Supporting information

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1 Supporting information Blue Quantum Dot Light-Emitting Diodes with High Electroluminescent Efficiency Lishuang Wang,, Jie Lin *, Yongsheng Hu, Xiaoyang Guo,Ying Lv, Zhaobing Tang,, Jialong Zhao *, Yi Fan, Nan Zhang, Yunjun Wang, Xingyuan Liu * State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun , Jilin, China University of Chinese Academy of Sciences, Beijing , China Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping , Jilin, China Suzhou Xingshuo Nanotech Co., Ltd. (Mesolight), Suzhou , Jiangsu, China * address: linj@ciomp.ac.cn; zhaojl@ciomp.ac.cn; liuxy@ciomp.ac.cn S-1

2 Experimental Section Materials. PVK (average molecular weight 25, ,000 g mol -1 ), chlorobenzene (99.8%) and hexane (99%) were purchased from Sigma Aldrich. Zinc acetate dehydrate (99%) was purchased from Xilong Chemical Co., Ltd (Guangdong, China), potassium hydroxide was acquired from Tianjin Chemical Reagent Factory (Tianjin, China) and normal butanol (99%) was purchased from Alfa-Aesar. The thick outer shell CdSe/ZnS core-shell QDs with an average size of nm and oleic acid surface ligand were synthesized under the help of Mesolight Inc. All materials were used as received without any further purification. Synthesis of ZnO NPs. The ZnO NPs were prepared according to the method reported in the literature S1,S2 with some modification. Dissolving potassium hydroxide (0.74 g) in methanol (30 ml) was added dropwisely into zinc acetate dihydrate (1.48 g) in methanol (60 ml) under vigorous stirring at 300 rpm. For ZnO NPs A, the dissolving potassium hydroxide was dropped 12 ml per time, and 5 times in 3 min. The mixture solution turned turbid, and then stirred few seconds. When the cloudy mixture solution clarified the second dropwise was carried on. While, for ZnO NPs B, the dissolving potassium hydroxide was dropped 6 ml per time, and 10 times in 3 min. The reaction mixture was stirred at 63 C for over 3.5 h. The reaction mixture was then cooled to room temperature and the solvent was removed from the precipitate. The precipitate was washed several times with methanol and separated by centrifugation. Then the particles were dissolved in normal butanol at a desirable concentration for experiments. S-2

3 Fabrication of QLEDs. The custom-made ITO electrodes on glass with sheet resistance of ~15 / were treated with the UV-ozone method for 15 min. PEDOT:PSS (Baytron PVPAl 4083, filtered through a 0.22 m filter) were spin-coated onto ITO electrodes at 3100 rpm for 30 s. Then the PEDOT:PSS-coated substrates were transferred into a nitrogen-filled glove box (O 2 <1 p.p.m., H 2 O<1 p.p.m.). PVK (7 mg ml -1, chlorobenzene), QDs (8 mg ml -1, hexane), and ZnO NPs (30 mg ml -1 ) were deposited layer by layer by spin-coating on the PEDOT:PSS layer. The PVK and ZnO layers were baked at 170 C for 30 min and 120 C for 10 min, respectively. The QD layer was spin-coated at 1140 rpm for 4 s, then at 2500 rpm for 30 s. Finally, Al electrodes (100 nm) were deposited using a thermal evaporation system through a shadow mask to form an active device area of 1 mm 2 under a high vacuum of Pa. The thickness of multiple layers in electron-only device are: ITO/ZnO NPs (30 nm)/qds (20 nm)/zno NPs (30 nm)/al and hole-only device are: ITO/PEDOT:PSS (20 nm)/pvk (30 nm)/qds (20 nm)/moo 3 (30 nm)/al. Characterizations. The cross-sectional Scanning Electron Microscope (SEM) measurements were performed on a field emission scanning electron microscopy instrument (S4800, Hitachi). Atomic force microscope (AFM) measurements were conducted using a scanning probe microscope (SPM-9700, Shimadzu) in the tapping mode. Transmission electron microscopy (TEM) images were recorded with a transmission electron microscope (JEM-2100F, JEOL) operating at 200 kv. The ultraviolet photoelectron spectroscopy (UPS) was measured using Thermo ESCALAB 250 surface analysis system. The PL spectra were measured with a fluorescence S-3

