PEROVSKITE LIGHTING SYSTEMS

Abstract

In one aspect, composite nanoparticles are provided. In some embodiments, a composite nanoparticle comprises a host matrix comprising A.sub.4BX.sub.6-ZY.sub.Z and ABX.sub.3-PY.sub.P inclusions dispersed within the host matrix of the composite nanoparticle, wherein A is an alkali metal, B is an element selected from the group consisting of transition metals, Group IVA elements and rare earth elements and X and Y are independently selected from Group VIIA elements, 0≤z≤6, and 0≤p<3.

Claims

1. A composite nanoparticle comprising: a host matrix comprising A.sub.4BX.sub.6-zY.sub.z; and ABX.sub.3-pY.sub.p inclusions dispersed within the host matrix of the composite nanoparticle, wherein A is an alkali metal, B is an element selected from the group consisting of transition metals, Group IVA elements and rare earth elements and X and Y are independently selected from Group VIIA elements, 0≤z≤6, and 0≤p<3.

2. The composite nanoparticle of claim 1, having a diameter of at least 100 nm.

3. The composite nanoparticle of claim 1, having a diameter of 100 nm to 500 nm.

4. The composite nanoparticle of claim 1, wherein the ABX.sub.3-pY.sub.p inclusions are dispersed in the host matrix bulk.

5. The composite nanoparticle of claim 1, wherein the ABX.sub.3-pY.sub.p inclusions are photoluminescent and/or electroluminescent.

6. The composite nanoparticle of claim 5, wherein the ABX.sub.3-pY.sub.p inclusions emit light in the visible region of the electromagnetic spectrum.

7. The composite nanoparticle of claim 5, wherein the ABX.sub.3-pY.sub.p inclusions have an emission full width half maximum of 20 nm or less.

8. The composite nanoparticle of claim 1, wherein ABX.sub.3-pY.sub.p inclusions have a size of 10 nm to 20 nm.

9. The composite nanoparticle of claim 1, wherein A is Cs and B is Pb.

10. The composite nanoparticle of claim 9, wherein X is Br or I and z and p are each 0.

11. The composite nanoparticle of claim 10, wherein the host matrix comprises Cs.sub.4PbBr.sub.6 and the inclusions are CsPbBr.sub.3.

12. The composite nanoparticle of claim 10, wherein the host matrix comprises Cs.sub.4PbBr.sub.6 and the inclusions are CsPbI.sub.3.

13. The composite nanoparticle of claim 9, wherein X is I, Y is Br, p=1 and z=6.

14. The composite nanoparticle claim 13, wherein the host matrix comprises Cs.sub.4PbBr.sub.6 and inclusions are CsPbI.sub.2Br.

15. The composite nanoparticle of claim 1, wherein the ABX.sub.3-pY.sub.p inclusions are crystalline.

16. The composite nanoparticle of claim 1, wherein the ABX.sub.3-pY.sub.p inclusions establish a Type I band offset with the A.sub.4BX.sub.6-zY.sub.z host matrix.

17. The composite nanoparticle of claim 1, wherein a capping layer resides between the ABX.sub.3-pY.sub.p inclusions and the A.sub.4BX.sub.6-zY.sub.z host matrix.

18. The thin film of claim 17, wherein the capping layer has composition different from the ABX.sub.3-pY.sub.p inclusions and the A.sub.4BX.sub.6-zY.sub.z host matrix.

19. The thin film of claim 18, wherein the capping layer exhibits a Br gradient.

20. An thin film comprising: a continuous host matrix comprising A.sub.4BX.sub.6-zY.sub.z; and ABX.sub.3-pY.sub.p inclusions dispersed within the continuous host matrix, wherein A is an alkali metal, B is an element selected from the group consisting of transition metals, Group IVA elements and rare earth elements and X and Y are independently selected from Group VIIA elements, 0≤z≤6, and 0≤p<3.

21. The thin film of claim 20, wherein the ABX.sub.3-pY.sub.p inclusions are dispersed in the host matrix bulk.

22. The thin film of claim 20, wherein the ABX.sub.3-pY.sub.p inclusions are photoluminescent and/or electroluminescent.

23. The thin film of claim 20, wherein the ABX.sub.3-pY.sub.p inclusions emit light in the visible region of the electromagnetic spectrum.

