Open Access
Add to BibSonomyAbstract
We proposed an optical simulation model for the quantum dot (QD) nanophosphor based on the mean free path concept to understand precisely the optical performance of optoelectronic devices. A measurement methodology was also developed to get the desired optical characteristics such as the mean free path and absorption spectra for QD nanophosphors which are to be incorporated into the simulation. The simulation results for QD-based white LED and OLED displays show good agreement with the experimental values from the fabricated devices in terms of spectral power distribution, chromaticity coordinate, CCT, and CRI. The proposed simulation model and measurement methodology can be applied easily to the design of lots of optoelectronics devices using QD nanophosphors to obtain high efficiency and the desired color characteristics.
© 2017 Optical Society of America
1. Introduction
Semiconductor colloidal quantum dots (QDs), one of the alternatives to general phosphors, exhibit favorable properties such as easy processing, tunable emission wavelength, narrow bandwidth emission, and reasonable quantum efficiency [1–3]. Because of their outstanding properties, QDs are used in many electroluminescence and photoluminescence devices, such as displays, lighting, solar cells, and biosensors, for their improved optical performance. When QDs are used as a color converter to make white LEDs (QD-WLEDs), the emission spectrum can easily be tuned and optimized due to their narrow bandwidth of emission spectra. One of the first demonstrations of the use of color converting QDs on LEDs employed CdSe/ZnS QDs on an InGaN/GaN quantum-well blue LED [4]. It was reported that the QD-WLEDs reached a CRI (color rendering index) of 81 while having an LER (luminous efficacy of optical radiation) of 323 lm/Wopt and a CCT (correlated color temperature) of 3190 K. The use of QD nanophosphor as a color converting phosphor based on the quantum confinement effect was also reported to improve the luminous efficiency, CRI, and CCT value for a white LED package [5–8]. In addition, several approaches have been proposed to enlarge the color gamut of LCD displays. Narrowing the bandwidth of color filters would lead to a larger color gamut, but the transmittance would be significantly reduced [9]. Thus some backlight technologies have been developed for high color gamut display with less reduced optical efficiency using QDs where a blue LED excites the green and red light using QDs to make white light [10–15]. The narrow bandwidth of QD emission enables the reproduction of high purity color in the display application. Recently, a QD-dispersed photoresist (PR) film was also applied to the white organic light-emitting diodes (OLEDs) to improve the optical efficiency where QDs absorb unnecessary blue and green light, which is cut by a color filter from the white OLED, and convert to green or red light [16].
To achieve high performance lighting and displays, lots of photometry studies on QD-based LEDs and backlight technologies have been reported [16–21]. In the case of LEDs, there are three performance metrics, i.e., CRI, CCT, and LER. By changing the peak emission wavelength, the full-width at half-maximum (FWHM), and the relative amplitude of each QD color component, spectral power distributions of QD-WLEDs were investigated and optimized [17–19]. In the case of the backlight, the efficiency and color gamut of the display for blue LED-pumped red and green QDs were analyzed [20, 21]. The optimal emission spectra of QDs are obtained for a wide color gamut of LCD display. While these researches provided an important guideline for better properties of LEDs and displays with QD nanophosphors, they could not predict the optical performance of QDs exactly to optimize their physical characteristics, i.e., type, size, concentration, and arrangement.
In the case of conventional inorganic phosphors of micro-scale size, various studies on optical modeling for the phosphor-converted white LED package have been published [22–25]. However, to the best of our knowledge, there is still no analogous optical simulation model for the QD nanophosphors to expect the properties exactly in many QD-based optoelectronic devices, including LEDs and displays. Also, since the dispersion and interaction characteristics of QD nanoparticles in polymer matrix are different from the case of conventional micro-size phosphors, conventional phosphor models cannot be directly applied to the QD nanophosphor simulation [26–28]. Thus, we propose new optical simulation model for the QD nanophosphors, which accurately predicts the optical performance of QDs in the real application such as the displays and white LEDs. We also develop the measurement methodology of QD characteristics which is considering the actual dispersion state in the polymer matrix to be incorporated into the proposed simulation model. Finally, the white OLED and LEDs based on the QD film of relatively high concentration were fabricated and evaluated to compare the simulation and experimental data to validate the proposed simulation model and methodology.
