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Effects of Graphene Monolayer Coating on the OpticalPerformance of Remote Phosphors
M. YAZDAN MEHR,1,2,3 S. VOLGBERT,2 W.D. VAN DRIEL,2
and G.Q. ZHANG2
1.—Materials Engineering Faculty, Isfahan University of Technology, Isfahan, Iran. 2.—EEMCSFaculty, Delft University of Technology, Delft, The Netherlands. 3.—e-mail: [email protected]
A graphene monolayer has been successfully coated on one side of a bisphenol-A-polycarbonate (BPA-PC) plate, used as a substrate for remote phosphorapplications in light-emitting diode (LED)-based products. Using a photoresisttransferring method, graphene sheet has been coated on BPA-PC plates. Theresults show that this graphene monolayer significantly improves the lifetimeand performance of LEDs mainly by protecting them against externaldegradation factors such as moisture and oxygen. Also, LED-based productscomposed of graphene-coated BPA-PC plates exhibit longer stability withcomparatively less loss of luminous efficiency. This method has great potentialto significantly improve the reliability of not only LED-based products but alsomany other microelectronics packaging and components, in which moistureand oxygen are the key causes of failures.
Key words: Light-emitting diode, graphene, remote phosphor, reliability
A solid-state lighting system is composed of anLED chip with electronic driver(s) integrated in apackage that provides optical functions, thermalmanagement and/or other functions. Currently,there are different technologies used to producewhite light high-power LED systems.4–8 The mostcommon one is combining a blue chip with yellowphosphor. In such a system, the phosphor layer canbe either deposited directly on the chip or incorpo-rated into a lens disk.9–13 Moreover, white LEDs aremultipart systems. Apparently, each of these com-ponents can break and induce failure, leading to areduction of light intensity and even early failurebefore the expected lifetime of the instrument.Numerous investigations have been performed onvarious parts of LED-based products; however, notmuch has been done on the lens/remote phosphorsof the LEDs.1–19 Recent reliability studies14–19 haveshown that the optical degradation of white LEDproducts is mainly due to the aging of the
encapsulants/lens. The encapsulants/lens aremainly used to prevent thermal shock and mechan-ical stress as well as humidity-induced corrosion.Both refractive index and good transparency of thelens are other important requirements for illumi-nation enhancement in LEDs. Encapsulants/lensmust also have high thermal conductivity, goodthermal stability, superior chemical resistance, andexcellent protection against moisture and degrada-tion agents in the environment.
In addition, effective heat dissipation is importantto improve the luminous output because the temper-ature of the device is high during operation. This hightemperature of the junction is due to the presence ofphosphor and also prolonged exposure to the light. Tohave LED lighting as a viable lighting source, manytechnical challenges have to be resolved. The lightextraction efficiency and the light output degradationare the key issues, which are related to the packagingmaterials. LEDs have to operate at different temper-atures and humidity environments ranging fromindoor conditions to outdoor climate changes. Mois-ture, ionic contaminants, heat, radiation, andmechanical stresses can be highly detrimental toLEDs, and may lead to device failures. Discoloration(Received December 20, 2016; accepted May 13, 2017;
published online May 26, 2017)
Journal of ELECTRONIC MATERIALS, Vol. 46, No. 10, 2017
DOI: 10.1007/s11664-017-5592-8� 2017 The Author(s). This article is an open access publication
and yellowing are the most common failure mecha-nisms. They lead to a decrease in the transparency ofencapsulants and also a decrease in the LED lightoutput.9
Although various studies13–19 have been per-formed to reduce the thermal and light degradationof bisphenol-A-polycarbonate (BPA-PC), there arestill many challenges to overcome, particularlywhen it comes to the design and production ofencapsulant/lens made by BPA-PC. High heat con-ductivity, as well as light and heat transparency arereally important for encapsulant/lens materials inthe LED package.
