Victor J. A. Wesselingh, Tim Meijer, Haci E. Halici and Caspar Y. J. Waterham
Abstract
In a world dominated by screens it is shocking to realise that today’s commonly used screens cannot display all the colours perceived by the human eye [1], [2], [3]. The world seen through our eyes is still more vivid than what is shown on screens, but new materials might change this [4]. This research examines whether blue-emitting LEDs with relatively new green-emitting perovskite nanocrystals rather than phosphor techniques used in standard high-definition television (HDTV) screens [1] can improve the colour range of displays. This is because perovskite nanocrystals are thought to be able to emit purer colours than phosphor techniques [3].
We found that replacing YAG-phosphors with perovskites improves the colour range of displays by 52-64% and purity by up to 27%, compared to HDTV standards. Therefore, screens with modified LEDs will be able to display more of the human-sensable colours and display colours more vividly than current screens.
Introduction
Display applications are ubiquitous in the modern world, ranging from television and computer monitors to smartphones and digital billboards. Since the colour range commonly used in most applications today is fairly limited when compared to the entire palette visible to the human eye, the demand arose for techniques to extend the colour range beyond the standard palette. Nanocrystals can offer great improvement in this area. Since the light emitted from nanocrystals offers a more narrow spectral distribution compared to today’s phosphor-based techniques, resulting in less energy loss when filtering emitted light and thus a higher efficiency [3], the use of nanocrystals is widely regarded as an improvement in display applications. Perovskites are a new type of nanocrystal that are also semiconductors and can be tuned to emit a specific dominant wavelength of light; hence, perovskites are interesting materials for applications in displays [4].
This article explores to which extent the colour properties of green-emitting perovskites in displays differ compared to the colour properties of green-emitting YAG-phosphor techniques used in HDTV screens [1], [3].
2.1 Semiconductor nanocrystals
Colloidal semiconductor nanocrystals (SCNCs) are tiny semiconductor particles that have atom-like properties. The band theory states that electrons in this material can only have energy values within certain specific ranges, called bands. Therefore electrons can only exist inside a band, and not between two bands (the so-called band gap). For an electron to jump to a higher band, i.e., from the valence band to the conduction band, an amount of energy that exceeds the energy that is needed to bridge the gap between the two bands has to be absorbed. In the most ideal energy state of the particle, the electrons are in the valence band.
The ability of SCNCs to absorb a photon (usually UV light) results in excitation of an electron from the valence band to the conduction band, leaving behind a ‘hole’ with a positive charge (Fig. 1a): the electron-hole pair forms an ‘exciton’ with a characteristic binding energy [3], [5].
When an exciton with energy way higher than the band gap energy is created (a so-called ‘hot exciton’), both the electron and the hole first relax to the band gap states in a thermalization process involving electron-phonon and electron-hole interactions. (Fig. 1b)
Finally, as the electron cannot exist inside the band gap, the electron jumps from the conduction band to the valence band, recombining the electron-hole pair and emitting a photon with an energy equivalent to the band gap. (Fig. 1c) [3], [5].
Figure 1. a) By absorbing a photon, an electron is excited from the valence band to the conduction band, leaving a hole in the conduction band. b) Through electron-phonon and electron-hole interactions, the exciton decays toward the band gap. c) The electron and hole recombine and emit a photon with an energy equivalent to the band gap. This illustration was adapted from Ref. [3].
A well-known property of SCNCs is that the emission colour can be tuned by changing their size and/or shape: a decrease in size leads to a greater band gap (Fig. 2), due to increased presence of a quantum confinement effect [6]. A greater SCNC size decreases the amount of energy released during exciton recombination, and thus increases the wavelength of the emitted photon. By tuning the size, the entire visible spectrum can be displayed [5].
Figure 2. Schematic representation of the electronic energy structure of SCNCs with decreasing size. This illustration was adapted from Ref. [3].
2.2 Perovskites
Perovskites are crystals with the chemical formula ABC3, where A and B are cations and C are anions. Some perovskites are SCNCs, e.g., crystals with one caesium ion, one lead ion and three anions in the unit cell. (Fig. 3a). The ratio of anions influences the size of the band gap and thus the wavelengths emitted by the material. It is possible to make two-dimensional nanoplatelets (NPL) out of these crystals and control the thickness of the NPLs as precise as a single monolayer (ML) [5]. As the perovskites get thicker, the emitted radiation shifts toward the red spectrum due to the increased absence of quantum confinement [7] (Fig. 3b). A procedure called ‘anion-exchange’ can be used to alter the ratio of anions in the perovskite and thereby change the emitted wavelengths [7].
