Nanoscale

PAPER

Cite this: Nanoscale, 2016, 8, 1117

Received 20th August 2015,Accepted 25th November 2015

DOI: 10.1039/c5nr05676d

www.rsc.org/nanoscale

A high quality liquid-type quantum dot whitelight-emitting diode

Chin-Wei Sher,a Chin-Hao Lin,b Huang-Yu Lin,b Chien-Chung Lin,*c

Che-Hsuan Huang,b Kuo-Ju Chen,b Jie-Ru Li,b Kuan-Yu Wang,c Hsien-Hao Tu,b

Chien-Chung Fua and Hao-Chung Kuo*b,d

This study demonstrates a novel package design to store colloidal quantum dots in liquid format and inte-

grate them with a standard LED. The high efficiency and high quality color performance at a neutral white

correlated color temperature is demonstrated. The experimental results indicate that the liquid-type

quantum dot white light-emitting diode (LQD WLED) is highly efficient and reliable. The luminous

efficiency and color rendering index (CRI) of the LQD WLED can reach 271 lm Wop−1 and 95, respectively.

Moreover, a glass box is employed to prevent humidity and oxygen erosion. With this encapsulation

design, our quantum dot box can survive over 1000 hours of storage time.

Introduction

Recently, colloidal quantum dots (QDs) have attracted indus-trial attention because of their unique properties such as highquantum yield, minimal backscattering, size dependenttunable bandgap, and narrow emission full width at halfmaximum.1–7 As a novel technology in this field, QDs offermore variety in color mixing. In the past, high quantumefficiency and an excellent color-rendering index were achievedby multishell QDs and co-doping QDs in a phosphor.3,8 Core–shell structures have been employed to enhance the quantumyield of QDs and derive high photoluminescence efficiency.8,9

QDs can also be applied in some premium products such asdisplays with a high color gamut and WLEDs.10–14 As for themethods to dispense QDs, several groups have reported suc-cessful results in transfer printing,12 pulsed spray,1 inkjetprinting11 and mist coating.14 Furthermore, the 30 nm emis-sion bandwidth of QDs can yield a higher degree of colorpurity compared with the estimated 100 nm bandwidth ofmonochromatic phosphor-converted LEDs.15,16

One major concern in these QD devices is the reduction inquantum efficiency when the solvent is dried up. When the

QDs are dispersed in solvent, their light emission capability ismuch better than in the solid phase, due to the self-aggrega-tion effect.17,18 Another issue accompanying solidification isthe so-called “coffee ring” effect which results from themigration of solutes toward the edge of droplets during thedrying process.19

In this study, a liquid-type QD white LED is demonstratedas an efficient color-conversion layer in UV LED packages. Weuse a glass material to protect the quantum dot material fromdrying. This method can increase quantum dot efficiency andreliability. The white liquid-type quantum dot LED is shown tohave a high color rendering index (CRI) at a correlated colortemperature (CCT) of 4360 K.

Experimental

In the sample preparation, two types of device were fabricatedat the same time for comparison: one is a remote QD type (asour reference sample), and the other is a liquid type QDdevice. Fig. 1(a) shows the structure and process flow of theremote structure QD. We use PDMS to fill the 5070 typepackage. After the PDMS material was cured, an illuminatinglayer which contains QDs and PMMA was dispensed onto theLED package surface. To pump these QDs, a UV LED chip wasinstalled in this 5070 package. The UV LED chips are firstattached to a sub-mount. Then, the gold-wire bonding methodwas used to connect the anode and cathode pads. The chipsize is 45 mil × 45 mil and the nominal power output of theUV chip (SemiLEDs EV-D45A) is 240 mW at 350 mA and theemission wavelength is 365 nm at a 350 mA driving current.Real device measurement yields a UV optical power (watts) to

aDepartment of Power Mechanical Engineering, National Tsing Hua University,

No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan.

E-mail: steven.sher@hotmail.combDepartment of Photonics & Institute of Electro-Optical Engineering, National Chiao

Tung University, Hsinchu 30010, Taiwan. E-mail: hckuo@faculty.nctu.edu.twcInstitute of Photonic System, National Chiao Tung University, Tainan 711, Taiwan.

