Optical Properties and Reliability Studies of Gradient Alloyed Green

Nov 9, 2017 - Luminescent materials become one of interesting issues for white LED-based lighting devices (WLEDs) due to their high performances for c...
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Optical Properties and Reliability Studies of Gradient Alloyed Green Emitting (CdSe)x(ZnS)1−x and Red Emitting (CuInS2)x(ZnS)1−x Quantum Dots for White Light-Emitting Diodes Rachod Boonsin,*,† Anthony Barros,† Florian Donat,‡ Damien Boyer,† Geneviève Chadeyron,*,† Raphael̈ Schneider,‡ Philippe Boutinaud,† and Rachid Mahiou† †

Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand (ICCF), 63000 Clermont-Ferrand, France ‡ Laboratoire Réactions et Génie des Procédés (LRGP), UMR 7274, Université de Lorraine, CNRS, 54001 Nancy Cedex, France S Supporting Information *

ABSTRACT: Luminescent materials become one of interesting issues for white LED-based lighting devices (WLEDs) due to their high performances for converting the monochromatic light from UV/blue LED chips into white light. Nevertheless, the availability of rare earth materials, the low color rendering index (CRI) and too-cold color temperature of the white LEDs remain some drawbacks to penetrate into the general LED lighting markets. Herein, we report the development of rare-earth-free luminescent nanocomposites combined with a UV/blue LED chip in order to provide white light. Gradient alloyed cadmium selenide/zinc sulfide (CdSe)x(ZnS)1−x and copper indium sulfide/zinc sulfide (CuInS2)x(ZnS)1−x quantum dots have been used to achieve the luminescent nanocomposite films in silicone as polymer matrix. The optical performances were investigated upon LED excitation. The photometric parameters of systems consisting of luminescent nanocomposites and LEDs including color rendering index (CRI), correlated color temperature (CCT), Commission Internationale de l’Eclairage (CIE) chromaticity coordinates, and luminous efficacy can be tuned by controlling the compositions and thickness of nanocomposite films. Furthermore, the thermal stability and the reliability of these luminescent nanocomposite films were investigated. KEYWORDS: CdSe, CuInS2, quantum dots, composite films, reliability, light-emitting diodes

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layer between the hole-transport layer (CBP, poly-TPD) and an electron-transport layer (Alq3)17−19 or ZnO20,21 for white QDs LEDs in order to improve the electroluminescent efficiency and the luminance, but the luminous efficiency of the system was quite low (up to 1.5 lm/W) for lighting applications. Since recently, some core/shell QDs have also been investigated as light converters for lighting applications in combination with UV/blue commercial light-emitting diodes (LEDs).22,23 Numerous studies demonstrated that the most common fabrication of QDs combined with a LED chip for the LEDs packaging is to prepare the homogeneous QDs-polymer hybrid material by dispersing QDs in the polymer matrix.24,25 The association of highly efficient green and red light-emitting core/shell CdSe/ZnS QDs was used in the fabrication of white light emitters under excitation at 445 nm of blue InGaN LEDs.26 The multishell-structured green CdSe/ZnS/CdSZnS QDs and red CdSe/CdS/ZnS/CdSZnS QDs were synthesized and coated in direct contact with blue LEDs for white QD-LED backlight showing high luminous efficacy of 41 lm/W, CIE coordinates of (0.24, 0.21) and good external quantum efficiencies