Figure 3.
a) Schematic representation of the crystal structure of CsPbBr3. b) CsPbBr3 NPLs with increasing monolayers from left to right luminated with UV-light. This illustration was adapted from Ref. [4].
2.3 CIE diagram
The Commission Internationale de l\’Éclairage (CIE) chromaticity diagram works by combining two factors: hue and saturation. The former describes how similar a colour is to stimuli described as red, yellow, green, blue, etc., while the latter describes the combination of the light intensity of the wavelength and how distributed it is on the spectrum of different wavelengths. Stimuli with maximum saturation (spectrum colours) are placed on the outside of the curve in the diagram. By determining the light intensity and distribution toward the other colours, it can be determined how closely the stimulus is located toward the centre (white) [8]. Fig. 4 shows a rendering of the CIE diagram with the colour triangle of the HDTV standards shown in Table 1 [1].
| xr | yr | xg | yg | xb | yb |
| 0,64 | 0,33 | 0,3 | 0,6 | 0,15 | 0,06 |
Table 1. The (x, y) parameter values for the CIE 1931 colour space for red, green and blue standards for HDTV screens as recommended by the ITU-R [1].
Figure 4. The CIE 1931 colour space with the HDTV standard [1] colour triangle. The GoCIE [9] software was used to map the colours.
Figure 5. The colour matching functions per wavelength [10], [8].
For determining the tristimulus values of a green sample, a spectrum range of 440-600 nm is sufficient, because of the limited emission range of the green sample: the values for the spectral distribution outside this range are negligible. The following functions [8] are thus used for determining the tristimulus values X, Y and Z for a spectral distribution of a green sample:
(1)
(2)
(3)
In the functions 1, 2 and 3, λ is the wavelength of the monochromatic light, S(λ) is the spectral distribution and and are the colour matching functions shown in Fig. 5. The coordinates for the CIE diagram can be calculated by the following functions [8] using the previously calculated X, Y and Z:
(4)
(5)
2.4 Screens
In current HDTV screens, multiple layers are stacked on top of each other. (Fig. 6). The first layer is a blue LED with a phosphor layer on top to convert the emitted blue light to white. Blue LEDs are used because they emit the most energetic photons, which can be converted into lower-energy photons, resulting in rendition of nearly the entire visible spectrum. The second layer is a polarizing filter to align the light rays. After this, the light passes through an RGB filter to get the desired colour. The last layer is another polarizing filter to align the light rays before they leave the screen. By combining different intensities of red, blue and green, all the colours within the triangle in Fig. 4 can be made [3].
Figure 6. Illustration of the different layers of which a HDTV screen consists [3].
2.4.1 HDTV versus SCNC techniques
Currently, the white light in liquid crystal displays (LCDs) is mostly generated by ‘YAG-rare-earth-based white LEDs. Methods with phosphors like YAG (yttrium aluminium garnet) emit a fixed spectrum and have low red and green intensity, due to strong electron-phonon coupling. This means that when the electron decays from the excited to the ground state, it can excite vibrations that take up part of the excited-state energy. The emitted photon carries the rest of the energy, which is a different amount depending on how many vibrations were excited. This results in peak broadening. Thus, SCNCs can be made to have higher intensities with a narrower wavelength distribution compared to YAG techniques where these electron-phonon interactions are more present.
By tuning and matching the backlight spectrum to the used RGB filter, it is possible to display brighter, more vivid colours with higher efficiency than in commonly used displays [3], [5].
Method and Material
To determine the difference between the colour properties of green perovskites and YAG-phosphor techniques, perovskites need to be synthesised and their emission measured. With this data, a comparison regarding emission can be made between the perovskites and the YAG-phosphors. The methodology was adapted from 9. Appendix.
3.1 Preparation of the precursors
36 mg Cs2CO3, 78 mg PbBr2 and 80 mg PbI2 were weighted up inside a secluded nitrogen atmosphere, sealed and then transported to a fume hood.
The Cs2CO3 precursor was made by dissolving the weighted amount of Cs2CO3 in 10 mL oleic acid (90%) (OA) after which the solution was continuously stirred at 500 rpm at 373 K for one hour.
The PbBr2 precursor was obtained by dissolving the weighted amount of lead(II) bromide, 200 μL OA (90%) and 200 μL oleylamine (80-90%) (OLAM) in 20 mL toluene, after which the solution was continuously stirred at 500 rpm at 373 K for one hour.
The PbI2 precursor was made by dissolving the weighted amount of PbI2, 200 μL OA (90%) and 200 μL OLAM (80-90%) in 20 mL toluene, after which the solution was continuously stirred at 500 rpm at 373 K for one hour.