E-mail: chienchunglin@faculty.nctu.edu.tw; Fax: +886-6-3032535;

Tel: +886-6-3032121#57754dDepartment of Electrical Engineering, Da Yeh University, Changhua 51591, Taiwan

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electrical power (watt)−1 ratio of 0.091 or 9.1%. Fig. 1(b) illus-trates the process flow of the liquid-type QD white LED. Theprocedure is as follows: first, the glass substrate was cut intothin strips with a size of 3.0 cm × 1.5 cm. Second, four of thesethin strips were sandwiched in between two larger pieces ofglass. Third, an appropriate gap was left such that the air canventilate when the liquid QDs are injected. Finally, the gapswere sealed with an epoxy-based glue to finish the liquid QDprocess. The thickness of the glass slide is 0.1 cm and thus theoverall volume which can accommodate the QD solution is1.5 cm × 3 cm × 0.1 cm = 0.45 cm3 = 0.45 ml. Silicone isapplied into the 5070 cup using a direct dispensing method.Under normal operation, the UV LED is based at 100 mA andthe normal power is 43 mW. Considering the area of the 5070package, which is 0.25 cm2, the pumping intensity is172.8 mW cm−2. Both the remote quantum dot structure andliquid-type quantum dot structure were fabricated with thesame quantum dots and characterized under the same con-ditions (i.e. the same UV chip initial brightness, and the samehumidity and temperature).

As for QD characterization, we used a Horiba, FL-3 systemto measure the spectrum and intensity. Fig. 2(a) and (b) showthe UV-visible absorption and photoluminescence (PL) spectraof the five QD species used in this experiment. Our QDs werepurchased from UT Dots, Inc. (web site: http://www.utdots.com). The QDs are dissolved in xylene and the different sizeddots can provide different emission wavelengths. To have theemission wavelength of each QD sitting within ±10 nm of thespecification, a certain level of control of the dot size distri-bution is necessary. The full width at half maximum (FWHM)of the emission spectrum is normally less than 35 nm for allproducts. The surface stabilizers of the dots are either octa-decylamine (green and red) or oleic acid (blue and blue-green)and the concentration is 5 mg mL−1. The photoluminescencequantum yield (PLQY) of all QDs is higher than 50% accordingto the vendor’s datasheet. From the experimental results, thewavelengths of the blue, blue-green, green, orange, and redcomponents were located at 450 nm, 490 nm, 535 nm, 590 nm

and 630 nm, respectively. The FWHM of these emissionspectra is approximately 20, 40, 40, 35, and 40 nm, respect-ively. Fig. 2(c) and (d) show the completed liquid type QD glasswithout and with UV pumping. The surrounding environmentwas totally dark when we took the picture shown in Fig. 2(d).So instead of white light, the strong scattering of the residualblue photons from the UV lamp (model number: UVGL-58)masked the weaker visible ones from the QDs and the wholefilm looked blue.

When the QDs with different colors were mixed, optimizedprocedures were carried out to obtain similar CCTs and thebest CRI values. The quantity of each color of QD can deter-mine the final color quality of the device. Microbalances wereused to precisely measure the weight of each type of QD. Theweight percentages for the reference and liquid QD samplesare listed in Table 1.

Measurement and analysis

Fig. 3(a) and (b) show the EL spectra of the remote QD typeand liquid QD type devices under injection currents from20 mA to 250 mA. The inset in Fig. 3(a) shows the current-dependent integrated intensity extracted from Fig. 3(a) and (b).All the measurements were performed in a calibrated inte-grated sphere. From the data, we see an overall improvementof 66.6% between liquid and remote samples at a 250 mA

Fig. 1 Process flow of (a) remote structure QD white LED and (b)liquid-type QD white LED.

Fig. 2 (a) Absorption spectra and (b) emission spectra of blue, blue-green, green, orange, and red QDs. (c) Liquid-type QDs in the glasspackage before UV LED pumping and (d) liquid-type QDs in the glasspackage with uniform UV light pumping.

