420 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al. Table 4. Device Structure, Luminance (Lv) and External Quantum Efficiency (ηext) <strong>of</strong> Reported Quantum Dot LEDs a QD material device structure Lv (cd/m2 ) EL color ηext (%) year ref CdSe ITO/QDs/PPV/Mg green to red 0.001-0.01 1994398 CdSe/CdS ITO/PPV/QDs/Mg/Ag 600 green to red ∼0.22 1997399 CdSe/ZnS ITO/TPD/QDs/Alq3/Mg:Ag/Ag 2000 ∼0.52 2002408 CdSe/ZnS ITO/TPD/QD(monolayer)/Alq3/Mg:Ag/Ag 7000 red >2 2005402 CdxZn1-xSe/CdyZn1-yS ITO/CBP/QD(monolayer)/TAZ//Alq3/Mg:Ag/Ag green
<strong>Colloidal</strong> <strong>Nanocrystals</strong> in <strong>Electronic</strong> Applications Chemical Reviews, 2010, Vol. 110, No. 1 421 Figure 36. White light emitting LED utilizing a mixture <strong>of</strong> red, green, and blue emitting semiconductor nanocrystals. (a) Normalized EL spectra <strong>of</strong> devices containing only blue-, green-, and red-emitting nanocrystals. (b) Device cross section, and (c) photograph <strong>of</strong> white QD-LED. Reprinted with permission from ref 93. Copyright 2007 American Chemical Society. core in emitting CdSe/ZnS core-shell NCs from 3.2 to 5.8 nm; ηext was improved to ∼1.1% by optimizing the thickness <strong>of</strong> ZnS shell. 407 Optimization <strong>of</strong> packing and ordering <strong>of</strong> NCs allowed enhancing ηext to over 2% (>1 lmW -1 ) with a maximum brightness <strong>of</strong> over 7000 cd m -2 . 402 The electroluminescence spectrum was dominated by the NC emission with excellent color purity. 402 Steckel et al. reported LEDs utilizing CdxZn1-xSe/CdyZn1-yS core-shell NCs integrated into a four-layered HTL/QDs/HBL/ETL structure. 403 The color saturated green emission was achieved with a peak ηext <strong>of</strong> 0.5% at low operating voltages (1100 cd m -2 ). 405 Uni<strong>for</strong>m and defect-free EL emission from the QDs over a large surface area (1.5 cm × 2.5 cm) was displayed, promising <strong>for</strong> the application <strong>of</strong> QD-LEDs in large-area displays. Other potential areas <strong>of</strong> application <strong>for</strong> thin film LEDs are large area lighting systems and backlighting <strong>for</strong> liquid crystal displays. Such applications require LEDs emitting white light. Here, colloidal NCs with their tunable emission colors and the possibility <strong>of</strong> inexpensive solution-based device fabrication have a potential to successfully compete with other technologies such as OLEDs. Li et al. fabricated white organic/NCs LEDs by combining blue emitting poly[(9,9dihexyloxyfluoren-2,7-diyl)-alt-co-(2-methoxy-5-phenylen- 1,4-diyl)] (PFH-MEH) polymer doped with red-emitting QDs and green-emitting Alq3. 413 The key <strong>for</strong> balanced white emission in the hybrid ternary systems PFH-MEH/QDs/Alq3 was the charge-transfer processes from PFH-MEH and Alq3 to the QDs. Maximum external quantum efficiency <strong>of</strong> 0.24% at1mAcm -2 and 11 V during operation in air was reported. Anikeeva et al. reported white QD-LEDs with the emitting layer assembled <strong>of</strong> a balanced mixture <strong>of</strong> red, green, and blue emitting NCs integrated into ITO/PEDOT:PSS/TPD/ QDs/TAZ/Alq3 device (Figure 36). 93 The mixed-monolayer QD-LED revealed uni<strong>for</strong>mly white luminescence with the international commission on illumination (CIE) coordinates (0.35, 0.41) at an applied voltage <strong>of</strong> 9 V. At 5 V applied bias (1.51 mA/cm 2 ), the peak ηext <strong>of</strong> white QD-LED was 0.36%, corresponding to 0.9 cd A -1 . The independent processing <strong>of</strong> HTL and emitting layer <strong>of</strong> NCs provided precise tuning <strong>of</strong> the emission spectrum by simply changing the color ratio <strong>of</strong> QDs without altering the device structure. In contrast to significant efficiency improvements <strong>of</strong> QD- LEDs, the lifetime <strong>of</strong> hybrid organic/NC devices is limited by the instability <strong>of</strong> the metal contacts and degradation <strong>of</strong> organic components under high current operation conditions. In an attempt to overcome these limitations, the organic hole transport and electron transport layers have been replaced with much more robust inorganic materials. Caurge et al. reported QD-LEDs with p-type inorganic NiO films as HTL. 266 Inorganic NiO films demonstrated higher chemical, thermal, and electrical stability as compared to TPD and other tested organic materials. A maximum ηext ≈ 0.18% and brightness ∼3000 cd m -2 were achieved by optimizing resistivity <strong>of</strong> the NiO layer. In the next step toward all-inorganic QD LEDs, same authors fabricated inorganic ETL by replacing TAZ and Alq3 layers with sputtered amorphous ZnO:SnO2 semiconductor. 266 The all-inorganic QD-LED combined HTL <strong>of</strong> p-type NiO, luminescent Cd1-xZnxSe NCs as the emitting layer, and n-type ZnO:SnO2 ETL as shown in Figure 37. These devices demonstrated pure QD emission with the peak luminance <strong>of</strong> 1950 cd m -2 and enabled very high injection currents <strong>of</strong> 3.5A cm -2 . The maximum ηext was about 0.1%. The ability <strong>of</strong> inorganic HTL and ETL to drive high currents into QD layers may one day open up bright perspectives <strong>for</strong> electrically pumped lasers on colloidal NCs. Highly luminescent spectrally tunable core-shell NCs can also be coupled to bright and efficient single-crystal LEDs. This approach utilizes NCs as the down-converting medium, which efficiently absorbs blue light emitted by, <strong>for</strong> example,
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