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Quantum Dots for White LEDs

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					Quantum Dot White LEDs


       Jennifer Asis
       EECS 277A
             Motivation
•Energy
 efficient                          •Small size
•Long life                          •Design
•Durable                             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.science24.com/resources/paper/15507/images/OLED_2.JPG
                     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
           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
   e-                         h+            Exciton Bohr Radius
        Exciton Bohr Radius                     – Discrete energy levels
                                                →Tunable band gap

                                   http://www.science24.com/resources/paper/15507/images/OLED_2.JPG
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
WLED from Ternary Nanocrystal
        Composites




                        Charge transfer mechanisms:
                        -Charge trapping
                        -Forster energy transfer


           QDs: CdSe/ZnS
           -Red λ =618 nm     At 13 V:
           -Green λ =540 nm   CIE (0.32, 0.45)
           -Blue λ =490 nm

                                Advanced Materials (2006) 18 2545-2548
            RGB Colloidal Quantum Dot
                   Monolayer
                      Cathode
                      Electron transport layer
                                                 Red: CdSe/ZnS
                                                 (λ=620 nm)
                      Hole blocking layer        Green: ZnSe/CdSe
                      Quantum dot layer          (λ=540 nm)
                      Hole transport layer       Blue: ZnCdS
                                                 (λ=440 nm)
                      Hole injection layer
                      Anode




                                                                       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
                             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, “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

				
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