Quantum Dot White LEDs Jennifer Asis EECS 277
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Quantum Dot White LEDs Jennifer Asis EECS 277 A
Motivation • Energy efficient • Long life • Durable • Small size • Design flexibility Replacement for incandescent and fluorescent lighting Improve White LED performance Quantum dot white LED www. reprap. org Science 2008 319 1776
White LEDS • Multichip devices (red, green-, blue-emitting chips) • Single-chip devices (phosphors) • Electroluminescence (EL) – Light emitted in response to an electric current – Result of radiative recombination (Charge injection) – Photon is released http: //www. meisemi. com/image/CIE. gif http: //www. science 24. com/resources/paper/15507/images/OLED_2. JPG
Quantum Dots • Colloidal inorganic semiconductor nanocrystal – II-VI semiconductor materials (i. e. Cd. S, Cd. Se) • 2 -10 nm in diameter – Exhibit strongly sizedependent optical and electrical properties – Quantum confinement effects http: //chem. ps. uci. edu/~lawm/Barriers%20 and%20 wells. pdf
Quantum Confinement • Light-Emitting Diode (LED) is a PN junction – Recombination of an electron and hole – Electron-hole pair known as an exciton e- h+ Exciton Bohr Radius • Size of semiconductor crystal on the order of Exciton Bohr Radius – Discrete energy levels →Tunable band gap http: //www. science 24. com/resources/paper/15507/images/OLED_2. JPG
In. Ga. N-Cd. Se-Zn. Se Quantum Dot White LEDs • Single-chip In. Ga. N used as excitation source • Cd. Se-Zn. Se QDs used as phosphor In. Ga. N Cd. Se-Zn. Se • Efficiency 7. 2 lm/W at 20 m. A – Commercial WLEDs (1530 lm/W) • CIE (0. 33, 0. 33) • CRI = 91 IEEE Photonics Technology Letters 2006 18 [1] 193
WLED from Ternary Nanocrystal Composites Charge transfer mechanisms: -Charge trapping -Forster energy transfer QDs: Cd. Se/Zn. S -Red λ =618 nm -Green λ =540 nm -Blue λ =490 nm At 13 V: CIE (0. 32, 0. 45) Advanced Materials (2006) 18 2545 -2548
RGB Colloidal Quantum Dot Monolayer Cathode Electron transport layer Hole blocking layer Quantum dot layer Hole transport layer Hole injection layer Anode Red: Cd. Se/Zn. S (λ=620 nm) Green: Zn. Se/Cd. Se (λ=540 nm) Blue: Zn. Cd. S (λ=440 nm) At 9 V: CIE (0. 35, 0. 41) CRI = 86 Brightness: 92 cd/m 2 Charge injection into blue QDs more efficient at higher applied biases Nano Letters (2007) 7 [8] 2196 -2200
Summary Size-dependent properties of Quantum Dots LEDs – PN Diode
References • • X. Zhao, “Commercialization of Quantum Dot White Light Emitting Diode Technology, ” M. Eng. Thesis (2006). A. P. Alivisatos, “Semiconductor Clusters, Nanocrystals, and Quantum Dots, ” Science, 271 [5251], 933 -937 (1996). Y. Li, A. Rizzo, R. Cingolani, and G. Gigli, “White-light-emitting diodes using semiconductor nanocrystals, ” Microchim Acta, 159, 207 -215 (2007). H. S. Chen, C. K. Hsu, and H. Y. Hong, “In. Ga. N-Cd. Se-Zn. Se Quantum Dots White LEDs, ” IEEE Photonics Technology Letters, 18 [1], 193 -195 (2006). Y. Li, A. Rizzo, R. Cingolani, and G. Gigli, “Bright White-Light-Emitting Device from Ternary Nanocrystal Composites, ” Advanced Materials, 18 2545 -2548 (2006). 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, 7 [8] 2196 -2200 (2007). http: //www. evidenttech. com