24. The thin film of claim 23, wherein the ABX.sub.3-pY.sub.p inclusions have an emission full width half maximum of 20 nm or less.

25. The thin film of claim 20, wherein ABX.sub.3-pY.sub.p inclusions have a size of 10 nm to 20 nm.

26-37. (canceled)

38. An electroluminescent device comprising: a first electrode and a second electrode; and a light emitting layer positioned between the first and second electrodes, the light emitting layer comprising a continuous host matrix comprising A.sub.4BX.sub.6-zY.sub.z and ABX.sub.3-pY.sub.p inclusions dispersed within the host matrix, wherein A is an alkali metal, B is an element selected from the group consisting of transition metals, Group IVA elements and rare earth elements and X and Y are independently selected from Group VIIA elements, 0≤z≤6, and 0≤p<3.

39. The electroluminescent device of claim 38, wherein the ABX.sub.3-pY.sub.p inclusions are dispersed within the host matrix bulk.

40. The electroluminescent device of claim 38, wherein the ABX.sub.3-pY.sub.p inclusions emit light in the visible region of the electromagnetic spectrum.

41. The electroluminescent device of claim 40, wherein the ABX.sub.3-pY.sub.p inclusions have an emission full width half maximum of 20 nm or less.

42. The electroluminescent device of claim 38, wherein ABX.sub.3-pY.sub.p inclusions have a size of 10 nm to 20 nm.

43-62. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1a illustrates composite nanoparticles according to some embodiments wherein CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix in a core-shell architecture.

[0025] FIG. 1b are transmission electron microscopy images (TEM) of the composite nanoparticles according to some embodiments.

[0026] FIG. 1c illustrates a Type I heterojunction between CsPbBr.sub.3 inclusions in a Cs.sub.4PbBr.sub.6 host matrix according to some embodiments.

[0027] FIG. 1d provide photoluminescence spectrum and absorption coefficient of the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 architecture according to some embodiments.

[0028] FIG. 1e optical clarity and photoluminescence of a perovskite film comprising CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix according to some embodiments.

[0029] FIG. 1f is an atomic force microscopy (AFM) image of a perovskite film comprising CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix according to some embodiments.

[0030] FIG. 1g illustrates a light emitting diode (LED) architecture employing a perovskite film comprising CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix according to some embodiments.

[0031] FIG. 1h illustrates a band diagram for the LED architecture illustrated in FIG. 1g.

[0032] FIG. 1i illustrates top emission from the LED architecture of FIG. 1g according to some embodiments.

[0033] FIG. 2a illustrates electroluminescence spectra (EL) of LED devices based on CsPbBr.sub.3 quantum dots and CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix, respectively according to some embodiments.

[0034] FIG. 2b illustrates the dependence of the current density and luminance on the driving voltage in the devices based on CsPbBr.sub.3 quantum dots and CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix, respectively according to some embodiments.

[0035] FIG. 2c illustrates current efficiency and power efficiency as a function of current density for devices based on CsPbBr.sub.3 quantum dots and CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix, respectively according to some embodiments.

[0036] FIG. 2d brightness of the devices based on CsPbBr.sub.3 quantum dots and CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix, respectively according to some embodiments.

[0037] FIG. 2e illustrates current efficiency over time for devices based on CsPbBr.sub.3 quantum dots and CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix, respectively at a current density of 62.5 mA/cm.sup.2.

[0038] FIG. 2f illustrates water resistant Cs.sub.4PbBr.sub.6 films with polydimethysulfoxide (PDMS) encapsulation according to some embodiments.

[0039] FIG. 2g illustrates transparency of CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin films before and after moisture aging.

[0040] FIG. 2h illustrates EL intensity and voltage of PDMS sealed CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LEDs under water according to some embodiments.

[0041] FIG. 2i illustrates water resistance testing of LEDs described herein according to some embodiments.

