UV-Green Emission from Organolead Bromide Perovskite

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 15041−15046

UV-Green Emission from Organolead Bromide Perovskite Nanocrystals Gopi C. Adhikari,† Hongyang Zhu,*,†,‡ Preston A. Vargas,† and Peifen Zhu*,† †

Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, Oklahoma 74104, United States School of Physics and Electronic Engineering, Linyi University, Linyi 276005, China

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‡

ABSTRACT: Ultraviolet (UV)-green emitting MAPbBr3 nanocrystals were synthesized at room temperature, employing a costeffective solution-based method. The size control of nanocrystals was achieved through varying ligand and solute concentrations, which resulted in a tunable band gap and emission spectrum. The growth mechanism as well as the effect of ligand concentration on the structural and optical properties were studied in detail. The excitation spectra extended from the blue to UV region. This indicates that these perovskites are promising photon down conversion materials, which can combine with III-nitride UV/ blue light-emitting diodes (LEDs) to emit white light. This work may bring III-nitride-based white LEDs one step closer to widespread adoption in general illumination market because the large emission range that has been produced with the ligand-assisted reprecipitation process is an important milestone in the path to justifying commercialization. abundant chemical components.11−25 A recent study showed that halide perovskites have narrower luminescence peaks, broader color gamut, lower cost, and higher quantum yield26 compared to typical Cd-based quantum dots (QDs). Perovskite QDs have been proven capable of greatly improving the performance of LEDs because of their unique properties.27−30 Therefore, halide perovskites have great potential for application in the next generation of light-emitting devices. In this work, the narrow line-width emissions were obtained from MAPbBr3 nanocrystals under near UV/blue excitation by employing a low-cost solution-based method. The MAPbBr3 nanocrystals with various thicknesses and sizes were synthesized by precisely controlling the amount of ligand used in the reaction, which leads to tunable emission due to quantum confinement. A cheap and facile solution-processable method to tune the emission from violet-green was demonstrated by varying the organic ligand and solute concentrations. Thus, this unique technique renders the use of chloride-based perovskites, most often used to emit blue light,31 redundant. This new process is the simplest synthesis method yet known. Thus, the bromide-based perovskites are promising photon down conversion materials, which can combine with III-nitride near UV/blue LED chips to generate white light.

1. INTRODUCTION High-efficiency ultraviolet (UV)/blue light-emitting diodes (LEDs) have been realized in recent decades because of tremendous efforts devoted to the improvement of material quality and device structure.1−4 There has been a rapid development of III-nitride-based LEDs for modern lighting and display technology because of their superior efficiency compared to conventional lighting technologies.5 The low efficiency of green LEDs because of the fundamental phase separation issue in the high In-content InGaN quantum well layer is a barrier to obtaining white LEDs with high efficiency, color rendering index, and chromaticity stability.6 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 the desired color quality.7−9 However, the limited material availability of the rare-earth elements used in the current generation of down conversion phosphors for LEDs increases the initial cost of white LEDs.10 Therefore, finding more common photon down conversion materials to couple with UV/blue LEDs is critical toward generating economical white light emission from LEDs. The key for obtaining photon down conversion materials is to acquire high quantum yield materials with emission in the blue, green, and red ranges while having a narrow full-width at half-maximum (FWHM). Organic−inorganic hybrid perovskites have been proven to be promising semiconductor materials for photovoltaic applications because of the tunable and direct band gap, high absorption coefficient, low carrier recombination rate, long charge diffusion length, good electrical conductivity, and earth© 2018 American Chemical Society