4 spectrophotometer (F-7000, Hitachi) at an excitation wavelength of 320 nm, and the slit width of light source and detector for ZnO NPs were set with the same value (4 nm and 3 nm) for QDs. The UV-abs spectrum of QD solution was recorded with a spectrophotometer (UV-3100PC, Hitachi). The thickness of the films was calibrated using a surface profiler (XP-1, Ambios). The current density-voltage (J-V) characteristics of the devices were determined using a computer-controlled source meter (2611, Keithley). The EL spectra of the QLEDs were obtained using a well-calibrated Spectral scanning colorimeter (PR 705, Photo Research). The luminance of the QLEDs measured with a luminance meter (LS-110, Konica Minolta) and cross-checked with the PR705 Spectral scanning colorimeter. Figure S1. TEM images of (a) 462 nm QDs and (b) 466 nm QDs. Figure S2. Particle size of (a) 462 nm and (b) 466 nm QDs. S-4

5 Figure S3. XRD patterns of ZnO NPs A (black), B (red) and bulk zincite ZnO (blue). Figure S4. PL spectra of ZnO NPs A and a structure of QDs/ZnO NPs A with an excitation wavelength of 320 nm (Under the same experimental condition, the PL intensity of ZnO NPs A is nearly two orders of magnitude lower than that of QDs). Figure S5. Current density-voltage (J-V) characteristic of the electron-only device with a structure of Al/ZnO NPs (40 nm)/al. S-5

6 The J-V curve clearly displays two regions, i.e. the ohmic region (J ~ V) and space charge limited current (SCLC) region (J ~ V 2 ). The electron mobility e of the ZnO NP films can be obtained by fitting the SCLC region according to Child s law J / 8 r 0 e V / d, where ε r and ε 0 are relative and vacuum permittivity, respectively, and d is the film thickness. By assuming ε r = 4, the fitting leads to a zero field electron mobility μ e of cm 2 V -1 s -1 for ZnO NPs A and cm 2 V -1 s -1 for ZnO NPs B, respectively. Figure S6. (a) UV-abs spectrum, (b) UPS spectrum of the PVK layer. Figure S7. The operational lifetime characteristics of device A. The luminance of QLEDs was measured during the continuous operation at a constant current density S-6

7 corresponding to an initial luminance of 1008 cd m -2. Lifetime test was conducted under accelerated conditions to shorten the testing period, as widely used and accepted for organic light-emitting devices (OLEDs). The T 50 lifetime of QLEDs at various luminances was 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 T50 50 ( H ) A L T H L The acceleration factor, A, for OLEDs usually ranges from 1.6 to 2. In our devices, for example, we test QLEDs at two different luminance data of and 499 cd m -2, respectively. Then the A factor was calculated to be So the T 50 lifetime at a display-relevant luminance of 100 cd m -2, for device A can be extrapolated by the equation above to be 47.4 hours (based on 30 1,008 cd m -2 ). There is still much room for further improvement in the device performance through the continuous fabrication progress with a glove box connected thermal evaporation system. Operational lifetime could be further improved by avoiding exposure to the air, optimization of thick-shell QD structure, and improvement of EL performance and encapsulate method of devices. L S-7

8 Table S1. Fitted results of PL exponential decay of QDs. A 1 t 1 (ns) A 2 t 2 (ns) 466 in solution on solid state in solution on solid state The decay of PL intensity of QDs can be fitted with a double exponential formula: y y Ae A e. x t1 x t REFERENCES S1 Kwak, J.; Bae, W. K.; Lee, D.; Park, I.; Lim, J.; Park, M.; Cho, H.; Woo, H.; Yoon do, Y.; S2 Char, K.; Lee, S.; Lee, C. Bright and Efficient Full-color Colloidal Quantum Dot Light-emitting Diodes Using an Inverted Device Structure. Nano Lett. 2012, 12, Wang, L.; Lv, Y.; Lin, J.; Fan, Y.; Zhao, J.; Wang, Y.; Liu, X. High-efficiency inverted quantum dot light-emitting diodes with enhanced hole injection. Nanoscale 2017, 9, S-8