emiconductor quantum dots (QDs) have gained great fundamental interest in a wide range of applications due to their attractive optical properties, such as broad absorption, narrow emission band, and highly luminescent efficiency.1−3 The absorption and emission characteristics of QDs can be tuned with respect to their size and chemical composition, which is related to their bandgap energy.4 In order to stabilize and enhance the photoluminescence, a wider bandgap material such as ZnS5 or CdS6 is generally used as shell for insulating the inner core structure. The ions composing the shell may diffuse in the core during the introduction of the shell yielding gradient alloyed QDs which generally exhibit a higher photochemical stability than conventional core/shell QDs.7 The shell at the periphery is advantageous for lighting applications in order to prevent contact with moisture and oxygen, which plays an important role in the degradation of QDs.8,9 White light-emitting diodes (WLEDs) have become alternative lightings due to their excellent features such as low power consumption, small packaging, and long lifetime over obsolete technologies.10−13 So far, the most common WLEDs consist of a blue-emitting InGaN LED combined with Ce-doped yttrium aluminum garnet (YAG: Ce) as a light converting material.14−16 The core/shell CdSe/ZnS QDs can be incorporated as emissive © XXXX American Chemical Society

Received: August 29, 2017

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DOI: 10.1021/acsphotonics.7b00980 ACS Photonics XXXX, XXX, XXX−XXX

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(EQEs) over long-term operation.27 Previously, several kinds of CdSe/ZnS nanocrystals have been combined with blue LEDs in order to obtain the desired optical parameters for white light.28 However, the color rendering index (CRI) of the system was still low and needed to be improved.28 Another type of core/ shell quantum dots called CuInS2/ZnS QDs were also studied for lighting applications because these cadmium-free materials are eco-friendly.29,30 The red-emitting alloyed (CuInS2)x(ZnS)1−x QDs have been used in order to improve the weak red emission in YAG:Ce phosphor and achieve the high color rendering index (91) of white light.31,32 Recently, Zn−Ag−In-S and Zn-Cu-In-S alloyed QDs incorporated with a 445 nm blue LED have been demonstrated to provide a moderate luminous efficacy of 31.2 lm/W with very high optical parameters (CRI solely = 97 and color temperature range of 2700−10000 K).33 Finally, the combination of CuInS2/ZnS QDs with polymer nanofibers has been investigated and the association of these materials with a blue LED can generate a white light with moderate luminous efficacy (11.5−32.3 lm/W) and high CRI values.34 However, the optical stability over the LEDs operating conditions is the step necessary to take the investigation. For lighting applications, the reliability and lifetime of light converting materials must be considered because LEDs are electronic components which produce heat.35 Therefore, it is also important to study the optical properties of QDs in order to ensure their stability over the LEDs operating conditions. In this study, we have investigated the photoluminescence properties of QDs composite films obtained by incorporating gradient alloyed (CdSe)x(ZnS)1−x and (CuInS2)x(ZnS)1−x QDs into the two-component silicone resin as a polymer matrix. The silicone resin is widely used for LED packaging due to its flexibility, free-standing material, high transparency over

UV−visible range, high thermal stability and compatibility with several organic solvents. The combination of mixed QDs composite films with a UV/blue LED to perform trials in remote phosphor configuration for white light emission was investigated as well as their optical and photometric parameters such as correlated color temperature (CCT) and color rendering index (CRI). The remote phosphor configuration were selected for the fabrication of WLEDs instead of conventional configuration (phosphors coated in direct contact with LEDs) in order to avoid high junction temperature generated from the LED chips (up to 150 °C) and allow the QDs composite films to work at much lower temperature. The reliability of QDs composite films was evaluated by collecting the photoluminescence (PL) intensity upon LED excitation at 375 or 450 nm, for different irradiation powers and different temperatures. The obtained behaviors are discussed.

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RESULTS AND DISCUSSION QDs Optical Characterization. The absorption and PL emission spectra of (CdSe)x(ZnS)1−x and (CuInS2)x(ZnS)1−x QDs dispersed in toluene and in silicone films are shown in Figure 1. The first exciton absorption of (CdSe)x(ZnS)1−x QDs is found at 489 nm, which gives a bandgap energy of approximately 2.54 eV. The absorption signal is much broader in the case of (CuInS2)x(ZnS)1−x QDs, which is typical of quaternary alloyed QDs, preventing the accurate determination of the first exciton peak. Considering that CuInS2 is a direct transition semiconductor,4,36 it is possible to estimate its bandgap energy by plotting the quantity (aℏν)2 against the photon energy ℏν, where a is the absorbance, ℏ is the Planck’s constant, and ν is photon frequency. Applying the Tauc method37−39 to the graph (Figure 2), we determined a bandgap energy of 2.22 ± 0.01 eV

