 Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 28 - 30 28 Color Stability Enhancement of White Organic Light-Emitting Diodes Using a Charge Control Layer Shui-Hsiang Su * , Yu-Cheng Lin, Meiso Yokoyama Department of Electronic Engineering, I-Shou University, Kaohsiung, Taiwan. Received 07 April 2016; received in revised form 01 May 2016; accept ed 26 May 2016 Abstract Blue phosphorescent iridium complexes iridium (III) bis [(4,6-di-fluorophenyl)-pyridinato-N,C2'] (FIrpic) and yellow phosphorescent iridium com- plexes Iridium(III) bis (4-phenylthieno [3,2-c] pyri- dinato-N,C2′) acetylacetonate (PO-01) was doped into the small molecular phosphorescent host N, N_-dicarbazolyl-3, 5-benzene (mCP) to fabricate white phosphorescent organic light-emitting diodes (white PHOLEDs). Device current efficiency is enhanced by inserting a charge control layer (CCL) into the emitting layer. The peaks of PHOLED e lectrolu minescent (EL) spectrum locate at 472 n m (FIrpic) and 560 nm (PO-01). A h igh current efficiency white PHOLED has been fabricated by the use of FIrp ic and Po -01 as the double e mitting layer (EM L), in wh ich the mCP is used as the CCL inserted. The doping concentration of PO-01 is optimized and the carrie r transport mechanis m of CCL is discussed. The optimized current efficiency is 30.06 cd/A. The CIE coordinates locate at (0.33, 0.41) and vary with in (± 0.01, ±0.01) under driving voltage of 5-15V. Keywor ds : OLED, charge control layer (CCL) 1. Introduction Organic light-e mitting diodes (OLEDs) have attracted much interest over the past few years because of their properties of self-e mission, fast response time, h igh lu minance, low cost, and ease of fabrication, a mong other characteris- tics . 1,2 White organic light-e mitting diodes (OLEDs) are one of the most pro mising tech- nologies which will like ly replace the e xisting liquid c rystal display (LCD). In particula r, white OLEDs (WOLEDs) have drawn particular at- tention because of their use in full-color displays combined with red-green-b lue (RGB) co lor filters, liquid crystal display (LCD) backlights, and next-generation light sources. 3-9 White light emission can be obtained by mixing light of two comple mentary colors (such as red/bluish green, blue/orange, or green/magenta) or the three prima ry colors (red, green, and blue) fro m sma ll mo lecules and/or polyme rs. To obtain high lu- minance of a typical layered OLED, the light-e mitting layer is generally doped with various fluorescent or phosphorescent dyes. In this paper, iridium (III) bis [(4, 6-di-fluorophenyl)-pyridinato-N, C2’] (FIrp ic) and Iridium (III) bis (4-phenylthieno [3,2-c] pyridinato-N,C2′)acetylacetonate (PO-01) are used as blue and yellow dopants. Doping con- centrations are optimized and the carrier trans- porting mechanism of CCL is discussed. 2. Experiment The structures of WOLED devices used in this study were ITO/ m-MTDATA/α-NPB/ mCP/ mCP:FIrpic/ mCP:PO-01/T PBi/ LiF/Al. A thin charge control layer (CCL) layer of mCP is sandwiched between mCP:FIrp ic and mCP:PO-01 EM L to study the optoelectronic performance of WOLED devices. Devices we re fabricated on glass substrates that had been precoated with a 150-n m-thick layer of indi- um-t in-o xide (IT O) with a sheet resistance of 10 /square. Cleaned and UV-ozone-treated IT O substrates were loaded into an evaporation sys- tem at a base pressure of under 10 -6 Torr. The WOLEDs were formed such that the intersec- tions between the ITO anode and the cathode stripes each had an area of 0.24 cm 2 . The current density-voltage (J-V), lu mi- nance-current density-luminous efficiency (L-J -ηL) and power effic iency-current dens i- ty-externa l quantum efficiency (ηp-J-ηext) char- acteristics were measured using a PR650 spec- troscan spectrometer and a Keithley 2400 p ro- *Corresponding aut hor. Email: shsu@isu.edu.t w Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 28 - 30 29 Copyright © TAETI grammable voltage-current source. The EL spectra and the Co mmission Internationale d’Ec la irage (CIE) coordinates of these devices were a lso obtained using a PR650 spectroscan spectrometer. 