2. Optical modeling of QD nanophosphors
Figure 1 describes the optical interaction of light with QD nanoparticles in the QD-dispersed polymer film. First, blue rays emitted from LED chip enter the QD film to meet some QD nanoparticles. Some rays may not strike any QD nanoparticles and exit directly through the QD film; these are defined as a transmitted rays. When incident rays strike QD nanoparticles, they are absorbed or scattered or transmitted depending on the absorption spectra. The absorbed rays can be used as excitation energy of the QD to be converted to longer wavelength rays and/or transferred to heat. The wavelength of the converted rays is determined by the emission spectra of QD nanoparticles with a characteristic of isotropic emission. Since some of the encountered rays are not absorbed but scattered randomly, the scattered direction can be determined by the measured scattering data to be used in the simulation model. For a precise QD nanophosphor simulation, optical parameters such as the mean free path, absorption spectra, emission spectra, quantum yield, and scattering distribution must be measured accurately in experimental.
Fig. 1 The simulation flow for optical interaction of light with QD nanoparticles in the polymer matrix.
2.1 Mean free path
The mean free path is defined as an average distance that a ray travels inside the QD film before striking a QD nanoparticle. In the general optical simulation of micro-size phosphor, the mean free path of phosphors can be calculated theoretically as a relative distance between micro sized particles. In this case, it is assumed that the phosphor particles are spherical and have a homogeneous distribution in the polymer matrix. In the case of QD nanoparticles, however the distribution of QDs is not uniform in the polymer matrix [26, 27]. Thus we cannot use the theoretical value of mean free path to simulate the optical characteristics of QD nanoparticles. For the exact value of mean free path of QD nanoparticles, a novel measurement methodology is proposed in this study after considering the actual dispersion state of QDs in the polymer.
To measure the real mean free path of QDs in the polymer matrix, we used the Beer-Lambert law, which relates the attenuation of light to the absorption property of the material through which the light is traveling. Assuming that the ray travels an infinitesimally thin slab with QD nanoparticles, the probability that a ray strikes the QD nanoparticles will be defined as the total cross-sectional area of the particles divided by the total area of the slab:
where σ is the effective cross-sectional area for collision of one particle, and N is the number of dispersed particles per unit volume, and A is the area of the slab, and dx is the thickness of slab. Expressing the number of rays stroked by the QD nanoparticles in slab as dIx, and the total number of rays incident on the slab as Ix, the number of rays absorbed or scattered by the QD nanoparticles in slab is given by,
Note that dIx is negative since only fewer rays can pass through the slab than the incident rays. The solution to this simple differential equation can be obtained by integrating both sides to obtain Ix as a function of x:
Using the Eq. (4), the intensity difference between I0 at x = 0 and I1 at x = t can be written as,
and
At Eq. (1), when the ray travels a distance of a mean free path, l in the slab, dx = l, the probability of striking to particles will be 1. Therefore, a mean free path can be expressed as follows:
So, Eq. (6) can be written as,
Finally, the mean free path of QD nanoparticles in polymer matrix can be expressed as,
where t is the thickness of QD-dispersed polymer film and T is the transmittance of QD-dispersed polymer film.
To acquire the mean free path from various QD-dispersed polymer films having a range of concentrations of QDs, the transmittance and thickness of QD films must be measured as a function of QD concentrations. In this measurement, two different polymers were used to disperse QD nanoparticles in the film, UV curable resin and photoresist. The thickness of UV curable polymer was around 100μm after preparing by blade coating while it was kept around 2μm for photoresist after preparing by spin coating. From the experimental measurement data, the mean free paths of QD nanoparticles were calculated and summarized in Table 1. The transmittance of the QD films was measured with a relatively long distance between the QD film and the detector to reduce the error caused by the scattered light from the QDs and detect only the propagated light after striking QD nanoparticles.
