Open Access
Add to BibSonomy
Abstract
In this paper, we report the synthesis, the structural and optical characterization of CdSe/CdS//CdS nanorods (NRs) and their exploitation in nanorod-based light-emitting diodes (NR-LEDs). Two kinds of NRs of CdSe/CdS and CdSe/CdS//CdS were incorporated into the structure of solution-processed hybrid NR-LEDs. Compared to CdSe/CdS, the efficiencies of CdSe/CdS//CdS NR-based LEDs are overwhelmingly higher, specifically showing unprecedented values of peak current efficiency of 19.8 cd/A and external quantum efficiency of 15.7%. Such excellent results are likely attributable to a unique structure in CdSe/CdS//CdS NRs with a relatively high quantum yield, thick CdS outer shell, and rod structure which minimize nonradiative energy transfer between closely packed NRs in emitting layer.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fluorescent semiconductor quantum dots (QDs) have emerged as an important kind of emitting material to fabricate quantum dot light-emitting diodes (QD-LEDs) because of their unique optoelectronic properties including high quantum yield, solution processability, tunability of emission wavelength by size-control and a narrow emission linewidth, which originate from quantum size effects [1–7]. Over the past two decades, QD-LEDs with emissive layers mainly composed of monodispersed spherical QDs have aroused extensive interests due to their outstanding performances in thin film lighting devices. Impressive peak external quantum efficiencies (EQE) of 21.4%, 27.6% and 23.1%have been achieved in blue, green and red QD-LEDs [8], respectively, owning to the well-controlled quantum dot materials, suitable device architectures and optimized fabrication processes.
However, as an emerging class of fluorescent materials with unique geometrical features, the progress of nanorods (NRs) with heterostructures based LEDs is still relatively few with respect to the traditional spherical QDs [9]. Like spherical QDs, the NRs show wide tunable range of emission wavelengths as well, which is mainly realized by the modulation of diameter or width of the NRs. Semiconductor NRs have several unique optical properties like linearly polarized emission, a larger Stokes shift, a faster radiative decay process and slower bleaching kinetics than spherical QDs [10,11]. These unique properties obtained from optical anisotropic NRs are possibly due to favorable band offset and enhanced light outcoupling [9]. Different from spherical QDs, which usually possess excellent quantum yields (QYs) in solution state but severely reduced QYs when being transformed to closely-packed thin films, the NRs can effective suppress nonradiative energy transfer between closely-packed NRs. This is because the rod geometry makes the spatial distribution of light emitting primitives relatively far away from each other and thus effectively suppresses the nonradiative inter-QDs Förster resonant energy transfer (FRET) [12,13]. These features manifest that non-spherical NRs show a good prospect of the application in the next-generation of displays and lighting.
To the best of our knowledge, only a few studies reported the electroluminescent (EL) devices based on nanorods. In 2015, red NR-LEDs based on CdS/CdSe/ZnSe double-heterojunction NRs with low photoluminescent (PL) QYs of 40% were fabricated by Nam et al. and demonstrated peak EQE of 12.05%, current efficiency(CE) of 27.52 cd/A and high brightness of 76000 cd/m2 [9]. Then, Castelli et al. combined CdSe/CdS dot-in-rod to demonstrate the first example of fully solution-based QD-LEDs incorporating all-organic injection/transport layers with EQE as high as 6.1% [14]. The relatively lower efficiency of NR-LEDs compared with QD-LEDs is mainly attributed to the low QYs of NRs used in the NR-LEDs, which results in low radiative recombination efficiency of the electrons and holes injected into the NRs emitting layer and eventually leads to poor performance in NR-LEDs [9]. Therefore, how to synthesize NRs with high QYs, uniform size, and thicker shell thickness simultaneously is the key factor to the fabrication of efficient NR-LEDs.
In this paper, CdSe QD cores were first synthesized and CdSe/CdS (F-NRs) with a structure of dot-in-rod were prepared by using a conventional fast hot injection reaction method to grow a thin CdS layer onto CdSe cores [15,16]. Then, CdSe/CdS//CdS (S-NRs) were obtained by coating an additional CdS shell on F-NRs using a seeded-growth approach. The PL emission peaks for F-NRs and S-NRs are 610 nm and 627 nm, respectively, and the QYs of as-synthesized S-NRs can reach up to 86% from 63% of F-NRs. The F-NRs and S-NRs were incorporated into the solution-processed hybrid NR-LEDs structures, in which the NRs emitting layer was sandwiched by poly(9,9-dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine) (TFB) and ZnO nanoparticles as hole- and electron-transport layers, respectively. It showed that the presence of an additional CdS shell made a profound impact on the performances of luminance and efficiencies. Compared to devices using F-NRs, the S-NRs based LEDs showed overwhelmingly high efficiencies with unprecedented values of peak current efficiency of 19.8 cd/A and EQE of 15.7%. Such excellent results are likely attributable to a unique composition proportion of CdSe/CdS//CdS with a relatively high QYs, thick CdS outer shell, effectively suppressed the Auger recombination of charged NRs, and rod structure which minimized nonradiative energy transfer between NRs in closely packed emissive layer (EML).
2. Experimental section
2.1. Chemicals
All reagents were used as received without further experimental purification. Cadmium oxide (CdO, 99.99%), octadecylphosphonic acid, (ODPA, 98%), trioctylphosphine oxide (TOPO, 99%), hexylphosphonic acid (HPA), sulfur (S, 99.99%, powder), 1-octanethiol (OT, 98%), trioctylphosphine (TOP, 97%) 1-octadecene (ODE, 90%), oleylamine (OAM, 70%), oleic acid (OA, 90%), selenium (Se, 99.99%, powder) and chlorobenzene (99%) were purchased from Aldrich. Acetone, toluene, hexane and isopropanol were obtained from Beijing Chemical Reagent Co. Ltd., China. Poly(methyl methacrylate) (PMMA, analytical grade) was purchased from Aladdin.
2.2. Synthesis of CdSe/CdS NRs
Synthesis of CdSe core: The CdSe cores were prepared by following the procedures reported by Carbone et al [16]. 0.06 g CdO, 0.28 g ODPA and 3.0 g TOPO were mixed in a three-neck flask, heated to 150 °C and exposed to vacuum for 1 hour. Then, under nitrogen flow, the mixture was heated to above 300 °C to dissolve the CdO until it turned optically clear and colorless. At this point, 1.5 g of TOP was injected into the flask and the temperature was allowed to recover to the value required for the injection of the Se:TOP solution (0.058 g Se + 0.360 g TOP). CdSe dots could be synthesized by injecting the Se:TOP at 370 °C and then stop the reaction after only 3 minutes. The final solution was precipitated using acetone followed by redispersion in hexanes.
First growth of CdS shell [15]: 3.0 g TOPO, 290 mg ODPA, 90 mg HPA, and 90 mg of CdO were introduced in another three-neck flask and degassed at 150 °C for 1 hour. The solution was then heated up to 350 °C under nitrogen, and 1.5 mL TOP was injected into the mixture. In a separate vial, 100 nmol of the CdSe cores were concentrated and mixed with 120 mg of S, and the solids were dissolved in 2 mL of TOP at ~80 °C. Finally, the CdSe/S solution was rapidly injected into the former solution containing CdO and the reaction was allowed to proceed for 10 min, and then cooled down to room temperature. The nanorods of F-NRs were precipitated using a mixture of methanol and butanol and then re-dispersed in hexanes.
Second growth of CdS shell: 33 mL of ODE, 3 mL of OAM, 3 mL of OA, 0.4 mmol Cd-oleate (2 mL of a 0.2 M solution of Cd-oleate in ODE) were added to a 100 mL round bottom flask. The solution was degassed for 20 min at 100 °C to remove the hexanes and water. The solution was then stirred under nitrogen and the temperature was raised to 310 °C. Then, 40 nmol of the aforementioned F-NRs and octanethiol (100 μL) which were dissolved in 0.5 mL hexane were injected. The mixture was maintained at 310 °C for 15 minutes. The final solution was then cooled down and the nanorods of S-NRs were precipitated using acetone and finally re-dispersed in hexanes.
2.3. Fabrication of NR-LEDs devices
For the fabrication of devices, a slightly modified process similar to that of our previously published reports was adopted [17]. ZnO nanoparticles were synthesized according to previous report. For a typical synthesis, a solution of zinc acetate dihydrate in DMSO (0.5 M) and 30 mL of a solution of tetramethylammonium hydroxide in ethanol (0.55 M) were mixed and stirred for 1 h in ambient conditions, then washed and dispersed in ethanol for device fabrication [3]. Typically, the pre-patterned indium tin oxide (ITO) glass substrates were thoroughly cleaned sequentially with deionized (DI) water, acetone, and isopropanol for 15 min each, and then treated with UV-ozone for 15 min. Subsequently, poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) (AI 4083) as hole injection layer (HIL) was spin-deposited onto the cleaned substrates and annealed at 150 °C for 15 min in air. Then, the coated substrates were transferred to a N2-filled glove box for spin-coating of the poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), NRs, and ZnO layers. The TFB serving as the hole transport layer (HTL) was spin-casted at a concentration of 8 mg/mL in chlorobenzene with a spin speed of 3000 rpm and then baked at 150 °C for 30 min. The as-prepared NRs were dissolved in toluene (18 mg/mL), and were then spin-coated on top of TFB at 2500 rpm. Poly(methyl methacrylate) (PMMA, 2 mg/mL in acetone) and ZnO (30 mg/mL in ethanol) were deposited at 2000 and 4000 rpm for 30 s, respectively to be used as interlayer and electron transport layer (ETL), and followed by baking at 60 °C for 30 min. Finally, 100-nm-thick Al top cathode was deposited by a thermal evaporation. All the devices are 2 mm × 2 mm in active area.
2.4. Characterization
Absorption and PL spectra were collected with an Ocean Optics spectrophotometer (model PC2000-ISA). Relative PL QYs of NRs were measured by an integrating sphere whose inner face was coated with BenFlect equipped with a spectrofluorometer. Transit-PL lifetime of QD solution was measured using a JY HORIBA FluoroLog-3 fluorescence spectrometer. Transmission electron microscope (TEM) images of CdSe cores, CdSe/CdS core/shell NRs and the cross-sections of the NR-LEDs were obtained using JEOL JEM-2100 and Tecnai G2 F30 S-TWIN electron microscopes, both operated at an accelerating voltage of 200 kV. The phase and the crystallographic structure of the NRs were investigated by X-ray diffraction (XRD, Bruker D8 Advance). The EQE is calculated according to the published references [18,19]. The calculation of the EQE can be described by the following equation of Eq. (1):
where e is the electron charge, h is the Planck’s constant, c is the velocity of light, and Km = 683 lm/W is the maximum luminous efficacy. J is the current density. I(λ) is the relative EL intensity at wavelength λ, obtained from the EL spectrum, and V(λ) is the normalized photonic spectral response function. L is the total luminance. The current–luminance–voltage (J-L-V) characteristics were recorded on a Keithley 2400 sourcemeter and a Keithley 6485 picoammeter coupled with a Si photodetector (Newport UV-818), which was calibrated using a Minolta CS-100 Chroma Meter under ambient conditions. EL spectra were recorded with an Ocean Optics spectrometer (USB 2000) and a Keithley 2400 power source.3. Results and discussion
The synthesis of CdSe/CdS//CdS nanorods was performed by using a seeded-growth approach as shown in Fig. 1(a). Red-emitting CdSe/CdS core/shell F-NRs were prepared via a single-step synthetic approach of fast hot-injection reaction to grow a thin elongated layer of CdS shell [15]. The final thick S-NRs of CdSe/CdS//CdS were obtained by a two-step process including a seeded-growth approach to form an additional CdS shell on F-NRs (Fig. 1(a), see the Experimental Section for synthetic details).
Fig. 1 (a) Schematic illustration of designed growth of CdS NRs over CdSe cores, and the growth of an additional CdS shell over the resulting CdSe/CdS NRs to form CdSe/CdS//CdS NRs. Evolution of UV-vis absorption spectra (b) and photoluminescence spectra of CdSe/CdS//CdS NRs upon shell growth (c).
The absorption and PL spectra and QYs of the starting spherical CdSe QDs, F-NRs and the final S-NRs are compared in Figs. 1(b) and 1(c). The average diameter of the starting CdSe QDs used as seeds to grow the NRs, was ∼3.5 nm (Fig. 2(a)), with PL peak of 583 nm and PL QYs of around 14%. The growth of first CdS shell to form F-NRs resulted in a large increase in PL QYs up to about 63%, which was accompanied by a red-shift for both the first absorption and PL peaks, and the PL peak for F-NRs moved to 610 nm, as shown in Figs. 1(b) and 1(c). The growth of an additional CdS shell around the F-NRs also resulted in the continuous red-shift of spectra and slightly broadened line shapes as expected from the known tendency of the delocalization of electron into the shell across the typical quasi-type-II barrier of CdSe/CdS heterostructures [20]. The PL QYs of S-NRs could be as high as 86% with an emission peak located at 627 nm, which proved a better passivation of the surface states of F-NRs by an additional CdS shell.
Fig. 2 (a) TEM image of CdSe core QDs. (b) TEM image of F-NRs. (c) TEM image of S-NRs. (d, e) High-resolution TEM images corresponding to F-NRs and S-NRs, respectively.
The TEM and HRTEM images of the F-NRs and S-NRs are shown in Figs. 2(b)–2(e). Based on the statistical analysis of a large number of nanorods from TEM and HRTEM images, we could tell the dimensions increase of the S-NRs compared with F-NRs. The increase in diameter is ∼1 nm, whereas the increase in length is ∼7 nm. The average diameter and length are estimated to be 4.2 nm and 63 nm, 5.3 nm and 70 nm for F-NRs and S-NRs, respectively. This result means that near 3-4 monolayers of CdS were grown over the side surface of the F-NRs, whereas around 10-12 monolayers were grown onto the rod tips during the second growth process of CdS shell, due to the higher reactivity and lower interfacial strain of the facet along the c axis [21]. The distinct lattice fringes of CdSe/CdS NRs presented in Figs. 2(d) and 2(e) reveal the relatively high crystallinity nature of as-prepared NRs.
Powder X-ray diffraction patterns (Fig. 3) acquired from the XRD characterization further prove the wurtzite structure of CdSe/CdS NRs. No shift in the peak position could be measured between samples of F-NRs and S-NRs in the whole angular range, indicating no induction of additional strain on the F-NRs with the growth of an additional CdS shell. However, the diffraction pattern shows clearly that the extra CdS shell cannot only reduce the peak width of (002), but also increase the intensity of (002). This suggests a preferential growth of the CdS shell along the <001> direction, mainly at the rod tips. The diameter increase of NRs is due to the epitaxial shell growth on the side surface of F-NRs, which is consistent with the above analysis of TEM results.
Fig. 3 Powder X-ray diffraction patterns of CdSe, CdSe/CdS core/shell F-NRs, and CdSe/CdS//CdS S-NRs.
Figure 4(a) shows the comparison of QYs of F-NRs and S-NRs in solution and solid film states. The QYs of F-NRs film was about 44%, which reduced about 30% compared to the corresponding value of solution form due to the FRET. For comparison, the QYs deviation of S-NRs in different forms of solution (86%) and solid film (72%) is much smaller than that of F-NRs. The S-NRs with additional CdS outer shell can significantly reduce the negative influence of NRs in close-packed solid films since the extra CdS shell serves as effective spacer that enables better suppression of non-radiative FRET between S-NRs [13]. At the same time, the PL lifetime results of F-NRs and S-NRs in solution and solid film states also confirmed a better suppression of FRET of S-NRs (Fig. 4(b)). The PL lifetime of the S-NRs decreased by ~25% as was transformed from solution (41.6 ns) to solid films (31.2 ns). Compared with the value of the F-NRs of ~32% (34.7 ns for solution and 23.6 ns for solid film), the smaller decrease of 25% was supposed to originate from the effective prevention of non-radiative FRET by the additional CdS shell [13]. As for high quality QDs with geometry of sphere, their PL lifetime usually suffers severe decrease of 40% when being transformed from solution to close-packed thin films due to the efficient nonradiative FRET [22]. The remaining lifetime of NRs is always >50% since the rod geometry can space the emitting centers of NRs relatively far away from each other and therefore minimize the energy transfer loss, and such characteristic is expected to cater the fabrication of high efficiency NR-LEDs.
Fig. 4 (a) Comparison of PL QYs of F-NRs and S-NRs in the forms of solution versus solid film. Variations of PL lifetime in solution versus film states of (b) F-NRs and (c) S-NRs.
To compare the EL properties of devices based on these two kinds of NRs, NR-LEDs containing the two types of emissive layers (F-NRs and S-NRs) were respectively fabricated. For the CdSe/CdS NRs, the small energetic offset between the conduction band edge of the CdS shell and the commonly used ZnO electron transport layer leads to virtually barrierless electron injection into the NRs, which complicates the balance between electron and hole injections [5]. To obtain balanced injection of electrons and holes, an insulating layer was introduced between electron transport layer and NRs layer which had been demonstrated in our previous work [17]. By the insertion of a PMMA interlayer between ZnO electron transport layer and NRs emitting layer, a multilayer structure of NR-LEDs has been fixed as demonstrated in Fig. 5(a). It exhibits the schematic structure of multilayered QLEDs, consisting of ITO (~90 nm), PEDOT:PSS/TFB/NRs/PMMA/ZnO/Al (100 nm). Except for Al cathode that is deposited by thermal vacuum deposition, all the other layers are sequentially spin-coated on glass substrates with a pre-patterned ITO transparent anode. The cross-sectional STEM image, together with the flat-band energy level diagram of different layers of the NR-LEDs are illustrated in Fig. 5b and 5c, with energy level values taken from published reports [4,23,24]. The thicknesses of the PEDOT:PSS, TFB, emissive NRs layer, and ZnO layers are 45 nm, 75 nm, 35 nm, and 120 nm, respectively. Atomic force microscope (AFM) images confirmed that the root mean squared roughness of the NRs film was 2.31 nm (Fig. 6). After the deposition of the PMMA layer, the root mean squared roughness decreased to 2.03 nm, which was slightly lower than that without PMMA.
Fig. 5 (a) Schematic structure illustration of the multilayered NR-LEDs. (b) The cross-sectional STEM image of a hybrid NR-LEDs device. (c) Energy level diagram of the multilayered NR-LEDs.
Fig. 6 AFM characterizations of surface of F-NRs as the emitting layer without (a) and with (b) PMMA interlayer.
EL properties of NR-LEDs integrated with 610 nm-emitting F-NRs versus 627 nm-emitting S-NRs core/shell structures were tested. Figure 7(a) presents the variations of current density and luminance with increasing applied voltage for these devices. The turn-on voltage (the voltage at the luminance of 1 cd/m2) of 1.9 V and 2.2 V and maximum brightness of 58000 and 104000 cd/m2 were achieved, respectively. The current densities of F-NRs LEDs are substantially higher than those of the S-NRs LEDs throughout the bias range of 1-7 V, for instance, 866 and 248 mA/cm2 at 5 V from the F-NRs and S-NRs based devices, respectively. Lower current densities observed from the S-NRs based devices indicate that the charge injection might be more hindered into the emitting layer by the presence of a thicker shell as a nontrivial energetic barrier, resulting in the reduced bulk current [25]. As a direct consequence of lower leakage currents along with higher luminance, NR-LEDs employing S-NRs overwhelmingly exceed F-NRs ones with respect to the device efficiency. The current efficiency and EQE characteristic curves of the devices as a function of luminance and applied voltage were plotted in Fig. 7(b). These devices show the maximum current efficiency of 17.5 and 19.8 cd/A, corresponding to the highest EQE of 9.4% and 15.7%. Power efficiency (PE) as a function of luminance for F-NRs versus S-NRs LEDs was plotted in Fig. 7(c), with the maximum PE of 17.19 and 27 lm/W, respectively. The efficiencies of devices based on S-NRs are tremendously higher, specifically yielding more than 160% increase than F-NRs based devices in EQE. These exceptional peak efficiencies of our red S-NRs LEDs can exceed the highest EQE values of 12% reported to date from solution-processed red (612 nm) NR-LEDs [9]. Two main factors of FRET and introduction of PMMA insulating layer are likely involved in determining the QDs luminescence efficiency of device. The beneficial role of a thick CdS shell of S-NRs in suppressing the inter-NR FRET event is reflected in the retention of high PL QYs even in solid-state film and is supported by the analysis of aforementioned PL dynamics. The PMMA interlayer can prevent direct contact of NRs with ZnO nanoparticles, and therefore effectively blocks the overflow of electrons and promotes the hole-electron injection balance [5,17].
Fig. 7 (a) Variation of current-density (J) and luminance (L) versus driving voltage (V). (b) Current efficiency (ηA) and EQE (ηEQE) as a function of luminance for F-NRs versus S-NRs LEDs.
Figure 8(a) depicts the PL spectra of S-NRs solution and EL spectra of the corresponding NR-LEDs. The red LEDs show slightly red shifted EL peaks at wavelength of 635 nm from the PL spectra and exhibit narrow spectral linewidth (37 nm and 42 nm for FWHM of PL and EL, respectively), which can be attributed to the FRET and/or the Stark effect [26,27]. Most of the emission was originated from the S-NRs (99% of the total EL emission). No noticeable parasitic emission from adjacent organic layers (i.e., TFB) can be observed during the device operation, which supports the good exciton confinement within the emitting layer. Figure 8(b) shows the EL spectra of the S-NRs LEDs measured under different voltages. A gradual shift of EL to lower energy side and broadened FWHM with increasing bias were exhibited owing to the so-called field-induced quantum confinement Stark effect (QCSE) and enhanced exciton polarization under steadily increasing electric field, resulting in the increased LO (longitudinal optical)-phonon coupling [26]. Hence, the resulting Commission Internationale del’Eclairage (CIE) chromaticity coordinates are only slightly varied in narrow range of red territory with fringe coordinates of (0.654-0.672, 0.312-0.316) under applied voltages of 4-7 V, as shown in Fig. 8(c). The inconspicuous change of CIE color coordinates is advantageous to applications requiring high color reproducibility under minor voltage fluctuations in the practical application of LED displays.
Fig. 8 (a) PL spectra of S-NRs solution versus corresponding EL spectra of devices. (b) Evolution of EL of S-NRs LEDs with increasing driving voltage. (c) CIE chromaticity coordinates of S-NRs LEDs.
4. Summary
In conclusion, we have developed a two-step method to grow a thick rod-shape CdS shell onto CdSe cores. The successful overgrowth of an additional CdS shell over the CdSe/CdS NRs led to the increase of PL quantum efficiencies up to 86%. The thick shell CdSe/CdS//CdS NRs also exhibited a much lower QYs decrease of 86% versus 72% for solution versus solid film, respectively, which was attributable to the effectively lowered FRET efficiency enabled by the additionally overgrown of CdS shell, and was also supported by the PL lifetime analysis. The efficiencies of devices based on CdSe/CdS//CdS NRs are overwhelmingly higher, specifically yielding more than 160% increase than CdSe/CdS NRs based devices in EQE. The high maximum luminance and EQE can reach up to 104000 cd/m2 and 15.7%. These exceptional values are at least 1.3-fold higher as compared to the highest values of luminance of 76000 cd/m2 and EQE of 12% reported to date. Such excellent results are likely attributable to a unique structure in CdSe/CdS//CdS NRs with a relatively high QYs and thick CdS outer shell that minimizes nonradiative energy transfer between closely packed EML NRs.
Funding
National Natural Science Foundation of China (61474037, 21671058, 61874039, 61504040); Key Project of National Natural Science Foundation of China (U1604261).
References
1. V. L. Colvin, M. C. Schlamp, and A. P. J. N. Alivisatos, “Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer,” Nature 370(6488), 354–357 (1994). [CrossRef]
2. Q. Sun, Y. A. Wang, L. S. Li, D. Wang, T. Zhu, J. Xu, C. Yang, and Y. Li, “Bright, multicoloured light-emitting diodes based on quantum dots,” Nat. Photonics 1(12), 717–722 (2007). [CrossRef]
3. L. Qian, Y. Zheng, J. Xue, and P. H. Holloway, “Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures,” Nat. Photonics 5(9), 543–548 (2011). [CrossRef]
4. H. Shen, W. Cao, N. T. Shewmon, C. Yang, L. S. Li, and J. Xue, “High-efficiency, low turn-on voltage blue-violet quantum-dot-based light-emitting diodes,” Nano Lett. 15(2), 1211–1216 (2015). [CrossRef] [PubMed]
5. X. Dai, Z. Zhang, Y. Jin, Y. Niu, H. Cao, X. Liang, L. Chen, J. Wang, and X. Peng, “Solution-processed, high-performance light-emitting diodes based on quantum dots,” Nature 515(7525), 96–99 (2014). [CrossRef] [PubMed]
6. Y. Yang, Y. Zheng, W. Cao, A. Titov, J. Hyvonen, J. R. Manders, J. Xue, P. H. Holloway, and L. Qian, “High-efficiency light-emitting devices based on quantum dots with tailored nanostructures,” Nat. Photonics 9(4), 259–266 (2015). [CrossRef]
7. W. Cao, C. Xiang, Y. Yang, Q. Chen, L. Chen, X. Yan, and L. Qian, “Highly stable QLEDs with improved hole injection via quantum dot structure tailoring,” Nat. Commun. 9(1), 2608 (2018). [CrossRef] [PubMed]
8. H. Zhang, S. Chen, and X. W. Sun, “Efficient Red/Green/Blue Tandem Quantum-Dot Light-Emitting Diodes with External Quantum Efficiency Exceeding 21,” ACS Nano 12(1), 697–704 (2018). [CrossRef] [PubMed]
9. S. Nam, N. Oh, Y. Zhai, and M. Shim, “High efficiency and optical anisotropy in double-heterojunction nanorod light-emitting diodes,” ACS Nano 9(1), 878–885 (2015). [CrossRef] [PubMed]
10. R. A. M. Hikmet, P. T. K. Chin, D. V. Talapin, and H. Weller, “Polarized-Light-Emitting Quantum-Rod Diodes,” Adv. Mater. 17(11), 1436–1439 (2005). [CrossRef]
11. S. Deka, A. Quarta, M. G. Lupo, A. Falqui, S. Boninelli, C. Giannini, G. Morello, M. De Giorgi, G. Lanzani, C. Spinella, R. Cingolani, T. Pellegrino, and L. Manna, “CdSe/CdS/ZnS double shell nanorods with high photoluminescence efficiency and their exploitation as biolabeling probes,” J. Am. Chem. Soc. 131(8), 2948–2958 (2009). [CrossRef] [PubMed]
12. B. N. Pal, Y. Ghosh, S. Brovelli, R. Laocharoensuk, V. I. Klimov, J. A. Hollingsworth, and H. Htoon, “‘Giant’ CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: strong influence of shell thickness on light-emitting diode performance,” Nano Lett. 12(1), 331–336 (2012). [CrossRef] [PubMed]
13. Z. Li, F. Chen, L. Wang, H. Shen, L. Guo, Y. Kuang, H. Wang, N. Li, and L. S. Li, “Synthesis and Evaluation of Ideal Core/Shell Quantum Dots with Precisely Controlled Shell Growth: Nonblinking, Single Photoluminescence Decay Channel, and Suppressed FRET,” Chem. Mater. 30(11), 3668–3676 (2018). [CrossRef]
14. A. Castelli, F. Meinardi, M. Pasini, F. Galeotti, V. Pinchetti, M. Lorenzon, L. Manna, I. Moreels, U. Giovanella, and S. Brovelli, “High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers,” Nano Lett. 15(8), 5455–5464 (2015). [CrossRef] [PubMed]
