Hybrid Quantum Dot Light-Emitting Diodes for White Emission Using Blue Phosphorescent Organic Molecules and Red Quantum Dots

micromachines

Article Hybrid Quantum Dot Light-Emitting Diodes for White Emission Using Blue Phosphorescent Organic Molecules and Red Quantum Dots

Aram Moon and Jiwan Kim *

Department of Advanced Materials Engineering, Kyonggi University, Suwon 16227, Korea; [email protected] * Correspondence: [email protected]

Received: 30 August 2019; Accepted: 13 September 2019; Published: 14 September 2019

Abstract: Hybrid quantum dot light-emitting diodes (QLEDs) with no buffer layer were developed to achieve white emission using red quantum dots by spin-coating, and blue phosphorescent organic molecules by thermal evaporation. These unique bichromatic devices exhibit two distinct electroluminescent peaks with similar intensities at 10.5 V. For white emission, these hybrid QLEDs present a maximum luminance of 6195 cd/m2 and a current efficiency of 2.02 cd/A. These results indicate that the unique double emission layers have the potential for bright and efficient white devices using fewer materials.

Keywords: quantum dots; phosphorescent molecules; white emission

1. Introduction Colloidal quantum dots (QDs) have an excellent photoluminescence quantum yield (up to 97%) [1,2] and their color can be tuned by controlling their size. Recently, electrically driven quantum dot light-emitting diodes (QLEDs) have become the next-generation display platform owing to their superior optical properties and convenient solution processability. Since the first QLEDs were reported in 1994 [3], many groups have studied the development of balanced charge transport layers as well as efficient QDs [4–7]. In particular, the adaptation of ZnO-based inorganic electron transport layers (ETLs) has dramatically improved the brightness and the device efficiency of QLEDs [8–10]. For practical full-color display applications, white emitting devices should be the next step over monochromatic devices. However, only limited white electroluminescence (EL) devices using QDs have been reported, for two main reasons. First, the emission layer (EML) using red, green, and blue mixed QDs showed inevitable energy loss to adjacent QDs [11]. Second, the stacked tandem structure is another way to achieve white emission, but its complicated fabrication cannot be avoided because of in-between buffer layers as well as multiple EMLs [12]. Our group reported highly efficient white QLEDs with blue and green mixed QDs and red organic phosphorescent molecules, which had a maximum luminance of 20,453 cd/m2 and an external quantum efficiency (EQE) of 9.19% [13]. However, there are other ways to reduce the number of emitting materials for white emission. Intriguingly, white light-emitting diodes by controlling the thickness of silicon nanocrystals and organic dye have been reported [14]. In this study, we fabricate white standard structured QLEDs with double EMLs using red QDs and blue organic phosphorescent molecules. After the red EML (rEML) is formed with only red QDs by spin-coating, the blue EML (bEML) is deposited by the thermal evaporation of blue organic phosphorescent molecules on rEML. Recently, a structure of QD color filters with organic light-emitting diodes (OLEDs) used by major display manufacturers, which mainly applies QDs to the

Micromachines 2019, 10, 609; doi:10.3390/mi10090609 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 609 2 of 6 color converting layer [15,16], has been attracting a great deal of attention from the display industry. This QD-OLED structure is different from our white QLEDs, but there is a common use of blue OLEDs in both structures. In this study, we use bis[2-(4,6-difluorophenyl)pyridinato-C2,N]iridium(III) (FIrpic) as blue emitting organic molecules. By using FIrpic, which has a broader emission spectrum than QDs, white emission with a high color rendering index (CRI) can be achieved with fewer materials, i.e., only two emitting materials. The device exhibits a maximum luminance of 6195 cd/m2, a current efficiency of 2.02 cd/A, and an EQE of 1.01%. For an applied voltage of 10.5 V, the Commission Internationale de l’Eclairage (CIE) coordinates are positioned to a white point of (0.35, 0.33) with a high CRI of 60. A shift of the color coordinates according to the applied voltage is observed owing to changes in the EL peaks.