Table 1. Mean free path calculated from the experimental data for red/green QDs in UV curable polymer and photoresist
Figure 2 shows the relationship between the mean free path for the QDs and the inverse concentration in a UV curable polymer Fig. 2(a) and a photoresist Fig. 2(b). The mean free paths for both red and green QDs are proportional to the inverse concentration, though the mean free path for red QDs is larger than that for green QDs. The trend-line equations for red and green QDs in the UV curable polymer were y = 46.708x + 0.4402 with a square correlation coefficient of R2 = 0.9976, and y = 36.462x + 1.1218 with R2 = 0.9984, respectively. The square correlation coefficients of both trend lines were close to 1.0000, indicating a good linearity. The trend-line equations for red and green QDs in the photoresist were also shown a similar linearity. From these results, we can calculate the mean free paths of QDs at various concentrations for both polymers, which also depend on the matrix materials for dispersion of QDs. Therefore, the proportionality factor of the trend line equation changes as a function of both the type of QDs and the matrix material for QDs as well as the concentration of QDs.
Fig. 2 The relationship between the mean free path and the inverse concentration of QDs, dispersed in (a) a UV curable polymer and (b) a photoresist.
2.2 Absorption spectra
Rays from an LED chip that strike a QD nanoparticle are absorbed or scattered, depending on the absorption spectrum of QDs. The absorption spectrum for QDs means the fraction of incident light absorbed by the QD nanoparticle over a range of wavelengths from an LED and QD nanoparticle. In the case of QD films, the final absorption spectrum is also dependent on the concentration of QDs, since higher concentration of QDs generally results in more absorption of light and lower quantum efficiency due to some aggregation of QD nanoparticles. It is required to use the absorption spectrum of single QD for the exact simulation; however it is difficult to measure the absorption spectrum directly in the QD film of high concentration. Thus a new measurement method for the absorption spectrum of single QD was proposed and established using the mean free path concept. As mentioned above, the mean free path is the average distance that a ray of light travels through the QD film without striking a QD nanoparticle. In other words, if light enters a QD film having a thickness equal to the mean free path, the ray of incident light strikes a QD nanoparticle once. Thus we can acquire the average absorption spectrum of single QD after measuring it from the QD film having a thickness of the mean free path. This absorption spectrum will be constant even at different concentrations of QDs in the film if the types of matrix polymer and QDs are fixed.
The schematics of the measuring system for the absorption spectrum in the QD film having a thickness equal to the mean free path, using an integrated hemi-sphere, are shown in Fig. 3. There are two steps to acquire the absorption spectrum of single QD from the QD film. First, the QD film is placed on the inlet of the integrated hemisphere to measure the characteristics of forward light during the excitation of QDs through the light irradiation. Second, the film is placed with the light absorber on the inner bottom side of the integrated hemisphere to measure the characteristics of backward light. Through these two experiments, the transmitted, scattered, and emitted light from the QD film can be measured. The light absorbed at the QD film can be calculated and expressed as:
Fig. 3 The schematics of a system for measuring the absorption spectra of QD films with thickness equal to the mean free path using an integrated hemi-sphere.
where Ex is the excitation light, and S is the scattered light, and T is the transmitted light.
Based on the calculated mean free paths for the green and red QDs for the concentrations of 0.2 wt%, 0.6 wt%, and 1 wt% as shown in Table 1, the QD films of thickness corresponding to the mean paths for the respective concentrations were fabricated using UV curable polymer resin. Then the characteristics of forward and backward light were measured by controlling the wavelength of the monochoromatic light source from 400 nm to 600 nm in a 20-nm interval. Figure 4 shows the calculated absorption spectrum using Eq. (10) from the measured data from the QD films having various concentrations of green and red QDs. The absorption spectra from the QD films prepared with the thicknesses of the mean free path showed similar absorption rate to each other; this result means that the QD films prepared in this way resulted in almost same amount of absorption light. It is understood that the absorption spectra for green and red QDs have the highest absorption rate of 0.23 and 0.48 at 400 nm, and slowly decrease to extend to 540 nm and 680 nm, respectively.