Table 1 The QD weight percentages for each type of sample

Wavelength (nm) 450 490 535 590 630Liquid QD 53.33% 13.34% 13.34% 13.34% 6.66%Remote 57.70% 11.34% 11.34% 13.80% 5.82%

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driving current. After analyzing in more detail, the incrementalpercentages of the blue, blue-green, green, orange, and redcolor bands are 14.6%, 37.8%, 38.9%, 29.4%, and 39.4%,respectively at a 250 mA driving current.

The luminous efficacy of radiation (LER) is one of the indi-cators to show how efficient the emitter is in terms of whitelight generation. To calculate the LER, one can use the follow-ing expression:20

LER ¼ 683 lmW�1

ÐVðλÞPwhiteðλÞdλÐPwhiteðλÞdλ ð1Þ

where 683 lm W−1 is a normalization factor, V(λ) is the humaneye sensitivity function, and Pwhite(λ) is the spectral powerdensity of the light source. In Fig. 4(a), the LER of the LEDdevice with liquid-type quantum dots is approximately 271 lmWop

−1, and this value is higher than the 78 lm Wop−1 of the

remote quantum dot structure. This improvement can beattributed to the high quantum yield of the liquid type device.In addition, Fig. 4(b) demonstrates the current-dependent CRIof both devices. On average, the CRI values are very stable andexceed 90 at different current levels. This characteristic isbrought about by the five colors of the QDs in the spectrawhich can fulfill the CRI requirement.

To verify the claim that the liquid type device can preservethe quantum yield, one key factor to be tested is the PLQY ofthe QD LED. Because the QD layer is optically pumped with a

UV source, the efficiency of converting the UV photons intovisible ones can be expressed as21,22

PLQY ¼Ðvisible band

λ

hc

� �� IQDem ðλÞ � IrefemðλÞ� �

dλ

ÐUV band

λ

hc

� �� Irefex ðλÞ � IQDex ðλÞ� �

dλð2Þ

where IQDex and Irefex are the integrated intensities of the UV exci-tation with and without the QD layer, and IQDem and Irefem are thespectral intensities of the visible band with and without theQD layer, respectively. By using the spectra of samples withand without the QD layer, we can estimate the PLQY values forboth types of device. The PLQY of the liquid QD device is33.87% and the reference remote QD sample is 31.32%. Thenumber for the liquid QD case can be re-adjusted to 34% ifthe UV absorption of the glass slides, which is about 10%usually, is taken into account. A higher PLQY value of theliquid type device also justifies our initial idea of keeping QDsfrom solidifying.

Fig. 5(a)–(c) show the thermal images of the surfaces ofregular LED, remote quantum dot and liquid-type quantumdot samples. Fig. 5(d) shows the current dependent surfacetemperatures among these three different types of device. Thesurface temperature can reach 200 °C at 250 mA in the remotecase. This will reduce the efficiency of the quantum dots andcause a reliability problem.23 On the other hand, the liquidtype QD LED can keep the surface temperature as low as 50 °C,which is advantageous towards both performance andreliability. As for the temporal behavior of the sample underUV excitation, surface thermal image measurements for asimilar setup which contains QDs wrapped in a silicone likematerial with a UV LED packaged together were performed.The stabilization of the surface temperatures both under high

Fig. 3 (a) Conventional remote QD and (b) liquid-type QD emissionspectra of blue, blue-green, green, orange, and red QDs. The inset in (a)is the current-dependent integrated intensity of the two types of device.

Fig. 4 Current-dependent (a) luminous efficacy of radiation (LER) ofthe UV LED with conventional remote QDs and liquid QDs; (b) color ren-dering index (CRI) of the UV LED with conventional remote QDs andliquid-type QDs.

Fig. 5 The IR images of surface temperature in a (a) LED withoutquantum dots, (b) remote quantum dot white LED and (c) liquid-typequantum dot white LED at a driving current of 100 mA. (d) The tempera-ture and current relationship of two different quantum dot structures.

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and low current injection is within 5 minutes based on the30-minute observation.