[0042] FIG. 3a illustrates EL intensity of a CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED after post thermal treatment at different temperatures according to some embodiments.

[0043] FIG. 3b illustrates quantum yield of a CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin film after post thermal treatment according to some embodiments.

[0044] FIG. 3c illustrates cooling processes of a CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin film through metal and air under UV illumination according to some embodiments.

[0045] FIG. 3d is the PL spectrum of a CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin film during the heating process according to some embodiments.

[0046] FIG. 3e is the PL spectrum of a CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin film during the heating process according to some embodiments.

[0047] FIG. 4a provide brightness bending test results of a CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED, according to some embodiments.

[0048] FIG. 4b displays voltage variation at a constant current of 62.5 mA/cm.sup.2 in the brightness bending test.

[0049] FIG. 4c is a dark field image of the Al-based CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED after 10,000 bends.

[0050] FIG. 4d illustrates cracks in the ITO/Al-based CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED after 10 bends.

[0051] FIG. 4e illustrates luminance and current variation as a function of LED bending angle according to some embodiments.

[0052] FIG. 4f illustrates EL spectra of the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED during bending testing, according to some embodiments.

[0053] FIG. 4g is a bright optical image of four pixels in an 8×8 CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED matrix according to some embodiments.

[0054] FIG. 4h illustrates a flexible CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED matrix display according to some embodiments.

DETAILED DESCRIPTION

[0055] Embodiments described herein can be understood more readily by reference to the following detailed description, examples and drawings. Elements, apparatus, and methods described herein, however, are not limited to the specific embodiments presented in the detailed description, examples and drawings. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations will be readily apparent to those of skill in the art without departing from the spirit and scope of the invention.

Example 1—Composite Peroskite Thin Film and LED

[0056] As described herein, composite nanoparticles comprise a host matrix comprising A.sub.4BX.sub.6-zY.sub.z, and ABX.sub.3-pY.sub.p inclusions dispersed within the host matrix of the composite nanoparticle, wherein A is an alkali metal, B is an element selected from the group consisting of transition metals, Group IVA elements and rare earth elements and X and Y are independently selected from Group VIIA elements, 0≤z≤6, and 0≤p<3. In some embodiments, the host matrix and inclusions present a core shell architecture. In the present example, CsPbBr.sub.3 nano-inclusions embedded Cs.sub.4PbBr.sub.6 matrix, is illustrated in FIG. 1a. The core-shell pervoskite films were fabricated from solutions made by dissolving PbBr.sub.2 and CsBr in dimethyl sulfoxide (DMSO) at molar ratios of 1:1.5 without additive. Films were spun on glass substrates at room temperature in a nitrogen-filled glovebox. The formation of Cs—Pb—Br core-shell perovskites was initiated by dripping acetone for fast crystallization during spin-coating followed by thermal post annealing for 10 min. Different with organic ligand terminated low dimensional perovskite (nanoparticles, nanowires, nanoplates, or quasi-2D), the CsPbBr.sub.3 nanocrystals (5˜7 cores) are imbedded in Cs.sub.4PbBr.sub.6 host crystals, which was demonstrated by TEM microscopy as shown in FIG. 1b. The sizes of CsPbBr.sub.3 nanoinclusions range from 10 nm to 15 nm as the Cs.sub.4PbBr.sub.6 shell layer is as thick as ˜20 nm. High-resolution TEM image shows the lattice spacing distances of CsPbBr.sub.3 (100) and Cs.sub.4PbBr.sub.6 (100) which are 8.5 Å and 3.8 Å respectively. The (010) plane of Cs.sub.4PbBr.sub.6 was also observed in an angle of 60° with (100) plane. In contrast, some fringe patterns on CsPbBr.sub.3 area were also recorded due to the overlapping the Cs.sub.4PbBr.sub.6 lattice. The electron affinity levels of CsPbBr.sub.3 and Cs.sub.4PbBr.sub.6 were successfully estimated to 3.2 eV and 4.1 eV respectively, indicating a type I heterojunction (FIG. 1c) at the interface together with the band gaps of CsPbBr.sub.3 (˜2.4 eV) and Cs.sub.4PbBr.sub.6 (˜3.7 eV) confirmed by the absorption edges in FIG. 1d. The CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 composite showed an emission peak at 520 nm which aligns with the intrinsic CsPbBr.sub.3 fluorescence.