Figure 1. UV−visible absorption (dashed) and PL emission spectra (solid) of (a) (CdSe)x(ZnS)1−x QDs and (b) (CuInS2)x(ZnS)1−x QDs dispersed in toluene. (c) 0.5 wt % (CdSe)x(ZnS)1−x QDs and (d) 0.5 wt % (CuInS2)x(ZnS)1−x QDs in silicone polymer. The inset shows the films under day light (left) and upon 365 nm excitation (right). B

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Figure 2. Determination of bandgap energy using the Tauc method for (CuInS2)x(ZnS)1−x QDs in solution. The value can be obtained at the x-intercept.

for (CuInS2)x(ZnS)1−x QDs, which is quite similar to the value obtained for the (CuInS2)x(ZnS)1−x composite film. It should be noted that the bandgap energy of (CdSe) x(ZnS)1−x composite film could not easily determined from the UV absorption spectrum compared with those in solution due to the unclear first exciton absorption. Under 365 nm excitation, the PL spectra consist of a band peaking at 2.41 eV (514 nm) and 2.02 eV (615 nm) for (CdSe)x(ZnS)1−x QDs and (CuInS2)x(ZnS)1−x QDs in toluene, respectively. The absolute PL quantum yields (PL QYs) were found to be 54% for (CdSe) x(ZnS) 1−x and 25% for (CuInS2)x(ZnS)1−x QDs, respectively. The PL QYs dropped to 27% and 18%, respectively, under 450 nm excitation. Incorporation of the QDs in silicone did not modify the position of the PL peak for (CdSe)x(ZnS)1−x QDs but led to a red shift of the PL peak to 1.94 eV (639 nm) in the case of (CuInS2)x(ZnS)1−x QDs. The absolute PL QYs were measured at 21% for the (CdSe)x(ZnS)1−x composite film and 11% for (CuInS2)x(ZnS)1−x composite film under 365 nm excitation, respectively. The PL QYs decreased to 10% and 7%, respectively, under 450 nm excitation. It should be noted that the absolute PL QYs under excitation at 365 nm are higher than those upon 450 nm excitation due to the UV absorption characteristics. Optical Characterization of Mixed QDs Composites. White light was generated by varying the weight ratio of green (G) QDs and red (R)-emitting QDs (sample A to F, as shown in the Figure 3a) in silicone resin, for a total concentration of QDs kept fixed at 0.5 wt %. We show the corresponding results under day light and under 365 nm excitation. This tunable luminescence is represented on the CIE color coordinates of Figure 3b under 365 nm excitation. Samples D and E possess CIE color coordinates close to the blackbody line, corresponding to warm white. Sample D has CIE coordinates of (0.437, 0.432), CRI of 65.5, CCT of 3220 K, and optical luminous efficacy of 45.6 lm/W at 100 mA. Sample E has CIE coordinates of (0.455, 0.413), CRI of 67.2, CCT of 2783 K, and low optical luminous efficacy of 39.8 lm/W at 100 mA. Figure 4 shows the PL spectra and the corresponding CIE chromaticity coordinates (x,y) in the red region of monolayered mixed QDs composite film containing larger amounts of QDs (3 and 7 wt %, respectively). The data were collected upon 365 nm LED excitation with different forward currents.

Figure 3. (a) Images of QDs composite films when varying the weight ratio of green-emitting and red-emitting QDs in silicone resin under day light (top) and under excitation at 365 nm UV lamp (bottom). (b) CIE chromaticity coordinates of QDs composite films (0.5 wt %) under 365 nm excitation.

