Influence of PMMA on All-Inorganic Halide Perovskite CsPbBr3 Quantum Dots Combined with Polymer Matrix

The poor stability of CsPbX3 quantum dots (QDs-CsPbX3) under wet conditions is still considered to be a key issue. In order to overcome this problem, this study presents a high molecular weight polymer matrix (polymethylmethacrylate, PMMA) incorporated into the QDs-CsPbBr3 to improve its stability and maintain its excellent optical properties. In this study, the Cs2CO3, PbO, Tetrabutylammonium Bromide (TOAB) powder, oleic acid, and toluene solvent were uniformly mixed and purified to prepare high-quality QDs powders. Then, hexane was used as a dispersing agent for the QD powder to complete the perovskite QDs-CsPbBr3 solution. Finally, a solution with different proportions of quantum dots CsPbBr3 and PMMA was prepared and discussed. In the preparation of thin films, firstly, a thin film with the structure of glass/QD-CsPbBr3/PMMA was fabricated in a glove box using a well-developed QDs-CsPbBr3 solution by changing the ratio of CsPbBr3:PMMA. The material analysis of QDs-CsPbBr3 thin films was performed with photoluminescence (PL), transmittance, absorbance, and transmission electron microscopy (TEM). The structures and morphologies were further examined to study the effect of doped PMMA on perovskite QDs-CsPbBr3.


Introduction
Colloidal quantum dots (QDs) have received extensive attention owing to their high photoluminescence quantum yield (PLQY), tunable emission wavelength, broad band absorption, and narrow emission band [1][2][3][4][5][6][7]. Due to its unique quantum confinement effect [7][8][9], researchers can achieve bandgap tuning by adjusting the emission color of QDs size and content [10,11]. Given these excellent optoelectronic properties, QDs are widely used in solar cells [12,13], light-emitting diodes (LEDs) [14,15], photodetectors [16,17], and lasers [18,19]. QDs are also used in nuclear medicine and in medical imaging [20][21][22][23]. In recent years, the power conversion efficiency of lead-halide perovskite in solar cells has rapidly climbed from 3.81% to 23.7% [24,25], and it took only ten years to achieve this breakthrough. The great success of lead-halide perovskite in the photovoltaic industry has also prompted researchers to explore the application of lead-halide perovskite in other related fields. The light absorbing material perovskite has a common structural formula ABX 3 , and A may be an organic cation CH 3 NH 3 + (MA), HC(NH 2 ) 2 + (FA), or an inorganic cation Cs + ; B is usually a divalent

Materials and Methods
The perovskite CsPbBr 3 solution was prepared by the chemical solution synthesis method. First, a Cs:Pb solution was prepared, and the Cs 2 CO 3 powder (162.9 mg) (Echo Chemical Co., Ltd., Miaoli, Taiwan) and PbO powder (233.2 mg) (Echo Chemical Co., Ltd., Miaoli, Taiwan) were added to 5 mL of oleic acid (Echo Chemical Co., Ltd., Miaoli, Taiwan). The Cs:Pb solution was stirred until it was transparent at 160 • C using a hot plate stirrer. The Cs:Pb solution was placed in a circulator oven and heated to 120 • C for 30 min to bake the solution. Following this, 5 mL of toluene (Echo Chemical Co., Ltd., Miaoli, Taiwan) was injected for dilution. A total of 1 mL of the Cs:Pb solution was taken out, which was then poured into 15 mL of toluene and stirred at room temperature for 5 min. The Tetrabutylammonium Bromide (TOAB) powder (54.68 mg) (Echo Chemical Co., Ltd., Miaoli, Taiwan), 0.5 mL of oleic acid, and 2 mL of toluene were added together to blend, and stirred at room temperature ambient to obtain a Br source solution. The resulting Br source was injected into the Cs:Pb solution to obtain the unpurified perovskite CsPbBr 3 solution. A total of 3 mL of ethyl acetate (Echo Chemical Co., Ltd., Miaoli, Taiwan) was added to the unpurified perovskite CsPbBr 3 solution, followed by the centrifugal process with 6000 rpm for 20 min to separate the green precipitate from the unpurified perovskite CsPbBr 3 precursor solution. The green precipitate was dried under vacuum for 12 h to remove the solvent to complete the purification step. The green precipitate CsPbBr 3 powder was then dissolved in 50 µL of hexane, and vortexed for 5 min using an ultrasonic oscillating machine to prepare a CsPbBr 3 perovskite QD solution. A mixture of 50 mg of PMMA powder (Uni-Onward Co., Ltd., New Taipei City, Taiwan) and 1 mL of toluene was stirred at 80 • C until the powder was completely dissolved. At this time, the solution was transparent and colorless, and it was left at room temperature for cooling to complete the PMMA solution. The QDs-CsPbBr 3 solution and the PMMA solution were mixed in different proportions, which were QDs-CsPbBr 3 :PMMA = 1:2, 1:1, 2:1, and 3:1. Finally, the QDs-CsPbBr 3 :PMMA mixed solution was coated on a slide glass at 1000 rpm for 15 seconds to complete the glass/QDs-CsPbBr 3 :PMMA film preparation, as shown in Figure 1.