[0057] The CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin film on glass substrate was clear with yellow tint in room light (FIG. 1e) and showed a strong green luminescence (520 nm, FWHM 20 nm) with UV excitation. AFM image in FIG. if indicates the compact and smooth film morphology with an RMS roughness of 3.94±0.52 nm. The CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 pervoskite thin film exhibited a QY as high as 20% and maintained 90% of the value for 9 month in ambient storage. The high quality and stability of the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin films enabled the development of multi-layer devices. The LED architecture [Al (60 nm)/ZnO (35 nm)/CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 (100 nm)/Poly-TPD (60 nm)/MoO3 (9 nm)/Au (15 nm)] chosen for this example is shown in FIG. 1g. The device operates via direct radiative e-h pair formation within the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 multi quantum wells (MQWs) by injecting charge carriers from neighboring charge transport layers (CTLs) as depicted in FIG. 1h. The emitted photons were out-coupled from the semi-transparent top Au electrode as demonstrated in FIG. 1i.

[0058] The CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 device showed an electroluminescence (EL) peak with the full width of half maximum (FWHM) of 20.9 nm solely from the imbedded CsPbBr.sub.3 nanoinclusions, which was sharper than the emission from CsPbBr.sub.3 NC device (FWHM=27.4 nm, FIG. 2a). The uniformity of CsPbBr.sub.3 NC size was attributed to the broad emission peak. Regardless of the emitter structure, the both of devices showed typical diode behavior. However, the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 device showed a turn-on voltage of 3.5 V close to the optical bandgap of Cs.sub.4PbBr.sub.6 indicating that the shell played a role of buffer layer to take over the charge injection into potential wells. At the same voltages, the brightness of the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 device (10,070 cd/m2) was improved by two orders of magnitude comparing to the CsPbBr.sub.3 NC device which led to a tenfold increase in current efficiency and power efficiency (1.64 cd/A and 0.77 lm/W). It was also found the CsPbBr.sub.3 NC device showed a higher break-down voltage up to 9 V. For the sake of date reproducibility and validity, the device measurements were conducted in on-off alternative mode (2 second for each acquisition) and averaged from 0.16 cm.sup.2 pixel by a CCD luminance meter. More importantly, operation stability was dramatically improved in CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 device. Under a constant current density (62.5 mA/cm2), 90% of the initial brightness (200 cd/m2) and current efficiency (1.4 cd/A) were maintained over 1000 hours in N.sub.2 glovebox. In contrast, the CsPbBr.sub.3 NC device showed a quick degradation to 10% in 20 hours.

[0059] Even though the solid cap significantly improved the device lifetime, the Cs.sub.4PbBr.sub.6 lattice was not chemically stable directly contacting water. As shown in FIG. 2f, the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 film was dissolved and significantly damaged in the presence of water. To further isolate H2O, the devices were finished with hydrophobic PDMS coating. Similarly, moisture is prone to change the microscopic morphology of CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin film leading to higher roughness and less coverage (Supplementary Figure S3) which reduced the transparency due to light scattering (FIG. 2g). However, optical changes were trivial with the PDMS coating. With sealed electrodes, the device could survive under the water for 500 s in FIG. 2h. Three devices were electrically connected in parallel (active area ˜0.48 cm.sup.2) for the water resistance demonstration in FIG. 2i.

