1. Quantum Dot White LEDs Jennifer Asis EECS 277A

2. Motivation • Energy • efficient • Small size • Long life • Design • flexibility • Durable Replacement for incandescent and fluorescent lighting Improve White LED performance Quantum dot white LED www.reprap.org Science 2008 319 1776

3. 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.science24.com/resources/paper/15507/images/OLED_2.JPG

4. Quantum Dots • Colloidal inorganic semiconductor nanocrystal • II-VI semiconductor materials (i.e. CdS, CdSe) • 2-10 nm in diameter • Exhibit strongly size-dependent optical and electrical properties • Quantum confinement effects http://chem.ps.uci.edu/~lawm/Barriers%20and%20wells.pdf

5. Quantum Confinement • Light-Emitting Diode (LED) is a PN junction • Recombination of an electron and hole • Electron-hole pair known as an exciton • Size of semiconductor crystal on the order of Exciton Bohr Radius • Discrete energy levels →Tunable band gap e- h+ Exciton Bohr Radius http://www.science24.com/resources/paper/15507/images/OLED_2.JPG

6. InGaN-CdSe-ZnSe Quantum Dot White LEDs • Single-chip InGaN used as excitation source • CdSe-ZnSe QDs used as phosphor InGaN CdSe-ZnSe • Efficiency 7.2 lm/W at 20 mA • Commercial WLEDs (15-30 lm/W) • CIE (0.33, 0.33) • CRI = 91 IEEE Photonics Technology Letters 2006 18 [1] 193

7. WLED from Ternary Nanocrystal Composites Charge transfer mechanisms: -Charge trapping -Forster energy transfer QDs: CdSe/ZnS -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

8. Cathode Electron transport layer Hole blocking layer Quantum dot layer Hole transport layer Hole injection layer Anode RGB Colloidal Quantum Dot Monolayer Red: CdSe/ZnS (λ=620 nm) Green: ZnSe/CdSe (λ=540 nm) Blue: ZnCdS (λ=440 nm) At 9V: CIE (0.35, 0.41) CRI = 86 Brightness: 92 cd/m2 Charge injection into blue QDs more efficient at higher applied biases Nano Letters (2007) 7 [8] 2196-2200

9. Summary Size-dependent properties of Quantum Dots LEDs – PN Diode

10. 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, “InGaN-CdSe-ZnSe 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