2. Materials and Methods The white QLEDs were fabricated on indium-tin-oxide- (ITO) coated glass substrates. The substrates were sequentially cleaned with isopropyl alcohol and then rinsed with deionized water. After the patterned ITO substrates were treated in ultraviolet-ozone for 15 minutes, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was spin-coated onto the ITO substrate as the hole injection layer (HIL) at 3000 rpm for 35 seconds and dried on a hot plate for 30 minutes at 150 ◦C. For the hole transport layer (HTL), poly[(9,9-dioctylﬂuorene-co-N- [4-(3-methylpropyl)]-diphenylamine] (TFB) was dissolved in chlorobenzene (8 mg/mL). The resulting mixture was spin-coated onto the PEDOT:PSS layer at 2000 rpm for 35 seconds. The resulting assembly was then dried on a hot plate at 150 ◦C for 30 minutes. For the fabrication of hybrid double EMLs, CdSeS/ZnS QDs (In-visible Inc., Suwon, Korea) for the rEML were dissolved in hexane with a concentration of 10 mg/mL and then spin-coated on top of the TFB layer at 2000 rpm for 5 seconds. The organic molecules and metals were deposited in continuance by thermal evaporation without breaking vacuum. In addition, 1,3-Bis(carbazol-9-yl)benzene (mCP) + FIrpic for the bEML, 20,20,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) for the ETL, and LiF/Al for the cathode were thermally evaporated with a deposition rate of approximately 2 Å/s for mCP + FIrpic, 1 Å/s for TPBi, 0.5 Å/s for LiF, and 3 Å/s for Al electrode, respectively. The current density–voltage–luminance (J-V-L) characteristics of the devices were measured using a spectroradiometer (CS-2000, Konica Minolta Holdings, Inc., Tokyo, Japan) with a source meter (Keithley-2400, Keithley Co. Ltd., Beaverton, Oregon, USA) under ambient conditions. From these J-V-L measurements, changes in the luminance and current eﬃciency of the fabricated devices as a function of the applied voltage were systematically studied.

3. Results and Discussion Figure1a illustrates white-emitting hybrid QLEDs with double EMLs comprising ITO /PEDOT:PSS (40 nm)/TFB (40 nm)/Red QDs (rEML, 10 nm)/mCP:FIrpic (bEML, 20, 30 nm)/TPBi(30, 40 nm)/ LiF(1 nm)/Al (100 nm). In this study, unique double EMLs were designed for simultaneous red and blue emissions from the QDs and FIrpic, respectively. The organic molecules, mCP and FIrpic, were selected as the host and dopant materials and deposited directly onto the red QD layer as the bEML using an evaporation method with no buffer layer. As shown in Figure1b, PEDOT:PSS was used as the HIL on an ITO anode to match with the anode work function. TFB was used as the HTL to transport holes efficiently from PEDOT:PSS to the highest occupied molecular orbital of the red QDs. Because of the chemical inertness of the TFB thin films to nonpolar solvents, red QDs in hexane were successfully spin-coated onto the TFB layer for the rEML with no chemical damage. The entire device features a sophisticated design for balanced charge transport from each electrode to central EMLs. Micromachines 2019, 10, x 3 of 6 Micromachines 2019, 10, 609 3 of 6 MicromachinesMicromachines 2019 2019, 10,, 10x , x 3 of 36 of 6