Fig. 4 The calculated absorption spectra for green & red QDs with concentrations of 0.2 wt%, 0.6 wt%, and 1.0 wt%: (a) green QDs (b) red QDs.
2.3 Other parameters
The emission spectrum shows wavelength information about the intensity of the light converted by QD nanoparticles. As the concentration of QDs or the optical path increases, the effect of self-absorption increases due to the overlap between absorption and emission spectrum of QDs. Since the QD nanoparticles have a size distribution, relatively larger QDs can absorb the light emitted by relatively smaller QDs in the QD film. Also, the aggregations of QDs due to their poor dispersion in the polymer may cause the Förster resonance energy transfer (FRET). As a result, the QD film of high concentration usually results in a red-shift of the emission spectrum and decreasing the quantum yield [28]. Since the FRET effect cannot be considered in the simulation based on the ray-tracing method, it should be contained to the emission spectrum and quantum yield. Thus, we prepared thin QD film with low concentration to measure the emission spectrum and quantum yield for minimizing the self-absorption effect and containing the FRET effect. The emission spectra and quantum yield are collected by the QE-1000 absolute quantum yield measurement system (Otsuka, Japan). The emission spectra of QDs with low concentrations of 0.1 wt% are measured and shown in Fig. 5(a) for both QDs in toluene solution and in UV curable polymer. It is observed that the emission spectra of the green and red QDs in polymer films resulted in a small amount of red-shift with approximately 6 to 9 nm compared to the emission spectra of the same QDs in toluene solution, in which no FRET occur. In this proposed model, we considered only monodispersed QD film to include the FRET effect. However, in case of blended QD film with different size of QDs, an extra consideration on the FRET effect must be considered to acquire the emission spectrum and quantum yield since the strong FRET effect from the green QDs to red QDs is usually expected in the blended QD films [28]. The quantum yield of red QD with concentration of 0.1wt% dispersed in photoresist and UV curable polymer was measured as 0.72 and 0.68, respectively. The reason for the difference in quantum yield is that the dispersion varies depending on the polymer matrix. The higher quantum yield for same QDs indicate that dispersibility in photoresist is better than that of UV curable polymer. Figure 5(b) shows the measured scattering characteristics of green and red QDs in the film with the concentration of 5 wt% and 10 wt%. The normalized angular intensity by scattering in the QD film with different types and concentrations of QDs are similar to each other.
Fig. 5 (a) Emission spectra for green/red QDs dispersed in toluene solution (solid line) and in a polymer film (dotted line) and (b) Angular distribution of the scattered light for green/red QDs with concentrations of 5 wt% and 10 wt%.
3. Verifications
In order to validate our proposed simulation model for the optical performance of optoelectronic devices using QD nanophosphors, we applied it to white LED and OLED structure. Optical properties such as mean free path, absorption and emission spectrum, scattering distribution and quantum yield were first measured and then used for the QD simulation. From the measured optical characteristics, the QD simulation was basically conducted using a LightTools software based on Monte Carlo ray tracing method. Finally, OLED display and white LED package were fabricated after integrating the QD film to compare the simulated and real measured optical properties from the optoelectronic devices.
3.1 Red QD based OLED
An OLED sample consisting of a white OLED, red color filter, and red QD-dispersed photoresist film, is prepared to understand the accuracy of the proposed simulation model, as shown in Fig. 6(a). This structure was developed to improve the optical intensity in the red color region through the down-conversion of blue and green light to the red light in the white OLED display [16]. The quantum yield for the red QD in the photoresist (PR) film was measured as 0.72 and the spectral intensity distribution of white OLED and the transmittance of red color filter were also measured. A refractive index matching resin was inserted to reduce reflection losses due to an air gap between two elements. Two different QD-dispersed PR films with the concentration of 10 wt% and 30 wt% QD were fabricated with a thickness of 2 μm. A brighter red light from the OLED having a QD-dispersed PR film was confirmed experimentally, as shown in Fig. 6(b) and 6(c) which were prepared without and with the QD-dispersed PR film, respectively.
Fig. 6 (a) Schematic of a white OLED with a QD-dispersed photoresist, and red light emitted from the white OLED (b) without and (c) with the QD-dispersed PR.