In addition, the 50 µL of QD-CsPbBr 3 solution was spin-coated on the glass substrate at 1000 rpm for 15 s to complete the preparation of the glass/QDs-CsPbBr 3 film. Following this, 100 µL of the PMMA solution was spin-coated on the glass/QDs-CsPbBr 3 sample at 1000 rpm for 15 s to complete the film preparation of the glass/QDs-CsPbBr 3 /PMMA, as shown in Figure 2.
QDs-CsPbBr3:PMMA mixed solution was coated on a slide glass at 1000 rpm for 15 seconds to complete the glass/QDs-CsPbBr3:PMMA film preparation, as shown in Figure 1. In addition, the 50 μL of QD-CsPbBr3 solution was spin-coated on the glass substrate at 1000 rpm for 15 s to complete the preparation of the glass/QDs-CsPbBr3 film. Following this, 100 μL of the PMMA solution was spin-coated on the glass/QDs-CsPbBr3 sample at 1000 rpm for 15 s to complete the film preparation of the glass/QDs-CsPbBr3/PMMA, as shown in Figure 2.
The photoluminescence (PL) spectra was measured using a Protrustech UniRAM low temperature Raman/PL spectrophotometer system. The transmittance and absorbance spectra were measured using a Hitachi U-4100 UV/Vis/NIR Spectrophotometer (Hitachi, Tokyo, Japan). The size of perovskite QDs was characterized by transmission electron microscopy (TEM, Tecnai F30, Philips, Amsterdam, Netherlands).

Results and Discussion
In order to investigate the color change of QDs-CsPbBr3-doped PMMA solution, different QDs luminescent color changes were produced at different QDs-CsPbBr3:PMMA solution ratios, which, from left to right, PMMA, QDs-CsPbBr3:PMMA = 1:2, QDs-CsPbBr3:PMMA = 1:1, QDs-CsPbBr3:PMMA = 2:1, QDs-CsPbBr3:PMMA = 3:1, CsPbBr3, are shown in Figure 3a. The excited light source is a CW semiconductor laser (Homemade, Taipei, Taiwan) with a 405 nm wavelength and 5 mW of light of output power. When laser excitation was not used, it could be observed that the color of the QDs-CsPbBr3 solution was semitransparent, and the PMMA solution was nearly transparent, indicating that both have high transparency characteristics. As the PMMA ratio was increased, the color of the solution of QDs-CsPbBr3 tended to be more transparent. When excited by a 405 nm laser, the color of the QDs-CsPbBr3 complete the glass/QDs-CsPbBr3:PMMA film preparation, as shown in Figure 1. In addition, the 50 μL of QD-CsPbBr3 solution was spin-coated on the glass substrate at 1000 rpm for 15 s to complete the preparation of the glass/QDs-CsPbBr3 film. Following this, 100 μL of the PMMA solution was spin-coated on the glass/QDs-CsPbBr3 sample at 1000 rpm for 15 s to complete the film preparation of the glass/QDs-CsPbBr3/PMMA, as shown in Figure 2.
The photoluminescence (PL) spectra was measured using a Protrustech UniRAM low temperature Raman/PL spectrophotometer system. The transmittance and absorbance spectra were measured using a Hitachi U-4100 UV/Vis/NIR Spectrophotometer (Hitachi, Tokyo, Japan). The size of perovskite QDs was characterized by transmission electron microscopy (TEM, Tecnai F30, Philips, Amsterdam, Netherlands).

