High-Efficiency Light-Emitting Devices based on Quantum Dots with Tailored Nanostructures Yixing Yang 1, Ying Zheng 1, Weiran Cao 2, Alexandre Titov 1, Jake Hyvonen 1, Jiangeng Xue 2*, Paul H. Holloway 1,2*, and Lei Qian 1 1 NanoPhotonica Inc., Gainesville, FL, 32601, USA 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL, 32611, USA This PDF file includes: Quantum Dots Synthesis Device Lifetime Test Figure S1 to S8 Tables S1 to S2 Electronic mail: lei.qian@nanophotonica.com (L.Q.); pholl@mse.ufl.edu (P.H.H.); jxue@mse.ufl.edu (J.X.). NATURE PHOTONICS www.nature.com/naturephotonics 1
Quantum Dots Synthesis. For a typical synthesis of CdSe/ZnS QDs, 0.2 mmol of CdO, 4 mmol of zinc acetate and 5 ml of oleic acid (OA) were placed in a 50 ml flask and heated to 170 C in flowing high-purity argon for 30 min. Then 15 ml of 1-octadecene (ODE) was added to the flask and the temperature was elevated to 300 C. A stock solution containing 0.1 mmol of Se and 3.5 mmol of S dissolved in 2 ml of trioctylphosphine (TOP) was quickly injected into the flask. The reaction temperature was kept for 10 min and then cooled to room temperature. The resulting QDs were washed several times and finally dispersed in toluene. The relative ratios of the precursors were varied in order to form QDs with different emissions and nanostructures. For a typical synthesis of Cd 1-x Zn x S/ZnS QDs, the procedure is the same as described above, until the stock solutions were injected twice. First, the sulfur powder dissolved in ODE was quickly injected into the flask and the temperature was increased to 310 C. After 8 min of reaction, sulfur powder dissolved in TOP was then introduced into the reactor for ZnS shell growth of 40 min at 310 C, and then cooled to room temperature. The synthesized QDs were performed the similar purification process as above. 2 NATURE PHOTONICS www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION Device Lifetime Test. T 50 lifetime is defined as the time over which the luminance from a device drops to 50% of the initial value. All lifetime tests were performed under ambient conditions of laboratory air temperature typically ranging from 23-26 C (no additional cooling applied) and relative humidity of 30-60%. Devices were driven by a Keithley sourcemeters at constant current and luminance was measured with a Minolta LS-110 luminance meter with a color correction factor for each color. Readings were taken at multiple points (minimum of 3) across the 2 2 mm 2 pixel and averaged. All QD-LEDs were encapsulated with commercially available UV-curing epoxy and cover glass. Lifetime test was conducted under accelerated conditions to shorten the testing period, as widely used and accepted for OLED devices. The T 50 lifetimes of QD-LEDs at various luminances were measured. The lifetime at low luminance, L L, can be extrapolated based on the lifetime of the same device operated at high luminance, L H, following the equation below: L H A T 50L T50H ( ) LL The acceleration factor, A, was obtained by sampling the device at certain stresses of luminance ranging from 500~4,000, and the respective lifetimes were measured. These data were fitted to obtain the value of A. The value of A for OLEDs usually ranges from 1.6 to 2. In our devices, for example, we test our green QD-LEDs at two different luminances of 1,800 and 3,300, and the corresponding T 50 lifetimes are 320 and 97 hours, respectively. Then the A factor was calculated to be 1.96. So the T 50 lifetime under 100 for our green QD-LEDs can be extrapolated by the equation above to be 92,360 hours (based on 320 hrs @ 1,800 ). NATURE PHOTONICS www.nature.com/naturephotonics 3
