbium thin layers improve the electron injection property, these layers have a high absorbance at the visible wavelength range. In recent years, many researchers have reported regarding the highly transparent electrode configuration such as thermally evaporable high refractive index materials/Ag/high refractive index materials. Yook et al. demonstrated the WO3∕Ag∕WO3 (WAW) structure with the high transparency over 80% [16]. The MoO3/Ag/MoO3 (MAM) stack with an optical transmittance of about 65–80% in the range 400–700 nm and a low sheet resistance (9 Ω/□) was also realized [17], while the 15-nm-thick Yb:Ag and Mg:Ag semi-transparent cathodes showed the transmission/reflection of 50.2%/21.7% (absorption 28.2%) and 20.4%/67.2% (absorption 12.4%) at 532 nm, respectively [18]. Some researchers proposed the metal mesh as the most promising candidates of transparent electrodes in TEOLEDs [19–21]. A 50 nm thick Ag transparent metallic network electrodes realized using the interconnected crack template masking method and thermal evaporation showed the low sheet resistance of 6.8 Ω/□ and the higher optical transparency over 80% [22]. In addition to these modified Ag film cathodes, Kim et al. reported an indium zinc oxide (IZO), deposited using a box cathode sputtering (BCS) technique, with a sheet resistance of 42.6Ω∕□ and average transmittance above 88% in the visible range as a cathode layer for TEOLEDs [23]. However, these results about the highly transparent cathode are accompanied with the unsolicited low reflectance which is negating the manifestation of strong micro-cavity effect.
In this study, we present an optimized Mg:Ag semi-transparent cathode for the highly efficient top emitting OLED with a strong micro-cavity effect. In order to reduce the light absorption, a ratio of magnesium in the cathode structure is minimized from 90 wt% to 10 wt% with an adequate electron injection property. Our optimized cathode with the Mg ratio of 10 wt% has an average transmittance of 49.7%, reflectance of 41.4%, and absorbance of 8.9% over the visible wavelength region (400~700nm). Also, a low sheet resistance of 5.2 Ω/□ which is suitable to display applications is demonstrated in this cathode. The fabricated phosphorescent red TEOLED with the optimized cathode (10 wt% of Mg) shows the current efficiency value of 50.1 cd/A, whereas the cathode device with 90 wt% of Mg has that of 32.5 cd/A. Our optimized cathode with reduced absorption attribute enhances the performance of phosphorescent red TEOLED by 1.5 times.
2. Experimental
N,N’-Di[4-(N,N’-diphenylamino)phenyl]-N,N’-diphenylbenzidine (DNTPD) and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) were used as an hole injection layer (HIL) and an additional HTL from EM Index, respectively. N,N’-Bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (NPB) as a hole transport layer (HTL), bis(10-hydroxybenzo [h] quinolinato)beryllium complex (Bebq2) as a red host and 4,7-diphenyl-1,10-phenanthroline (Bphen) as an electron transport layer (ETL) were procured from Jilin OLED Materials Tech. Iridium(III)bis(4-methyl-2-(3,5-dimethylphenyl)quinolinato-N,C2’)acetylacetonate (Ir(mphmq)2(acac)) was synthesized by using our previously reported method [24]. Mg procured from Kurt J. Lesker, and silver (Ag) from Taewon Scientific were used to fabricate the semi-transparent cathode layer. The NPB was used as a capping layer (CPL) on thin Mg and Ag alloy cathode. Bare glass and the strong reflective anode coated glass substrates were cleaned using sonification in an acetone and isopropylalcohol (IPA) for 10 minutes each. The cleaned substrates subsequently were rinsed using deionized (DI) water, followed by UV-ozone treatment for 10 minutes. All organic materials and cathode units were deposited on the pre-cleaned substrates using vacuum evaporation technique under a pressure of 10−7 Torr. The deposition rate of all organic layers for HTL, EML, and ETL was about 0.5~0.7 Å/s. The deposition rates of Mg and Ag were 2.0 Å/s and 0.2 Å/s, respectively. All devices were encapsulated in a glass-to-glass epoxy sealed package with desiccant and later treated by thermal annealing at 90 °C for 60 min.