Results and Discussion
In order to investigate the color change of QDs-CsPbBr3-doped PMMA solution, different QDs luminescent color changes were produced at different QDs-CsPbBr3:PMMA solution ratios, which, from left to right, PMMA, QDs-CsPbBr3:PMMA = 1:2, QDs-CsPbBr3:PMMA = 1:1, QDs-CsPbBr3:PMMA = 2:1, QDs-CsPbBr3:PMMA = 3:1, CsPbBr3, are shown in Figure 3a. The excited light source is a CW semiconductor laser (Homemade, Taipei, Taiwan) with a 405 nm wavelength and 5 mW of light of output power. When laser excitation was not used, it could be observed that the color of the QDs-CsPbBr3 solution was semitransparent, and the PMMA solution was nearly transparent, indicating that both have high transparency characteristics. As the PMMA ratio was increased, the color of the solution of QDs-CsPbBr3 tended to be more transparent. When excited by a 405 nm laser, the color of the QDs-CsPbBr3 The photoluminescence (PL) spectra was measured using a Protrustech UniRAM low temperature Raman/PL spectrophotometer system. The transmittance and absorbance spectra were measured using a Hitachi U-4100 UV/Vis/NIR Spectrophotometer (Hitachi, Tokyo, Japan). The size of perovskite QDs was characterized by transmission electron microscopy (TEM, Tecnai F30, Philips, Amsterdam, The Netherlands).

Results and Discussion
In order to investigate the color change of QDs-CsPbBr 3 -doped PMMA solution, different QDs luminescent color changes were produced at different QDs-CsPbBr 3 :PMMA solution ratios, which, from left to right, PMMA, QDs-CsPbBr 3 :PMMA = 1:2, QDs-CsPbBr 3 :PMMA = 1:1, QDs-CsPbBr 3 :PMMA = 2:1, QDs-CsPbBr 3 :PMMA = 3:1, CsPbBr 3 , are shown in Figure 3a. The excited light source is a CW semiconductor laser (Homemade, Taipei, Taiwan) with a 405 nm wavelength and 5 mW of light of output power. When laser excitation was not used, it could be observed that the color of the QDs-CsPbBr 3 solution was semitransparent, and the PMMA solution was nearly transparent, indicating that both have high transparency characteristics. As the PMMA ratio was increased, the color of the solution of QDs-CsPbBr 3 tended to be more transparent. When excited by a 405 nm laser, the color of the QDs-CsPbBr 3 solution was observed to be green, and the PMMA solution was not luminescent. As the proportion of PMMA increased, the luminous intensity of QDs-CsPbBr 3 weakened, but it still retained a vivid green light. Figure 3b shows the PL excitation spectra of perovskite QDs-CsPbBr 3 solution, which was a mixture of CsPbBr 3 and PMMA with four different doping ratios of QDs-CsPbBr 3 :PMMA solution. It can be observed that the photoexcitation wavelength barely changes after doping with PMMA. They are within the limits of error. The location of the PL peak of the QDs-CsPbBr 3 solution was 513 nm. The QDs-CsPbBr 3 :PMMA with four different doping ratios of 1:2, 1:1, 2:1, and 3:1 were 512, 512, 512, and 513 nm, respectively. It can be observed that when the QDs-CsPbBr 3 solution was doped with various ratios of PMMA solution, the location of the PL peak barely shifted. This means that the PMMA does not influence the band structure of the QDs-CsPbBr 3 , but surrounds and protects the QDs-CsPbBr 3 to stabilize the material structure [41,42].
