We have developed a white organic light-emitting diode featuring a double emission layer comprising a blue light-emitting conductive polymer as a host material for Cadmium Selenide (CdSe) quantum dots as red light emitters and tris-(8-hydroxyquinoline) aluminium thin layer for green light emission. The Commission Internationale de l'Eclairage coordinates of the emitting light of the device were found to be (0.32,？0.40) which were only slightly changed over a range of applied voltages between 5 and 10 volts. The use of CdSe nanocrystalline quantum dots (surface-stabilized with hexadecylamine/trioctylphosphine oxide ligands) in the hybrid heterostructure with poly(9,9-di-n-octylfluorenyl-2,7-diyl) conductive polymer was studied for a variety of CdSe concentrations developing the performance of the device in means of overcoming segregation problems in the blend. Besides, constituents' ratio was further examined for the exploration of possible energy transfer from polymer host material to the CdSe quantum dots as a key factor for well-balanced emission in the electroluminescent devices. 1. Introduction Electroluminescence as a result of radiative recombination of electrons and holes in an inorganic semiconducting material is a well-known phenomenon for many years with a variety of applications usually in common LEDs [1]. The use of organic semiconductors either in form of conjugated polymers or small molecules is a more recent technological development of the initial idea. Since the first announcement of organic light-emitting diodes (OLEDs) [2] and polymer-based light-emitting diodes (PLEDs) [3] an extensive research has been established which led these devices to the commercialization stage [4, 5]. The basic advantage of these devices is the easy fabrication process using a variety of small molecules [6] or conductive polymers [7] where common deposition techniques such as dip coating, spin casting, and inkjet printing are used. Furthermore, the variety of conductive organic materials and new trends in materials’ modification has led the OLED technology to present high-efficiency devices with long-term stability in a variety of emitting colours and turn-on voltages [8–10]. However, the most attractive emitting colour in LED industry is the white colour as it may offer many benefits in lighting technology including energy saving (e.g., flashlights, architectural lighting, displays backlighting, and large area displays). The fabrication of a white emitting LED based on organic materials (WOLED) is an attractive idea which was developed in very recent years
References
[1]
D. G. Fink and H. W. Beaty, Standard Handbook for Electrical Engineers, McGraw-Hill, New York, NY, USA, 11th edition, 1978.
[2]
C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes,” Applied Physics Letters, vol. 51, no. 12, pp. 913–915, 1987.
[3]
J. H. Burroughes, D. D. C. Bradley, A. R. Brown et al., “Light-emitting diodes based on conjugated polymers,” Nature, vol. 347, no. 6293, pp. 539–541, 1990.
[4]
S. Logothetidis, “Flexible organic electronic devices: Materials, process and applications,” Materials Science and Engineering B, vol. 152, no. 1–3, pp. 96–104, 2008.
[5]
H. Wu, G. Zhou, J. Zou et al., “Efficient polymer white-light-emitting devices for solid-state lighting,” Advanced Materials, vol. 21, no. 41, pp. 4181–4184, 2009.
[6]
Y. J. Pu, M. Higashidate, K. I. Nakayama, and J. Kido, “Solution-processable organic fluorescent dyes for multicolor emission in organic light emitting diodes,” Journal of Materials Chemistry, vol. 18, no. 35, pp. 4183–4188, 2008.
[7]
S. Y. Ryu, J. T. Kim, J. H. Noh et al., “Polymeric tandem organic light-emitting diodes using a self-organized interfacial layer,” Applied Physics Letters, vol. 92, no. 10, Article ID 103301, 2008.
[8]
L. Yan, J. Y. Zhang, Y. Cui, and Y. Qiao, “Voltage-dependent electroluminescence from colloidal CdSeZnS quantum dots,” Applied Physics Letters, vol. 91, no. 24, Article ID 243114, 2007.
[9]
I. Ugarte, W. Cambarau, C. Waldauf, F. L. Arbeloa, and R. Pacios, “Precisely voltage tunable polymeric light emitting diodes by controlling polymer chemical oxidation and adding inorganic semiconducting nanoparticles. From blue to red stopping at white in the same device,” Organic Electronics, vol. 10, no. 8, pp. 1606–1609, 2009.
[10]
C. W. Lee, C. H. Chou, J. H. Huang, C. S. Hsu, and T. P. Nguyen, “Investigations of organic light emitting diodes with CdSe(ZnS) quantum dots,” Materials Science and Engineering B, vol. 147, no. 2-3, pp. 307–311, 2008.
[11]
G. Tu, C. Mei, Q. Zhou et al., “Highly efficient pure-white-light-emitting diodes from a single polymer: polyfluorene with naphthalimide moieties,” Advanced Functional Materials, vol. 16, no. 1, pp. 101–106, 2006.
[12]
Q. J. Sun, B. H. Fan, Z. A. Tan, C. H. Yang, Y. F. Li, and Y. Yang, “White light from polymer light-emitting diodes: utilization of fluorenone defects and exciplex,” Applied Physics Letters, vol. 88, no. 16, Article ID 163510, 2006.
[13]
J. H. Jou, W. B. Wang, C. C. Chen et al., “High-efficiency phosphorescent and fluorescent pure-white organic light-emitting diodes by incorporating small nano-dot in non-emissive layer,” IEICE Transactions on Electronics, vol. E91, no. 10, pp. 1532–1535, 2008.
[14]
A. L. Rogach, N. Gaponik, J. M. Lupton et al., “Light-emitting diodes with semiconductor nanocrystals,” Angewandte Chemie. International Edition, vol. 47, no. 35, pp. 6538–6549, 2008.
[15]
V. L. Colvin, M. C. Schlamp, and A. P. Alivisatos, “Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer,” Nature, vol. 370, no. 6488, pp. 354–357, 1994.
[16]
P. O. Anikeeva, J. E. Halpert, M. G. Bawendi, and V. Bulovi？, “Electroluminescence from a mixed red-green-blue colloidal quantum dot monolayer,” Nano Letters, vol. 7, no. 8, pp. 2196–2200, 2007.
[17]
F. Teng, A. Tang, B. Feng, and Z. Lou, “The optical properties of the blends of CdSe nanocrystals and poly(N-vinylcarbazole),” Applied Surface Science, vol. 254, no. 20, pp. 6341–6345, 2008.
[18]
Z. S. Guo, L. Zhao, J. Pei et al., “CdSe/ZnS Nanoparticle composites with amine-functionalized polyfluorene derivatives for polymeric light-emitting diodes: synthesis, photophysical properties, and the electroluminescent performance,” Macromolecules, vol. 43, no. 4, pp. 1860–1866, 2010.
[19]
I. H. Campbell and B. K. Crone, “Efficient, visible organic light-emitting diodes utilizing a single polymer layer doped with quantum dots,” Applied Physics Letters, vol. 92, no. 4, Article ID 043303, 2008.
[20]
S. Nizamoglu and H. V. Demir, “Excitation resolved color conversion of CdSe/ZnS core/shell quantum dot solids for hybrid white light emitting diodes,” Journal of Applied Physics, vol. 105, no. 8, Article ID 083112, 2009.
[21]
S. B. Shin, S. C. Gong, J. K. Jang et al., “Properties of polymer light-emitting diodes coated on surface-treated ITO/glass substrates,” Journal of Applied Polymer Science, vol. 110, no. 6, pp. 3678–3682, 2008.
[22]
R. A. M. Hikmet, P. T. K. Chin, D. V. Talapin, and H. Weller, “Polarized-light-emitting quantum-rod diodes,” Advanced Materials, vol. 17, no. 11, pp. 1436–1439, 2005.
[23]
L. Bakueva, S. Musikhin, M. A. Hines et al., “Size-tunable infrared (1000–1600？nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer,” Applied Physics Letters, vol. 82, no. 17, pp. 2895–2897, 2003.
[24]
M. Zorn, W. Ki Bae, J. Kwak et al., “Quantum dot-block copolymer hybrids with improved properties and their application to quantum dot light-emitting devices,” ACS Nano, vol. 3, no. 5, pp. 1063–1068, 2009.
[25]
S. I. Hintschich, C. Rothe, S. Sinha, A. P. Monkman, P. Scandiucci de Freitas, and U. Scherf, “Population and decay of keto states in conjugated polymers,” Journal of Chemical Physics, vol. 119, no. 22, pp. 12017–12022, 2003.
[26]
A. P. Monkman, H. D. Burrows, L. J. Hartwell, L. E. Horsburgh, I. Hamblett, and S. Navaratnam, “Triplet energies of π-conjugated polymers,” Physical Review Letters, vol. 86, no. 7, pp. 1358–1361, 2001.