Monolayer graphene is a one-atom-thick carbonlayer which has received great attention over thepast few years20–26 because of its high light trans-mittance, great thermal and electrical conductivi-ties, outstanding thermal and photochemicalstabilities, excellent mechanical strength, andexceptional stability against heat and light. Usinga graphene coating, optical and thermal stabilitiesof BPA-PC can be improved. Therefore, thisresearch aims at developing a reliable method forincorporating graphene coating(s) into encapsulant/lens components to produce a more reliable andstable LED-based product.
MATERIALS AND METHODS
Thick BPA-PC plates with industrial purity weremanufactured by injection molding. Plates of1 cm 9 1 cm were then coated with a monolayer ofgraphene using photoresist. Three samples werecoated with graphene and used in the aging tests.The structure of the remote phosphor componenttogether with the schematics of expected function-ality from the graphene coating are shown in Fig. 1.To coat the graphene on the samples, photoresistwas spin-coated on 500 nm Cu at 1600 rpm in 30 s.Graphene was already deposited on the Cu by achemical vapor deposition (CVD) method, in whichit was coated on 500 nm of Cu on 100 nm of SiO2 onSi(100) wafers as substrate. The AIXTRON Black-Magic at 900�C was performed using CH4/H2/Ar asfeedstock with a growth time of 300 s. A gel-packfilm was applied after spin coating and then wetetching was performed by 5% FeCl3 for almost 20 h.The graphene was then stamped to the BPA-PC on ahot plate at 150�C for 5 min. Subsequently, the gel-pack was removed by applying heat. Finally, thephotoresist was dissolved by acetone in 70�C for10 min. The process is schematically shown inFig. 2. The transferring of the graphene and thecoated graphene was performed on SiO2 usingphotoresist. Figure 3 illustrates the Raman spectraof the Cu before and after transferring the grapheneon the SiO2.
Both plates with and without a graphene mono-layer were aged in a furnace at 120�C up to 1500 h.This process was performed to check the effects ofthe graphene monolayer on the chemical and optical
properties of the samples, as well as the reliabilityand lifetime of the BPA-PC. Testing temperaturesfor the aging test was determined in such a way thatthe temperature did not exceed the glass transitiontemperature (Tg) of the plastics. The glass transi-tion temperature of BPA-PC is 150�C, therefore themaximum accelerated temperature was chosenbelow the Tg. All optical and chemical tests on thedegraded specimens were performed at room tem-perature. Infrared spectra of the aged specimenswere also recorded using a Perkin-Elmer Spectrum100 series spectrometer in the attenuated totalreflection (ATR) mode for 200 scans at a resolutionof 4 cm�1. Spectral power distribution (SPD) ofBPA-PC plates and the yellowing index of thethermally-aged plates were also measured by anintegrated sphere. Transmission spectra of BPA-PC,in the range of 300–1200 nm, were recoded with theLambda 950 spectrophotometer (PerkinElmer 950).
RESULTS AND DISCUSSION
High thermal and optical transparencies are themain characterises of lens/substrates in remotephosphor plates. High transmittance of the encap-sulant is one of the most important expectedproperties because it has a direct influence on thepackage efficiency and the reliability of the finalproduct. As the graphene monolayer is very thin(about 0.34 nm), and it has high light transparency,it can be used in an encapsulant/lens with hardlyany adverse influence on the optical properties ofthe BPA-PC plate. The transparency of the BPA-PCsamples with and without graphene was measuredby LAMBDA spectroscopy (shown in Fig. 4). It isobvious that the effect of the graphene on thetransparency of the plate is insignificant, and theBPA-PC coated with graphene has high transmit-tance values over 400–700 nm.
