Supporting Information

Transcription

1 Supporting Information Over cd/a Efficient Green Quantum Dot Electroluminescent Device Comprising Uniquely Large-Sized Quantum Dot Ki-Heon Lee, Jeong-Hoon Lee, Hee-Don Kang, Byoungnam Park, Yongwoo Kwon, Heejoo Ko, Changho Lee, Jonghyuk Lee, and Heesun Yang*, Department of Materials Science and Engineering, Hongik University, Seoul , Korea Display Research Center Samsung Display Co., Ltd. Yongin, Kyunggi-do , Korea 1

2 Synthesis of ZnO NPs: For a typical synthesis of nm-sized ZnO NPs, 3 mmol of Zn acetate hydrate was dissolved in 3 ml of dimethyl sulfoxide (DMSO). 5 ml of tetramethylammonium hydroxide (TMAH) dissolved in ml of ethanol was dropwisely introduced in a rate of ~8 ml/min to the above Zn solution at room temperature, and then the reaction proceeded at that temperature for 1 h. The resulting ZnO NPs were precipitated with an excessive amount of acetone and then completely redispersed in ethanol for spin-deposition of ETL. Hydrophobic-to-Hydrophilic Ligand Exchange: For a ligand exchange processing with MPA, ml of MPA was added to 3 ml of hydrophobic CdSe@ZnS/ZnS QDs dispersed in chloroform. This mixture was placed in sonication for 1 h at room temperature. The surfacemodified QDs were precipitated with the addition of excess acetone and collected by centrifugation (8 rpm, min). These QD precipitates were purified repeatedly with a solvent combination of sodium tetraborate buffer solution (ph=9)/acetone (1:4 in volume ratio) by centrifugation ( rpm, min) and finally re-dispersed in distilled DI water. Characterization: Absorption and PL spectra were collected with UV visible absorption spectroscopy (Shimadzu, UV-245) and a 5 W Xe lamp-equipped spectrophotometer (PSI Co. Ltd., Darsa Pro-5), respectively. Relative PL QYs of QDs were calculated by comparing their integrated emissions with that of a standard dye solution of Rhodamine 6G (QY of ~96%) in ethanol with an identical OD of ~.5 at 45 nm. In addition, PL QYs of solid-state QD films were measured in an integrating sphere with an absolute PL QY measurement system (C99-2, Hamamatsu). TEM images of QDs were obtained using JEOL JEM- electron microscope 2

3 operated at an accelerating voltage of kv. For PL lifetime measurements, the QD samples in solution or film states were excited at 3. ev by 3 ps pulses from Ti:Sapphire laser operating at a repetition rate of 76 MHz and PL decay dynamics were resolved using a time-correlated single photon counting method. Field emission-sem (Hitachi S-43) operated at kv was employed to obtain information on the surface morphologies of QD EML and ZnO NP ETL as well as the thicknesses of constituent layers in multilayered QLED. EL spectra and luminance current density voltage characteristics of green QLEDs were recorded with a Konica-Minolta CS- spectroradiometer coupled with a Keithley voltage and current source under ambient conditions. 3

4 (a) CdZnSeS CdZnSeS/ZnS (b) CdZnSeS CdZnSeS/ZnS Absorbance (a.u.) PL Intensity (a.u.) Wavelength(nm) Wavelength (nm) Figure S1. Comparison of (a) absorption and (b) PL spectra of CdSe@ZnS versus CdSe@ZnS/ZnS QDs synthesized with the injection of 2. ml (Se+S)-TOP. (a) (b) Figure S2. Low-magnification TEM images of (a) CdSe@ZnS and (b) CdSe@ZnS/ZnS QDs (scale bar, 5 nm). 4

5 PL QY (%) 8 (a) CdSe@ZnS ZnCdSeS CdSe@ZnS/ZnS ZnCdSeS/ZnS PL Intensity (a.u.) (b) water Hydrophobic QDs in chloroform before ligand exchange after ligand exchange chloroform Hydrophilic QDs in water Number of purification Wavelength (nm) Figure S3. (a) Variations of solution PL QY of CdSe@ZnS versus CdSe@ZnS/ZnS QDs against a repeated number of purification and (b) PL spectral comparison between original hydrophobic versus MPA-capped hydrophilic CdSe@ZnS/ZnS QDs dispersed in chloroform and DI water, respectively (inset). The QD samples tested in (a,b) were prepared with the injection of 2. ml (Se+S)-TOP. Normalized Intensity -1-2 nm nm nm PL Intensity (a.u.) Wavelength (nm) Time (ns) Figure S4. PL decay curves of CdSe@ZnS/ZnS QD film sample collected at different emission wavelengths, i.e., 497, 516, and 535 nm, which are also indicated with the arrows in the inset of PL spectrum. 5

6 (a) (b) (c) ~3 nm QDs ~ nm PEDOT//PVK 1 nmito ~ nm CdSe@ZnS/ZnS QDs ~5 nm ZnONPs nm nm nm (d) Energy (ev) ITO PEDOT: PSS PVK CdSe@ZnS/ZnS QDs ZnS CdSe@ZnS ZnS ZnO NPs Al Figure S5. Cross-sectional SEM images of (a) ITO // PEDOT:PSS // PVK // CdSe@ZnS QDs, and the same multilayered device consisting of CdSe@ZnS/ZnS QDs (b) without and (c) with ETL of ZnO NPs. (d) Proposed energy levels of multilayered device with CdSe@ZnS/ZnS QDs. External Quantum Efficiency (%) (a) (b) CdSe@ZnS Voltage (V) CdSe@ZnS CdZnSeS CdSe@ZnS/ZnS CdZnSeS/ZnS Current efficiency (cd/a) Voltage (V) CdZnSeS CdSe@ZnS/ZnS CdZnSeS/ZnS Figure S6. Variations of (a) EQE and (b) CE with increasing applied voltage of CdSe@ZnS versus CdSe@ZnS/ZnS QLEDs. 6

7 External Quantum Efficiency (%) 5 3 External Quantum Efficiency Current Efficiency Device 1 Device 2 Device 3 Device 4 Device 5 Device Current Efficiency (cd/a) Figure S7. Efficiency variation in peak EQE and peak CE among 6 devices of CdSe@ZnS/ZnS QLED. Current Efficiency (cd/a) Current Efficiency Power Efficiency Luminance (cd/m 2 ) Power Efficiency (lm/w) Figure S8. Variations of CE and PE as a function of luminance of CdSe@ZnS/ZnS QLED. 7