The photometric parameters are compiled in Table 1. We note significant increase of the luminous efficacy of the films upon increasing the QDs content. The maximum values are obtained for a forward luminous efficacy of the films upon increasing the QDs content. The maximum values are obtained for a forward current of 100 mA. Raising the forward current degrades the efficacy, the color temperature that moves progressively from warmer white to colder white, and the color rendering index. This is the consequence of an increase of LED power, especially regarding the red color emitted by (CuInS2)x(ZnS)1−x QDs that seems to show saturation upon high values of forward current (Figure 5a,c). This aspect will be examined in more details in the following sections. Stability of (CdSe) x (ZnS) 1−x Composite Films. A representative transmission electron micrograph (TEM) of (CdSe)x(ZnS)1−x QDs shows the dots are well-dispersed, of near-spherical shape and that their average diameter is of ca. 6.5 ± 0.7 nm (Figure S1a). First, we have investigated the temperature and atmospheric effects on the luminescence behavior of QDs composite films (for a total concentration of QDs kept fixed at 0.5 wt %) by placing samples in darkness at 85 °C for 72 h either under air or under vacuum. Then, we measured the absorption and PL emission spectra before and after these treatments (Figure 5a and 5b). Only a slight decrease in PL intensity (3%) was observed under excitation at 365 nm, C

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Table 1. Evolution of Performance Parameters of Light (Optical Luminous Efficacy, Correlated Color Temperature (CCT), and Color Rendering Index (CRI) of Monolayer Mixed QDs Film Containing 3 and 7 wt % QDs with Different Forward Currentsa optical luminous efficacy (lm/W)

a

forward current (mA)

3 wt %

100 200 300 400 500

82 76 72 69 67

CCT (K)

7 wt % 3 wt % 136 130 126 124 122

2307 2499 2678 2800 2981

7 wt % 1941 2255 2581 2935 3232

CRI 3 wt % 7 wt % 76.0 73.3 71.7 70.8 70.1

75.0 70.0 67.4 66.6 67.9

Excitation at 365 nm.

only a slight bathochromic shift of the emission spectrum is observed at 22 °C/48 W/m2 (0.12 nm/h). This shift is amplified (0.16 nm/h) under 85 °C/48 W/m2. This red shift could be the consequence of some photoinduced charges at the surface of the QDs40,41 resulting in a quantum-confined Stark effect.40 Stability of (CuInS2)x(ZnS)1−x Composite Films. After dispersion in the silicone matrix, the (CuInS2)x(ZnS)1−x QDs were found to self-associate to form filaments with lengths up to 45 nm (Figure S1b). Similarly to the previous section, the (CuInS2)x(ZnS)1−x composite films were placed in the darkness at 85 °C for 72 h either under air or under vacuum. In this case, we observed a drop of the absorption and of the correlated PL intensity (Figure 5c,d). The decrease appeared much more pronounced under air than under vacuum (∼0 mbar). In contrast to the previous case, no spectral shift of the PL spectrum is evidenced (Figure 7a,b). These results suggest that the physical integrity of the (CuInS2)x(ZnS)1−x QDs is altered in the silicone film by the action of oxygen and/or moisture, both factors being usually involved in photooxidation and/or photobleaching processes.8,9 In other words, some QDs simply “died” in the composite films and do not emit anymore. In conjunction with these results, we show in Figure 7 the time dependence of the PL spectra and of their corresponding integrated areas, upon continuous excitation at 375 nm for temperatures fixed at 22 and 85 °C, respectively, and UV LED power tuned in the range 20−48 W/m2. As previously, we observe a clear depreciation of the luminescence of the (CuInS2)x(ZnS)1−x composite films upon the action of temperature and of the incoming irradiation power. These degradations are presumably caused by the damage to the dodecanethiol (DDT) capping ligand, which generates defects on the surface of QDs42 and quenches the luminescence. The degradation of CuInS2/ZnS QDs may occur by the ligand detachment followed by the oxidation of ZnS shell and CuInS2 core.43 However, the blue-shift of emission spectra due to the reduced size of the core QDs was not observed. Moreover, the self-organization of the dots in the silicone matrix was not altered (Figure S1d). Figure 7c,d reveal two quenching regimes: a fast decay with complex behavior up to ≈10 h, then a single exponential decay that is well reproduced by a first order kinetics of the type:44