3. Results and Discussion An OLED device without a CCL has the structure of IT O/ m-MT DATA/α-NPB/ mCP/ mCP:FIrpic/ mCP:PO-01/TPBi/ LiF/Al. The op- timized current effic iency (CE) is 20.1 cd/A and lu minance (L) is 16200 cd/m 2 . A thin CCL layer of mCP sandwiched between mCP:FIrpic and mCP:PO-01 EM L increase the CE to 24.1 cd/A. The thickness of each emitt ing layer is modified to observe the luminous characteristics. The optimized current efficiency of 30.06 cd/A and lu minance of 17000 cd/m 2 have been achieved. Its CIE coordinates locate at (0.33, 0.41) and vary within (±0.01, ±0.01) under driving voltage of 5-15V. Fig. 1 shows the triplet e xc iton transfer mechanism in white PHOLED. Fig. 1 The triplet states of T EBL, CCL and EM Ls . The trip let e xciton transfer mechanism in white PHOLED 4. Conclusions In this paper, the fabrication of high co lor stability WOLEDs with mCP as a CCL layer is demonstrated. The CIE coordinates locate at (0.33, 0.41) and vary within (± 0.01, ±0.01) u n- der driving voltage of 5-15V. The optimized current efficiency reaches 30.06 cd/A. Expe ri- mental results reveal that we have successfully fabricated high efficiency and color stability WOLED by optimizing the doping concentra- tion of PO-01 and the thickness of CCL. Acknowledgement The authors would like to thank the Ministry of Science and Technology of the Republic of China, fo r financially supporting this research under contract No. MOST 104-2221-E-214 -041 References [1] C. W. Joo, J . W. Sh in, J. Moon, J. W. Huh, D. H. Cho, J. Lee, S. K. Park, N. S. Cho, J. H. Han, H. Y. Chu, J. I. Lee , “Highly effi- cient white transparent organic light e mit- ting diodes with nano-structured substrate,” Org. Electron., vol. 29, pp. 72, 2016. [2] R. Joyce, K. Singh, S. Va rghese, J. Akhtar, “Effective c leaning process and its influ- ence on surface roughness in anodic bond- ing for semiconductor device packaging ,” Mater. Sc i. Se micond. Process., vol. 31, pp. 84, 2015. [3] B. Liu , J. Zou , Z. Zhou , L. Wang, M. Xu, H. Tao, D. Gao, L. Lan, H. Ning, J. Peng, “Efficient single-e mitting layer hybrid white organic light-e mitting diodes with low efficiency roll-off, stable color and ex- treme ly high lu minance,” J. Ind. and Eng. Chem., vol. 30, pp. 85, 2015. [4] C. W. Tang and S. A. Vanslyke, “ Organic electrolu minescent diodes ,” Appl. Phys. Lett., vol. 51,p p. 913, 1987. [5] G. T . Chen, S. H. Su, C. C. Hou, and M. Yo koya ma, “ Effects of Thermal Annealing on Performance of Organic Light-Emitt ing Diodes ,” J. Electrochem. Soc., vo l. 153, pp. J159, 2007. [6] J. F. Li, S. F. Chen, S. H. Su, K. A. Hwang, and M. Yo koya ma, “Full-Wavelength White Organic Light-Emitt ing Diodes with Blue Fluorescence and Phosphorescent Iridiu m Co mple xes ,“ J. Electrochem. Soc., vol. 153, pp. H195, 2006. [7] X. Du, Y. Huang, S. Tao, X. Yang, X Ding, X. Zhang, “Highly efficient wh ite fluores- cence/phosphorescence hybrid organic light emitting devices based on an effic ient hole-transporting blue emitter,” Dyes Pigments, vol. 115, pp. 149, 2015. [8] Y. S. Tsai, A. Ch ittawanij, L. A. Hong, C. Y. Ou, F. S. Juang, C. C. Wang, S.H. La i, “Adjusting dopant concentrations in solu- tion process to optimize the white pho s- phorescent organic light-e mitt ing diodes ,” Microelectron. Eng., vol. 138, pp. 31, 2015. http://www.engineeringvillage2.org/controller/servlet/Controller?CID=quickSearchCitationFormat&searchWord1=%7bChen%2C+Guan-Ting%7d§ion1=AU&database=1&yearselect=yearrange&sort=yr http://www.engineeringvillage2.org/controller/servlet/Controller?CID=quickSearchCitationFormat&searchWord1=%7bSu%2C+Shui-Hsiang%7d§ion1=AU&database=1&yearselect=yearrange&sort=yr http://www.engineeringvillage2.org/controller/servlet/Controller?CID=quickSearchCitationFormat&searchWord1=%7bHou%2C+Cheng-Chieh%7d§ion1=AU&database=1&yearselect=yearrange&sort=yr http://www.engineeringvillage2.org/controller/servlet/Controller?CID=quickSearchCitationFormat&searchWord1=%7bYokoyama%2C+Meiso%7d§ion1=AU&database=1&yearselect=yearrange&sort=yr http://www.ecsdl.org/vsearch/servlet/VerityServlet?KEY=JESOAN&possible1=Li%2C+Jian-Feng&possible1zone=author&maxdisp=25&smode=strresults&aqs=true http://www.ecsdl.org/vsearch/servlet/VerityServlet?KEY=JESOAN&possible1=Su%2C+Shui-Hsiang&possible1zone=author&maxdisp=25&smode=strresults&aqs=true http://www.ecsdl.org/vsearch/servlet/VerityServlet?KEY=JESOAN&possible1=Yokoyama%2C+Meiso&possible1zone=author&maxdisp=25&smode=strresults&aqs=true Proceedings of Engineering and Technology Innovation , vol. 2, 2016, pp. 28 - 30 30 Copyright © TAETI [9] B. S. Kim, J. Y. Lee, “Interlayer free hybrid white organic light-e mitting diodes with red/blue phosphorescent e mitters and a green therma lly activated delayed fluores- cent emitter,” Org. Electron., vol. 21, pp. 100, 2015.