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Abstract
Comprehensive studies were carried out to investigate potential applications of hybrid metal halide perovskites in next-generation white light-emitting diodes (LEDs). We investigated the effect of spectral power distributions on the color quality of white light to provide guidelines for designing white LED devices. The white light was obtained by combining appropriate ratios of blue, green, yellow, and red light emitted from hybrid halide perovskites [MAPb(BrxI1-x)3]. The color characteristics of white light were evaluated by calculating CIE 1931 chromaticity coordinates, correlated color temperature (CCT), general color rendering index (Ra), special color rendering indices (R9-R15), Duv, and luminous efficacy of radiation (LER). The high tunability of CCT from 2298 K (warm white) to 8270 K (cool white) with CRI up to 95.4 has been achieved by tuning the ratios of integrated areas of different emissions with extremely small Duv (0.00002-0.0043) indicating the neutral appearance (not greenish or pinkish) of the obtained white light. High LER above 300 lm/W demonstrates that the high vision performance of white LEDs based on perovskite materials. This work provides strong motivation and guidelines for further developing this type of materials for white LEDs with the goal of realizing widespread adoption of white LEDs in general illumination market.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Light-emitting diodes (LEDs) have been developing for modern lighting and display technology due to their high energy efficacy compared to conventional light sources such as incandescent light bulbs and fluorescence light tubes [1–4]. The development of the highly efficient UV/blue LEDs coupled with the advances in the phosphor materials has directly resulted in the revolution of white light technology. The white LEDs are typically obtained by coating the Ce-doped YAG yellow phosphors on the blue LEDs and the luminous efficacy of these LEDs is high because the portion of light in the green part of the emission spectra overlaps largely with the eye sensitivity curve [7], which covers the spectral range of 370-750 nm and has a maximum at 555 nm. However, the high correlated color temperature (CCT∼6000 K) and the low color rendering index (CRI < 75) are undesirable for indoor lighting applications. The white light with CCT < 4000 K and CRI > 80 is suitable for indoor lighting applications [8,9]. To achieve warm white light (low CCT), additional red phosphor needs to be added, such as the red-emitting quantum dots (QDs) [10]. Alternatively, coating UV LEDs with blue, green, and red phosphors or coating blue LEDs with green and red phosphors is one of the most promising approaches to achieving high-efficiency white LEDs with high-color quality that are suitable for indoor lighting applications [11–15]. However, the limited material availability of the rare-earth elements [16,17] used in the current generation of down conversion phosphors for white LEDs increases the initial cost of white LEDs. Therefore, finding low-cost photon down conversion materials to couple with UV/blue LEDs is critical towards generating cost-effective white light from LEDs. The key for seeking photon down conversion materials is to obtain high quantum yield materials with emission in the blue, green, and red regions while having a narrow full-width at half-maximum (FWHM).
Organic/inorganic metal halide perovskites have attracted tremendous attention in recent years due to the earth-abundant elements, cost-effective production process, and remarkable structural, electrical, and optical properties. The use of these materials in various optoelectronic devices has been extensively studied compared to all other chalcogenides nanocrystals including CdS, CdTe, Cu:Zn-Cd-S, and Cu:ZnS/Zn-Cd-S QDs [5,6,18–21]. Despite their promising optoelectronic properties, the intrinsic toxicity of Cd sheds doubts on the practical applicability of these QDs. In some other report of Cd-free as InP@ZnSeS, and Cu-doped (Cu:Zn-In-Se) [22] or Mn-doped [23] system, the obtained dopant emission wavelength range covers only 540-660 nm or 580-600 nm with an intermediate emission efficiency of 25-30%, demanding the necessity of nanocrystals with a wide emission range feasible for various optoelectronic applications. Previous work showed that perovskite nanocrystals are of extremely high photoluminescence quantum yield (PLQY, above 90%), narrow linewidth emission, and tunable band gap and emission. These materials are suitable candidates to replace expensive rare-earth doped phosphors in conventional white LEDs except for the concerns regarding the toxicity of lead [24–41]. In the view of addressing the hindrance impose by the inclusion of toxic Pb2+ element in the perovskite structure, the synthesis of various lead-free perovskites has been investigated [42,43]. Particularly, the widely studied tin-based halide perovskite exhibits a decrease in the PLQY along with the low stability, and small band gap, intrinsic defects of the nanocrystals compared to the lead-based perovskites, which limited their practical applications [42,43]. The use of halide perovskites in white LEDs has been reported. Li et al. reported that the use of all-inorganic red and green halide perovskite QDs on blue LED chip resulted in tunable CCT between 2500 and 11500 K, and the CIE color coordinates has been optimized to (0.33, 0.30), which are very close to the standard coordinates of white color (0.33, 0.33) [34]. Pathak et al. reported that by stacking green-emitting and red-emitting organic metal halide perovskite thin films over the blue LED chip, white light with CRI of 86 and CCT of 5229 K was obtained [35]. Ma et al. incorporated green-emitting CsPbBr3 perovskites and red-emitting CdSe QDs into the blue LED chip to achieve white light emission with CRI of 89.2 and color coordinates of (0.34, 0.33) [33]. Zhang et al. coated halide perovskite composite films on a UV LED chip and obtained white light with CRI of 85 [32]. Palazon et al. placed all-inorganic halide perovskites over a 365 nm UV LED chip to achieve white light emission with tunable CCT and high stability upon continuous illumination at high power [41]. These studies demonstrate the potential applications of organic/inorganic perovskites in white LEDs, while systematic studies of these materials for white LED applications are needed to find out what is the optimum combination to achieve the best color quality and luminous efficacy.
