Supporting Information

Transcription

1 Supporting Information Size Tunable ZnO Nanoparticles to Enhance Electron Injection in Solution Processed QLEDs Jiangyong Pan,, Jing Chen,, Qianqian Huang, Qasim Khan, Xiang Liu, Zhi Tao, Zichen Zhang, Wei Lei, * Arokia Nathan,,* School of Electronic Science and Engineering, Southeast University, Nanjing, China, State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing , China. Electrical Engineering Division, Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, CB3 0FA, Cambridge, UK.

2 The relationship between size and bandgap The consistency becomes apparent when we note the tendency of band gap enlargement with decreasing size based on experimental observations and predictions based on the effective mass approximation as shown in Figure S1. It can be found that the band gap enlargement with decreasing size is consistent with the curve calculated from the effective mass model. Figure S1. The dependence of the band gap enlargement versus the ZnO NPs radius as calculated from the effective mass model and corresponding experimental data

3 UPS spectrum of ZnO film E F was determined from the Au surface, and all other spectra are plotted with respect to this value. In other words, the abscissa is the binding energy relative to the E F of Au as displayed in Figure S2. As the result, the valence band maximum (VBM) can be calculated as 7.45 ev, 7.36 ev, and 7.30 ev for the 2.9 nm, 4.0 nm and 5.5 nm sized ZnO NPs. The band gap can be calculated as 3.65 ev, 3.47 ev and 3.35 ev for 2.9 nm, 4.0 nm and 5.5 nm sized ZnO NPs determined by UV-Vis absorption. Based on the VBM and the bandgap of the ZnO, the conduction band minimum (CBM) of ZnO can be calculated as 3.80 ev, 3.89 ev, and 3.95 ev for 2.9 nm, 4.0 nm and 5.5 nm sized ZnO NPs, as summarized in Table S1 Figure S2. UPS spectra of a) the secondary cut off region and b) the HOMO region of ZnO NPs with different size.

4 Table S1 Energy level of different size of ZnO NPs ZnO diameter E cutoff (ev) a E onset (ev) b VBM(eV) c CBM (ev) d (nm) a The high binding-energy cutoff. b The onset relative to the Fermi level (E F ) of Au (at 0 ev), where the E F was determined from the Au substrate. c Calculated values according to VBM =hυ (E cutoff E oneset ), where hυ is the incident photon energy, hυ= 21.2 ev. d Calculated values from the VBM level and optical band gap obtained from UV vis absorption spectra.

5 Mobility Characterization of ZnO Nanoparticle Films by Field Effect Thin film transistor (TFT) structures were considered with fabrication process as follows: top-contact bottom-gate geometry was prepared on heavily doped Si substrates used as the common gate electrode with thermally grown 300 nm-thick SiO 2 gate dielectric layer (unit area capacitance: 11.5 nf cm -2 ). Then ZnO nanoparticle films were deposited on the SiO 2 /Si substrates by spin-casting at a rate of 4000 rpm for 30s. Three kinds of ZnO nanoparticle dispersions comprising different particle sizes were considered. Patterned Al source drain electrodes were thermally evaporated on top of the ZnO nanoparticle films through a shadow mask. The channel length and width of TFT is 0.1 mm and 2 mm respectively. The aspect ratio (i.e. ratio of channel width to length, W/L) of the TFTs is 20. The equation for mobility calculation is as follow: L did µ device = V C W dv d ox g Where L and W are the device channel length and width, V d =30 V, and C ox is the gate capacitance per unit area. The mobility of the ZnO nanoparticle was retrieved as follows: cm 2 /V s, cm 2 /V s and cm 2 /V s for 2.9 nm, 4.0 nm and 5.5 nm ZnO NP, respectively.

6 Figure S3. TFT transfer characteristics of ZnO nanoparticle film, the fitting line for linear region is also shown. Inset shows TFT structure. Table S2. The calculation point, turn on/off ratio and turn on voltage of TFT based on different size of ZnO NPs ZnO diameter (nm) Calculation point Turn on/off ratio Turn on voltage (V) 2.9 (20.1, ) (24.9, ) (24.7, )

7 Stability of device The improved device stability based on smaller ZnO NPs can be deduced from measurement of luminance versus time for an unencapsulated QD-LED based on different size of ZnO NPs operated at a constant voltage of 5V as shown in Figure S4. It can be seen from the figure that the QLED based on 2.9 nm ZnO decreases most slowly compared with the other NP sizes. Only 13% of luminance loss occurs in the 2.9 nm ZnO NPs based QLED without encapsulation after 150 hours, compared with 34% and 75% luminance loss in the 4.0 nm and 5.5 nm ZnO NPs based QLED, respectively. Thus it is clear that the smaller ZnO NPs lead to better device stability. Figure S4. Luminance of QLEDs based on different size ZnO NPs as a function of time at constant driving voltage

8 EL spectra of QLED devices The EL spectra for sample A is presented in Figure S5 Figure S5. EL spectra for sample A

9 Characterization of Quantum Dots Room temperature UV-Vis absorption and photoluminescence (PL) spectra were measured with a U-4100 UV-visible and NIR-300 spectrophotometer, respectively. The absorption spectrum clearly displays the first excitonic transition peak at 520 nm. The PL spectrum shows a Gaussian-shaped peak located at 540 nm with a narrow FWHM of 30 nm, indicating green emission from the QD solution. Figure S6. UV-Vis & PL spectra of ZnCdSeS green QDs.