Figure 1. (a) Device structure and (b) energy band diagram of white standard structured quantum FigureFigure 1. (a 1.) Device(a) Device structure structure and and (b )(b energy) energy band band diagram diagram of of white white standardstandard structured quantum quantum dot dot light-emitting diodes (QLEDs) with double emission layers (EMLs) [17]. light-emittingFiguredot light-emitting1. (a) Device diodes diodesstructure (QLEDs) (QLEDs) and with ( b doublewith) energy double emission band emission diagram layers layers (EMLs) of (EMLs) white [17 [17].standard]. structured quantum dot light-emitting diodes (QLEDs) with double emission layers (EMLs) [17]. FigureFigureFigure 2 presentspresents 2 presents thethe the ELEL EL spectrumspectrum spectrum ofof of thethe the white-emitting white-emittingwhite-emitting hybrid hybridhybrid QLEDs. QLEDs.QLEDs. During During the thefabrication fabricationfabrication process,process,process,Figure the the devicethe2 device presents device was was was thetransferred transferred transferredEL spectrum to to tothe theof the thermal thethermal thermal white-emitting evaporator evaporator forhybrid for the the for formation QLEDs. formation the formation During of bEMLof bEML the ofafter fabrication bEML after spin- spin- after coatingspin-coatingprocess,coating rEML. the rEML. device rEML. Contrary Contrary was Contrary to transferred theto the expected to expected the to expected thebehavior behavior thermal behavior thth atevaporator phosphorescent that phosphorescentfor the organic formationorganic molecules molecules organic of bEML are molecules sensitiveare after sensitive spin- are tosensitivecoating theto solution the rEML. to solution the process,Contrary solution process, narrow to process, narrowthe expected red narrowred emission emission behavior red from emissionfrom that QDs phosphorescent frominin the the QDsrEML rEML in and organic theand traditional rEML traditional molecules and blue traditional blue areemission sensitive emission blue fromemissionto the fromFIrpic solution fromFIrpic in the FIrpic inprocess, bEMLthe bEML in thewerenarrow were bEML simultaneously simultaneouslyred were emission simultaneously fromobserv observ QDsed forfor observedin an anthe applied applied rEML for voltage anand voltage applied traditional of 10.5of voltage10.5 V. Inblue V. Figure In of emission Figure 10.5 3, V. 3, In blueFigurefrom blueOLED 3FIrpic, blueOLED devices in OLED thedevices bEML devices(ITO/PEDOT:PSS/TFB/mCP:FIrpic (ITO/PEDOT:PSS/TFB/mCP:FIrpic were (ITO simultaneously/PEDOT:PSS/TFB observ/mCP:FIrpiced/TPBi/LiF/Al)/TPBi/LiF/Al) for an/ TPBiapplied/ LiFwere werevoltage/Al) separately wereseparately of separately10.5 fabricated V. fabricatedIn Figure fabricated to 3, to evaluate the feasibility of phosphorescent organic materials on the solution-processed TFB layer. evaluatetoblue evaluate OLED the the feasibilitydevices feasibility (ITO/PEDOT:PSS/TFB/mCP:FIrpic of ofphosphorescent phosphorescent organi organicc materials materials/TPBi/LiF/Al) on on the the weresolution-processed solution-processed separately fabricated TFB TFB layer. layer. to Moreover, stable optical/electrical properties were demonstrated. Moreover,Moreover,evaluate thestable feasibility optical/electrical optical /electricalof phosphorescent properties organi were c demonstrated. materials on the solution-processed TFB layer. Moreover, stable optical/electrical properties were demonstrated.

Figure 2. Electroluminescence (EL) spectrum of hybrid QLEDs with double EMLs for an applied voltage of 10.5 V. Figure 2. Electroluminescence (EL) spectrum of hybrid QLEDs with double EMLs for an applied FigureFigure 2.2. ElectroluminescenceElectroluminescence ( (EL)EL) spectrum spectrum of of hybrid hybrid QLEDs QLEDs with with double double EMLs EMLs for for an an applied applied voltage of 10.5 V. voltagevoltage of of 10.5 10.5 V. V.

Figure 3. (a) EL spectrum and (b) current density–luminance with applied voltage of OLEDs with 1,3- Bis(carbazol-9-yl)benzene (mCP): bis[2-(4,6-difluorophenyl)pyridinato-C2,N]iridium(III) (FIrpic) as an EML.

FigureFigureFigure 3. 3. 3. (a ()a )EL EL spectrum spectrum spectrum and and and ( (bb) () bcurrent current) current density–luminancedensity–luminance density–luminance withwith with appliedapplied applied voltage voltage voltage of of OLEDs OLEDs of OLEDs with with 1,3- with 1,3- Bis(carbazol-9-yl)benzene1,3-Bis(carbazol-9-yl)benzeneBis(carbazol-9-yl)benzene (mCP):(mCP): (mCP): bis[2-(4,6-difluorbis[2-(4,6-difluor bis[2-(4,6-diﬂuorophenyl)pyridinato-C2,N]iridium(III)ophenyl)pyridinato-C2,N]iridium(III)idium(III) (FIrpic) (FIrpic) (FIrpic) as asas ananan EML. EML. EML.

