Polymer-Free Films of Inorganic Halide Perovskite Nanocrystals as UV-to-White Color-Conversion Layers in LEDs.

Being considerably more efficient than traditional incandescent bulbs or fluorescent tubes, light-emitting diodes (LEDs) are becoming the main technology for general lighting applications.[1,2] In the last years, a variety of white-LEDs have been demonstrated employing, for example, organic molecules (OLEDs)[3−7] or inorganic quantum dots (QDs) of different composition.[8] A common way of achieving a white-light emission is to couple a near-UV or blue LED with down-conversion materials such as phosphors.[9] More recently, QDs have been incorporated into LEDs, replacing conventional phosphors to tune finely the emission spectrum.[10] Among the different available QDs, lead halide-based perovskite nanocrystals have recently emerged as very promising candidates for many optoelectronic applications.[11−15] Colloidal perovskite nanocrystals can be synthesized and/or transformed postsynthesis, so that samples emitting at different wavelengths throughout the whole visible spectrum, with high photoluminescence quantum yield (PLQY), can be prepared easily. This can be achieved either by changing the chemical composition (by anion-exchange for example)[16,17] or the shape (cubes, platelets, sheets, wires).[18−21] Also, electroluminescence has been observed from bulk perovskite films,[22−25] blended perovskite-in-polymer films,[26] as well as from nanocrystals-only films[27,28] leading to the first perovskite-based LEDs.

Perovskite nanocrystals have also been used as color-conversion materials for white-emitting LEDs. Recent reports use a commercial blue GaN LED on top of which green-emitting bromide-based nanocrystals are placed.[29,30] The spectrum is then completed either with conventional red phosphors or with iodide-based perovskite QDs.[29,30] In the latter case, both bromide- and iodide-based perovskite nanocrystals are embedded in a polymer matrix to prevent spontaneous interparticle anion-exchange reactions. Recent reports have shown that shape control over perovskite nanocrystals can be used to tune the emission wavelength: for example, quantum confinement in bromide-based nanoplatelets shifts the emission wavelength from green (around 515 nm for “standard” nanocubes) to blue.[18] This avoids the need for a blue-emitting source. Additionally, we have recently demonstrated that X-ray irradiation under vacuum inhibits anion-exchange reactions on perovskite nanocrystals, making it unnecessary to embed them in a polymer matrix.[31] Eventually, “fully inorganic” cesium-based perovskites are reported to be more stable than their organic–inorganic (methylammonium or other organic cation-based) counterpart.[32]

We report here various concepts of polymer-free films made solely of inorganic quantum dots as a color-conversion layer on a commercial UV (365 nm) LED. First, we demonstrated single color blue, green, orange and red films with CsPbBr3 nanoplatelets (Br-NPL), CsPbBr3 nanocubes (Br-NC), CsPbI3 nanosheets (I-NS) and CsPbI3 nanocubes (I-NC), respectively. Then, we showed how tunable bicolor (blue-green and orange-red) films could be easily prepared by mixing NPLs, NSs and NCs of the same chemical composition. Finally, we fabricated a full UV-to-white color conversion film combining a mixture of CsPbBr3 NPLs (blue) and NCs (green) and additional nanocrystals emitting in the orange/red region of the spectrum. The latter were either CsPbI3 NSs (all perovskite approach) or nonperovskite emitters, for example Cu–In—Zn–S (CIZS)[33] or giant-shell (GS) CdSe/CdS nanocrystals[34] (mixed perovskite–chalcogenide approach). In the all perovskite approach, we used X-ray stabilization to inhibit anion-exchange reactions between the bromide and iodide nanocrystals. X-ray irradiation was also found to improve the stability of the whole film in the case of CsPbI3, which otherwise would degrade upon illumination, even when embedded in a polymer.[29] In all the cases, we were able to prepare films displaying white light emission with different correlated color temperature (CCT),[35] which could be applied as a color-conversion layer on top of a commercial UV LED.