2. Spectral optimization of white light emission
In this work, comprehensive studies were carried out to investigate the color quality as well as the luminous efficacy of radiation (LER) of white light obtained by combining blue-, green-, yellow-, and red-emitting organic metal halide perovskites in multilayer device structure [31]. The perovskites investigated in this work were synthesized using a modified ligand assisted re-precipitation method as reported in our previous works [44,45]. The blue-green tunable emission was obtained from MAPbBr3 by tuning the ligand concentration. The green-red tunable emission was obtained from MAPb(BrxI1-x)3 through composition engineering. The normalized spectral power distributions of blue (SB), green (SG), yellow (SY), and red (SR) emissions are shown in Fig. 1. The peak emission wavelengths are 457.4 nm, 529.8 nm, 569.3 nm, and 627.8 nm, respectively. The corresponding FWHMs are 17.3 nm, 25.0 nm, 35.2 nm, and 41.4 nm, respectively. The narrow photoluminescence (PL) emission line width due to the uniform distribution of particle size indicates the color purity of the emission and it tends to increase the quality of white light. The calculations of optical parameters were carried out by combining four emission spectra with various ratios by using the following equations:
where, ${S_C}$ is the combined spectral power distribution. The ratios $(B:G:Y:R)$ shown in Table 1 and Table 2 were calculated using the following equation:
Fig. 1. Photoluminescence spectra of blue-, green-, yellow-, and red-emitting organic metal halide perovskites excited by UV light (λ∼400 nm).
Table 1. Color characteristics of selected LEDs with tunable CCT from warm white to cool white.
Table 2. Color characteristics (R9-R15) of selected LEDs.
3. Results and discussion
One of the most important characteristics of light sources for general lighting is color rendering. The estimated CRIs of the spectra obtained by combining four different emissions are shown in Fig. 2. The maximum CRI of 95.4 has been achieved by tuning the power ratio of four emissions to 1: 1.3: 2.5: 5.7. The corresponding CCT is 2509 K (see LED-2 in Table 1), which is warm light. The high CCT and high CRI (> 90) have also been achieved. Overall, the tunable CCT from warm white to cool white has been estimated with higher CRI (> 90). Note that the value for perfect color rendering is 100 [48]. Based on LED color characteristic datasheet published by The U.S. Department of Energy (DOE) [46], a light source with a CRI in the 70s is acceptable for indoor lighting; scores in the 80s is good; scores in 90s is excellent. Therefore, we have obtained LEDs with tunable CCT while having excellent color rendering using organic halide perovskites. The color rendering of white light obtained by using perovskite is comparable or even better than other materials, such as nitride phosphors as reported in the previous work [49–51].
Fig. 2. General CRI (Ra), and CCT of different combinations of blue, green, yellow, and red light. The background color represents the appearance of a blackbody at different temperatures.
The high energy efficiency of white LEDs is one of the primary driving forces to widespread adoption of this technology in the general illumination market. Therefore, LER was also calculated for the LEDs investigated in this work, which was plotted together with CCT and CRI in Fig. 3. Note that the color of points represents the appearance of the light. Our calculated results show that the LER decreases with the increase in CRI which is demonstrated by arrows ‘a’ and ‘c’. CRI and LER are in a trade-off, which is consistent with the previous report [51]. CRI is best achieved by broadband spectra distributed throughout the visible region, while the luminous efficacy is highest with monochromatic radiation at 555 nm [50]. Therefore, a high CRI corresponds to a low LER.
Fig. 3. LER, CRI (Ra), and CCT of different combinations of blue, green, yellow, and red light. The color of points represents the color temperature.
The challenge in obtaining LEDs for illumination purpose is to provide the highest possible energy efficiency while achieving the best color rendering possible. In addition, LER also decreases with an increase in CCT (arrow ‘b’). This is because larger spectra power distribution in the blue region is necessary to obtain high CCT, while the blue component has a very low lumen contribution compared to green and red as we can see from color matching function [52]. The values of the LER we obtained are above 300 lm/W which are superior to the previously reported results on inorganic halide QD-white LEDs, or a combination of a blue chip and RGB CdTe QDs, and phosphor-coated white LEDs [53,54].