Micromachines 2019, 10, 609 4 of 6 Micromachines 2019, 10, x 4 of 6 Micromachines 2019, 10, x 4 of 6 FigureFigure4 a4a presents presents the the voltage-dependent voltage-dependent variationsvariations of the luminance luminance and and current current density density of of the 2 thehybridFigure hybrid QLEDs 4a QLEDs presents with with thedouble doublevoltage-dependent EMLs EMLs.. A Apeak peak variatio luminance luminancens of the of of luminance 6195 6195 cd/m cd/m and2 was current achieved density atat of 11.511.5 the V V 2 2 hybrid(corresponding(corresponding QLEDs with to to a a current doublecurrent density densityEMLs. of Aof 269.58 269.58peak mA luminancemA/cm/cm2).). Owing Owingof 6195 to to the thecd/m thick thick was double double achieved EMLs, EMLs, theat the 11.5 turn-on turn-on V 2 (correspondingvoltagevoltage was was shifted shifted to a current to to a a higher higher density voltage voltage of 269.58 region region mA/cm than than). general generalOwing monochromatic tomonochromatic the thick double QLEDs. QLEDs. EMLs, In In the Figure Figure turn-on4 b,4b, a a voltagemaximummaximum was EQEshifted EQE of of 1.01%to 1.01% a higher and and a voltagea current current region e ﬃefficiencyciency than of ofgeneral 2.02 2.02 cd cd/A /monochromaticA were were obtained obtained QLEDs. for for a a current currentIn Figure density density 4b, a of of 2 maximum0.150.15 mA mA/cm/cm EQE2. . of 1.01% and a current efficiency of 2.02 cd/A were obtained for a current density of 0.15 mA/cm2.

Figure 4. Characteristics in (a) current density–luminance with applied voltage and (b) external FigureFigurequantum 4. 4. Characteristics Characteristicsefficiency (EQE)—current in in(a ()a current) current efficiency density–luminance density–luminance with applied with with voltage applied applied of hybridvoltage voltage QLEDsand and (b ()bwith )external external double quantumquantumEMLs. efficiency efficiency (EQE)—current eefficiencyfficiency with with applied applied voltage voltage of hybridof hybrid QLEDs QLEDs with with double double EMLs. EMLs. TheThe spectral spectral variations variations of of the the EL EL spectra spectra and and the the shift shift of of the the CIE CIE coordinates coordinates of of the the hybrid hybrid QLEDs QLEDs withwithThe double double spectral EMLs EMLs variations at at di differentfferent of the voltages voltEL spectraages are are shownand shown the in shift in Figure Figures of the5a,b, CIE 5a respectively. and coordinates 5b, respectively.Firstly, of the hybrid hybrid Firstly, QLEDs QLEDs hybrid withemittedQLEDs double redemitted EMLs light, red at then different light, shifted then volt shifted toages white areto aswhite shown the as voltage inth eFigures voltage increased 5a increased and up 5b, to respectively.up 10.5 to 10.5 V. As V. demonstrated AsFirstly, demonstrated hybrid in QLEDsFigurein Figure emitted5a, this5a, this shiftred light,shift resulted resulted then fromshifted from the to the later white later emissions as emissions the voltage of FIrpicof increasedFIrpic in in bEML bEML up to at 10.5 at a highera V.higher As voltagedemonstrated voltage region. region. inTherefore, Therefore,Figure 5a, the thisthe CIE CIEshift coordinates coordinates resulted from shifted shifted the fromlater from emissions (0.49, (0.49, 0.32) 0.32) of at atFIrpic 6.5 6.5 V V in to to bEML (0.35, (0.35, 0.33)at 0.33) a higher at at 10.5 10.5 voltage V V with with region. a a CRI CRI of of Therefore,60.60. Both Both blue theblue CIE and and coordinates red red emissions emissions shifted decreased decreased from after(0.49, after exhibiting 0.32)exhibi atting 6.5 balanced balancedV to (0.35, white white 0.33) emission emission at 10.5 V at at with 10.5 10.5 a V, V, CRI but but of the the 60.decrementdecrement Both blue of andof blue blue red emission emission emissions was was decreased dominant dominant after at at a a higherexhibi higherting applied applied balanced voltage. voltage. white Therefore, Therefore, emission the theat CIE10.5 CIE coordinatesV, coordinates but the decrementreturnedreturned toof to redblue red at atemission 12.5 12.5 V. V. The wasThe hybrid dohybridminant QLEDs QLEDs at a emittedhigher emitted applied white white lightvoltage. light at at a aTherefore, lower lower applied applied the CIE voltage voltage coordinates than than our our returnedpreviousprevious to results redresults at [12.5 13[13]] becauseV. because The hybrid QLEDs QLEDs QLEDs with with red emittedred QDs QD swhite showed showed light a a relatively atrelatively a lower lower lowerapplied turn-on turn-on voltage voltage voltage than our than than previousthosethose with with results blue blue [13] QDs, QDs, because owing owing toQLEDs to their their smallwith small red bandgap bandgap QDs showed [8 [8,18].,18]. a relatively lower turn-on voltage than those with blue QDs, owing to their small bandgap [8,18].