Different halide perovskite nanocrystals (Br-NPL, Br-NC, I-NS and I-NC) were colloidally synthesized (Figure ). These had different PL emission spectra, with main peaks around 465, 515, 610 and 680 nm, respectively (Figure a). In a recent report,[18] we showed how we could tune the thickness of CsPbBr3 NPLs in a regime where carriers were strongly confined (3–5 monolayers), such that we could shift the emission peak form its bulk position. Figure a reveals the presence of different thicknesses on the Br-NPL and I-NS samples, which was beneficial for application in white light emission. First, each of these samples could be used to prepare single-color (blue, green, orange, red) emitting layers. Furthermore, by mixing platelets/sheets and cubes of the same chemical composition we could obtain a broadband emission spectrum (Figure ). CsPbBr3 NPLs and NCs (Figure b) and CsPbI3 NSs and NCs (Figure c) were mixed in different ratios (1:0, 3:1, 1:1, 1:3, 0:1) to achieve a tunable emission spectrum between 450 and 550 nm (Figure b) and between 550 and 750 nm (Figure c). Figure d shows the CIE 1931 coordinates of the different solutions. This diagram, introduced by the Commission Internationale de l’Eclairage in 1931 maps all the visible colors by the human eye (“standard observer”) in terms of hue and saturation. The point at coordinates x = 1/3; y = 1/3 is defined as the equal-energy point (the optical spectrum corresponding to this point has a constant spectral distribution) and can be used for a definition of white light, though illuminants corresponding to other coordinates in a loosely defined region around this point can be used as reference white lights as well.[35]Figure shows that even though significant changes in the iodide-based mixture spectra are shown in Figure c when varying the NSs to NCs ratio, these changes were quite minor in terms of CIE coordinates. On the other hand, slight changes in the Br-NPL/Br-NC blue-green spectra (Figure b) resulted in significant differences in the CIE diagram (Figure d).

Figure 1

TEM images of different perovskite nanocrystals. Scale bars are 100 nm.

Figure 2

PL spectra of 4 different perovskite nanocrystals solutions (a) as well as different mixtures of Br-NPL and Br-NC (b) and I-NS and I-NC (c). Corresponding CIE coordinates (d).

One would assume that white light emission could be achieved by simply mixing Br- and I-based crystals in solution. However, as already reported, in such a mixture interparticle anion-exchange would eventually yield a single emission peak solution at intermediate wavelength.[16,17] Anion exchange occurred even between the particles deposited on a film, so that casting a film of iodide-based particles on top of a previously casted film of bromide-based ones (or vice versa) led as well to a single emission peak and not to a broadband emission (Figure S1). Although broadband emission has been addressed by others by encapsulating particles in a polymer matrix,[29] we used here an X-ray stabilization process that inhibits anion-exchange reactions.[31] Following this strategy, we spin-coated a first layer of blue/green emitting CsPbBr3 nanocrystals (mixed NCs and NPLs) and then irradiated the film before depositing a second layer of CsPbI3 NSs. As the final setup was illuminated from the bottom (Figure a), it was important to place the material with lower bandgap (I-NS here) as the top layer, so that it could be excited both from direct UV illumination and by Förster transfer[36−38] from the bottom layer with higher bandgap (CsPbBr3).[30] Furthermore, the top CsPbI3 layer has to be protected from degradation that occurs in different environments, even when embedded in a polymer.[29] Thus, we additionally carried out a second irradiation step which was found to protect the nanocrystals from degradation, as already reported by us.[31] A sketch of the all perovskite approach with X-ray stabilization is reported in Figure a.

Figure 3

(a) Sketch of the preparation of an all perovskite white-emitting bilayer. (1) Deposition of CsPbBr3 NPL and NC; (2) X-ray irradiation to inhibit further anion-exchange; (3) deposition of a second layer of CsPbI3 NS and (4) final X-ray irradiation preventing degradation of CsPbI3 nanocrystals under illumination. Final bilayer system, illuminated with a 365 nm UV LED emits at different emission wavelengths corresponding to perovskite nanocrystals excited by direct illumination or by Förster transfer. (b) CIE coordinates and (c) efficiency curves of perovskite bilayer films with (stabilized) and without (not stabilized) a final X-ray irradiation step. Both samples were irradiated between the first and second deposition to avoid anion-exchange.

Figure b,c presents the CIE coordinates and efficiency of bilayer films that have been irradiated by X-rays only once after the first layer (not-stabilized) or twice (stabilized). In the not-stabilized case, the initial emission was dominated by the second layer (CsPbI3) which had not been exposed to X-rays. These particles were however degraded upon UV illumination, leading to a blue shift of the overall color (Figure b). This degradation of iodide-based nanocrystals and the resulting blue-shift was already observed by other groups on particles embedded in a polymer.[29] In the stabilized case, the overall efficiency (Figure c) was lower (we calculated a luminous intensity of 1.19 Cd at the highest excitation power of 800 mW), which can be attributed to the longer exposure to X-rays in vacuum. As already reported by us,[31] this irradiation may result in partial photoluminescence quenching. Nonetheless, it is worth noting that the resulting color (Figure c) was maintained after UV illumination at high power. As this all-perovskite approach requires two different deposition steps, each step followed by an X-ray irradiation step in a vacuum (Figure a), the whole process is somewhat unpractical and can introduce homogeneity and reproducibility issues, especially since all different particles cannot be premixed in solution. It must also be noted that CsPbI3 alone (as raw synthesis solution, under inert atmosphere) already suffered from instability problems and we saw degradation occurring at varying times from batch to batch. Indeed, degradation of CsPbI3 is a major problem observed in different fields.[39−42] We therefore explored other means of obtaining white light emission from mixtures of perovskites and chalcogenide nanocrystals. To avoid the use of iodide-based perovskite nanocrystals, we mixed the stable blue and green-emitting CsPbBr3 nanocrystals (NPL and NC) together with different orange/red-emitting, nonperovskite based nanocrystals. In this case, because no anion or cation exchange takes place spontaneously in solution, there was the added benefit that the emission color could easily be tuned in solution by simply adding the different particles in various ratios (Figure S2). In the present work, we investigated mixtures of CsPbBr3 nanocrystals with CIZS nanocrystals as well as with giant-shell (GS) CdSe/CdS nanocrystals (Figure ).

Figure 4

PL spectra (a, c) and corresponding CIE coordinates (b) of different solutions of Br-NPL and Br-NC mixed with CISZ (a) and CdSe/CdS (b) nanocrystals. In the CIE diagram, pure composition solutions are marked with a star, dots correspond to CdSe/CdS-based mixtures and triangles to CIZS-based mixtures. Spectra in (a,c) correspond to rounded markers of same color in panel b. (d) Efficiency of white LED consisting of a commercial UV LED with thin films of mixed perovskite/CIZS and perovskite/GS as color-conversion layers.

As can be seen from the color diagram (Figure b), the use of both types of chalcogenide nanocrystals, in combination with CsPbBr3 nanocrystals lead to a white light emission with different color-correlated temperatures. Both black-rounded spots (corresponding to the black spectra in Figure a,c) have CIE coordinates close to the theoretical ideal white (x = 1/3; y = 1/3): x = 0.31467; y = 0.34251; CCT = 6317 K for the CIZS-based solution and x = 0.32396; y = 0.35315 and CCT = 5856 K for the CdSe/CdS-based solution. However, as can be seen in Figure a,c, the emission spectra leading to these similar white lights were significantly different. The CIZS particles had a broad PL emission centered around 610 nm, whereas the GS ones had a sharper emission at longer wavelength. This had an impact on the color-rendering capabilities of both white lights. Color-rendering refers to the capability of a white light to reveal colors of illuminated objects faithfully (in comparison with an ideal or natural light source of same CCT). To assess this objectively, the so-called CRI (color rendering index)[35] can be computed from the emission spectra. In terms of color rendering, an ideal white light has a CRI of 100, which, for artificial lighting, is only achieved by incandescent bulbs, whereas current white LEDs commonly claim a CRI higher than 80.[35] We found that CIZS-based solutions could achieve a CRI as high as 85, whereas GS-based solutions had a poor CRI of 20 at best. However, it must be noted that color-rendering is crucial only for some applications: common low pressure sodium lamps for outside lighting, for example, have a negative CRI. Different solutions of mixed perovskite/CIZS and perovskite/GS nanocrystals were deposited as a thin film on fused silica substrates to evaluate their stability as a conversion layer on top of the commercial UV LED. As can be seen in Figure d, the CIZS-containing solution had poor efficiency, as CIZS nanocrystals were degraded upon illumination (28.6 Cd at maximum excitation power). As with the nonstabilized CsPbI3, this resulted in a blue-shift of the overall emission spectrum, arising from the stable CsPbBr3 nanocrystals (Figure S3). In the case of the perovskite/GS solutions, the efficiency was higher (114 Cd at maximum excitation power) and the emission was stable even at high illumination power (Figure d). As both CsPbBr3 and GS nanocrystals were stable upon illumination, the emission spectrum of the film did not change significantly (see Figure S4).

In conclusion, we have demonstrated how inorganic perovskite nanocrystals can be used as efficient UV-to-white color-conversion materials on a commercial LED. Using a mixture of CsPbBr3 nanoplatelets and nanocubes as a basis for blue/green emission, white light could be obtained either in combination with CsPbI3 nanosheets (all perovskite approach) or using different chalcogenide based nanocrystals (hybrid perovskite-chalcogenide approach) including Cu–In—Zn–S (CIZS) and giant-shell CdSe/CdS nanocrystals. The first case required the sequential deposition of two nanocrystal thin films (bromide- and iodide-based) with an intermediate X-ray irradiation step under vacuum to inhibit anion-exchange. Furthermore, a final X-ray irradiation step was needed to stabilize the CsPbI3 nanocrystals which otherwise were degraded under UV illumination. The hybrid approach had the benefit of allowing mixing and optimization of the different particles in solution before spin-coating a single mixed-composition film to act as a color-conversion layer. In the approach with giant-shell CdSe/CdS nanocrystals, the films showed very high stability upon illumination at high power. In all cases, white light emission with tunable color correlated temperature could be obtained.