QDs-CsPbBr3 solution, which was a mixture of CsPbBr3 and PMMA with four different doping ratios of QDs-CsPbBr3:PMMA solution. It can be observed that the photoexcitation wavelength barely changes after doping with PMMA. They are within the limits of error. The location of the PL peak of the QDs-CsPbBr3 solution was 513 nm. The QDs-CsPbBr3:PMMA with four different doping ratios of 1:2, 1:1, 2:1, and 3:1 were 512, 512, 512, and 513 nm, respectively. It can be observed that when the QDs-CsPbBr3 solution was doped with various ratios of PMMA solution, the location of the PL peak barely shifted. This means that the PMMA does not influence the band structure of the QDs-CsPbBr3, but surrounds and protects the QDs-CsPbBr3 to stabilize the material structure [41,42].     Figure 5a show the structural diagrams and photographs excited without and with a UV laser (405 nm, 5 mW) of the QDs-CsPbBr3 film, QDs-CsPbBr3/PMMA film, and QDs-CsPbBr3:PMMA film. It can be observed that the QD-CsPbBr3 film excited by a laser exhibits a bright pure-green light. The luminance of the QD-CsPbBr3/PMMA film and QD-CsPbBr3:PMMA film excited by the laser seem darker than that of the QD-CsPbBr3 film owing to the total reflection effect and Snell's law. The refractive indexes of QD-CsPbBr3 and PMMA are 2.3 and 1.47, respectively [43,44]. Therefore, only around 12% of the light inside the CsPbBr3 film from the surface radiates to the outside, according to the light escape cone [45]. Figure 5b shows the PL spectrum of the QDs-CsPbBr3 film, QDs-CsPbBr3/PMMA film, and QD-CsPbBr3:PMMA film. The QD-CsPbBr3 film and QD-CsPbBr3/PMMA film peak locations were observed at 515 nm and 514 nm, respectively. Four different doping ratios of QD-CsPbBr3:PMMA films = 1:2, 1:1, 2:1, and 3:1 were 514, 515, 515, and 515 nm, respectively.  Figure 5a show the structural diagrams and photographs excited without and with a UV laser (405 nm, 5 mW) of the QDs-CsPbBr 3 film, QDs-CsPbBr 3 /PMMA film, and QDs-CsPbBr 3 :PMMA film. It can be observed that the QD-CsPbBr 3 film excited by a laser exhibits a bright pure-green light. The luminance of the QD-CsPbBr 3 /PMMA film and QD-CsPbBr 3 :PMMA film excited by the laser seem darker than that of the QD-CsPbBr 3 film owing to the total reflection effect and Snell's law. The refractive indexes of QD-CsPbBr 3 and PMMA are 2.3 and 1.47, respectively [43,44]. Therefore, only around 12% of the light inside the CsPbBr 3 film from the surface radiates to the outside, according to the light escape cone [45]. Figure 5b shows the PL spectrum of the QDs-CsPbBr 3 film, QDs-CsPbBr 3 /PMMA film, and QD-CsPbBr 3 :PMMA film. The QD-CsPbBr 3 film and QD-CsPbBr 3 /PMMA film peak locations were observed at 515 nm and 514 nm, respectively. Four different doping ratios of QD-CsPbBr 3 :PMMA films = 1:2, 1:1, 2:1, and 3:1 were 514, 515, 515, and 515 nm, respectively. Therefore, it can be observed that the PL peaks of the QD-CsPbBr 3 /PMMA film and QD-CsPbBr 3 :PMMA film almost do not shift when the PMMA doping ratio increases. This result was the same as the solution preparation characteristics. laser (405 nm, 5 mW) of the QDs-CsPbBr3 film, QDs-CsPbBr3/PMMA film, and QDs-CsPbBr3:PMMA film. It can be observed that the QD-CsPbBr3 film excited by a laser exhibits a bright pure-green light. The luminance of the QD-CsPbBr3/PMMA film and QD-CsPbBr3:PMMA film excited by the laser seem darker than that of the QD-CsPbBr3 film owing to the total reflection effect and Snell's law. The refractive indexes of QD-CsPbBr3 and PMMA are 2.3 and 1.47, respectively [43,44]. Therefore, only around 12% of the light inside the CsPbBr3 film from the surface radiates to the outside, according to the light escape cone [45]. Figure 5b shows the PL spectrum of the QDs-CsPbBr3 film, QDs-CsPbBr3/PMMA film, and QD-CsPbBr3:PMMA film. The QD-CsPbBr3 film and QD-CsPbBr3/PMMA film peak locations were observed at 515 nm and 514 nm, respectively. Four different doping ratios of QD-CsPbBr3:PMMA films = 1:2, 1:1, 2:1, and 3:1 were 514, 515, 515, and 515 nm, respectively. Therefore, it can be observed that the PL peaks of the QD-CsPbBr3/PMMA film and QD-CsPbBr3:PMMA film almost do not shift when the PMMA doping ratio increases. This result was the same as the solution preparation characteristics.  Figure 6a,b show the transmittance and absorbance spectrums of perovskite QDs-CsPbBr3 film, which were the QDs-CsPbBr3 film, QDs-CsPbBr3/PMMA film, and QDs-CsPbBr3:PMMA films with four different doping ratios. It can be found that the transmittance of the QDs-CsPbBr3 film and QDs-CsPbBr3/PMMA film was about 80%-90% in  Figure 6a,b show the transmittance and absorbance spectrums of perovskite QDs-CsPbBr 3 film, which were the QDs-CsPbBr 3 film, QDs-CsPbBr 3 /PMMA film, and QDs-CsPbBr 3 :PMMA films with four different doping ratios. It can be found that the transmittance of the QDs-CsPbBr 3 film and QDs-CsPbBr 3 /PMMA film was about 80%-90% in the visible range. When the proportion of doped PMMA increases, the transmittance of the QDs-CsPbBr 3 :PMMA film also becomes higher. When the QDs-CsPbBr 3 :PMMA film = 1:2, the transmittance at a wavelength of 400-700 nm was higher than about 90%. However, when the QDs-CsPbBr 3 :PMMA film = 3:1, the transmittance rate drops below 80%, indicating that the QDs concentration ratio is too high, which affects the optical properties of the QDs-CsPbBr 3 :PMMA film. On the other hand, the absorption steps of QDs-CsPbBr 3 film, QDs-CsPbBr 3 /PMMA film, and four different ratios of QDs-CsPbBr 3 :PMMA films prepared as thin films were observed at 520 nm, corresponding to the band gap of the QDs-CsPbBr 3 . This result was also the same as the solution samples.