[0060] To investigate the thermal robustness, a CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 device was heated and maintained for 30 s. Due to the half micrometer of the total device thickness, the heat could spread over the sample in a short period of time. The devices were then cooled back to 25° C. (room temperature) before the luminous and electrical measurements. As shown in FIG. 3a, the device showed no clear sign of brightness reduction when the temperature was below 100° C., which indicates the EL could be fully recovered from the heat impact. However, the brightness decreased linearly with temperature as the annealing went over 100° C. to 200° C. A single layer of CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 emitter was fabricated on glass substrate and put in a further investigation. The QY of the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 film was subsequently measured with post-treated temperature ascending from 40° C. to 200° C. and the results are shown in FIG. 3b. Similarly, a relative constant QY (˜25%) was achieved while the heat cycles were conducted below 100° C. This QY recoverability was attributed by the promoted dissociation of e-h pairs in CsPbBr.sub.3 lattice with extra thermal energy, which can be visualized by cooling the preheated sample on an edge of a steel table in FIG. 3c. Since metal has a higher thermal conductivity, the steel contact half of the sample showed a faster PL recovery than the half in the air. Above 100° C., the QY reduced linearly with annealing temperature and was gradually losing its recoverability blamed to perovskite crystalline phase transition or decomposition. The PL spectra at real-time temperatures were useful to understand the contributions from the phase change (permanent decay) and the e-h pair dissociation (reversible decay). In FIG. 3d, the CsPbBr.sub.3's 520 nm emission peak was diminishing as the heating temperature was tuned to 120° C. The PL intensity recovered partially in FIG. 3e when the sample was cooling back down to 25° C. The reversible portion thanks to the promoted e-h dissociation probability with more thermal energy at high temperature. The PL loss is rationalized by CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 composition change confirmed by XRD data.

[0061] Instead of rigid glass, the flexible PET was also used as substrate to build the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED. Device flexibility on plain and ITO coated PET substrates was investigated. In the irrigative bending tests with a constant current (62.5 mA/cm.sup.2), the ITO free device maintained over 90% of the initial brightness (70 cd/m.sup.2) up to 4,000 bending cycles as shown in FIG. 4a. The good flexibility is owing to the good ductility of the Al and Au electrodes. In contrast, 55% of the initial brightness was sacrificed within only 10 bends on ITO coat PET substrate and the device was died within 100 bends. This directly resulted from the significant decay of the voltage (FIG. 4b) due to the current leakage through the deep cracks initiated by the bottom ITO. The morphology of ITO-free device barely changed even after 10,000 bending cycles (FIG. 4c) while destructive ITO damages were created with only 10 bends and ripped the other function layers in FIG. 4d. FIG. 4e shows the luminance and current variation as a function of bending angle which ranges from −90° (compression) to +90° (tensile strain). The device exhibited stable luminescent output (100±5 cd/m2 at 0°, 95±2 cd/m2 at −90°, 85±5 cd/m2 at +90°) and current flow (48.75±5.63 mA/cm2 at 0°, 30.62±1.88 mA/cm2 at −90°, 33.13±6.88 mA/cm2 at +90°). The EL spectra (FIG. 4f) also remained intact and stable in the different bending positions. For demonstration, an 8×8 CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LED dot matrix was developed by patterning the bottom Al and top Au electrodes. Each display pixel was controlled to 1×1 mm.sup.2 (FIG. 4g). The pixel rows were selectively illuminated in different bending positions with no significant brightness change as shown in FIG. 4h.

[0062] The ligand assisted CsPbBr.sub.3 nanocrystals have a good stability in liquid suspension. However, once the nanocrystals are processed into the thin film, it suffers the stability issue and the surface agents become unwanted. There are three reasons: (1) The remained surface agents are normally sensitive to air not as stable as in solution. The instability of nanocrystal emitter in solid form is one of the major problems leading to the short lifetime of EL device. The loss of surface ligands could also cause the nanocrystals fusion or phase transition, which ultimately have negative impacts on radiative efficiency. (2) The excessive organic ligands in chemical synthesis make it difficult to dry the film because of the strong bonds between solvent molecules and the surface agents. The bottom “wet” film is a major issue for the deposition of top functional layer. (3) The organic ligands normally have high resistivity which increase the barrier for carrier injection to emitter under electric field. The consequences are higher driving voltage and low power efficiency which accelerate the device degradation.