The transmittance and reflectance spectra were measured using a UV-visible spectrophotometer with a diffuse reflectance accessory (the absolute specular reflectance accessory with the VN-type) in air at room temperature. In order to evaluate the exact thickness and the element ratio, Transmission Electron Microscope (TEM, JEOL JEM 2100F) – Focused Ion Beam (FIB, FEI scios) and Energy Dispersive X-ray Spectroscopy (EDS, Oxford X-Max 80T) were performed. The sheet resistance was measured using the four-point probe method. The work function of the fabricated cathode was measured using a surface analyzer (RIKEN KEIKI, AC-2) in the atmospheric environment. The current density–voltage–luminance (J–V–L) characteristics and electroluminescence (EL) spectra with CIE color coordinate of TEOLED devices were measured using a Keithley 236 and a PR-705 spectrophotometer (Photo Research). All the devices were encapsulated in a glove box under nitrogen atmosphere prior to measurement.
3. Results and discussion
3.1 Semitransparent cathode
We fabricated the Mg:Ag cathode samples in order to analyze the optical and electrical properties with their compositions as follows:
Sample 1: ETL (40 nm) / Mg:Ag (20 nm, 10:1) / Capping Layer (60 nm)
Sample 2: ETL (40 nm) / Mg:Ag (20 nm, 5:1) / Capping Layer (60 nm)
Sample 3: ETL (40 nm) / Mg:Ag (20 nm, 1:1) / Capping Layer (60 nm)
Sample 4: ETL (40 nm) / Mg:Ag (20 nm, 1:5) / Capping Layer (60 nm)
Sample 5: ETL (40 nm) / Mg:Ag (20 nm, 1:10) / Capping Layer (60 nm).
The total thickness of the Mg:Ag layer was maintained at a value of 20 nm in all samples. Figures 1 shows the transmittance (T), reflectance (R), absorbance (A) spectra and the sheet resistance of the fabricated Mg:Ag cathode samples. Results displayed in Table 1 clearly demonstrate that the transmittance, reflectance, absorbance, and sheet resistance are strongly dependent on the compositions of Mg and Ag in Mg:Ag films. The average T, R, and A values are determined for the visible wavelengths region 400-700 nm. The average transmittance increases with the decrease of Mg concentration (or increase of Ag concentration) in Mg:Ag mixed layer; the increase was from 33.4% to 49.7% as Mg decreases. Further, the lowest absorbance is observed with the minimized magnesium ratio (less than 10 wt %). Such performance is attributed to (i) the reduced Mg ratio which has a high absorbance, and (ii) the increase in Ag ratio with the superior reflectance and the low absorption [25]. Whereas, the average reflectance is almost similar however the slope of reflectance increases as the Ag ratio increases as displayed in Fig. 1(b). These results are ascribed to the high reflectivity of Ag at the long wavelength range. Figure 1(d) shows the sheet resistance with different compositions of Mg and Ag. The Mg:Ag (10:1) film has a sheet resistance value as high as 32.2 Ω/□. The sheet resistance decreases with the increase of Ag concentration except the Mg and Ag ratio of five to one. Exceptionally high sheet resistance (41.4 Ω/□) is noticed in sample 2 which is higher than the sample 1. The observance of somewhat strange results in Mg:Ag 5:1 ratio is unclear. It appears to have a turning point of Ag homogenous distribution in this Mg:Ag film. The Mg:Ag film with the ratio of 1:10 exhibits the lowest sheet resistance of 5.2 Ω/□ due to the superior conductivity of Ag than Mg in the 20 nm thick mixed metal layer. As described above, Mg:Ag (1:10) films shows a best tradeoff between the optical transparency, reflectance, absorbance, and sheet resistance. Evidently, the thin Mg:Ag (1:10) electrode is an appropriate candidate for a semitransparent electrode in the visible wavelength region.