Received: May 26, 2018 Revised: June 2, 2018 Published: June 4, 2018 15041

DOI: 10.1021/acs.jpcc.8b05049 J. Phys. Chem. C 2018, 122, 15041−15046

Article

The Journal of Physical Chemistry C

2. EXPERIMENTAL METHOD In a typical synthesis process, 0.05 g of MABr and 0.1 g of PbBr2 were added into the mixture of 1 mL of anhydrous N,Ndimethylformamide (DMF), 17 mL of γ-butyrolactone (GBL), and 30 μL of octylamine under magnetic stirring. The solution became clear after the solids were dissolved completely. Then, this precursor solution was dropped into 10 mL of toluene with vigorous stirring at room temperature. The obtained colloidal solution was centrifuged at 5000 rpm for 5 min to separate the aggregates of precipitates. The supernatant was collected in a clean vial and centrifuged again at 16 000 rpm for 15 min. The precipitate was collected by discarding the remaining supernatant. The obtained nanocrystals were then redispersed into toluene to obtain a nanocrystal suspension. The concentrations of the solutes and the ligand were varied to obtain a plethora of distinct sizes of nanocrystals, which resulted in emissions of different wavelengths from UV-green. Here, octylamine was used as an organic ligand to control the growth of the nanocrystals. GBL is used here as an antisolvent, which facilitates easier precipitation of nanocrystals. The changing ligand and solute concentrations during the synthesis process are summarized in Table 1.

3. RESULTS AND DISCUSSION The crystal structures of these samples were confirmed by XRD patterns, as shown in Figure 1. The diffraction peaks at ∼14.9°,

Figure 1. XRD patterns of MAPbBr3 perovskites synthesized with various ligand concentrations. The patterns are ordered such that the descending patterns are indicative of increasing ligand concentration.

∼30.08°, and ∼45.9° belong to MAPbBr3 perovskites and are assigned to miller indices of (100), (200), and (300), respectively.22,32 The strong diffraction peaks at low angles (below 10°) in Figure 1 are due to periodic superstructures.33 The peak present at 12.2° does not correspond to a perovskite structure, but arises as a result of MABr in the sample.34 When the structure of the nanocrystals transitions to that of nanoplatelets, the superlattice behavior of these nanocrystals becomes relevant to the XRD patterns and begins to show stronger peaks than the individual perovskites themselves. This causes the main perovskite peaks to become substantially subdued in comparison to the superlattice peaks. TEM images were also taken to investigate the effect of ligand and solute concentrations on the structural properties of these samples. The transformation of the perovskite structure from three-dimensional at a low ligand concentration to quasi two-dimensional with decreasing number of perovskite monolayers was observed when the concentration of ligand increased. Cube-shaped MAPbBr3 crystals were obtained without adding octylamine in the reaction, as shown in Figure 2a. The dimensions of these crystals are ∼200 nm. These large cubes appear in this synthesis type because this is the natural result of unrestricted crystal growth without the use of a ligand. With the relatively large growth because of the low-ligand environment, the quantum confinement effect would be marginal for these larger nanocubes, and a clear band would be formed, which means that the emission wavelength is maximum. This also means the emission line of this specific material has the lowest energy. The size of cube-shaped MAPbBr3 crystals is significantly decreased by adding 30 μL of octylamine and decreasing the solute concentration. The relatively restricted crystal growth because of an increased ligand concentration resulted in the formation of nanocubes in Figure 2b in comparison to the cubes in Figure 2a. The nanoplates in Figure 2c were obtained by increasing the ligand concentration (60 μL) and decreasing the solute concentration. A mixture of nanoplates and nanoplatelets in Figure 2d was obtained when the ligand concentration was further increased (90 μL). Pure nanoplatelets were obtained when the amount of ligand used was increased to 105 μL (Figure 2e) and 120 μL

Table 1. Amount of Ligand, Solutes, and Solvents Used in the Experimentsa

a

sample ID

octylamine (μL)

MABr (g)

PbBr2 (g)

DMF (mL)

GBL (mL)

(a) (b) (c) (d) (e) (f)

0 30 60 90 105 120

0.05 0.05 0.05 0.05 0.05 0.05

0.10 0.10 0.10 0.10 0.10 0.10

4.5 1 2 2 2 2

0 17 34 34 34 34

All figures will be labeled by the same sample ID as noted below.

Note that blue emission is commonly obtained from chloride-based perovskites.31 Here, blue emission was obtained from bromide-based perovskites. Bromide-based perovskites are cheaper to synthesize than chloride-based perovskites. The chemicals used for synthesizing chloride-based perovskite materials are comparatively expensive. Also, it takes more time to completely dissolve the solutes, and the reaction requires significantly more heat to complete, thus making the replacement of chloride-based perovskites with bromide-based perovskites not only feasible, but preferable. X-ray diffraction (XRD) measurements were carried out using a Rigaku SmartLab diffractometer with Cu Kα1 radiation, λ = 1.54 Å operating at 40 kV and 44 mA to study the crystal structure of the as-grown samples. These patterns were obtained at room temperature at an angular range (2θ) of 2°−70° with a step of 0.01°. Transmission electron microscopy (TEM) measurements were performed using a Hitachi H-7000 transmission electron microscope at 75 kV to visually characterize the structure of the nanocrystals. Photoluminescence (PL) spectra were measured using a spectro-fluorophotometer (Shimadzu, RF-6000), with a xenon lamp as the excitation source. Absorption spectra were taken using a Varian Carry 50 Scan UV spectrophotometer in the range of 200−800 nm. 15042


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Figure 2. TEM images of MAPbBr3 nanocrystals synthesized at various ligand concentrations. The ligand concentration increases from (a−f).

(Figure 2f). In Figure 2e,f, the nanoplatelets are stacked with an average spacing of 1.30 and 1.83 nm, length of 19.08 and 15.6 nm, and thickness of 2.28 and 1.73 nm, respectively. As the increasing ligand concentration forces the attached ligands to occupy more space between the nanoplatelets, the thickness and the lateral dimension of the nanoplatelets decrease. TEM images revealed periodic and well-ordered nanoplatelets, thus confirming the presence of superlattices. In these images, some nanocrystals are brighter and some are darker. The darker places on the image indicate that there is overlapping between the nanocrystals. The absorption spectra were measured to study the effect of ligand concentration on the band gap of those nanocrystals. The band gap of this material was tuned by changing the ligand and solute concentrations. This tuning was observed across the UV−visible range in the absorption spectra, as shown in Figure 3, which indicates that the synthesized compounds have good absorption in the visible region. The peak in the absorption spectrum is attributed to exciton absorption. The peak is blueshifted with increasing ligand and decreasing solute concentrations because of the quantum confinement effect. Quantum

confinement is a change in the optical and electronic properties, which occurs when the dimension of nanocrystals is in the order of the exciton Bohr radius (the distance between electrons and holes) or less. This effect is also called size quantization. In this effect, the nanocrystals experience changes to their band structure because of the particle size being comparable to the spatial extent of the wave function. Then, the valence and conduction bands split into discrete and quantized energy levels because of the confinement of charge carriers. The corresponding band gaps were calculated from these absorption spectra, and the obtained band gap is tunable from ∼2.31 to 2.83 eV. In Figure 4, the optical band gap has an approximately

Figure 4. Band gap of MAPbBr3 perovskites as a function of the amount of ligand used in the reaction. The linear fit shows that there is a clear correlation between ligand use and band gap.

linear correlation with the ligand concentration. Thus, the band gap can be controllably tuned. This demonstrates a broadly variable band gap. The emission spectra of MAPbBr3 nanocrystals are shown in Figure 5. The peak is red-shifted with decreasing ligand and increasing solute concentrations. When increasing the solute concentration, it allows for more reaction collision and therefore more growth. Allowing for more reaction collision leads to larger crystal growth and red-shifted emission because of diminishing quantum confinement.

Figure 3. Absorption Spectra of MAPbBr3 perovskites synthesized at various ligand concentrations. The ligand concentration increases from (a−f). The peak absorption wavelength is blue-shifted with an increase in the amount of ligand present in the reaction. 15043


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Figure 6. Excitation spectra of halide perovskites at various ligand concentrations. The predominantly broad-band profiles for the excitation show that this material is a reasonable partner to InGaN LEDs because InGaN emission is contained by MAPbBr3 excitation.

Figure 5. Emission Spectra of MAPbBr3 perovskites synthesized at various ligand concentrations. The emission profile becomes thinner with increasing ligand use.

Conversely, increasing the ligand concentration causes the crystal growth to be throttled by the ligand, thereby resulting in blue-shifted emission due to a waxing quantum confinement effect.29,35−37 This occurs because of the ligands present in the final solution binding to the surface of the growing nanocrystals as they develop, forcing a stop to growth once the ligands bind to all sides of the nanocrystals. The difference between the absorption and emission peak wavelengths is attributed to Stokes shift. If we further increase the amount of ligand to 180 μL, the emission peak is further blue-shifted and limited to 407 nm, as shown in Figure 5. The peak wavelength of the emission spectra of the nanocrystals as well as the band gap with various ligand and solute concentrations are summarized in Table 2.