Figure 4. PL emission spectra and corresponding CIE color coordinates for different forward currents of monolayer mixed QD composite films containing 3 wt % (a, b) and 7 wt % (c, d) QDs. Excitation at 365 nm.

whatever the composition of the atmosphere. This confirms that the (CdSe)x(ZnS)1−x composite film is stable under these conditions, which is further corroborated by TEM experiments (Figure S1c). Figure 6 shows the time dependence of the PL spectra and of their corresponding integrated areas, upon continuous excitation at 375 nm for temperatures fixed at 22 and 85 °C, respectively, and UV LED power tuned in the range 16−48 W/m2. Graphs (c) and (d) relate to normalized luminescence intensity I(t)/I(t0), where I(t) is the intensity at time t and I(t0) is intensity at the origin of time. These results confirm the good stability of the QDs under these conditions,

I(t )/I(t0) = A ·exp( −k(P , T )t )

(1)

where the intensity ratio I(t)/I(t0) is considered proportional to the surviving rate of QDs. A is a pre-exponential factor and k(P,T) is a degradation rate depending on the temperature T D

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Figure 5. Effect of UV−visible absorption (dash line) and PL spectra (solid line) under darkness condition at 85 °C and 3 days for (a, b) (CdSe)x(ZnS)1−x QDs composite films and (c, d) (CuInS2)x(ZnS)1−x QDs composite films under heating in air (for a, c) and in vacuum (for b, d).

Figure 6. Typical PL spectra of (CdSe)x(ZnS)1−x composite film under continuous excitation at 375 nm for 25 h measured at (a) 22 °C and (b) 85 °C. Normalized PL intensity of (CdSe)x(ZnS)1−x composite film under continuous excitation at 375 nm (c) at different LED power, fixed temperature at 22 °C and (d) at different temperatures, fixed LED power at 48 W/m2. E

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Figure 7. Typical PL spectra of (CuInS2)x(ZnS)1−x composite film under continuous excitation at 375 nm for 25 h measured at (a) 22 °C and (b) 85 °C. Normalized PL intensity of (CuInS2)x(ZnS)1−x composite film (c) at different LED power, fixed temperature at 22 °C and (d) at different temperatures, fixed LED power at 48 W/m2.

(CdSe)x(ZnS)1−x composite film on the reliability and the photostability are under investigation and will be reported in an incoming report.

and on the LED power P. The degradation rate follows the modified Eyring equation:45 k(P , T ) = B ·(P /P0)n ·exp( −Ea /kBT )

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(2)

