1. Quantum Dot White LEDs Jennifer Asis EECS 277A

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

3. White LEDS Multichip devices (red-,green-,blue-emitting chips) Single-chip devices (phosphors) http://www.meisemi.com/image/CIE.gif Electroluminescence (EL) –Light emitted in response to an electric current –Result of radiative recombination (Charge injection) –Photon is released http://www.science24.com/resources/paper/15507/images/OLED_2.JPG

4. Quantum Dots http://chem.ps.uci.edu/~lawm/Barriers%20and%20wells.pdf 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

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

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

7. WLED from Ternary Nanocrystal Composites Advanced Materials (2006) 18 2545-2548 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)

8. RGB Colloidal Quantum Dot Monolayer Nano Letters (2007) 7 [8] 2196-2200 Electron transport layer Cathode Hole blocking layer Quantum dot layer Hole transport layer Hole injection layer Anode Red: CdSe/ZnS (λ=620 nm) Green: ZnSe/CdSe (λ=540 nm) Blue: ZnCdS (λ=440 nm) Charge injection into blue QDs more efficient at higher applied biases At 9V: CIE (0.35, 0.41) CRI = 86 Brightness: 92 cd/m 2

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