To confirm the accuracy of proposed simulation model, we designed the optimum physical parameters for the red QD-dispersed PR film and to measure the optical characteristics after applying to the white OLED. Figure 7 shows the simulation and experimental results for the optical intensity of the OLED structure prepared with red QDs at a concentration of 10 wt% and 30 wt%, respectively. The simulation and experimental results are very similar regardless of the concentration of the QDs. A comparison for the uniform chromaticity coordinates are also summarized in Table 2, showing that the color differences between the simulated and experimental results are within 0.0046. Consequently, this good agreement in both optical intensity and uniform chromaticity coordinate indicates that the proposed simulation model is very accurate to optimize the properties of the QD based white OLED structure.
Fig. 7 Comparison of spectral power distribution between experimental (black solid line) and simulation results (red dots) for QD concentrations of 10 wt% (a) and 30 wt% (b).
Table 2. Comparison of the uniform chromaticity coordinates for QDs based on a white OLED with a red color filter
3.2 Green/red QD based White LED
White LED packages were fabricated with a blue LED chip, a lead frame, an encapsulation resin, and a QD-dispersed film with a layer-by-layer (LBL) structure of separate green and red QD distributions, as shown in Fig. 8(a). The thicknesses of LBL QD polymer film was controlled at 125 μm for green QD layer and 85 μm for red QD layer, by blade coating process using QD dispersed polymer solution having a concentration of 3 wt% for green QDs and 1.5 wt% for red QDs, respectively. In the simulation, a blue LED chip with a peak wavelength of 452 nm was used with Lambertian distribution. The outer size of the lead frame is 5 mm x 5 mm x 1 mm, and the top and bottom diameters of the package cup are 4.41 mm and 3.2 mm, respectively. The refractive index of the UV curable polymer was fixed as 1.53 and the reflectance of the surface of the lead frame was assumed to be 90% with a diffusion reflection. In this case, the refractive index of both green and red QD films was kept as a same value to delete the effect of refractive index deference at the interface. The quantum yield for green and red QD in UV curable polymer was used as a measured value of 0.65 and 0.68, respectively.
Fig. 8 (a) Schematic of LED with a LBL structure of separate QD polymer layers, (b) cross-sectional image of QD polymer layer, and (c) white light emitted from the LEDs with QD-dispersed film.
The spectral intensity distributions for white LEDs based on the QD-dispersed film are shown in Fig. 9. The results show that the simulated spectrum for the QDs based white LEDs is very close to the measured one. A comparison for the other optical performance from the experiment and the simulation is also summarized in Table 3, with small deviations of the CCT and CRI as 187 K and 3.1, respectively. We also confirm that the color differences between the simulated and experimental results are within 0.0061. The simulation results using a proposed simulation model after employing the QD properties obtained by the proposed measurement methodology have a good agreement with the experimental data.
Fig. 9 Comparison of emission spectra between the fabricated sample (solid line) and simulation results (dotted line).
Table 3. Comparison of the optical performances between the fabricated sample and simulation results
4. Conclusions
An optical simulation model for the quantum dot nanophosphor based on the mean free path concept was proposed to understand precisely the optical performance of optoelectronic devices and validated by the comparison with the fabricated both white LEDs and an OLED structure. A measurement methodology was also developed to get the desired optical characteristics such as the mean free path, absorption spectra for QD nanophosphors which are to be incorporated into the simulation model. The mean free path according to the QD concentration was calculated using a Beer-Lambert law after confirming the proportional relationship between the inverse concentration and the mean free path. The absorption spectra of the QDs were determined by measuring the forward and backward light of samples prepared with a thickness equal to the mean free path. The simulation results show good agreement with the measured optical performance such as spectral intensity distribution, the chromaticity coordinates, CCT, and CRI in both white LED packages and OLED structure prepared with the QD-dispersed film. Thus, we believe that the proposed simulation model using optical characteristics obtained by the proposed measurement methodology can be easily applied to the optimum design for lots of optoelectronic devices with QD nanophosphors to realize high optical intensity and the desired color characteristics.