EXPERIMENTAL SECTION Synthesis of the Blue/Green-Emitting (CdSe)x(ZnS)1−x QDs. (CdSe)x(ZnS)1−x QDs were prepared according to published procedures, with slight modifications.3,19 For a typical preparation of (CdSe)x(ZnS)1−x QDs, cadmium oxide (CdO, 0.2 mmol), zinc acetate (Zn-(OAc)2, 4 mmol), and oleic acid (OA, 5 mL) were mixed in a 50 mL three-necked flask and then heated to 150 °C under argon for 30 min. At this state, the reaction proceeded to form cadmium oleate (Cd-(OA)2) and zinc oleate (Zn-(OA)2). When the solution was clear, 15 mL of 1-octadecene (ODE) were added to the reaction flask and the mixture was heated to 300 °C under argon. At this temperature, selenium (Se, 0.2 mmol) and sulfur (S, 4 mmol) dissolved in trioctylphosphine (2 mL) were quickly injected into the reaction flask and the mixture was kept at 300 °C for 8 min. 1-DDT (0.5 mL) used as capping ligand was then added. After stirring for 10 min, the reaction mixture was allowed to cool down to room temperature. Finally, the resulting (CdSe)x(ZnS)1−x QDs were purified by precipitation with excess acetone and dried under vacuum before further characterizations. The amount of purified (CdSe)x(ZnS)1−x QDs obtained is about 80 mg. Synthesis of the Red-Emitting (CuInS2)x(ZnS)1−x QDs. The synthesis of (CuInS2)x(ZnS)1−x QDs was conducted according previous reports.19,46 Step I: Preparation of CuInS2 Cores. Copper iodide (CuI, 0.14 mmol), indium acetate (In-(OAc)3, 0.2 mmol), and ODE (16 mL) were loaded into a three-necked flask under vacuum. The reaction mixture was heated to 80 °C for 20 min in order to remove the traces of water and oxygen from the

where B is a pre-exponential factor, P/P0 is the variation of LED power with respect to a reference power P0 (20 W/m2), n is the constant factor, Ea is the activation energy (eV) of degradation reaction, kB is the Boltzmann gas constant (eV/K), and T is the absolute temperature (K). From Figure 7d and eq 1, the temperature dependence of the degradation rate at a LED power of 48 W/m2 was obtained. Using eq 2, an activation energy Ea = 0.23 eV was determined. The power dependence of the degradation rate at 22 °C is obtained from Figure 7c and eq 1. Using eq 2, we obtain a constant factor of 0.34 under irradiation at 375 nm. According to literature,44 the constant factor n of 0.2 and the activation energy (Ea) of 0.31 eV have been reported for the remote phosphor configuration consisting of YAG: Ce powder and nitride red phosphor (CaSn-Eu) under 450 nm blue light source.

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CONCLUSION In summary, we have investigated the optical properties of gradient alloyed (CdSe)x(ZnS)1−x and (CuInS2)x(ZnS)1−x QDs. The optical properties of composite films based on each QDs were also studied. The combination of both QDs revealed that it is possible to produce a warm white light forcombination with UV commercial LED. However, the reliability studies showed the great influence of LED intensity and temperature on the photoluminescence properties of (CuInS2)x(ZnS)1−x composite film, which could limit its performance for combining with commercial LEDs. The (CdSe)x(ZnS)1−x composite film presented a good reliability for short observation times. The long-termed study of F

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perature was adjusted at 22, 45, and 85 °C, respectively. The emission spectra of the QDs composite films were collected every 30 mi during 25 h. Their area was integrated to obtain the total emission intensity for each selected set of temperature and irradiation power. The LED power was measured using a Scientech Model Mentor MA 10 with a MC2501 calorimetric head unit (25.4 mm aperture). The measurement was performed by centering the head unit over the LED source and measuring the LED power of light emitted through the aperture. The LED powers were 1.2 mW for UV @ 375 nm and 183 mW for blue @ 450 nm, respectively. The power density of LED can be expressed in W/m2 and was calculated by LED power (in watt) per unit surface of sample (∼0.25 cm2). According to our experiment, the power density of LED would be 48 W/m2 for UV @ 375 nm and 7320 W/m2 for blue @ 450 nm, respectively. TEM images were recorded using a Jeol ARM 200F instrument operating at 200 kV.

reaction flask. Then the reaction mixture was filled with argon. 1-DDT (8.3 mmol) was injected and the mixture was heated to 210 °C for 20 min. The CuInS2 QDs obtained were used in the following step without purification. Step II: Preparation of Alloyed (CuInS2)x(ZnS)1−x QDs. A solution of zinc acetate (Zn-(OAc)2, 2.2 mmol), ODE (12 mmol), and oleylamine (OA, 5 mmol) was prepared under an inert atmosphere. After 20 min of CuInS2 QDs growth, 0.5 mL of the aforementioned solution was injected. Similar injections were repeated each 15 min during 135 min. Afterward, the reaction mixture was allowed to cool down to room temperature. The (CuInS2)x(ZnS)1−x QDs were separated by precipitation with absolute ethanol. The precipitate was collected by centrifugation, the solid was redispersed in toluene and reprecipitated with absolute ethanol. The centrifugation and the precipitation steps were repeated 8× for the purification of (CuInS2)x(ZnS)1−x QDs. Finally, the dots were dried under vacuum before further characterizations. The amount of purified (CuInS2)x(ZnS)1−x QDs obtained is about 100 mg. Preparation of QDs/Silicone Composite Films. The free-standing QDs/silicone composite films were obtained as follow. First, the mixture was prepared by dispersing the QDs in a small quantity of toluene before adding the silicone resin/curing agent (Silicone Bluesil RTV 141 A&B). For the mixed QDs, the ratios of each QDs were calculated using each emission spectra via LED color calculator. Second, the mixture was stirred at room temperature in order to partially evaporate the solvent and then the mixture was mixed using a mechanical mixer to achieve homogeneous particles/silicone mixture. Next, the QDs/silicone composite films were prepared by casting onto a Teflon surface using an Elcometer 4340 automatic film applicator. The blade knife height was fixed at 400 μm and the casting speed was 20 mm.s−1. The composite film was allowed to dry in an oven at 60 °C overnight. Finally, the film thickness at final was measured using an Elcometer 456 coating thickness gauge. The film thickness of the composite films was varied between 200 and 250 μm depending on the loading amounts of quantum dots. Characterization Methods. Quantum yields efficiencies were measured using C9920−02G PL-QY measurement system from Hamamatsu. The setup consisted of a 150 W monochromatized Xe lamp, an integrating sphere (Spectralon coating, ⌀ = 3.3 in.) and a high sensibility CCD camera. Photoluminescence excitation (PLE) spectra were obtained by exciting the composite films from 250 to 500 nm with 5 nm increment and measuring their absolute QY. The absolute PL QYs were calculated by combining the QY values with the absorption coefficient (also measured by the apparatus) to plot the excitation spectra. Commission Internationale de l’Eclairage (CIE) color coordinates and CRI of the QDs/silicone WLEDs were measured at room temperature under different forward currents of 100, 200, 300, 400, and 500 mA (corresponding to applied voltages of 3.5, 3.8, 3.9, 4.1, and 4.2 V, respectively) in an integrating sphere with a diode array rapid analyzer system (GL Optic integrating sphere GLS 500). The thermal treatment of QD composite films was performed in darkness and left in air (Memmert ULE 400) or placed under vacuum oven (Heraeus VTR 5036) at temperature up to 85 °C for 3 days. The reliability studies were carried out using an homemade setup consisting of a power-controlled LED (either UV @ 375 nm or blue @ 450 nm) as excitation source and a HR4000 high resolution spectrometer (Ocean Optics) as PL analyzer. The samples were positioned on a heating element whose tem-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00980. TEM images of (CdSe)x(ZnS)1−x and (CuInS2)x(ZnS)1−x QDs after dispersion in the silicone matrix; TEM images correspond to the same composites after continuous excitation at 375 nm for 25 h at 85 °C (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +33 (0)4 73 40 91 50. E-mail: [email protected]. *Tel.: +33 (0)4 73 40 71 09. E-mail: genevieve.chadeyron@ sigma-clermont.fr. ORCID

Rachod Boonsin: 0000-0003-4579-3115 Raphaël Schneider: 0000-0002-6870-6902 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsphotonics.7b00980 ACS Photonics XXXX, XXX, XXX−XXX

ACS Photonics

Article

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DOI: 10.1021/acsphotonics.7b00980 ACS Photonics XXXX, XXX, XXX−XXX