10 Reproducible test For every type of QLED, we fabricated 3 to 5 samples and tested each sample at least three times. In our system, we found that the best device performance can be achieved using the 2.9 nm size ZnO NPs. Figure S7 presents the performance of the QLED with repeated experiments. Only slight difference between experiments is noted. Figure S7. Reproducibility of current efficiency and power efficiency for QLEDs based on (a) 2.9 nm ZnO, (b) 4.0 nm ZnO and (c) 5.5 nm ZnO. (d) Reproducibility test of stability for the QLED devices.

11 The performance comparison between our paper and previously published paper (1) Table S3 shows the optical properties of three different sizes of ZnO NPs, Table S3. The average size d, PL band maximum energy E PL and band gap Eg of ZnO NPs reported here and in other references. No. d (nm) E PL (ev) E g (ev) Our result 2.9, 4.0, , 2.37, , 3.47,3.35 Ref [1] 3.2, 4.0, , 2.34, , 3.54, 3.38 Ref [2] 3.06, , , 3.45 Ref [3] 3.9, , , 3.52 Ref [1]: Panasiuk Y V, et al. Nanotechnology, 2014, 25, Ref [2]: Zhang L, et al. The Journal of Physical Chemistry C, 2010, 114, Ref [3]: Stroyuk O L, et al. The Journal of Physical Chemistry C, 2009, 114, Conclusion: The size dependent characteristics of ZnO NPs reported here is very similar to that in previously published papers, which testifies to the reliability and accuracy of our experiment results.

12 (2) Table S4 shows the electronic properties of ZnO NP-based QLEDs. Table S4. The performance comparison of our results and that reported in the indicated references. The parameters include ZnO NP size, emission peak wavelength λ max, turn-on voltage V on, FWHM, luminous efficiency η A and power efficiency η P of the ZnO nanoparticle-based QD-LEDs. No. d (nm) EL λmax (nm) FWHM (nm) V on (V) η A (cd A -1 ) η P (lm W -1 ) Our results 2.9, 4.0, , 3.8, , 1.06, , 0.40, 0.06 Ref [4] Ref [5] Not mentioned Ref [6] Not mentioned Not mentioned Ref [7] Not mentioned none 1.26 Not mentioned Ref [4] Qian L, et al. Nature photonics, 2011, 5, Ref [5] Yang X, et al. ACS nano, 2014, 8, Ref [6]Kwak J, Bae W K, et al. Nano letters, 2012, 12, Ref [7] Kim H H, et al. Scientific reports, 2015, 5. Conclusion: Because there is no published paper on the size effect of ZnO NPs on QLED

13 performance, we compare our results with that of published paper for a specific size of ZnO NP. As seen in Table S4, the performance of our QLED is comparable to the reported results, for example for the FWHM, turn on voltage and device efficiency, which indicates the consistency of our experimental results.

14 Attributing PL quenching mainly to charge separation It is found that the charge separation can be considered as one of the major origins of the lifetime changes according to the experiments below, the steady state PL spectra of QD with different size of ZnO NP was measured shown in Figure S8 (a). It can be seen that the PL spectral shape is independent of ZnO nanoparticles size, ruling out the formation of new surface states or change of surface states, which could be induced by processes (i) photochemical reaction on the QD surface, (ii) change on the QD surface due to chemicals remaining on the ZnO NP layer and (iii) changes in surrounding permittivity. (Neuman D, et al. Journal of the American Chemical Society, 2008, 130(1): ) On the other hand, It is unlikely of processes (i), (ii) and (iii) because they do not seem to be size dependent, considering only the size of ZnO NPs were changed in the experiment. (2) Förster resonance energy transfer (FRET), fluorescence resonance energy transfer (FRET), resonance energy transfer (RET) or electronic energy transfer (EET) is a mechanism describing energy transfer between two light-sensitive molecules (chromophores) as displayed in Figure S8 [Handbook Of Biological Confocal Microscopy (3rd ed.). New York, NY: Springer. pp ]. A donor chromophore, initially in its electronic excited state, may transfer energy to an acceptor chromophore through nonradiative dipole dipole coupling [Principles of Computational Cell Biology. Weinheim: Wiley-VCH. p. 202.ISBN ]. The efficiency of this energy transfer is inversely proportional to the sixth power of the distance between donor and acceptor, making FRET extremely sensitive to small changes in distance["applications of Spectrophotometry".Quantitative Chemical Analysis (8th ed.). New York: W. H. Freeman and Co. pp ]. FRET may happen under very some restricted conditions as follows: a. The distance between the donor and the acceptor (typically in the range of 1-10 nm) b. The spectral overlap of the donor emission spectrum and the acceptor absorption spectrum.

15 c. The relative orientation of the donor emission dipole moment and the acceptor absorption dipole moment. Compared with the conditions, in our work, firstly, It is displayed in Figure S8 (b) that there is slight spectral overlap between the ZnO absorbance spectra and the QD PL spectra. So it is unlikely of FRET from QD to ZnO NPs according to the condition b mentioned above. Meanwhile, in this structure, QD is compactly deposited on ZnO, the distance between ZnO and QD cannot be estimated. The chance for FRET is very slight. (3) From the discussion in our work, it has been demonstrated that the PL quenching of QD with charge transport layer is caused by the charge separation (exciton dissociation) in a number of papers. (Wu X, Yeow E K L. Chemical Communications, 2010, 46(24): ; Jin S, Lian T. Nano letters, 2009, 9(6): ; Zhang Y, et al. The Journal of Physical Chemistry C, 2009, 113(5): ; Mashford B S, et al. Nature photonics, 2013, 7(5): ) In conclusion, it is reasonable to attribute a PL quenching on the nanosecond timescale mainly to charge separation (exciton dissociation). Figure S8. (a) PL spectra of QDs with different size of ZnO NP (b) UV-Vis spectra of ZnO NP and PL spectra of QDs