Fig. 1 (a) Transmittance, (b) reflectance, (c) absorption and (d) sheet resistance characteristics of Mg:Ag cathodes with different compositions.
Table 1. Parameters of Mg:Ag cathodes with different compositions
The physical distribution of each elements in co-deposited metals cathode layer plays significant role in determining the performance of semitransparent cathode. Therefore, we performed the lift-out technique in FIB-TEM and EDS to investigate the distribution of each elements in all Mg:Ag samples as shown in Fig. 2. The lift-out FIB-TEM technique is advantageous to directly measure the physical distribution of each elements from the bulk samples without any cutting and polishing process. The TEM images show the exact thickness of the cathode unit samples. For FIB-TEM investigations, the thickness of Mg:Ag layers deposited with various mixed ratio are about 20 ± 2 nm. In order to analyze the exact distribution of each element in the cathode unit, the EDS measurement with area scan methods were conducted on the fabricated cathode. In EDS area scan (mapping), the width is the representative of the cathode thickness. Figure 2 shows the Mg and Ag atom distributions. In case of sample 1, the Ag is mainly gathered at the bottom side of aggregated region as indicated by the EDS image. Therefore, the thin black line is detected in the TEM image. An island type at the more widespread area of 10 nm than its real content may be formed due to the low Ag concentration. In samples 1 and 2 with low Ag ratio in mixed layer, the Ag atoms are aggregated in the middle region (~10 nm) of Mg:Ag layer due to its strong aggregation property, which forms the structure similar to Mg/Mg:Ag/Mg multi-layer [26–30]. Therefore, they may have the similar transmittance characteristics. On the other hands, the spread black area is displayed in TEM images of almost all samples except sample 1 as the Ag is spaciously converged in Mg:Ag mixed film. The uniform distribution of Ag from sample 2 to sample 5 is more broaden as showed in EDS images of Fig. 2. In case of sample 4 and 5, small amount of Mg is mainly distributed in the upper part of aggregated Ag mass due to the difference of density between Mg and Ag. Due to the immiscible nature of each metal layer, the lower transmittance and the higher reflectance of sample 5 (Mg:Ag ratio of 1:10) than sample 4 (Mg:Ag ratio of 1:5) may be occurred due to a slightly thicker layer of Ag. Transmittance, reflectance, absorbance parameters of Mg:Ag mixed cathode are summarized as in Table 1.
In order to analyze the influence of these components distribution on the electron injection property, the electron only devices (EODs) using sample 1 and 5 cathode units (having the highest and lowest absorption) were fabricated as following structure: ITO/ Liq (3 nm)/ Bebq2: 3% Ir(mphmq)2(acac) (20 nm)/ Bphen (40 nm) /cathode unit (60 nm). Mg:Ag (10:1), Mg:Ag (1:10) and LiF(1 nm)/ Mg:Ag (1:10) were implemented as a cathode unit structure of EODs. Figure 3 shows the current density versus applied voltage characteristics of fabricated EODs. At the given constant current density of 1 mA/cm2, the operational voltages are 1.5 and 2.2 V for Mg:Ag (10:1) and Mg:Ag (1:10) cathodes, respectively. The low value of operational voltage with Mg:Ag (10:1) cathode unit indicates that the electron injection is facilitated without any additional electron injection layer (EIL) due to the large amount of Mg which has a trivial work function of 3.7 eV. On the other hands, the electron is injection from the Mg:Ag (1:10) cathode is rather difficult due to the formation of Ag agglomerate as revealed in Fig. 2. Therefore, the operational voltage difference appearance is reasonable. At the current density of 10 mA/cm2, both devices show the driving voltage of 1.6 and 2.4 V, respectively. These results corroborate that the mixed ratio of Mg and Ag substantially influences the electron injection property although Mg:Ag (1:10) cathode has a lower sheet resistance of 5.2 Ω/□ than that of the Mg:Ag (10:1) cathode (32.2 Ω/□). In order to investigate this unusual result, the energy barrier of cathode sample 1 and 5 at Bphen/Mg:Ag interface were measured. The electron injection barrier of 1.2 and 1.9 eV are observed in samples 1 and 5, respectively (see Table 1 for work function comparison). Clearly, the higher electron injection barrier with Mg:Ag (1:10) cathode is contributing to the inferior electron injection characteristics. Therefore, the EIL layer is really necessary to improve the electron injection properties for Mg:Ag (1:10) cathode. Earlier, the LiF buffer layer has been used widely at both low work function [31–33] and high work-function [34–38] electrodes because of the significant improvement of electrode properties. With the insertion of thin LiF buffer layer (1 nm), the electron injection properties are considerably improved. The current density versus voltages performance of EOD with LiF/Mg:Ag (1:10) is almost similar to that of only Mg:Ag (1:10) cathode device. However in case of LiF/Mg:Ag (10:1) cathode, the device performance was no more better than only Mg:Ag (10:1) cathode as the Mg:Ag (10:1) has a good electron injection characteristics without LiF.
Fig. 3 Current density versus applied voltage of EODs with various cathode structures of Mg:Ag (10:1), Mg:Ag (1:10) and LiF(1 nm)/ Mg:Ag (1:10).
3.2 TEOLEDs device fabrication and performance studies
To evaluate the performance of low absorption cathode unit, we fabricated phosphorescent red TEOLEDs as follows:
Device A: Ag (100 nm)/ ITO (10 nm)/ DNTPD (75 nm)/ HATCN (7 nm)/ NPB (123 nm)/ Bebq2 with 3% Ir(mphmq) (20 nm)/ Bphen (40nm)/Mg:Ag (10:1, 16 nm) / NPB (60 nm).
Device B: Ag (100 nm)/ ITO (10 nm)/ DNTPD (75 nm)/ HATCN (7 nm)/ NPB (123 nm)/ Bebq2 with 3% Ir(mphmq) (20 nm)/ Bphen (40nm)/Mg:Ag (1:10, 16 nm) / NPB (60 nm).
Device C: Ag (100 nm)/ ITO (10 nm)/ DNTPD (75 nm)/ HATCN (7 nm)/ NPB (123 nm)/ Bebq2 with 3% Ir(mphmq) (20 nm)/ Bphen (40nm)/LiF (1 nm)/Mg:Ag (1:10, 16 nm) / NPB (60 nm).
The energy diagram and molecular structure of the materials used are shown in Fig. 4. Indium tin oxide (ITO) of 10 nm was deposited on the silver as a strong reflective anode to enhance the hole injection property from Ag to DNTPD as a hole injection layer. Thin HATCN layer as a deep lowest unoccupied molecular orbital (LUMO) of 6.0 eV was inserted between the DNTPD and NPB layers to minimize the impact of any increase in driving voltage due to the thick HTL layer (~more than 200 nm). The characteristics of fabricated red TEOLEDs are shown in Fig. 5. From the results, the device B without any EIL materials has the highest operational voltage of 4.9 V at the current density of 10 mA/cm2 while the devices A and C are measured at lower voltage of 4.2 V. At the same current density condition, the luminance of fabricated devices are 4,064, 1,977 and 4,289 cd/m2, respectively. Improved performance with LiF (1nm)/ Mg:Ag (1:10) cathode is attributed to the enhanced electron injection properties. The current efficiency of devices A, B and C are 36.0, 46.1 and 54.3 cd/A at a given constant luminance of 3,000 cd/m2. Table 2 shows the characteristics of each OLED device at 10,000cd/m2. With the same device structure, the spectral peak positions of the fabricated TEOLEDs are at 616, 610 and 608 nm as shown in Fig. 5(c) and Table 2. It is argued that the different component ratio by the reflectivity difference of cathode changes the selective wavelength of micro-cavity condition. The full width at half maximum (FWHM) of electroluminescence (EL) spectra are also influenced due to the micro-cavity effect which are 44, 34 and 29.5 nm, respectively. Indeed, these results implied that Devices B and C with Mg:Ag (1:10) cathode generate strong micro-cavity effect owing to the higher reflectance at the long wavelength range than Mg:Ag (10:1) cathode (see Fig. 1(b)). Furthermore, the similar FWHM of emission light in Devices B and C indicates that the thin LiF layer may maintain the almost same cavity condition.
Fig. 4 Energy level diagram and molecular structure of the used material for the fabricated red TEOLEDs.
Fig. 5 (a) Current density versus voltage characteristic, (b) current efficiency and (c) electroluminescence spectra of the fabricated phosphorescent red TEOLEDs.
Table 2. OLED device characteristics of various Mg:Ag ratio
4. Conclusion
Suitable transparent conducting electrode materials with a low sheet resistance value, and excellent transmission and reflectance for top emitting OLED devices are gaining momentum. In that context, the semitransparent metal cathode plays the significant role in realization of efficient top emission devices. The fabricated Mg:Ag (1:10 wt%) semitransparent cathode shows a sheet resistance value as low as 5.2 Ω/ㅁ, an average transmittance of 49.7%, reflectance of 41.4%, and low absorbance of 8.9% over the visible spectral region (400~700nm). The red light emitting TEOLEDs device realized using LiF (1nm)/Mg:Ag (1:10) cathode shows excellent electrical properties (the operational voltage value of 4.17V at a current density of 10.00 mA/cm2, and current efficiencies variation from 55.29 to 50.09 cd/A over the brightness range 2000 – 12000nit). It may be mentioned here that the present investigation is not limited to the red emitting TEOLEDs. The described Mg:Ag electrode could be implemented in other TEOLEDs such as blue, green, etc. In conclusion, the results obtained in our fabricated TEOLEDs using LiF (1nm)/Mg:Ag (1:10) semitransparent cathode are useful to realize the pure and saturated color TEOLEDs devices for display applications.
Funding
Human Resources Development program (no. 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and Industrial Technology Innovation Program.No.10063289 'Development of High Temperature Negative tone Photosensitive Black Resin and fabrication process for Pol-less AMOLED Devices.
Acknowledgments
This work was supported by LG Display Co. Ltd.
References and links
1. H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieb, “Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling,” Appl. Phys. Lett. 82, 466 (2003).
2. S. F. Hsu, C. C. Lee, A. T. Hu, and C. H. Chen, “Fabrication of blue top-emitting organic light-emitting devices with highly saturated color,” Curr. Appl. Phys. 4, 663–666 (2004).
3. Z. Wu, Y. Zhai, R. X. Guo, and J. Wang, “Red top-emitting organic light-emitting device with improved efficiency and saturated color,” J. Lumin. 131, 2042–2045 (2011).
4. Y. Ji, F. Ran, H. Xu, W. Shen, and J. Zhang, “Improved performance and low cost OLED microdisplay with titanium nitride anode,” Org. Electron. 15, 3137–3143 (2014).
5. G. Gu and S. R. Forrest, “Design of flat-panel displays based on organic light-emitting devices,” IEEE J. Sel. Top. Quantum Electron. 4, 83–99 (1998).
6. E. Cantatore, Applications of Organic and Printed Electronics: a Technology-Enabled Revolution (Springer, 2013) chap. 3.
7. M. J. Park, Y. H. Son, G. H. Kim, R. Lampande, H. Woo Bae, R. Pode, Y. K. Lee, W. J. Song, and J. H. Kwon, “Device performances of third order micro-cavity green top-emitting organic light emitting diodes,” Org. Electron. 26, 458–463 (2015).
8. Y. H. Son, M. J. Park, R. Pode, and J. H. Kwon, “High efficiency top-emission organic light emitting diodes with second and third-order microcavity structure,” ECS J. Solid State Sci. Technol. 5(1), R3131–R3137 (2016).
9. R. Meerheim, M. Furno, S. Hofmann, B. Lussem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97, 253305 (2010).
10. C. C. Wu, C. W. Chen, C. L. Lin, and C. J. Yang, “Advanced organic light-emitting devices for enhancing display performances,” J. Disp. Technol. 01, 248–266 (2005).