Funding
Ministry of Trade, Industry and Energy (10042178); Ministry of Science, ICT and Future Planning (2016R1A2B4008869); Yonsei University (2015-12-0220).
References and links
1. J. Lee, V. C. Sundar, J. R. Heine, M. G. Bawendi, and K. F. Jensen, “Full color emission from II-VI semiconductor quantum dot-polymer composites,” Adv. Mater. 12(15), 1102–1105 (2000). [CrossRef]
2. P. Reiss, J. Bleuse, and A. Pron, “Highly luminescent CdSe/ZnSe core/shell nanocrystals of low size dispersion,” Nano Lett. 2(7), 781–784 (2002). [CrossRef]
3. R. Xie, D. Battaglia, and X. Peng, “Colloidal InP nanocrystals as efficient emitters covering blue to near-infrared,” J. Am. Chem. Soc. 129(50), 15432–15433 (2007). [CrossRef] [PubMed]
4. S. Nizamoglu, T. Ozel, E. Sari, and H. V. Demir, “White light generation using CdSe/ZnS core-shell nanocrystals hybridized with InGaN/GaN light emitting diodes,” Nanotechnology 18(6), 065709 (2007). [CrossRef]
5. S. Nizamoglu, T. Erdem, X. W. Sun, and H. V. Demir, “Warm-white light-emitting diodes integrated with colloidal quantum dots for high luminous efficacy and color rendering,” Opt. Lett. 35(20), 3372–3374 (2010). [CrossRef] [PubMed]
6. S. Nizamoglu, T. Erdem, and H. V. Demir, “High scotopic/photopic ratio white-light-emitting diodes integrated with semiconductor nanophosphors of colloidal quantum dots,” Opt. Lett. 36(10), 1893–1895 (2011). [CrossRef] [PubMed]
7. M.-H. Shin, H.-G. Hong, H.-J. Kim, and Y.-J. Kim, “Enhancement of optical extraction efficiency in white LED package with quantum dot phosphors and air-gap structure,” Appl. Phys. Express 7(5), 052101 (2014). [CrossRef]
8. K.-J. Chen, Y.-C. Lai, B.-C. Lin, C.-C. Lin, S.-H. Chiu, Z.-Y. Tu, M.-H. Shih, P. Yu, P.-T. Lee, and X. Li, H.-F. MG.-C Chi, T.-M Chen, and H.-C Kuo, “Efficient hybrid white light-emitting diodes by organic-inorganic materials at different CCT from 3000K to 9000K,” Opt. Express 23, A201–A210 (2015).
9. S. H. Ji, H. C. Lee, J. M. Yoon, J. C. Lim, M. Jun, and E. Yeo, “J. C. Lim, M. Jun, and E. Yeo, “P.91: adobe RGB LCD monitor with 3 primary colors by deep green color filter technology,” Dig. Tech. Pap. 44(1), 1332–1334 (2013). [CrossRef]
10. E. Jang, S. Jun, H. Jang, J. Lim, B. Kim, and Y. Kim, “White-light-emitting diodes with quantum dot color converters for display backlights,” Adv. Mater. 22(28), 3076–3080 (2010). [CrossRef] [PubMed]
11. S. Coe-Sullivan, W. Liu, P. Allen, and J. S. Steckel, “Quantum dots for LED downconversion in display applications,” J. Solid State Sci. Technol. 2(2), R3026 (2012). [CrossRef]
12. Z. Luo, Y. Chen, and S.-T. Wu, “Wide color gamut LCD with a quantum dot backlight,” Opt. Express 21(22), 26269–26284 (2013). [CrossRef] [PubMed]
13. Z. Luo, D. Xu, and S.-T. Wu, “Emerging quantum-dots-enhanced LCDs,” J. Disp. Technol. 10, 526–539 (2014). [CrossRef]
14. C. B. J. Steckel, W. Liu, J. Xi, C. Hamilton, and S. Coe-Sullivan, “Quantum dots: The ultimate down-conversion material for LCDs,” Dig. Tech. Pap. 45, 294 (2014). [CrossRef]