Effective heat dissipation is of crucial significanceto improve the luminous output and discolorationresistance. Heat generation inside the device cancause discoloration and also a reduction in the lightemission and therefore a shorter working lifetime.The monolayer of graphene does not seem todecrease the temperature on the top of the plate,either in a conductive or a convective manner. Thetemperature of the plates was measured by an IRcamera, and thermocouple. It was found that thedifference between the coated and un-coated sam-ples was insignificant. In an attempt to enhance thethermal conductivity of the coating, a multilayercoating of graphene was applied to the plates. Thisdid not really work since the multilayer of grapheneis black and it dramatically decreases the lighttransmission.
Figure 5 illustrates the FTIR-ATR spectra of asample aged at 120�C for 1500 h in both the coatedand non-coated conditions. In this figure, theappearance of the peak at 1840 cm�1 is an indica-tion of thermal oxidation. It has already been
Effects of Graphene Monolayer Coating on the Optical Performance of Remote Phosphors 5867
reported that 1840 cm�1 (cyclic anhydrides)14,16 inthe carbonyl region would appear in all tempera-tures as a result of thermal oxidation. These
oxidation products could significantly contribute tothe discoloration of thermally-aged specimens.Since samples are commercial ones and have differ-ent additives and stabilizers, and the increase in thepeak intensity is not large compared to pure sam-ples.16 However, it is obvious that the rate ofoxidation in samples coated with graphene is com-paratively smaller than that of the samples withoutthe coating. One can therefore conclude that thesamples with the graphene coating are compara-tively less oxidized.
The ability to protect a device from the externalenvironment is essential for long-term operationand high luminous efficiency. Encapsulants/lenshaving poor protective properties are prone to earlyoxidation. The power conversion efficiency of awhite LED (including the encapsulant) as a functionof the graphene layer was studied using moisturetests. The moisture barrier behavior was deter-mined by measuring the weight increase of a 2-mm-thick specimen after being saturated in water at45�C for 100 h (100% RH). It has previously beenreported that, by increasing the exposure time in
Fig. 1. (a) Schematic illustrating the remote phosphor white LED device structure. The remote phosphor is a protective BPA-PC coated by YAG:Celayer designed to convert blue light to yellow light. (b) Schematic summary of the function of graphene monolayer in the remote phosphor.
Fig. 2. Schematic of transferring graphene to the BPA-PC plates.
Yazdan Mehr, Volgbert, van Driel, and Zhang5868
100% humidity condition, the weight of the samplesincreases due to water absorption.24 The amount ofabsorbed water in the polymer is measured by thefollowing equation:
Water Content ¼ ðMt �M0ÞM0
where Mt is the weight of the sample saturated inwater at time t, and M0 is the initial weight of thesample. With increasing the time, the weight ofwater absorption increases in both samples. How-ever, the graphene coating reduces the weight gaincompared to the un-coated samples, as shownFig. 6. A clear difference between the kinetics ofwater uptake in the un-coated and graphene-coatedspecimens is observed, with the latter showing aremarkably slower kinetics. It has been reportedthat graphene is a good barrier against water andgas.26,27 Water molecules cannot easily enterthrough the graphene. As a result, graphene canact as an obstacle against moisture penetration.
Moreover, the optical properties of thermally-aged plates, i.e. luminous flux depreciation, werestudied at room temperature using an integratedsphere. The Commission International de l’Eclai-rage (CIE) system is the common method to char-acterize the composition of any color in terms ofthree primaries.12 (x,y) is usually used to representthe color. To obtain the reasonably equidistantchromaticity scales, which are better than the CIE
Fig. 3. Spectra of transferring graphene to the SiO2 plates (a) Beforetransferring, (b) After transferring by photoresist.
Fig. 4. Effect of graphene on transparency of BPA-PC.
Fig. 5. Oxidation of PC plates with and without graphene.