To investigate the huge difference in the surface tempera-tures, we must understand more of the thermal transportcharacteristics of the package materials. The thermalconductivity of the glass is 1.05 W (m K)−1 and PMMA is0.167–0.25 W (m K)−1. Apparently the glass container of theliquid type device is a much better material for heat dissipa-tion and the glass slides also possess a much larger area.Invoking Fourier’s law of heat conduction, we could obtain thethermal resistance (R) calculated as:24

heat-transfer-rate ¼ ΔTR

; R/ LκA

ð3Þ

where ΔT is the temperature difference between two endpointsand R is the thermal resistance. In this simplified model, wecould observe the significant difference made by the thermalconductivity and area of the device. From the calculation ofthe effective emission area, the heat dissipation capacitybetween remote QDs and liquid QDs is very different. Theeffective area of the liquid QD sample is 1.34 cm2 and for theother device it is only 0.25 cm2. On combining with a differentmaterial, a much larger thermal resistance (33.7 times underone-dimensional analysis) in the remote QD case is expected.This is the major cause of the much higher surface tem-perature detected. High levels of heat trapped inside thepackage can certainly reduce the efficiency of the QDs, whilethe liquid QDs will have a larger footprint and take up morespace for setup.

Fig. 6(a) and (b) show the white light images of the remotequantum dot structure and liquid-type quantum dot structureunder UV LED excitation. The pictures of Fig. 6(a) and (b) weretaken using a generic digital camera (HTC® One®) andimported into Matlab® to extract the two-dimensional inten-sity profile shown in Fig. 6(c). By appropriately choosing theratio between different QDs, both devices can achieve a similarCCT (4500 K) and high CRI (>90). The effective emission areasfor the liquid QD and reference QD samples are quitedifferent. From the 2D intensity profile as shown in Fig. 6(c),the emission area of the liquid QD device can be estimated as1.34 cm2, which is about five times that of the reference. But atthe same time, due to the controlled output power of UV LEDs,the excitation source is kept the same and thus the compari-son between the two types of samples should solely depend onthe emission efficiency of the quantum dots. A larger emissionarea is helpful for thermal dissipation (as can be observed inFig. 5) and further maintaining the quantum yield of the QDsolution in our case, which eventually contributes to a betterLER. The uniformity of the light intensity, if defined by the10% variation of the maximum, can be as high as 91.2% of theemission area. Meanwhile, the luminous efficiency of theliquid QD device is about 7 lumen watt (electrical power)−1.

After the initial characterization of the devices, the nextimportant task is to test the longevity of this liquid QD LED.In the past, many results have shown serious degradation ofthe QD performance when the device is exposed to air.7,17,25 In

our design, the glass container was sealed to prevent thesolvent from drying and also to keep oxygen and water fromreacting with the QDs. We hope this can help to preserve thequantum yield of the QDs over a long period of time. Fig. 7(a)and (b) show the emission spectra of the remote QD structureand liquid-type QD structure taken at various times when bothdevices are stored at room temperature. The measurement con-ditions for both devices is a 350 mA driving current and themeasurements were tested at 0, 0.15, 0.3, 0.4, 0.5, 1, 20, 40, 72,350, 500 and 1000 hours. The experimental results demon-strate that the liquid-type quantum dot structure has a stableand sustainable performance over a long period of time, asshown in Fig. 7(c). The efficiency reductions of the remotequantum dot structure and liquid-type quantum dot structureare 75% and 5% after 1000 hours of storage, respectively. Thecolor rendering index shifts of the remote quantum dot struc-ture and liquid-type quantum dot structure at a 350 mAcurrent from 0 to 1000 hours are shown in Fig. 7(d). It is foundthat the color rendering indices of both structures quicklybecome stable after 0.5 hours. From these results, the liquidtype of package not only maintains its intensity but also itscolor quality over a long storage time.

One concern about a possible change in the QD size distri-bution arises when the QDs are stored in the liquid phase.Under the current test conditions, which do not constantlyshine the QD samples with UV photons, the interference fromUV heating could be minimized. On the other hand, the self-

Fig. 6 Image of the (a) remote quantum dot structure under UV exci-tation, (b) liquid-type quantum dot structure under UV excitation. (c)The 2D intensity profile of the liquid QDs in (b).