[0063] The Cs.sub.4PbBr.sub.6 solid capping is an efficient and facile method to stabilize CsPbBr.sub.3 nanocrystals without additives. Unlike fuzzy organic ligands, the Cs.sub.4PbBr.sub.6 shell was demonstrated to be thick and compact which seals the CsPbBr.sub.3 nanocrystals air tight. There is barely moisture and air penetrations into the emission centers which significantly improved stability of CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 system. Crystalized by anti-solvent or/and thermal annealing. The CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 thin film is hard and flat in terms of morphology which eases the deposition of top layers. The solvent annealing is also a low temperature process which enables the devices on flexible plastic substrates. Meanwhile, the solid Cs.sub.4PbBr.sub.6 shell acts as a mechanical shield to reduce the friction between CsPbBr.sub.3 nanocrystals in distortional substrates. The type I heterojunction at the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 interface constructs a quantum well for the efficient exciton radiative recombination. Unlike carrier tunneling through organic ligands, the e-h pairs are formed in Cs.sub.4PbBr.sub.6 (˜3.7 eV) and transferred to CsPbBr.sub.3 (˜2.4 eV). The e-h pairs are trapped in CsPbBr.sub.3 confinement, then recombine leading to the photon emission. However, more works are required to carry out for a deep understanding of the energy transfer mechanism in the heterojunction to further improve the CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 LEDs' quantum efficiency.

[0064] Top-emitting structure of thin film LEDs allows driving transistors implemented under the LEDs, facilitating display contrast as well as its compatibility with the active driving circuits. Furthermore, due to the metal electrode replacing ITO, the top-emitting structure used dual metal electrodes which has a significantly improvement on the stabilities such as luminance, emission efficiency, and color accuracy.

[0065] Since OLED technology was first commercialized in 1997 by Tohoku Pioneer, the OLED display market has been well developed in the past two decades and is expected to garner $37.2 billion by 2020. However, the cost of OLED technology is still higher than its competitors such as LCD display. One of the main holds is the cost of high-purity, efficient, and stable organic semiconductors that require extremely high standard synthetic conditions and purification processes. For instant, the green emissive pi-conjugated materials are costly about 10 USD per m.sup.2 in an industrial scale comparing to 0.6 USD per m.sup.2 of Cs—Pb—Br materials. Regardless, the Cs—Pb—Br perovskite LED provides a dramatic high color purity that the organic semiconductor cannot compete. Even comparing with the state-of-the-art CdSe or InP quantum dots, the Cs—Pb—Br perovskite shows narrower FWHM and low cost, more importantly the ease of in-situ preparation. Nonetheless, typical type-I CdSe or InP QDs with rather thin (1-2 nm) shells are susceptible to becoming charged in actual devices, particularly at high current densities, resulting in substantial efficiency decrease and poor device stabilities.

[0066] Materials and Methods

Preparation of Cs—Pb—Br Precursor Solution

[0067] Cs—Pb—Br precursor was prepared by adding PbBr.sub.2 and CsBr in 5 ml vials with the molar ratio of 1:1.5 into Dimethyl sulfoxide (DMSO). The mixture was stirred for 2 hours at 65° C. until completely dissolved.

Cs—Pb—Br Thin Film Fabrication

[0068] The Cs—Pb—Br precursor solution was spin-coated onto the substrates in N.sub.2 filled glove box. The spin speed was accelerated to final speed of 4000 rpm through 1000 rpm in 3 seconds. A wet layer of Cs—Pb—Br perovskite is formed in the first 30 second. Then, two droplets of anti-solvent acetone was applied on the top of the spinning substrates to create a fast crystallization for CsPbBr.sub.3—Cs.sub.4PbBr.sub.6 formation. The duration of the entire spinning coat process is 60 seconds. For devices on glass, the samples were heated to 250° C. for 10 mins to improve the crystallization. For devices on PET substrate, 75° C. was used in post annealing to remove solvent residue.

Photoluminescence Quantum Yield (PLQY)

[0069] An integrating sphere was employed to determine the quantum yield (QY) of perovskite samples. The sample was added in a thin cuvette and was excited with a 405 nm blue LED light. The scattered excitation, along with emission, were collected by using a fiber optic coupled spectrometer (FLAME-S-XR1-ES). The whole spectrum was collected for both a reference (cuvette with DMSO only) and for the sample.