15. J. H. Oh, K.-H. Lee, H. C. Yoon, H. Yang, and Y. R. Do, “Color-by-blue display using blue quantum dot light-emitting diodes and green/red color converting phosphors,” Opt. Express 22(S2), A511–A520 (2014). [CrossRef] [PubMed]
16. H.-J. Kim, M.-H. Shin, H.-G. Hong, B.-S. Song, S.-K. Kim, W.-H. Koo, J.-G. Yoon, S.-Y. Yoon, and Y.-J. Kim, “Enhancement of optical efficiency in white OLED display using the patterned photoresist film dispersed white quantum dot nanocrystals,” J. Disp. Technol. 12(6), 526–531 (2016). [CrossRef]
17. T. Erdem, S. Nizamoglu, X. W. Sun, and H. V. Demir, “A photometric investigation of ultra-efficient LEDs with high color rendering index and high luminous efficacy employing nanocrystal quantum dot luminophores,” Opt. Express 18(1), 340–347 (2010). [CrossRef] [PubMed]
18. T. Erdem, S. Nizamoglu, and H. V. Demir, “Computational study of power conversion and luminous efficiency performance for semiconductor quantum dot nanophosphors on light-emitting diodes,” Opt. Express 20(3), 3275–3295 (2012). [CrossRef] [PubMed]
19. P. Zhong, G. He, and M. Zhang, “Optimal spectra of white light-emitting diodes using quantum dot nanophosphors,” Opt. Express 20(8), 9122–9134 (2012). [CrossRef] [PubMed]
20. Z. Luo, Y. Chen, and S. T. Wu, “Wide color gamut LCD with a quantum dot backlight,” Opt. Express 21(22), 26269–26284 (2013). [CrossRef] [PubMed]
21. K. Masaoka, Y. Nishida, and M. Sugawara, “Designing display primaries with currently available light sources for UHDTV wide-gamut system colorimetry,” Opt. Express 22(16), 19069–19077 (2014). [CrossRef] [PubMed]
22. C.-C. Sun, C.-Y. Chen, H.-Y. He, C.-C. Chen, W.-T. Chien, T.-X. Lee, and T.-H. Yang, “Precise optical modeling for silicate-based white LEDs,” Opt. Express 16(24), 20060–20066 (2008). [CrossRef] [PubMed]
23. C.-H. Hung and C. H. Tien, “Phosphor-converted LED modeling by bidirectional photometric data,” Opt. Express 18(S3Suppl 3), A261–A271 (2010). [CrossRef] [PubMed]
24. S.-L. Hsiao, N.-C. Hu, and H. Cornelissen, “Phosphor-converted LED modeling using near-field chromatic luminance data,” Opt. Express 21(S2Suppl 2), A250–A261 (2013). [CrossRef] [PubMed]
25. S.-W. Jeon, J. H. Noh, K. H. Kim, W. H. Kim, C. Yun, S. B. Song, and J. P. Kim, “Improvement of phosphor modeling based on the absorption of Stokes shifted light by a phosphor,” Opt. Express 22(S5Suppl 5), A1237–A1242 (2014). [CrossRef] [PubMed]
26. O. O. Matvienko, Y. N. Savin, A. S. Kryzhanovska, O. M. Vovk, M. V. Dobrotvorska, N. V. Pogorelova, and V. V. Vashchenko, “Dispersion and aggregation of quantum dots in polymer-inorganic hybrid films,” Thin Solid Films 537, 226–230 (2013). [CrossRef]
27. Y.-S. Seo, C. J. Raj, D.-J. Kim, B. C. Kim, and K.-H. Yu, “Effect of CdSe/ZnS quantum dots dispersion in silicone based polymeric fluids,” Mater. Lett. 130, 43–47 (2014). [CrossRef]
28. C.-J. Chen, C.-C. Lin, J.-Y. Lien, S.-L. Wang, and R.-K. Chiang, “Preparation of quantum dot/polymer light conversion films with alleviated Förster resonance energy transfer redshift,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(1), 196–203 (2015). [CrossRef]