Afterwards, the PbBr2 and the Cs2CO3 precursor were placed in a water bath at 294 K until cooled to room temperature, and the PbI2 precursor was placed in an ultrasonic bath at 294 K for 10 minutes.
3.2 Synthesis of the perovskite
To fabricate the six monolayer (green) nanoplatelets, 250 μL of the finished Cs2CO3 precursor was added to 300 μL acetone. Then, 1 mL of the finished PbBr2 precursor was added to the acetone solution while continuously stirred at 1200 rpm. After five seconds, an additional 2.5 mL acetone was added and the solution stirred at 1200 rpm at 294 K for one minute.
The supernate was then centrifuged in a Rotina 380R at 2000G for three minutes. The supernate was separated from the precipitation. 2 mL of PbBr2 precursor was added to the precipitation and vortexed in order for the precipitation to dissolve. The solution was then centrifuged at 1200G for three minutes, and the precipitation was separated from the resulting solution.
3.3 Purification of the perovskite
In order for the anion exchange [7] (Introduction 2.2) to take place, the solution was divided into six bottles, each containing 300 μL. Consecutively, 0, 60, 120, 180, 240 and 300 μL PbI2 was added to the bottles, respectively named Groen0 to Groen5 and filled up with toluene to a total of 2 mL.
3.4 Emission measurement
Finally, the resulting solutions were transferred into quartz cuvettes and placed into an Eddington spectrometer with an excitation light of 405 nm. The spectrometer measured an emission spectrum of the solutions in steps of 1 nm at a range of 440-600 nm, with a measuring time of one second per nm.
3.5 Colour triangle
With three chromaticity points in the CIE diagram (Introduction 2.3), a triangle can be made of which the area represents the colour range achievable by additive mixing of varying proportions of these three colours. The area can be described as
(6)
where the coordinates of the samples and of the green HDTV are represented by (xg, yg) and the blue and red points are respectively (xb, yb) and (xr, yr). Different colour triangles were made using the obtained chromaticity points of the samples, as well as using the standard chromaticity points of the HDTV screen [1],[2],[8].
3.6 Excitation purity
How ‘pure’ a given colour point (x, y) in the CIE 1931 colour space (Introduction 2.3) is, i.e., how much colour distribution is present in the given colour, can be analysed by determining the excitation purity (pe). This is defined as the ratio between the distance of the sample point to the white point (the spectrum locus) and the distance between the sample point and its dominant wavelength, located at the perimeter of the diagram [11]. The coordinates of the dominant wavelength were determined by extrapolating a straight line between the white point and the sample point, so that it intersects with the perimeter. Then, the excitation purity was calculated with the following function:
(7)
where (xn, yn) is the chromaticity of the white point, (x, y) is the point of the sample and (xI, yI) is the point on the perimeter with the dominant wavelength of the sample [2].
3.6.1 Excitation purity calculation
The dominant wavelength for each sample was calculated using curve fitting of the perimeter of the CIE 1931 colour space [10], [8], done with Google Spreadsheets and using an eighth degree polynomial function. Calculation of the intersections between the perimeter and formula 7 was done on a TI-84 Plus CE-T.
Results
After measuring the samples with the spectrometer at the range of 440 to 600 nm, the results are plotted in Fig. 7.
Figure 7. Graph of measured light intensity (photons/m2) per wavelength (nm) per sample. 
With the emission spectra (Fig. 7), the coordinates of the samples in the CIE diagram are calculated (Fig. 8) using the tristimulus formulas (Introduction 2.3).
Figure 8. CIE diagram with calculated coordinates of the emitted light. The GoCIE [9] software was used to map the colours.
Using the coordinates of HDTV standard LEDs (Table 1), Fig. 4 and Fig. 8 are combined, resulting in Fig. 9.
Figure 9. CIE diagram with coordinates of the samples and HDTV LEDs [1] and the triangles between them. The GoCIE [9] software was used to map the colours.
4.1 Triangle area
With these coordinates, a triangle can be made between the blue and red HDTV coordinates (Table 1) and the sample coordinates as shown in Fig. 9. The areas of the triangles are calculated using Formula 6 and are compared to the area of the HDTV triangle. Both results are displayed in Table 2.
| Triangle area | Triangle area in relation to HDTV area (%) | |
| HDTV | 0,0918 | 100 |
| Groen0 | 0,140695 | 153,3 |
| Groen1 | 0,1482 | 161,4 |
| Groen2 | 0,1504 | 163,8 |
| Groen3 | 0,15015 | 163,6 |
| Groen4 | 0,1461 | 159,2 |
| Groen5 | 0,1396 | 152,1 |
Table 2. Table showing the area of the triangle made between the sample, the red and the blue coordinates compared to the HDTV triangle in percentual size.