[0070] The ratio of the number of emission photons over the number of absorbed photons was used to determine the QY as shown in the equation below,

[00001]$\mathrm{QY}=\frac{S_{em}-R_{em}}{R_{exc}-S_{exc}}$

where Sem and Sexc are the integrated signal in the emission and excitation region, respectively, for the sample, and Rem and Rexc are the integrated signal in the emission and excitation region, respectively, for the reference. As a check for the accuracy of the system, the QY of Rhodamine 6G in ethanol was found to be 94.4%, close to the literature value of 95%. (Kubin, R. F.; Fletcher, A. N. Fluorescence quantum yields of some rhodamine dyes. J Lumin. 1982, 27, 455-462.)

X-Ray Powder Diffraction (XRD)

[0071] The crystalline phase analysis was performed by an X-ray powder diffractometer (Bruker D2 PHASER) in ambient.

Steady-State Photoluminescence (PL) Measurement

[0072] The PL of Cs—Pb—Br perovkite layers in different molar ratio was measured by a spectrofluorometer (PerkinElmer LS50B) on glass substrates.

Scanning Electron Microscopy (SEM)

[0073] A scanning electron microscope (JEOL 6330) was utilized to analyze the morphology of different molar ratio thin films and CsPbBr.sub.3 and Cs.sub.4PbBr.sub.6 microcrystals.

Transmission Electron Microscopy (TEM)

[0074] TEM images of Cs.sub.4PbBr.sub.6 nanograins were taken on a JEOL JEM-1200EX. CsPbBr.sub.3 and Cs.sub.4PbBr.sub.6 nanograins thin film was peeled off by a blade and placed on copper grid as imaging sample. The selected area electron diffraction patterns were achieved as well.

High-Resolution Transmission Electron Microscopy (HR-TEM)

[0075] HT-TEM measurements were performed on an aberration-corrected FEI Titan S 80-300 STEM/TEM microscope equipped with a Gatan OneView camera at an acceleration voltage of 300 kV.

LED Fabrication

[0076] Devices were built on a 2.54 cm×2.54 cm plain glass substrate. The glass substrates are cleaned in an ultrasonic bath with acetone followed by methanol and isopropanol for 1 hour each. A 100 nm thick layer of Al was pre-coated by thermal evaporator as bottom reflective electrode. The substrates were dry-cleaned for 30 min by exposure to an UV-ozone ambient. ZnO nanoparticle suspensions (˜35 nm) were spin-coated onto the Al-coated glass substrates at 4000 rpm for 30 s and baked at 100° C. for 30 min in vacuum oven. The substrates were transferred into glove box for further depositions. CsPbBr.sub.4—Cs.sub.4PbBr.sub.6 perovskite layers were deposited by spin-coating at 4000 rpm for 60 s from 1:1.5 PbBr.sub.2-to-CsBr precursor solution. Poly-TPD chlorobenzene solutions (concentration: 10 mg/mL) at 2000 rpm for 30 s were applied for hole transport layer. After baking, MoO3 (9 nm) and Au semi-transparent electrode (15 nm) were deposited using a thermal evaporation system through a shadow mask under a vacuum of 2×10.sup.−7 Torr.

LED Characterization

[0077] The LED devices in this report are measured in on-off mode inside of glove box at room temperature (25° C.) without encapsulation. Current versus voltage characteristics were measured using a Keithley 2400 source measure unit. Simultaneously, luminance (in cd/m.sup.2) was directly measured using a photometer (ILT 1400-A) with an optical fiber facing light-emitting pixel. The EL spectra for the devices were collected by an ILT 950 spectroradiometer (International Light Technologies).

TABLE-US-00006 TABLE VI Materials employed in the Examples Abbreviation Chemical name Supplier CsBr Cesium bromide Sigma-Aldrich PbBr.sub.2 Lead bromide Strem chemicals DMOS Dimethyl sulfoxide VWR International Poly-TPD Poly(4-butylphenyl- American Dye diphenyl-amine) Source ZnO

[0078] Various embodiments of the invention have been described in fulfillment of the various objectives of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention.