Inverted Device Architecture for Enhanced Performance of Flexible

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Inverted Device Architecture for Enhanced Performance of Flexible Silicon Quantum Dot Light-Emitting Diode Batu Ghosh, Hiroyuki Yamada, Shanmugavel Chinnathambi, #rem Nur Gamze Özbilgin, and Naoto Shirahata J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02278 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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The Journal of Physical Chemistry Letters

Inverted Device Architecture for Enhanced Performance of Flexible Silicon Quantum Dot Light-Emitting Diode Batu Ghosh,*,†,‡ Hiroyuki Yamada,†,§ Shanmugavel Chinnathambi,┴ Đrem Nur Gamze Özbilgin,†,# and Naoto Shirahata,*,†,§,# †

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials

Science (NIMS), 1-1 Namiki, Tsukuba, 305-0044, Japan ‡

Department of Physics, Triveni Devi Bhalotia College, Raniganj, West Bengal, India

§

Department of Physics, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan

┴ #

International Center for Young Scientists (ICYS), NIMS, 1-2-1 Sengen, Tsukuba 305-0047, Japan

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0814,

Japan

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

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ABSTRACT Here we report for the first time highly flexible quantum dot light-emitting diodes (QLEDs), in which a layer of red-emitting colloidal silicon quantum dots (SiQDs) works as the optically active component, by replacing a rigid glass substrate with a thin sheet of polyethylene terephthalate (PET). The enhanced optical performance for electroluminescence (EL) at room temperature in air is achieved by taking advantage of the inverted device structure. Our QLEDs do not exhibit parasitic EL emissions from the neighboring compositional layers or surface states of QDs over a wide range of driving voltages, and a shift in the EL peak position as the operational voltage increases. Compared to the previous Si-QLEDs with a conventional device structure, our QLED has a longer device operational lifetime and a longlived EQE value. The currently obtained brightness (~5,000 cd/m2), the 3.1% external quantum efficiency (EQE), and a turn-on voltage as low as 3.5 V are sufficiently high to encourage further developments of Si-QLEDs.

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Flexible display technologies are attracting increased market attention due to their potential applications, including portable, foldable, and wearable photoelectronics.1-3 Organic light emitting diodes (OLEDs) take advantage of a self-luminous layer to construct a device multilayer. Reducing its entire thickness yields mechanical flexibility (e.g., bending, stretching, and folding). However, device instabilities upon exposure to air, which are inherent to the insulating nature of organics, hinder operations at high applied voltages.4 Therefore, the flexibility of current commercialized OLEDs is limited by the encapsulation layer to protect from air. Colloidal quantum dots (QDs), which are semiconductor crystals nanostructured in three dimensions, provide advantageous photoluminescence (PL) properties, including color purity, narrower spectra for emission (full-width at half-maximum, FWHM< 30 nm), and spectral tunability of efficient emissions over a broad wavelength range. Moreover, QDs have superior thermal and air stabilities relative to common organics.4,5 Therefore, LEDs with an optically-active layer of QD (QLEDs) can provide excellent device performances such as an electroluminescence (EL) spectrum as narrow as 30 nm, a high luminance (~200,000 cd/m2), an operation voltage as low as 2V, a stable emission under long-term usage and high-current-density conditions, and a solution-based processability.6-9 Most QLEDs adopt a conventional (or normal) device structure where indium tin oxide (ITO) and aluminum (Al) work as the anode and the cathode, respectively. These QLEDs have a multilayer structure composed of an electron injection layer (EIL), an electron transportation layer (ETL), an optically active layer, a hole transportation layer (HTL), and a hole injection layer (HIL). Unlike the normal structure, the inverted device structure of QLEDs (iQLEDs) adopts the ITO as the cathode. Recently, the inverted structure has received much attention due to (i) its high compatibility with nchannel field-effect transistors for pixel driver backplanes in active-matrix displays,10,11 (ii) the availability of the transparent bottom cathode and a high work function (i.e., air stable) top anode, (iii) a


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broad range of choice of inorganic crystals working as the ETL (e.g., ZrO2, TiO2, and ZnO),12,13 or HTL (e.g., V2O5, WO3, and MoO3),14,15 which shield organic interlayers from air, and (iv) the available for EIL/ETL use of ZnO that provides the deep energy level of the highest occupied molecular orbital (HOMO), serving as an efficient hole blocker. Additionally, iQLEDs are advantageous from the viewpoint of processing toward improved device performance. For example, in (v) the combination of ZnO/QD provides a robust platform for consecutive deposition of the HTL/HIL and the anode, yielding an efficient hole injection and high hole mobility from a vacuum and avoiding damage or solvent penetration to the underlying layer. Baigent et al. first exploited the inverted structure to fabricate OLEDs. They referred to it as an “upside-down” structure.16 Although there are many combinations of materials for enhanced device performance of inverted OLEDs,17-19 the device structure of QLEDs is still dominated by a conventional (or normal) architecture. However, the number of inverted device structures have grown rapidly in the past five years.11,20-26 The state-of-the-art external quantum efficiencies (EQEs) of red, green, and blue (RGB) QLEDs with conventional device structures are 20.5%, 21% and 19.8%, whereas those of iQLEDs are 18.0%, 15.6%, and 4.0%, respectively.27-33 The high values of EQE require optically active (or emission) layers of cadmium-based QDs such as CdSe and CdS. However, Hazardous Substances (RoHS) strongly restrict the use of toxic elements, including Cd, for electronic products. Due to the complete ban of these elements in the future, recent efforts have shifted toward fabricating heavy-metalfree QLEDs.21,34 Silicon (Si), which is abundant, is poised to become a safe alternative to Cd-based QDs. Many studies have concluded that Si is not toxic to the environment and the human body, and that is suitable for SiQD.35-39 To date, the EL spectra over a wavelength range from 630 nm to 850 nm from Si-QLED devices have been reported. The EQE values are as high as 8.6% for near-infrared (NIR),40 6.2% for


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red,41 0.03% for orange,42 and 0.03% for white EL emissions.43 One current limitation is that such devices lack stability during operation. Thus, a device architecture appropriate for the optically active layer of SiQD is necessary to further develop nontoxic-QLEDs. In this paper, we report a new device architecture for Si-iQLEDs. We found that a combination of MoO3/4,4’-bis(carbazole-9-yl)biphenyl (CBP) HTL/HIL and ZnO EIL/ETL adapted for Si-iQLEDs results in a good band alignment, which allows for efficient charge injection and charge balance as well as overcomes the previous drawback. Of particular importance is that the energy level of the HOMO for the MoO3 film strongly depends on the deposition conditions. In accordance with our previous protocol,44 Figure 1a overviews the fabrication of a toluene-soluble SiQD, which consists of five steps: (i) Hydrogen silsesquioxane (HSiO1.5) is synthesized via hydrolysis/polymerization of triethoxysilane (TES) at pH 3. (ii) Thermal disproportionation under a H2/Ar 5%/95% atmosphere at 1100°C for 2h forms a composite of SiO2/Si powder. (iii) Hydrofluoric etching liberates the QDs terminated with hydrogen atoms from the SiO2 matrix. (iv) 1-Decene undergoes thermal hydrosilylation SiQD-De. (v) Finally, the product is centrifuged, and subjected to the gel permeation chromatography (GPC) for complete removal of the 1-decene. Details about the synthesis, characterization, and device fabrication are available in the Supporting Information. The Xray powder diffraction (XRD) pattern is composed of a single phase of the diamond cubic Si lattice structure (Figure 1b). Scherrer broadening analysis results in crystalline Si with a ~2.1-nm diameter, which is consistent with the result of the HR-TEM observation (Figure 1c). Figure 1d shows representative optical absorbance and emission spectra of the SiQD-De. Both spectra were obtained on a solid powder specimen to avoid the solvent effect.45,46 The combined UV−vis absorbance and scattering were studied via the Kubelka−Munk analysis. The very broad spectrum with a peak at 432 nm may be due to a size polydispersion of SiQD-De and the


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strong size-dependence of the peak of the absorbance/scattering ratio in the 720–840 nm wavelength range.47 The PL spectrum peak at 720 nm exhibits a narrow emission line but has a tail, possibly due to the polydispersion. The relationship between the size and the PL peak energy is consistent to those reported in the literature.44,48,49 The absolute quantum yield (QY) of this emission is around 40%. To fabricate a device, this red-emitting SiQD-De was redispersed in toluene. Figure 2 schematically illustrates the conventional and inverted QLEDs as well as their flat energy band diagrams. In Figure 2(a), the conventional QLED, which was used as a standard, was prepared by adopting PEDOT:PSS (10 nm) for HIL and HTL, Poly-TPD (10 nm) for HTL, 1,3,5-tris (Nphenylbenzimidazole-2yl) (TPBi, 20 nm) for EIL and ETL. The device was constructed on a 150-nm thick layer of the ITO anode (resistivity= 10~14 Ω/sq.). The cathode was a 150-nm-thick Al deposited under a vacuum. Devices with similar multilayer structures are frequently reported in the literature.42,47,50 SiQD-De has an average diameter of 2.1 nm, corresponding to the PL band peak at 720 nm and a PL quantum yield (QY) of 40%. The estimated PL lifetimes was 105 µsec, consistent with the reported values.48,51 The spin-coated SiQD-De film is smooth and uniform with a root-mean-square roughness below 1.2 nm (see Figure S1, Supporting Information). In the conventional device structure, electrons and holes are injected from the Al and ITO electrodes, respectively (see Figure 2b). The inverted device structure developed here has constituent layers in the following order: ITO (150 nm)/ZnO (20 nm)/SiQD-De (20 nm)/CBP (40 nm)/MoO3 (30 nm)/Al (150 nm) as illustrated in Figure 2(c). In contrast to the conventional QLED, electrons and holes are injected from the ITO cathode and the Al anode, as shown in Figure 2(d). We modified a previous method to synthesize ZnO nanocrystals,52 and prepared its nanocrystals with diameters of 5.4±0.7 nm (Figure S2, Supporting Information). The spin-coated ZnO film provides a flat platform for QD deposition. The ZnO film as an EIL/ETL takes advantage of its excellent inherencies,


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including a high transparency in the visible spectral region, a high electron mobility, a valence band (VB) edge energy level significantly lower than that of the QD to suppress hole leakage from the adjacent optically active layer, robustness for successive spin-coating of the QD film, and a low conduction band (CB) edge energy level for efficient injection and transportation of electrons. The film of MoO3 used as the HIL and the HTL provides a shallow work function for efficient injection of holes. Its surface is very smooth on the atomic scale with a grain boundary in small quantity (Figure S3, Supporting Information). A CBP film thermally deposited under a vacuum presents an advantage. According to the energy band diagram illustrated in Figure 2(d), a shallow HOMO level facilitates hole transportation between the MoO3 and QD layers while suppressing electron leakage from the QD to the MoO3 films due to the LUMO level, which produces a large barrier height between the QD and the MoO3 layers. Local surface work functions of the QD and the MoO3 films were experimentally measured by photoelectron yield spectroscopy (PYS). Both films were prepared via a deposition process in a manner very similar to the device fabrication process. Figure 3 shows the work function regions of the PYS results to define the VB maximum for the QD (Figure 3a) and the MoO3 (Figure 3b). Each line was determined as a pair consisting of the base and the rise to calculate the onset. For the MoO3 film, it should be noted that the electronic structure of MoO3 is strongly influenced by the stoichiometric oxygen composition in the film.53,54 For example, Yao et al. reported that a MoO3 film deposited under high vacuum conditions presents the deep HOMO and LUMO levels at 9.5 eV and 6.7 eV, respectively.25 Upon exposure to air the HOMO and LUMO levels are reduced to 5.3 eV and 2.3 eV, respectively.55 To facilitate hole injection from the Al anode to the QD layer through MoO3, employing a vacuum condition (~10-4 Pa) to deposit a 30 nm-thick MoO3 film provides an optimal work function of as low as ~6.2 eV.


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Figure 4 illustrates a typical performance of the Si-iQLED in terms of the device current-voltage, the photocurrent-voltage, the luminance-voltage, and the EL spectrum compared to the PL spectrum. Figure 4a shows the current-voltage (I-V) characteristics along with the photodiode J-V characteristics. A calibrated Si photodetector (Hamamatsu S1336 8BQ) coupled with Keithley 2423 was used for this measurement. The number of photons emitted from the ITO side, which were collected directly with the photodetector, increases with the current. The turn-on voltage, which is defined as the minimum applied bias where the iQLED starts to emit light, was estimated from the photodiode J-V characteristics. The estimated turn-on voltage is 3.5 V and a brightness as low as 0.01 Cd/m2 is produced according to the calculation from the calibrated photodiode current. A value of 0.01 Cd/m2 is the lower limit for optical detection of our optical setup. Recently, Xue, et al. reported the blue-violet ZnCdS/ZnS QD-LEDs with a turn-on voltage as low as 2.5V.56 For Si-QLED, Maier-Flaig et al. reported a turn-on voltage of ~4V.42 A slightly higher value (~4.6V) was reported by Mastronardi and co-workers.57 Liu et al. reported recently a turn-on voltage as low as 3V using a conventional device structure.41 The observation of a low turn-on voltage for our device suggests the presence of a small barrier height for charge injection into the QD layer from the electrodes. The decent deep HOMO level provided by the well-controlled MoO3 layer leads to a reduce operation applied voltage. Further lowering turn-on voltage for device operation will be achieved by reducing the energy-barrier between ITO and ZnO; one possible technique makes the work function of ITO shallow via its surface modification. Figure 4b plots the typical luminance curves as a function of the applied voltage. The luminance reaches a value as high as 5,000 cd/m2 at 5V. The EL spectrum has a peak at 720 nm and is as narrow as 150 nm FWHM (Figure 4c). These spectral characteristics resemble the PL characteristics of the corresponding SiQD-De specimen. The EL spectrum was slightly narrower than the PL spectrum, but


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further study is needed for clarification of the mechanism. As shown in the inset of Figure 4(c), a mechanical flexibility is realized by adopting a thin sheet of polyethylene terephthalate (PET) as a substrate. In addition, the light emitted from the iQLED is sufficiently bright and vivid to be visible by the naked eye even in an illuminated room (Figure 4c). This is the first time that a Si-QLED is fabricated with sufficient flexibility to wear or attach to a curved 3D surface because the device structure is also constructed on a thin sheet of PET. This device is a step toward wearable display technologies. Despite recent progress of Si-QLEDs, an in-depth study on the device performance stability upon device operation is lacking in the literature. That is, a study has yet to examine the stability of the EL color in a high acceleration voltage regime, the device operation lifetime, the parasitic emission from neighboring compositional layers or the QD surface states over the entire range of the driving voltages, and the leveling off of the EQE during device operations. It is noteworthy that the inverted device structure has the potential to overcome problems that are usually observed in conventional QLEDs. We prepared our Si-QLED with a normal device structure to compare its optical performance with that of an iQLED (see Figure 2 for device designs). Figure 5 compares the optical performance between the conventional and the inverted device structures. The EQE values for the conventional and the inverted structures are plotted as functions of applied voltage in Figures 5a and b, respectively. For the conventional device structure, the maximum (~1.4%) drastically decreases as the current density increases. The critical current density, where the EQE is reduced to half of its peak value, is about 0.01 mA/cm2. There is an 80% reduction from the peak value at 0.1 mA/cm2. The iQLED presents a very slow decreasing trend of the EQE as the current density rises. The best EQE performance is 3.1%. Unlike the conventional structure, a rapid reduction of the EQE is not observed over the entire current density range. Instead the EQE for the QLED levels off in a 10-4~10-5 A/cm2 regime. A noticeable reduction begins at J= 0.03 mA/cm2. The critical current density is approximately 0.3 mA/cm2, which is


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30 times larger than that for a conventional device. The observed stability of the EQE might appear due to the improved charge balance and reduced exciton quenching.59 Figures 5c and 5d compare the EL spectra measured at different voltages. For the conventional structure, the EL peak shifts to the blue by 50 nm at 11 V (Figure 5e). The observed blueshift of the EL peak with an increased driving voltage is common for Si-QLEDs with conventional structures. Mayier-Flaig et al. reported a ~15-nm blueshift of the EL peak by raising the operation voltage from 3.5 to 10 V.42 Liu et al. reported ~50-nm blue shift in the EL peak when the driving voltage is increased from 3.5 to 6.5 V although the EQE is as high as 6.2%.41 An NIR-emitting Si-QLED exhibits a blueshift of the EL peak position with a similar magnitude.40,60 One plausible origin of the blueshift might be due to the quantum confined stark effect and/or band filling at higher current density.58 Another possibility is that the parasitic emission from the neighboring compositional layers of the QD, as evidenced in the EL spectrum at 11 V in Figure 5c, leading to the EL spectral blueshift. As expected, there is no unwanted emission other than the EL spectra of the QDs in the inverted structure (see Figure 5d). The EL spectral shape is independent of the driving voltage. As evidenced in Figure 5e, the shift of the EL peak is sufficiently small to be ignored, indicating that the EL emission originates solely from the QD layer even in the high applied-voltage range. Further study is needed to provide a clear-cut evidence of such a very small spectral-shit, one thing to note is that a good conductivity in the good band alignment of HTL leads to the difficulty in built-up of the band-filling. Therefore the amplitude of the high photon-energy component of the EL spectrum would become lesser. The absence of parasitic emissions from neighbors, which might be due to the fact

that the electrons and holes injected from the electrodes in the iQLEDs recombine only within the optically active layer for efficient EL, leads to the lesser spectral shift. In this work, the operational lifetime of the Si-QLED with a continuous emission of light was examined in air for each device structure without sealing, encapsulating, or packaging to protect from


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reactions with oxygen and water molecules. A constant bias was applied to the device, and the photocurrent due to emitted photon was measured by the photodiode simultaneously. Figure 5f shows the values of the EL intensity, which are normalized by the initial intensity, as a function of the operation time. For the iQLED, a reduction in the intensity is not observed until 12 hrs, but the initial intensity is reduced by ~30% after 26 hrs. On the contrary, for the conventional device, the EL intensity is rapidly reduced to ~40% of the initial intensity on a time scale of only 1 hr. OLED studies often report device degradation during operations. In most cases, device failure occurs due to undesired damage of the device constituents during operations, including (i) migration or diffusion of the elements from neighboring layers to form a nonconductive barrier in the multilayer stack and (ii) oxidation of Al cathode. Device failure investigations for QLEDs are still in their infancy. Recently, Maier-Flaid et al. tried to elucidate the degradation process of Si-QLEDs upon device operations through advanced TEM observations of the conventional Si-QLED before and after operations and energy dispersive X-ray spectroscopic analysis.61 According to their paper, a shorter operation lifetime in a conventional QLED may arise from (i) the presence of a defective and/or poorly surface-functionalized SiQD in the active layer at a low voltage for operations, (ii) drastic morphological changes, including electromigration of the SiQD to the ETL (i.e., TPBi) for layer-intermixing, and (iii) microscopic defects that remain after the atomic migration at a high voltage, leading to the fast degradation of the device.61 Although the observed degradation behavior for the conventional structure matches the reported one, the iQLED has a longer device lifetime even in the high voltage regime, suggesting that most of the problems observed in the normal structure can be solved. However, an in-depth microscopic analysis is necessary for clarification. In particular, the use of two metal oxide layers (i.e., ZnO and MoO3) for the ETL/EIL and the HTL/HIL takes advantage of the inherent robustness and protection of the interlayers


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from oxidation. Decreasing the degree of the charged QDs, which leads to carrier loss due to Auger recombination, should contribute the enhanced EQE.62 Next, we compared our inverted device to the Si-QLEDs reported in the literature in terms of the EQE and the EL peak stability as the operation voltage increases. Figure 6a shows the EQE values versus the EL peak wavelength. Most of the device EQE are still in the 1% range. A couple of papers report EQE values that exceed 6% for red-EL and 8% for NIR-EL,40,41 but they lack of the EL spectral stability observed in both QLEDs. This may be because these previous reports used a normal device structure. The EQE of the iQLED is still 3.1% at maximum, but a high stability of the EQE is observed over a broad range of driving voltage. Figure 6b shows the EL peak wavelengths at different operation voltages for our device, and contains additional data points extracted from literatures. Our device shows a blueshift of the EL spectral peak as small as 4 nm when the voltage is varied from the turn-on voltage to 11 V. However, for the reported devices, the EL peaks shift toward the blue wavelength (~50 nm at maximum) as the applied voltage increases, resulting in the advantages of our device structure. In summary, we report the first investigation on the advantages of the inverted device structure for SiQLEDs by comparing with the optical performance of the conventional device structures. Adopting the optimal CBP/MoO3 ETL/EIL layers drastically enhances the optical performance of our iQLED. Typical observations include (i) a low turn-on voltage for light emission, (ii) a leveling off of the EQE over a wide driving-voltage regime, (iii) an unchanged EL spectral shape and position even at a high driving voltage (~11V), and (iv) longer device lifetimes. To our knowledge, our device outperforms previous SiQLEDs in terms of various optical performances. Moreover, this paper demonstrates a highly flexible Si-QLED for the first time by adopting a sheet of PET as a substrate instead of a glass substrate toward the widespread and convenient use of Si-QLEDs in flexible technologies.


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ACKNOWLEDGMENT The authors thank Dr. Yoshiyuki Nakashima (RIKEN KEIKI Co., Ltd.) for PYS measurements using a model AC-3. This study was supported by the WPI-Program, JST A-step (AS282I006e), Sumitomo Foundation. BS and NS thank to the JSPS International Fellowship for Research in Japan (Long term).

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis and structural characterization of SiQD and ZnO nanocrystals, preparation of light emitting diodes, characterization of optical performance of SiQD, calculation of EQE, AFM images of SiQD-De film and MoO3 film, and XRD of ZnO nanocrystals. REFERENCES (1) Choi, M. K.; Yang, J.; Hyeon, T.; Kim, D.-H. Flexible Quantum Dot Light-Emitting Diodes for Next-Generation Displays. npj Flexible Electronics (review) 2018, 2, 10. (2) Kim, T.-H.; Cho, K.-S.; Lee, E. K.; Lee, S. J.; Chae, J.; Kim, J. W.; Kim, D. H.; Kwon, J.-Y.; Amaratunga, G.; Lee, S. Y. Full-Colour Quantum Dot Displays Fabricated by Transfer Printing. Nat. Photon. 2011, 5, 176. (3) Tan, Z.; Xu, J.; Zhang, C.; Zhu, T.; Zhang, F.; Hedrick, B.; Pickering, S.; Wu, J.; Su, H.; Gao, S. Colloidal Nanocrystal-based Light-Emitting Diodes Fabricated on Plastic Toward Flexible Quantum Dot Optoelectronics. J. App. Phys. 2009, 105, 034312. (4) Shirasaki, Y.; Supran, G. J.; Bawendi, M. G.; Bulović, V. Emergence of Colloidal Quantum-Dot Light-Emitting Technologies. Nat. Photon. 2013, 7, 13. (5) Tyan, Y.-S. Organic Light Emitting-Diode Lighting Overview. J. Photoni. Energy 2011, 1, 011009. (6) Caruge, J. M.; Halpert, J. E.; Bulovic, V.; Bawendi, M. G. Colloidal Quantum-Dot Light-Emitting Diodes with Metal-Oxide Charge Transport Layers. Nat. Photon. 2008, 2, 247–250. (7) Bendall, J. S.; Paderi, M.; Ghigliotti, F.; Pira, N. L.; Lambertini, V.; Lesnyak, V.; Gaponik, N.; Visimberga, G.; Eychmüller, A.; Torres, C. M. S., et al. Layer-by-Layer All-Inorganic Quantum-Dotbased LEDs: A Simple Procedure with Robust Performance. Adv. Funct. Mater. 2010, 20, 3298–3302. (8) Yang, J.; Choi, M. K.; Kim, D.-H.; Hyeon, T. Designed Assembly and Integration of Colloidal Nanocrystals for Device Applications. Adv. Mater. 2016, 28, 1176–1207. (9) Dai, X.; Deng, Y.; Peng, X.; Jin, Y. Quantum-Dot Light-Emitting Diodes for Large Area Displays: towards the Dawn of Commercialization. Adv. Mater. 2017, 29, 1607022. (10) Zhang, H.; Li, H.; Sun, X.; Chen, S. Inverted Quantum-Dot Light-Emitting Diodes Fabricated by All-Solution Processing. ACS Appl. Mater. Interf. 2016, 8, 5493-5498.


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(63) Gu, W.; Liu, X.; Pi, X.; Dai, X.; Zhao, S.; Yao, L.; Li, D.; Jin, Y.; Xu, M.; Yang, D. SiliconQuantum-Dot Light-Emitting Diodes with Interlayer-Enhanced Hole Transport. IEEE Photon. J. 2017, 9, 1-10.


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Figure 1. (a) Scheme for thermal hydrosilylation of 1-decene to yield SiQD-De, (b) A typical XRD of the SiQD-De specimen which is compared with that of bulk-crystalline Si as a standard, (c) A representative HR-TEM photograph with a digital photograph of toluene dispersion of SiQD-De in the room light, (d) PL and Plots of F[R∞] vs wavelength for the powder form of the SiQD-De specimen. F[R∞] of the powder form is the Kubelka−Munk function, with F[R∞] = (1 − R∞)2/2R∞. PL spectrum was obtained at the excitation maximum (~390 nm).


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Figure 2. Schematic representations (upper) and flat band diagrams (lower) of the conventional (a,b) and inverted (c,d) device structures of the flexible, red-light emitting Si-QLED.


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Figure 3. Photoelectron yield spectra of (a) SiQD-De, (b) MoO3 films on a quarts glass substrate. The values in the figure show the negative values of the experimental HOMO energy.


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Figure 4. (a) Device I-V characteristics (black-line) and photodiode I-V characteristics (red-line) and (b) luminance−current density characteristics, and (c) a typical EL spectrum at the operation voltage of 4V (PL spectrum of the corresponding SiQD-De dispersed in chloroform). A photograph demonstrates a representative red-light emitting QLED folded in hand during operation.


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0

−6 1E-6 10

66

1E-5 10−5

−4 1E-4 10 2) 2) Current (A/cm Current density density (A/cm

(b)

Inverted device structure

Inverted

44

22

1E-3 10−3

Conventional

400 400

ELELpeak (nm) peak position Position (nm)

4V 6V 8V 11 V

500 600 700 800 900 1000 500 600 700 800 900 1000 Wavelength (nm) Wavelength (nm)

(d)

4V 5V 6V 7V 8V 11V

1E-3 10−3 2) 2 Currentdensity density (A/cm Current (A/cm ) 1E-4 10−4

Inverted

500

400

600

700

800

900

(e)

1000

700 (nm) 500 600 800 900 1000 Wavelength Wavelength (nm)

Normal Si-QDLED Conventional Inverted Si-QDLED Inverted

720 720 700 700 680 680 660 660

33

44

1 1.0

(f)

0.1

10 11 11 12 12 55 66 77 88 99 10 Voltage (V) (V) Applied voltage

Conventional Inverted

Normal Si-QD LED Inverted Si-QDLED

0.5 0.5

00 1E-5 10−5

740 740

Normalized Intensity (a.u.) NormalizedEL EL intensity (a.u)

11

760 760

(c) EL (a.u) EL Intensity (a.u.)

Normal Device Structure Conventional

EL (a.u) EL Intensity (a.u.)

(a)

EQE(%)

EQE (%)

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EQE (%) EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1 0.1

1 1.0 Time (hrs) Time (hour)

10 10

Figure 5. Summary of different optical performance between the conventional and the inverted QLEDs in terms of (a), (b) EQE-current density (c), (d) voltage variance of the EL spectra at different applied bias, and (e) EL peak positions as a function of driving voltage, and (f) operational device lifetime depicted by EL intensities at each time which is normalized with respect to the initial EL intensity.


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Figure 6. (a) Peak values of EQE versus EL peak wavelengths and (b) the spectral blueshift of EL peaks with increasing driving voltage for our work and a number of literature reports of Si-QLED. In the graph (b), the EL peak wavelengths are estimated by naked-eyes.


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Inverted Device Architecture for Enhanced Performance of Flexible

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