Figure 5. Voltage-dependent (a) EL spectra and (b) Commission Internationale de l’Eclairage Figure 5. Voltage-dependent (a) EL spectra and (b) Commission Internationale de l’Eclairage (CIE) (CIE) coordinates of hybrid QLEDs with double EMLs. The inset shows the white emission from Figurecoordinates 5. Voltage-dependent of hybrid QLEDs (a) withEL spectra double and EMLs. (b) TheCommission inset shows Internationale the white emission de l’Eclairage from 2.5 (CIE) mm × 2.5 mm 2.5 mm pixels at 10.5 V. coordinates2.5 mm× pixels of hybrid at 10.5 QLEDs V. with double EMLs. The inset shows the white emission from 2.5 mm × 2.5 mm pixels at 10.5 V.

Micromachines 2019, 10, 609 5 of 6

Micromachines 2019, 10, x 5 of 6 Figure6 shows the EL spectra of the hybrid QLEDs with various thicknesses of bEML and ETL Figure 6 shows the EL spectra of the hybrid QLEDs with various thicknesses of bEML and ETL at the same applied voltage of 10.5 V. In standard structured QLEDs, the transport of electrons from at the same applied voltage of 10.5 V. In standard structured QLEDs, the transport of electrons from the cathode can be slowed down as ETL becomes thicker. Therefore, the strong blue emissions of the the cathode can be slowed down as ETL becomes thicker. Therefore, the strong blue emissions of the hybrid QLEDs with 40 nm ETL result from an upward shift of the recombination zone (RZ) to bEML. hybrid QLEDs with 40 nm ETL result from an upward shift of the recombination zone (RZ) to bEML. Interestingly, for hybrid QLEDs with 30 nm bEML and 30 nm ETL, the entire EL emission was very Interestingly, for hybrid QLEDs with 30 nm bEML and 30 nm ETL, the entire EL emission was very weak even though the combined thickness of bEML and ETL is the same as 60 nm. It is assumed weak even though the combined thickness of bEML and ETL is the same as 60 nm. It is assumed that that the thickness of bEML is more critical than that of ETL for the eﬀective transport of electrons to the thickness of bEML is more critical than that of ETL for the effective transport of electrons to the the RZ, and it is also supported by the dramatic change of red emission from QDs in rEML with the RZ, and it is also supported by the dramatic change of red emission from QDs in rEML with the same same thickness of ETL. Consequently, hybrid QLEDs with 20 nm bEML and 40 nm ETL are ideal for thickness of ETL. Consequently, hybrid QLEDs with 20 nm bEML and 40 nm ETL are ideal for achieving balanced white emission due to the adequate location of the RZ at the interface of rEML achieving balanced white emission due to the adequate location of the RZ at the interface of rEML and bEML. and bEML.