indicates that these materials are favorable to be excited by a wide range of wavelengths. The highly tunable emission spectrum is also capable of covering a significant portion of the visible spectrum. Therefore, these perovskites can serve as photon down converters in III-nitride-based white LEDs. There are some differences in the profile of the excitation spectra, which are due to quantum confinement. The nanoplatelet samples in Figure 6e,f tend to have higher excitation behavior at the higher energy end of their spectra, whereas the nanoplates in Figure 6b,c have an excitation preference for the middle of their spectra, and cubes in Figure 6a have an excitation maximum at the lower energy bounds of their spectra. The blue shift in the excitation spectra for nanocrystals experiencing quantum confinement occurs in a manner that is logically consistent with that of the effect of quantum confinement at the emission spectra. The quantization of energy levels at the bottom end of the conduction band artificially increases the band gap of the material, leading to blue-shifted emission. The same effect can be applied to the higher end of the conduction band, forcing photons to hit higher energy levels than normal to excite an electron in the perovskite, thereby causing the blue shift in the peak of the excitation spectra. Furthermore, the XRD and TEM data were used to study the stability of these kind of nanocrystals. These materials were kept in air at ambient conditions (humidity ≈ 50−60%) for 15 days. No changes were observed in the XRD patterns and TEM images, as shown in Figure 7. This stability would allow for these perovskites to be layered onto optoelectronic devices without an inert gas glovebox.

Table 2. Peak Emission Wavelength, FWHM, and Band Gap with Ligand Amount sample ID peak emission wavelength (nm) (a) (b) (c) (d) (e) (f)

544 529 510 478 458 440

FWHM (nm)

band gap (eV)

22 21 25 26 16 15

2.31 2.36 2.52 2.73 2.78 2.83

The pure and sharp color spectra extended from UV to green (407−544 nm) with narrow line-width emission. The narrow emission is demonstrated by the FWHM shown in Table 2. For pure blue and green emissions, the FWHM is ∼15 and ∼21 nm, respectively. These values are significantly smaller than the previously reported values.11,38−40 This is attributed to the narrow size distribution of these nanocrystals and makes them exceptionally advantageous for optoelectronic applications. This narrow emission is characteristic of significantly strong and pure light.22,41−43 Note that UV-blue-green tunable emission was achieved from MAPbBr3 nanocrystals by simply adjusting the ligand concentration in this work. We will investigate the quantum yield and PL lifetimes of these nanocrystals in the future work. Figure 6 shows the excitation spectra as related to the corresponding peak emission wavelengths. These spectra are broad band and almost the same shape. Broad-band excitation

4. CONCLUSIONS In summary, using a low-cost solution-based synthesis method, MAPbBr3 nanocrystals were found to have tunable emission (UV-green) and band gap by varying the ligand and solute concentrations. The band gap energy of MAPbBr3 was found to have an approximately linear relationship with the ligand concentration. The particular innovation here is the extraordinarily wide range of possible emission spectra that can now be obtained from organic−inorganic bromide perovskites. By taking the simple idea of ligand-assisted reprecipitation and substantially expanding the boundaries of ligand application to the synthesis of nanocrystals, the possible emission range of 15044



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bromide perovskites has now expanded to encompass 54% of the visible range of wavelengths by only adjusting the direct amount of ligands. This extension of the reprecipitation process will allow for the production of highly versatile optoelectronic devices with a significantly lower amount of materials, thus justifying the excitement behind the perovskite material.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +1 (918) 631-5477 (H.Z.). *E-mail: [email protected]. Phone: +1 (918) 631-5125 (P.Z.). ORCID

Peifen Zhu: 0000-0002-5269-541X Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 7. (i) XRD patterns and (ii) TEM images of selected samples after 15 days in air.

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ACKNOWLEDGMENTS

This work was supported in part by The University of Tulsa through a Startup Fund, in part by The University of Tulsa Faculty Development Summer Fellowship, in part by the National Natural Science Foundation of China under grant 11774128, and in part by the Natural Science Foundation of Shandong Province under grant ZR2018JL003. The XRD patterns were measured by Dr. Alexei Grigoriev. The absorption spectra were measured in Dr. Parameswar Harikumar’s laboratory. Both are from the Department of Physics and Engineering Physics at The University of Tulsa. The authors would like to acknowledge the support and the help. 15045


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UV-Green Emission from Organolead Bromide Perovskite

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