Materials 2019, 12, x FOR PEER REVIEW 6 of 10 the visible range. When the proportion of doped PMMA increases, the transmittance of the QDs-CsPbBr3:PMMA film also becomes higher. When the QDs-CsPbBr3:PMMA film = 1:2, the transmittance at a wavelength of 400-700 nm was higher than about 90%. However, when the QDs-CsPbBr3:PMMA film = 3:1, the transmittance rate drops below 80%, indicating that the QDs concentration ratio is too high, which affects the optical properties of the QDs-CsPbBr3:PMMA film. On the other hand, the absorption steps of QDs-CsPbBr3 film, QDs-CsPbBr3/PMMA film, and four different ratios of QDs-CsPbBr3:PMMA films prepared as thin films were observed at 520 nm, corresponding to the band gap of the QDs-CsPbBr3. This result was also the same as the solution samples. In order to observe the lattice structure, actual dispersion, and particle size of QDs, TEM analysis was employed to compare the changes before and after doping with PMMA. The preparation of the samples was then applied as a droplet on a copper grid using a dropper, directly, and baked at 40 °C for 20 min for TEM analysis. The TEM images of perovskite QDs-CsPbBr3 and QDs-CsPbBr3:PMMA are shown in Figure 7. It can be seen from Figure 7a-d that the particle size of QDs-CsPbBr3 was about 15 nm, of which the average particle size was In order to observe the lattice structure, actual dispersion, and particle size of QDs, TEM analysis was employed to compare the changes before and after doping with PMMA. The preparation of the samples was then applied as a droplet on a copper grid using a dropper, directly, and baked at 40 • C for 20 min for TEM analysis. The TEM images of perovskite QDs-CsPbBr 3 and QDs-CsPbBr 3 :PMMA are shown in Figure 7. It can be seen from Figure 7a-d that the particle size of QDs-CsPbBr 3 was about 15 nm, of which the average particle size was obviously larger, and the morphology and arrangement were less irregular. After doping with PMMA, the particle size of QDs-CsPbBr 3 :PMMA was about 5-10 nm, of which the average particle size was significantly smaller, and its morphology and arrangement were more regular, as shown in Figure 7e-h.

Conclusions
In conclusion, we successfully used Cs2CO3, PbO, TOAB powder and oleic acid, toluene, and other solvents to prepare high-quality QDs powder, and then used hexene as a dispersing agent for QDs powder to complete the perovskite QDs-CsPbBr3 solution. By doping PMMA, the optical transparency of QDs-CsPbBr3 can be increased. It can be seen from the TEM image that the particle size presented is smaller, and its morphology and arrangement are also more regular. It is particularly clear that the geometry of QD changes from rectangular to square. Due to the low photoluminescent background of PMMA, the excellent optical properties of QDs-CsPbBr3 can be maintained.

Conclusions
In conclusion, we successfully used Cs 2 CO 3 , PbO, TOAB powder and oleic acid, toluene, and other solvents to prepare high-quality QDs powder, and then used hexene as a dispersing agent for QDs powder to complete the perovskite QDs-CsPbBr 3 solution. By doping PMMA, the optical transparency of QDs-CsPbBr 3 can be increased. It can be seen from the TEM image that the particle size presented is smaller, and its morphology and arrangement are also more regular. It is particularly clear that the geometry of QD changes from rectangular to square. Due to the low photoluminescent background of PMMA, the excellent optical properties of QDs-CsPbBr 3 can be maintained.