4.2 Purity
Using the coordinates of the dominant wavelength of each sample, the excitation purities of the samples are calculated using Formula 7 and compared to the excitation purity of the HDTV standard green point in Fig. 10. The purest sample gives a 27% increase compared to the HDTV standard [1].
Figure 10. Bar chart comparing excitation purity of the samples to the purity of the HDTV standard [1] green point. 
Discussion
Our results are in line with other research [4], [12], [7], [3], [5]: with SCNC’s, samples with a narrower emission spectrum and thus a higher purity can be achieved. This is due to less electron-phonon interactions, which are strongly present when using YAG-phosphor techniques. The saturation determines part of the area of the colour triangle and therefore the colour range of the hypothetical screen. It therefore enables a colour gamut larger than the HDTV standard. The larger colour range of the hypothetical screens achievable with the samples instead of the HDTV YAG-techniques is therefore also in line with other research.
5.1 Shortcomings
There was a limited amount of samples used over the range we chose to test from. This could have impacted the accuracy of the results. Moreover, the research was not done multiple times. Because the platelets were only produced once, there was no way to verify the results and see if they were accurate.
In addition, the synthesis took place under a fume hood without any further prevention of the surrounding air reacting with the substance, which could have influenced the durability (further discussed under 5.2 Relevance). Also, a human error could have been made at the determination of a time frame of five seconds during the synthesis; this affects the reproducibility.
5.2 Relevance
Perovskite SCNCs can be used to display a greater array of colours on a screen compared to the colour range of a standard HDTV screen. This could be used for commercial purposes regarding screens and for special instrumentation which requires a larger colour palette or a higher excitation purity than a regular HDTV screen.
Furthermore, we noticed that the SCNCs oxidised and, as a result, decomposed rather quickly (some within the time span of one day). This is concerning if we look at a possible implementation for industry-scaled purposes. Assuming these types of SCNCs would be commonly used in display devices with a durability of years, durability of the SCNCs should certainly be taken into consideration [5].
A further problem with implementing SCNCs in industry-scaled products is that of heavy metals, such as lead, integrated into the crystal structure, because the use of heavy metals is restricted in customer products for their toxicity [3].
5.3 Further research
As previously shown, more research is needed before SCNCs can be used for industry-scaled purposes.
Research into the difference between the colour properties of red- or blue-emitting perovskites and the colour properties of the appurtenant YAG-phosphor techniques could lead to purer red and blue samples and would therefore enlarge the colour range of screens [2], [8]. A potential research question could be: “To which extent do the colour properties of red-emitting perovskites in displays differ compared to the colour properties of red-emitting YAG-phosphor techniques used in HDTV screens?”
A study looking into preserving SCNCs for long term usage would also be essential before this technology could be used in any commercial application (see relevance). In this regard, “To which extent can the negative effect of oxidation on the durability of SCNCs be inhibited?” would also be a potential research question.
Conclusion
6.1 Area
Because the area of a triangle made between the red, blue and green coordinates in the CIE diagram is the colour range of the screen (Methodology 3.5) and the samples result in a larger area compared to the HDTV area (Table 2), we can conclude that hypothetical displays using the samples have a 52%-64% increase of colour range compared to YAG-phosphor techniques in HDTV.
6.2 Purity
With perovskites, a higher excitation purity can be achieved compared to those achieved with YAG-phosphor techniques used in HDTV (Fig. 10): the relative purity of the purest sample is 27% greater than the standard HDTV. This can be supported visually: as shown in Fig. 9, the coordinates of the samples are closer to the perimeter of the CIE diagram than those achieved with YAG-phosphor techniques.
6.3 Perovskites versus YAG-phosphors
To answer the research question “To which extent do the colour properties of green-emitting perovskites in displays differ compared to the colour properties of green-emitting YAG-phosphor techniques used in HDTV screens?”: the colour of green-emitting perovskites in displays is up to 27% purer and results in an increase of the colour range with 52-64% compared to the standard HDTV that uses YAG-phosphor techniques.
References
1. “Recommendation ITU-R BT.709-6 Parameter Values for the HDTV Standards for Production and International Programme Exchange.” Geneva: ITU-R, 2015.
2. Joseph, Charles Lynn., and Santiago Bernal. “Image Display Systems.” Essay. In Modern Devices: The Simple Physics of Sophisticated Technology, 295–99. Hoboken, New Jersey: Wiley, 2016.