Effects of Graphene Monolayer Coating on the Optical Performance of Remote Phosphors 5869
1931 diagram, the CIE 1976 uniform chromaticityscale (UCS) diagram was used. This diagram iscalled (u¢,v¢). The (u¢,v¢) coordinates are related tothe (x, y) coordinates by the following equations:
U0 ¼ 4x�2x� 12y� 3
V 0 ¼ 9y�2x� 12y� 3
Du¢v¢, which defines the color shifting at any twopositions (0 and 1), can be calculated using thefollowing formula,
u01 � u00� �
þ v01 � v00� �
Energy Star specifies that color maintenancemust not exceed Du0v0 ¼ 0:007 on the CIE u0v0
diagram, after 6000 h of operation. Color shifting inthermally-aged specimens, calculated by Eq. 3, isshown in Fig. 7. As can be seen in this figure, thecolor shifting of un-coated and graphene-coatedBPA-PC plates is less than the criteria of colorshifting in white light. However, yellowing anddiscoloration of samples coated with graphene arecomparatively lower than for the un-coatedsamples.
The effect of graphene on the thermal aging ofBPA-PC is shown in Fig. 8. Stress at a hightemperature level can cause thermal aging andconsequently depreciation of light output. This isshown in Fig. 8 for the case of thermal aging for anon-coated sample and a coated one. One can conferfrom this figure that the reduction of radiant powerin the coated sample is less than that in the non-coated one.
Luminous flux reduction in both samples is shownin Fig. 9. It can be seen in this figure that grapheneconsiderably improves the optical output of BPA-PC. As explained before, the lifetime of LEDs isdefined as the time to reach 70% of initial lumenoutput based on the ASSIST standard.15 Therefore,the extrapolation of experimental data is needed.Given that the reaction rate is assumed to beconstant at each temperature, a at temperature Tis calculated as follows:
a ðTÞ ¼ � ln ½/ðtÞ�t
To calculate a at each temperature, t is taken equalto the time when lumen decays to 0.9. This t isobtained experimentally. Having the reaction rateat each temperature, the time for 70% lumen decayFig. 6. Water content of PC plates with and without graphene.
Fig. 7. Variation in discoloration of BPA-PC plates with and without graphene.
Yazdan Mehr, Volgbert, van Driel, and Zhang5870
can then be easily calculated. Consequently, thecalculated a can be used to extrapolate the lumendecay until 70% at each temperature (see dashedlines in Fig. 9). Based on the experimental resultsand the extrapolation procedure explained above, itis shown that the time needed to achieve 70% oflumen output is 6500 h for un-coated BPA-PC. Thistime is around 12,000 h for graphene-coated BPA-
PC plates, which would imply that the monolayer ofgraphene can improve the lifetime of the BPA-PC bya factor of 2.
Significant improvements in optical properties aswell as the reliability and lifetime of BPA-PC plates
Fig. 8. Relative radiant power of remote phosphor plates at 120�C.
Fig. 9. Normalized flux of light as a function of aging hours for un-coated and coated samples.
Effects of Graphene Monolayer Coating on the Optical Performance of Remote Phosphors 5871
are observed by applying a graphene layer on theBPA-PC. A graphene monolayer acts as a barrieragainst oxygen and moisture diffusion. The gra-phene-coated PC was then used as lens/substrate inremote phosphor in the LED-based products. Thisresulted in a slower kinetics of oxidation anddiscoloration. The addition of relatively smallamount of graphene can improve lthe ong-termstability and reliability of LEDs.
The authors would like to acknowledge the ‘‘TNOinnovation for life’’ company for SPD measure-ments. They would also like to thank Prof. Pickenfor his useful suggestions and discussions. Theygratefully acknowledge Isfahan University ofTechnology (IUT) and Nokhbegan foundation fortheir support.
This article is distributed under the terms of theCreative Commons Attribution 4.0 InternationalLicense (http://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, andreproduction in any medium, provided you giveappropriate credit to the original author(s) and thesource, provide a link to the Creative Commons li-cense, and indicate if changes were made.
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Effects of Graphene Monolayer Coating on the Optical Performance of Remote PhosphorsAbstractIntroductionMaterials and MethodsResults and DiscussionConclusionsAcknowledgementsReferences