11. P. E. Burrows, G. Gu, S. R. Forrest, E. P. Vicenzi, and T. X. Zhou, “Semitransparent cathodes for organic light emitting devices,” J. Appl. Phys. 87, 3080 (2000).
12. M. Y. Chan, S. L. Lai, M. K. Fung, S. W. Tong, C. S. Lee, and S. T. Lee, “Efficient CsF/Yb/Ag cathodes for organic light-emitting devices,” Appl. Phys. Lett. 82, 1784 (2003).
13. R. B. Pode, C. J. Lee, D. G. Moon, and J. I. Han, “Transparent conducting metal electrode for top emission organic light-emitting devices: Ca–Ag double layer,” Appl. Phys. Lett. 84, 4614 (2004).
14. C. J. Lee, R. B. Pode, J. I. Han, and D. G. Moon, “Red electrophosphorescent top emission organic light-emitting device with Ca/Ag semitransparent cathode,” Appl. Phys. Lett. 89, 253508 (2006).
15. D. G. Moon, R. B. Pode, C. J. Lee, and J. I. Han, “Transient electrophosphorescence in red top-emitting organic light-emitting devices,” Appl. Phys. Lett. 85, 4771 (2004).
16. K. S. Yook, S. O. Jeon, C. W. Joo, and J. Y. Lee, “Transparent organic light emitting diodes using a multilayer oxide as a low resistance transparent cathode,” Appl. Phys. Lett. 93, 013301 (2008).
17. B. Tian, G. Williams, D. Ban, and H. Aziz, “Transparent organic light-emitting devices using a MoO3/Ag/MoO3 cathode,” J. Appl. Phys. 110, 104507 (2011).
18. W. F. Xie, K. C. Lau, C. S. Lee, and S. T. Lee, “Transparent organic light-emitting devices with LiF/Yb:Ag cathode,” Thin Solid Films 515, 6975–6977 (2007).
19. S. Hong, J. Yeo, G. Kim, D. Kim, H. Lee, J. Kwon, H. Lee, P. Lee, and S. H. Ko, “Nonvacuum, maskless fabrication of a flexible metal grid transparent conductor by low-temperature selective laser sintering of nanoparticle ink,” ACS Nano 7(6), 5024–5031 (2013). [PubMed]
20. W. Zhou, J. Chen, Y. Li, D. Wang, J. Chen, X. Feng, Z. Huang, R. Liu, X. Lin, H. Zhang, B. Mi, and Y. Ma, “Copper mesh templated by breath-figure polymer films as flexible transparent electrodes for organic photovoltaic devices,” ACS Appl. Mater. Interfaces 8(17), 11122–11127 (2016). [PubMed]
21. J. H. Han, D. H. Kim, E. G. Jeong, T. W. Lee, M. K. Lee, J. W. Park, H. Lee, and K. C. Choi, “Highly conductive transparent and flexible electrodes including double-stacked thin metal films for transparent flexible electronics,” ACS Appl. Mater. Interfaces 9(19), 16343–16350 (2017). [PubMed]
22. K. Pei, Z. Wang, X. Ren, Z. Zhang, B. Peng, and P. K. L. Chan, “Fully transparent organic transistors with junction-free metallic network electrodes,” Appl. Phys. Lett. 107(3), 033302 (2015).
23. H. K. Kim, K. S. Lee, and J. H. Kwon, “Transparent indium zinc oxide top cathode prepared by plasma damage-free sputtering for top-emitting organic light-emitting diodes,” Appl. Phys. Lett. 88, 012103 (2006).
24. D. H. Kim, N. S. Cho, H. Y. Oh, J. H. Yang, W. S. Jeon, J. S. Park, M. C. Suh, and J. H. Kwon, “Highly efficient red phosphorescent dopants in organic light-emitting devices,” Adv. Mater. 23(24), 2721–2726 (2011). [PubMed]
25. K. S. Lee, I. Kim, C. B. Yeon, J. W. Lim, S. J. Yun, and G. E. Jabbour, “Thin Metal Electrodes for Semitransparent Organic Photovoltaics,” ETRI J. 35, 587–593 (2013).