Figure S1. Electronic energy levels of several group II-VI semiconductor materials: CdS, CdSe, ZnSe and ZnS. The valence and conduction bands values are taken from literature. (111) CdSe ZnSe ZnS (a) (b) Intensity (a.u.) (220) (c) (311) (d) 20 30 40 50 60 70 2 (degree) Figure S2. X-ray diffraction spectrum of ZnSe-rich green QDs. The stick pattern shows the standard positions of bulk zinc blende CdSe (red), ZnSe (blue) and ZnS (green). The XRD spectrum indicates a zinc blende crystal structure of QDs with characteristic (111), (220) and (311) Bragg s. Inset: (a-d) High-resolution TEM images of ZnSe-rich green QDs, indicating high crystallinity of individual QD (scale bar is 2 nm). 4 NATURE PHOTONICS www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION Figure S3. Histogram of current efficiencies (η A ) of 76 devices for green QD-LEDs based on ZnSe-rich QDs. The average current efficiency is 59.6 cd/a. Figure S4. Electroluminescence performance of red QD-LEDs. a, Normalized EL spectrum, b, Current density-luminance -voltage (J L V) characteristics, c, Current efficiency (η A ) and external quantum efficiency (η EQE ) as a function of luminance of red QD-LEDs (champion devices) based on ZnSe-rich (red) and CdS-rich (blue) intermediate shell. NATURE PHOTONICS www.nature.com/naturephotonics 5
Table S1. Comparison of EL emission wavelength λ max, FWHM, external quantum efficiency η EQE, power efficiency η P and current efficiency η A of red QD-LEDs (champion Intermediate Shells of red QDs devices) based CdS-rich and ZnSe-rich intermediate shell. λ max (nm) FWHM (nm) η EQE (%) η P (lm/w) η A (cd/a) CdS-rich 629 39 5.4 5.3 5.7 5.2 5.2 5.1 ZnSe-rich 625 25 12.0 12.0 18 17 15 15 Figure S5. Histogram of current efficiencies (η A ) of 56 devices for red QD-LEDs based on ZnSerich QDs. The average current efficiency is 13.1 cd/a. 6 NATURE PHOTONICS www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION 10 3 TFB 10 1 PVK J (ma/cm 2 ) 10-1 10-3 10-5 -2 0 2 4 6 8 10 V (V) Figure S6. Current density-voltage (J-V) characteristics of TFB or PVK based hole-only devices with blue QDs. Device structure: ITO/PEDOT:PSS/HTLs/Blue QDs/NPB/Al. Electron injection is less efficient than hole injection in blue QD-LEDs with TFB as HTL due to largely upward shifted conduction band of blue QDs, which leads to a considerable energy barrier at the ETL/QD interface for electron injection. PVK has a hole mobility of μ h ~10-5 cm 2 V - 1 s -1, which is much lower than that of TFB (μ h ~10-2 cm 2 V -1 s -1 ). As a result, the hole injection is suppressed for blue QLED with PVK as HTL compared to TFB HTL based device, as supported by the comparison of TFB and PVK based hole-only devices with blue QDs shown in Supplementary Figure S6. Much better charge balance is therefore achieved for blue QLED based on PVK HTL, which leads to the efficiency enhancement. This approach actually is similar to the recently published paper (X. Dai et al. Nature 515, 86 (2014)), in which a thin interlayer of PMMA is introduced to reduce the electron injection of red QD-LEDs for better charge balance and device efficiency. NATURE PHOTONICS www.nature.com/naturephotonics 7
Figure S7. Current (η A ) and external quantum (η EQE ) efficiencies as a function of luminance of blue QD-LEDs (champion devices) based on PVK (blue squares) or TFB (red triangles) as HTL in devices. Table S2. Comparison of EL emission wavelength λ max, FWHM, external quantum efficiency η EQE, power efficiency η P and current efficiency η A of blue QD-LEDs (champion devices) based TFB or PVK as HTL. HTLs η EQE (%) η P (lm/w) η A (cd/a) λ max FWHM (nm) (nm) PVK 10.7 10.3 2.7 2.5 4.4 4.3 TFB 455 20 4.1 4.0 1.2 1.0 1.7 1.6 8 NATURE PHOTONICS www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION Figure S8. Histogram of current efficiencies (η A ) of 52 devices for blue QD-LEDs. The average current efficiency is 4.0 cd/a. NATURE PHOTONICS www.nature.com/naturephotonics 9