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aggregation of QDs in the liquid phase over a long storagetime should be discussed. To evaluate this without performingTEM, the emission spectrum of the QD layer holds the key.The emission spectrum of a QD ensemble usually representsthe size distribution of this ensemble,26 because the size of ananoparticle can determine the emission peak wavelength andthe population of the same sized particle is proportional to theemission intensity. So the direct comparison between the peakpositions and FWHM of the emission spectra can help us tounderstand the size distribution of the dots and thus the degreeof aggregation. Fig. 7(b) provides a great opportunity for evaluat-ing each QD’s variation during this long storage time. By usingappropriate curve fitting, the position and FWHM of each peakcan be found and the results are summarized in Table 2.

From this table, we could observe very little change betweenthe 0th and 1000th hour of testing among the five differentQDs we put into the glass slide. Certainly the nature of thisstorage test minimized the extreme thermal and UV illumina-tion issues for the sample, but the constant performance andminimum aggregation that can be implied from this tabledemonstrate a potential solution for the long-term stability inthis QD type device.

For the quantum dot white LED, the color deviation withdifferent structures is used to evaluate the color stability forhigh-quality lighting applications. Therefore, the chromaticitycoordinate shifts of the remote quantum dot and liquid-typequantum dot structures are recorded and shown in Fig. 8(a)and (b). As the time passed by, the chromaticity coordinatesfor the liquid-type quantum dot structure were stable. More-over, the color deviation of a lighting system, (Δu′v′) is calcu-lated as follows:27

u′ ¼ 4x=ð�2xþ 12yþ 3Þ ð4Þv′ ¼ 9y=ð�2xþ 12yþ 3Þ ð5Þ

Δu′v′ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðΔu′Þ2 þ ðΔv′Þ2

qð6Þ

where u′ and v′ are the chromaticity coordinates in the CIE1976 diagram, and x and y are the chromaticity coordinates inthe CIE 1931 diagram. The Δu′v′ value indicates color shiftswhich might alter the CCT eventually. From the experimentalresults, the remote QD sample showed a moderate CIE coordi-nate-shift while the liquid QD sample remained almost thesame after 1000 hours of storage. This clearly demonstratesthe superiority of our design in terms of light quality whichshould be crucial for use as a next generation light source.

Conclusion

This study demonstrates a hybrid white LED with a liquid-typequantum dot structure to produce a stable and high color ren-

Fig. 7 (a) The relative emission spectra of the remote quantum dotwhite light LED structure with various life times (0, 0.15, 0.3, 0.4, 0.5, 1,20, 40, 72, 350, 500 and 1000 hours, from top to bottom curves) (b) Therelative emission spectra of the liquid-type quantum dot white light LEDstructure with various life times (0, 24, 48, 72, 350, 500 and 1000 hours,from top to bottom curves). The life time character of the (c) relativeluminous efficiency and (d) CRI of remote quantum dot and liquid-typequantum dot structure white LEDs from 0 hours to 1000 hours.

Fig. 8 (a) The color shift character of the remote quantum dot struc-ture white LED from 0 hours to 1000 hours. (b) The color shift characterof the liquid-type quantum dot structure white LED from 0 hours to1000 hours.

Table 2 The comparison of the linewidths and peaks among different QDs and elapsed time

Parameters Elapsed time QD1 QD2 QD3 QD4 QD5

Linewidth (nm) 0th hour 22.67 45.94 44.67 26.84 24.351000th hour 23.51 46.99 43.88 26.84 25.21

Peak (nm) 0th hour 451.93 484.72 541.27 585.10 627.211000th hour 451.46 485.36 542.25 585.13 627.40

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dering index light source. We use a glass-slide-formed cavity toprotect the quantum dots from environmental influences. Theliquid-type quantum dot structure can maintain the quantumdot efficiency and reduce the thermal effect. This design canmodify the emission spectrum easily and enhance thequantum yield of the QDs. The finished WLED, when pumpedwith UV light, can achieve a LER of 271 lm Wop

−1 with a 5%decrease after 1000 hours of storage. We believe that theliquid-type quantum dot WLED with high luminous efficiencyis suitable for various high quality lighting applications in thenear future.

Acknowledgements

This work was funded by the National Science Council inTaiwan under grant numbers MOST 101-2221-E-009-046-MY3,MOST104-3113-E-009-002-CC2, MOST102-2221-E-009-131-MY3,and MOST103-2622-E-009-008-CC3.

Notes and references

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