15. L. Carbone, C. Nobile, M. De Giorgi, F. D. Sala, G. Morello, P. Pompa, M. Hytch, E. Snoeck, A. Fiore, I. R. Franchini, M. Nadasan, A. F. Silvestre, L. Chiodo, S. Kudera, R. Cingolani, R. Krahne, and L. Manna, “Synthesis and micrometer-scale assembly of colloidal CdSe/CdS nanorods prepared by a seeded growth approach,” Nano Lett. 7(10), 2942–2950 (2007). [CrossRef] [PubMed]
16. L. Carbone and P. D. Cozzoli, “Colloidal heterostructured nanocrystals: Synthesis and growth mechanisms,” Nano Today 5(5), 449–493 (2010). [CrossRef]
17. Q. Lin, L. Wang, Z. Li, H. Shen, L. Guo, Y. Kuang, H. Wang, and L. S. Li, “Nonblinking Quantum-Dot-Based Blue Light-Emitting Diodes with High Efficiency and a Balanced Charge-Injection Process,” ACS Photonics 5(3), 939–946 (2018). [CrossRef]
18. S. Okamoto, K. Tanaka, Y. Izumi, H. Adachi, T. Yamaji, and T. Suzuki, “H. Adachi1, T. Yamaji and T. Suzuki, “Simple measurement of quantum efficiency in organic electroluminescent devices,” Jpn. J. Appl. Phys. 40(Part 2, 7B), L783–L784 (2001). [CrossRef]
19. S. R. Forrest, D. D. C. Bradley, and M. E. Thompson, “Measuring the Efficiency of Organic Light-Emitting Devices,” Adv. Mater. 15(13), 1043–1048 (2003). [CrossRef]
20. S. Brovelli, R. D. Schaller, S. A. Crooker, F. García-Santamaría, Y. Chen, R. Viswanatha, J. A. Hollingsworth, H. Htoon, and V. I. Klimov, “Nano-engineered electron-hole exchange interaction controls exciton dynamics in core-shell semiconductor nanocrystals,” Nat. Commun. 2(1), 280 (2011). [CrossRef] [PubMed]
21. D. V. Talapin, R. Koeppe, S. Gotzinger, A. Kornowski, J. M. Lupton, A. L. Rogach, O. Benson, J. Feldmann, and H. Weller, “Highly emissive colloidal CdSe/CdS heterostructures of mixed dimensionality,” Nano Lett. 3(12), 1677–1681 (2003). [CrossRef]
22. K. H. Lee, J. H. Lee, H. D. Kang, B. Park, Y. Kwon, H. Ko, C. Lee, J. Lee, and H. Yang, “Over 40 cd/A efficient green quantum dot electroluminescent device comprising uniquely large-sized quantum dots,” ACS Nano 8(5), 4893–4901 (2014). [CrossRef] [PubMed]
23. Z. Tan, F. Zhang, T. Zhu, J. Xu, A. Y. Wang, J. D. Dixon, L. Li, Q. Zhang, S. E. Mohney, and J. Ruzyllo, “Bright and color-saturated emission from blue light-emitting diodes based on solution-processed colloidal nanocrystal quantum dots,” Nano Lett. 7(12), 3803–3807 (2007). [CrossRef] [PubMed]
24. B. S. Mashford, M. Stevenson, Z. Popovic, C. Hamilton, Z. Zhou, C. Breen, J. Steckel, V. Bulovic, M. Bawendi, S. Coe-Sullivan, and P. T. Kazlas, “High-efficiency quantum-dot light-emitting devices with enhanced charge injection,” Nat. Photonics 7(5), 407–412 (2013). [CrossRef]
25. K. H. Lee, J. H. Lee, W. S. Song, H. Ko, C. Lee, J. H. Lee, and H. Yang, “Highly efficient, color-pure, color-stable blue quantum dot light-emitting devices,” ACS Nano 7(8), 7295–7302 (2013). [CrossRef] [PubMed]
26. V. Wood, M. J. Panzer, J. M. Caruge, J. E. Halpert, M. G. Bawendi, and V. Bulović, “Air-stable operation of transparent, colloidal quantum dot based LEDs with a unipolar device architecture,” Nano Lett. 10(1), 24–29 (2010). [CrossRef] [PubMed]
27. Y. Shirasaki, G. J. Supran, W. A. Tisdale, and V. Bulović, “Origin of efficiency roll-off in colloidal quantum-dot light-emitting diodes,” Phys. Rev. Lett. 110(21), 217403 (2013). [CrossRef] [PubMed]