3. Panfil et al. “Colloidal Quantum Nanostructures: Emerging Materials for Display Applications.” Angewandte Chemie International Edition 57, no. 16 (2018): 4274–95. https://doi.org/10.1002/anie.201708510.
4. Bohn, Bernhard J. et al. “Boosting Tunable Blue Luminescence of Halide Perovskite Nanoplatelets through Postsynthetic Surface Trap Repair.” Nano Letters 18, no. 8 (2018): 5231–38. https://doi.org/10.1021/acs.nanolett.8b02190.
5. Shirasaki et al. “Emergence of Colloidal Quantum-Dot Light-Emitting Technologies.” Nature Photonics 7, no. 1 (2012): 13–23. https://doi.org/10.1038/nphoton.2012.328.
6. Efros, Al. L., and M. Rosen. “The Electronic Structure of Semiconductor Nanocrystals.” Annual Review of Materials Science 30, no. 1 (2000): 475–521. https://doi.org/10.1146/annurev.matsci.30.1.475.
7. Nedelcu et al. “Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I).” Nano Letters 15, no. 8 (2015): 5635–40. https://doi.org/10.1021/acs.nanolett.5b02404.
8. Schanda, Janos. Colorimetry: Understanding the CIE System. Hoboken, New Jersey: Wiley, 2007.
9. Justin Thomas, K. R. “GoCIE.” Computer software. GoCIE – CIE Plot Utility. Department of Chemistry, Indian Institute of Technology Roorkee, 2002. http://faculty.iitr.ac.in/~krjt8fcy/gocie.
10. Giorgianni, Edward J., and Thomas E. Madden. “Color Encoding in the Photo CD System.” Color for Science, Art and Technology AZimuth, 1998, 389–422. https://doi.org/10.1016/s1387-6783(98)80017-2.
11. “Excitation Purity [Pe].” eilv. Commission Internationale de l\’Éclairage, 2014. http://eilv.cie.co.at/term/408.
12. Dursun et al. “Perovskite Nanocrystals as Color Converters for Record-Breaking Visible Light Communications.” SPIE Newsroom, 2017. https://doi.org/10.1117/2.1201611.006756.
8. Acknowledgement
This research was funded by U-Talent, part of the Freudenthal Institute at Utrecht University. Furthermore, we want to thank Dr F. T. Rabouw, M. J. J. Mangnus, W. Koning and M. E. Gonzalez for their help and review of the article.
9. Appendix
Cesium Lead Halide Perovskite NPLs synthesis
Chemicals
Cesium carbonate Cs2CO3
Lead Bromide PbBr2
Lead Iodide PbI2
Octadecene ODE, Sigma-Aldrich, 90%
Oleic acid OA, Sigma-Aldrich, 90%
Oleylamine OLAM, Acros 80-90%
Toluene
Acetone
Lab equipment
Thermocouple
Hot plate with magnetic stirrer + stage
Reaction protocol CsPbBr3 perovskite NPLs
Cs-oleate precursor solution
Cs2CO3 (32.6 mg, 0.1 mmol) was dissolved in 10 mL oleic acid at 100 C under continuous stirring.
PbBr2 precursor
PbBr2 (73.4 mg, 0.2 mmol), 200 uL oleic acid and 200 uL OLAM were dissolved in 20 mL toluene at 100 C.
PbI2 precursor
PbI2 (92.2 mg, 0.2 mmol), 200 uL oleic acid and 200 uL OLAM were dissolved in 20 mL toluene at 100 C.
Reaction CsPbBr3 NPLs
Cs-OA precursor is added under vigorous stirring into a toluene solution containing the PbBr2-precursor at room temperature. For 5 or 6 ML NPLs, 0.2 or 0.3 mL of acetone are added to the PbBr2-precursor prior to addition of Cs-OA. After 5 s, acetone is added to initiate NPL formation. After 1 min of stirring, the solution is centrifuged at 2000 g for 3 mins and the precipitate is redispersed in toluene or TOL/PbX2 solution. Dispersion is centrifuged at 1200 g for 3 mins and the precipitate is discarded.
Anion exchange
Add a mixture of PbBr2/PbI2-ligand solution in the desired ratio.
About the Author
Caspar, Emre, Tim and Victor are four students which take part in the Utrecht University’s U-Talent programme. This programme is specifically made for high-school students with an extra passion for everything relating to science. To end this programme the students did research with Dr. F.T. Rabouw looking into the colour purity and intensity of nanocrystals.