Figure 6.6. EL spectraspectra ofof QLEDsQLEDs withwith variousvarious thicknessesthicknesses of blueblue emissionemission layerlayer (bEML)(bEML) andand electronelectron transporttransport layerlayer (ETL)(ETL) atat 10.510.5 V.V. These hybrid QLEDs demonstrated two distinct emissions simultaneously with no buffer layer These hybrid QLEDs demonstrated two distinct emissions simultaneously with no buffer layer between rEML and bEML, which is typically required to protect the delicate organic molecules. between rEML and bEML, which is typically required to protect the delicate organic molecules. More More importantly, hybrid QLEDs have unique heterogeneous EMLs with inorganic QDs and organic importantly, hybrid QLEDs have unique heterogeneous EMLs with inorganic QDs and organic mCP:FIrpic. As shown in Figure2, the asymmetric shape of the EL peak from FIrpic helped to cover mCP:FIrpic. As shown in Figure 2, the asymmetric shape of the EL peak from FIrpic helped to cover the green region in the entire visible spectrum. Therefore, the white emission with a high CRI of the green region in the entire visible spectrum. Therefore, the white emission with a high CRI of 60 60 achieved with hybrid QLEDs was mostly due to the broad blue emission from FIrpic. Unlike our achieved with hybrid QLEDs was mostly due to the broad blue emission from FIrpic. Unlike our previous report [13], fewer materials (only one red QD and one blue OLED) were used to accomplish previous report [13], fewer materials (only one red QD and one blue OLED) were used to accomplish white emission at a relatively low applied voltage and it provided a huge advantage in terms of white emission at a relatively low applied voltage and it provided a huge advantage in terms of reduced manufacturing costs and fabrication steps. reduced manufacturing costs and fabrication steps. 4. Conclusions 4. Conclusions Hybrid white-emitting QLEDs with blue phosphorescent organic molecules and red QDs were Hybrid white-emitting QLEDs with blue phosphorescent organic molecules and red QDs were successfully fabricated. The EL spectra of the device exhibited two emission peaks from each layer successfully fabricated. The EL spectra of the device exhibited two emission peaks from each layer simultaneously. The hybrid QLEDs with 20 nm bEML and 40 nm ETL had a maximum luminance of simultaneously. The hybrid QLEDs with 20 nm bEML and 40 nm ETL had a maximum luminance of 6195 cd/m2, an EQE of 1.01%, and a current efficiency of 2.02 cd/A, while emitting white light with a 6195 cd/m2, an EQE of 1.01%, and a current efficiency of 2.02 cd/A, while emitting white light with a high CRI of 60 at 10.5 V. The thickness of bEML is more important than that of ETL for balanced white high CRI of 60 at 10.5 V. The thickness of bEML is more important than that of ETL for balanced emission. On increasing the applied voltage, the emission arises from the red QDs firstly, followed by white emission. On increasing the applied voltage, the emission arises from the red QDs firstly, blue EL from FIrpic. Therefore, the CIE coordinates moved from the red to the white region according followed by blue EL from FIrpic. Therefore, the CIE coordinates moved from the red to the white to the increment of the applied voltage. region according to the increment of the applied voltage.

AuthorAuthor Contributions:Contributions: Investigation, A.M. and J.K.; writing—original draft prepar preparation,ation, A.M.; writing—review and editing, J.K.; supervision, J.K.; funding acquisition, J.K. and editing, J.K.; supervision, J.K.; funding acquisition, J.K. Funding: This work was supported by the ITECH R&D program of MOTIE/KEIT [project No. 20004978] andFunding: the National This work Research was supported Foundation by the of ITECH Korea (NRF)R&D prog grantram funded of MOTIE/KEIT by the Korean [project government No. 20004978] (MSIT) and (No. the 2017R1A2B4012274).National Research Foundation This work was of alsoKorea supported (NRF) bygrant Kyonggi funded University’s by the Korean Graduate government Assistantship (MSIT) 2019. (No. 2017R1A2B4012274). This work was also supported by Kyonggi University’s Graduate Assistantship 2019.

Conflicts of Interest: The authors declare no conflict of interest.