The color characteristics of selected LEDs with tunable CCT ranging from 2298 K (warm white) to 8270 K (cool white) are summarized in Table 1. The ratios in this table are the ratios of the integrated area of the emission spectra of an individual color. The R9 to R12 represent four saturated colors (red, yellow, green, and blue). The R13, R14, and R15 represent Caucasian complexion, green leaf, and oriental complexion [55]. The R9 is especially pertinent, as the rendition of saturated red is particularly important for the appearance of skin tones. An R9 score greater than 0 is generally considered acceptable [46]. As seen in Table 2, the R9 is in the range of 82.4-95.9 for the selected nine LEDs. Furthermore, the CRIs (R10-R15) are all relatively high, which means that the color can be precisely reproduced for any color objects by using the white light obtained from organic metal halide perovskites.
As we discussed earlier, color temperature is an important aspect of color appearance related to how “cold’ (bluish) or how “warm” (yellowish) nominally white light appears. By tuning the power ratios of each emission, tunable CCT (from warm white to cool white) has been achieved as seen in Table 1. The power was normalized based on the blue emission. The CCT decreases with the increase in green, yellow, and red components (see Fig. 4). Tunable CCT is especially important for practical applications. Light with low CCT creates a relaxing, cozy feeling. Light with high CCT is energizing and uplifting. Warm white light (∼3000 K) is often preferred in living rooms and bedrooms to create a cozy atmosphere. Neutral (∼4000 K) and cool white light (∼5000 K) has an energizing effect on people and are the good choice for offices and studies. Therefore, white light obtained using organic metal halide perovskites can be used in various lighting applications.
While CCT characterizes the appearance of light and CRI characterizes the color rendering, two light sources with the same CCT can look very different. The CRI does not account for the shift in chromaticity coordinates across the Planckian locus well. Thus, CRI alone is not the trustable metric [48] and the additional index, Duv, has been introduced to fully characterize light quality by ANSI. Duv is the distance from the chromaticity coordinates of the source to the Planckian locus on the CIE chromaticity diagram. CRI hardly changes with a change of light source chromaticity from Duv=0 to Duv=+0.015 [48] and it is important that the CIE (x, y) of the light source is very close to the Planckian locus as the greenish or pinkish white light is not favorable for general illumination purpose. The Duv for the selected LEDs ranges from 0.00002 to 0.0043, which is considered small. As a comparison, the Duv of the fluorescent lamp is typically controlled to less than 0.005 [47]. Therefore, the white lights obtained from organic metal halide perovskites are perceived as neutral in appearance. With the use of CCT and Duv, the two numbers can provide the full information of white light chromaticity of light in an intuitive manner [56]. The chromaticity coordinates (x, y) of these LEDs are shown in Table 1 and they are also plotted in Fig. 5. The CIE coordinates of white light emission for the samples are very close to the Planckian locus, which indicates that the color temperature we obtained is comfortable for the human eyesight. Furthermore, the CIE coordinate of the neutral white light is (0.32, 0.33), which is very close to the standard neutral white light (0.33, 0.33). Therefore, low cost, scalability of fabrication, and the excellent optical properties make these materials promising for white LED applications as replacement materials for conventional rare-earth doped phosphors.
4. Conclusion
In conclusion, spectral optimization was carried out to study the color characteristics and luminous efficacy of white light obtained from organic metal halide perovskites. We demonstrated the tunability of CCT from 2298 K (warm white) to 8270 K (cool white) by combining different ratios of blue, green, yellow, and red emissions from perovskite nanocrystals. The maximum CRI of 95.4 was obtained at CCT of 2509 K. We also obtained very high special CRIs (R9-R15). This indicates that by using the emissions from organic metal halides, the color can be precisely reproduced for any color objects. In addition, the high luminous efficacy of radiation above 300 lm/W was obtained in this work will lead to LER of LEDs based on perovskite materials. These properties indicate good prospects of these materials to be used as a replacement of conventional rare-earth doped phosphors in the next-generation white LEDs. The combination of the earth-abundant elements, low-cost synthesis and fabrication process, and superior structural, electrical optical properties place this type of materials in a unique position in next-generation of optoelectronic devices. The application of these materials in white LEDs will lead to a significant reduction in the initial cost of the white LEDs while having high energy efficacy and superior color quality. However, the material instability of perovskite LEDs is hindering their continuous-working lifetime compared to conventional white LEDs. The widespread adoption of white LEDs in general illumination market with the goal of achieving significant energy saving will speed up by improving the stability issue in the near future.
Funding
University of Tulsa (TU) (Faculty Startup Fund).
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