26. X. J. Liu, X. Cai, J. S. Qiao, H. F. Mao, and N. Jiang, “The design of ZnS/Ag/ZnS transparent conductive multilayer films,” Thin Solid Films 441, 200–206 (2003).
27. D. Y. Yang, S. M. Lee, W. J. Jang, and K. C. Choi, “Flexible organic light-emitting diodes with ZnS/Ag/ZnO/Ag/WO 3 multilayer electrode as a transparent anode,” Org. Electron. 15, 2468–2475 (2014).
28. T. Winkler, H. Schmidt, H. Flugge, F. Nikolayzik, I. Baumann, S. Schmale, T. Weimann, P. Hinze, H. H. Johannes, T. Rabe, S. Hamwi, T. Riedl, and W. Kowalsky, “Efficient large area semitransparent organic solar cells based on highly transparent and conductive ZTO/Ag/ZTO multilayer top electrodes,” Org. Electron. 12, 1612–1618 (2011).
29. T. Winkler, H. Schmidt, H. Flugge, F. Nikolayzik, I. Baumann, S. Schmale, H. H. Johannes, T. Rabe, S. Hamwi, T. Riedl, and W. Kowalsky, “Realization of ultrathin silver layers in highly conductive and transparent zinc tin oxide/silver/zinc tin oxide multilayer electrodes deposited at room temperature for transparent organic devices,” Thin Solid Films 520, 4669–4673 (2012).
30. Y. C. Han, M. S. Lim, J. H. Park, and K. C. Choi, “Optical effect of surface morphology of Ag on multilayer electrode applications for OLEDs,” IEEE Electron Device Lett. 35, 238–240 (2014).
31. X. Yang, P. Gao, Z. Yang, J. Zhu, F. Huang, and J. Ye, “Optimizing ultrathin Ag films for high performance oxide-metal-oxide flexible transparent electrodes through surface energy modulation and template-stripping procedures,” Sci. Rep. 7, 44576 (2017). [PubMed]
32. C. J. Brabec, S. E. Shaheen, C. Winder, N. S. Sariciftci, and P. Denk, “Effect of LiF/metal electrodes on the performance of plastic solar cells,” Appl. Phys. Lett. 80, 1288 (2002).
33. D. Grozea, A. Turak, X. D. Feng, Z. H. Lu, D. Johnson, and R. Wood, “Chemical structure of Al/LiF/Alq interfaces in organic light-emitting diodes,” Appl. Phys. Lett. 81, 3173 (2002).
34. L. S. Hung, C. W. Tang, and M. G. Mason, “Enhanced electron injection in organic electroluminescence devices using an Al/LiF electrode,” Appl. Phys. Lett. 70, 152 (1997).
35. J. M. Zhao, S. T. Zhang, X. J. Wang, Y. Q. Zhan, X. Z. Wang, G. Y. Zhong, Z. J. Wang, X. M. Ding, W. Huang, and X. Y. Hou, “Dual role of LiF as a hole-injection buffer in organic light-emitting diodes,” Appl. Phys. Lett. 84, 2913 (2004).
36. Y. Zhao, S. Y. Liu, and J. Y. Hou, “Effect of LiF buffer layer on the performance of organic electroluminescent devices,” Thin Solid Films 397, 208–210 (2001).
37. F. R. Zhu, B. L. Low, K. R. Zhang, and S. J. Chua, “Lithium–fluoride-modified indium tin oxide anode for enhanced carrier injection in phenyl-substituted polymer electroluminescent devices,” Appl. Phys. Lett. 79, 1205 (2001).
38. Z. Sun, S. Shi, Q. Bao, X. Liu, and M. Fahlman, “Role of thick-lithium fluoride layer in energy level alignment at organic/metal interface: unifying effect on high metallic work functions,” Adv. Mater. Interfaces 2, 1400527 (2015).