Micromachines 2019, 10, 609 6 of 6

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Greytak, A.B.; Allen, P.M.; Liu, W.; Zhao, J.; Young, E.R.; Popović,Z.; Walker, B.J.; Nocera, D.G.; Bawendi, M.G. Alternating layer addition approach to CdSe/CdS core/shell quantum dots with near-unity quantum yield and high on-time fractions. Chem. Sci. 2012, 3, 2028–2034. [CrossRef][PubMed] 2. Lee, K.-H.; Lee, J.-H.; Song, W.-S.; Ko, H.; Lee, C.; Lee, J.-H.; Yang, H. Highly efficient, color-pure, color-stable blue quantum dot light-emitting devices. ACS Nano 2013, 7, 7295–7302. [CrossRef][PubMed] 3. Colvin, V.L.; Schlamp, M.C.; Alivisatos, A.P.Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer. Nature 1994, 370, 354–357. [CrossRef] 4. Caruge, J.M.; Halpert, J.E.; Wood, V.; Bulovic, V.; Bawendi, M.G. Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers. Nature 2008, 2, 247–250. [CrossRef] 5. Cho, K.S.; Lee, E.K.; Joo, W.J.; Jang, E.J.; Kim, T.H.; Lee, S.J.; Kwon, S.J.; Han, J.Y.; Kim, B.K.; Choi, B.L.; et al. High-performance crosslinked colloidal quantum-dot light-emitting diodes. Nat. Photonics 2009, 3, 341–345. [CrossRef] 6. Qian, L.; Zheng, Y.; Xue, J.; Holloway, P.H. Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures. Nat. Photonics 2011, 5, 543–548. [CrossRef] 7. Han, C.-Y.; Yang, H. Development of colloidal quantum dots for electrically driven light-emitting devices. J. Korean Ceram. Soc. 2017, 54, 449–469. [CrossRef] 8. Kwak, J.H.; Bae, W.K.; Lee, D.G.; Park, I.S.; Lim, J.H.; Park, M.G.; Cho, H.D.; Woo, H.J.; Yoon, D.Y.; Char, K.H.; et al. Bright and efficient full-color colloidal quantum dot light-emitting diodes using an inverted device structure. Nano Lett. 2012, 12, 2362–2366. [CrossRef][PubMed] 9. Mashford, B.S.; Stevenson, M.; Popovic, Z.; Hamilton, C.; Zhou, Z.; Breen, C.; Steckel, J.; Bulovic, V.; Bawendi, M.G.; Coe-Sullivan, S.; et al. High-efficiency quantum-dot light-emitting devices with enhanced charge injection. Nat. Photonics 2013, 7, 407–412. [CrossRef] 10. Dai, X.L.; Zhang, Z.X.; Jin, Y.Z.; Niu, Y.; Cao, H.J.; Liang, X.Y.; Chen, L.W.; Wang, J.P.; Peng, X.G. Solution-processed, high-performance light-emitting diodes based on quantum dots. Nature 2014, 515, 96–99. [CrossRef][PubMed] 11. Bae, W.K.; Lim, J.; Lee, D.; Park, M.; Lee, H.; Kwak, J.; Char, K.; Lee, C.; Lee, S. R/G/B/natural white light thin colloidal quantum dot-based light-emitting devices. Adv. Mater. 2014, 26, 6387–6393. [CrossRef][PubMed] 12. Lee, K.-H.; Han, C.-Y.; Jang, E.-P.; Jo, J.-H.; Hong, S.; Hwang, J.Y.; Choi, E.; Hwang, J.-H.; Yang, H. Full-color capable light-emitting diodes based on solution-processed quantum dot layer stacking. Nanoscale 2018, 10, 6300–6305. [CrossRef][PubMed] 13. Oh, S.; Han, C.-Y.; Yang, H.; Kim, J. Highly efficient white electroluminescent devices with hybrid double emitting layers of quantum dots and phosphorescent molecules. Nanoscale 2019, 11, 9276–9280. [CrossRef] [PubMed] 14. Ghosh, B.; Masuda, Y.; Wakayama, Y.; Imanaka, Y.; Inoue, J.; Hashi, K.; Shirahata, N. Hybrid white light emitting diode based on silicon nanocrystals. Adv. Funct. Mater. 2014, 24, 7151–7160. [CrossRef] 15. Smeeton, T.M.; Angioni, E.; Boardman, E.A.; Izumi, M.; Iwata, N.; Nakanishi, Y.; Ishida, T. Development of electroluminescent QD-LED displays. SID Int. Symp. Dig. Tech. Pap. 2019, 50, 742–745. [CrossRef] 16. Hu, Z.; Zhang, S.; Peng, W.; Zhang, S.; Li, Y.; Li, D.; Jiao, S.; Chen, S.-J.; Lee, C.-Y.; Zhou, H. Inkjet-printed quantum dot display with blue OLEDs for next generation display. SID Int. Symp. Dig. Tech. Pap. 2019, 50, 1075–1078. [CrossRef] 17. Bae, W.K.; Kwak, J.; Park, J.W.; Char, K.; Lee, C.; Lee, S. Highly efficient green-light-emitting diodes based on CdSe@ZnS quantum dots with a chemical-composition gradient. Adv. Mater. 2009, 21, 1690–1694. [CrossRef] 18. Zhang, H.; Chen, S.; Sun, X.W. Efficient red/green/blue tandem quantum-dot light-emitting diodes with external quantum efficiency exceeding 21%. ACS Nano 2018, 12, 697–704. [CrossRef][PubMed]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).