Uniform PMMA-CH₃NH₃PbBr₃ Nanoparticle Composite Film for Optoelectronic Application

Vol. 27, No. 6 (2017)

307

Uniform PMMA-CH

3

NH

3

PbBr

3

Nanoparticle Composite Film for Optoelectronic Application

Artavazd Kirakosyan, Seokjin Yun and Jihoon Choi

^†

Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea

(Received February 23, 2017 : Revised April 3, 2017 : Accepted April 18, 2017)

Abstract

Organometal halide perovskite materials, due to the tunability of their electronic and optical properties by control of composition and structure, have taken a position of significant importance in optoelectronic applications such as photovoltaic and lighting devices. Despite numerous studies on the structure - property relationship, however, practical application of these materials in electronic and optical devices is still limited by their processability during fabrication. Achieving nano-sized perovskite particles embedded in a polymer matrix with high loading density and outstanding photoluminescence performance is challenging. Here, we demonstrate that the careful control of nanoparticle formation and growth in the presence of poly(methyl methacrylate) results in perovskite nanoparticle - polymer nanocomposites with very good dispersion and photoluminescence. Furthermore, this approach is found to prevent further growth of perovskite nanoparticles, and thus results in a more uniform film, which enables fabrication using the perovskite nanoparticles.

Key words

^CH3NH₃PbBr₃, methyl ammonium bromide, perovskite nanoparticle, luminescence, composite.

1. Introduction

In recent years, the organometal halide perovskite ma- terials have attracted considerable attention due to the tunability of the physical properties arising from their chemical diversity.

^1-6)

AMX

₃

general formula is commonly used to present the family of these materials, where A is an organic ammonium cation(methyl ammonium, ethyl ammonium, etc.),

^1,2)

M is a metal cation(Pb, Sn, etc.),

^3,4)

X is a halogen atom(Cl, Br, and I).

^5,6)

Owing to its vast variety of chemical composition, those materials exhibit a wide range of unique electronic and optical properties that are highly required in novel electronic and optical applications. The other powerful tool to tune the properties of these material is their size

^5-8)

and shape diversity. For example, CH

₃

NH

₃

PbBr

₃

shows high photoluminescence efficiency and color tunability as the particle size goes to nano-scale range,

^6,7)

thus enabling its application in light emitting devices,

^8-14)

solar cells,

^7,15)

sensors,

^16,17)

piezoelec- tric devices,

^18-20)

etc.

For lighting, photovoltaic, and piezoelectric devices with a planar structure, where perovskite particle - polymer

composite is required as an active layer,

8,12-15,21-23)

a precise control of particle dispersion and their concentration in polymer matrix, and film uniformity are essential to pre- vent their aggregation and achieve a uniform coating.

^8,12,15)

Typically, perovskite - polymer composites have been prepared by combining a polymer with the precursors of the desired perovskite material in a good solvent such as N,N-dimethylformamide(DMF) and spin-coating on a sub- strate. For example, Wang et al. demonstrated perovskite nanoparticle-based composite can be used as an active layer in light emitting diode, however, it was limited to very low particle concentration.

²²⁾

Therefore, this method would result in micro-scale large aggregates when high density precursor solution is used to achieve dense particle coverage.

^8,14)

Here, we report on a simple method to fabricate a thin film of poly(methyl methacrylate) (PMMA)-CH

₃

NH

₃

PbBr

₃

perovskite nanoparticle composites. We showed that the presence of PMMA during the formation of perovskite NPs suppresses their aggregation, and thus results in a uniform film with higher luminescence, compared to the conventional perovskite composites.

†Corresponding author

E-Mail : [email protected] (J. Choi, Chungnam Nat'l Univ.)

©Materials Research Society of Korea, All rights reserved.

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creative- commons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

& Metals. CH

₃

NH

₃

Br was synthesized by the previously reported method.

^6,24,25)

The reaction was carried out in an ice bath (0

^o

C, 2 h) to effectively remove the heat generated during the reaction and to prevent reagent evaporation.

The water solvent was evaporated by a rotary evaporator at 40~45

^o

C. Finally, the obtained precipitate was washed with diethyl ether for three times and dried under vacuum (60

^o

C, 6 h) condition. The precursor solution for CH

₃

- NH

₃

PbBr

₃

was prepared by dissolving CH

₃

NH

₃

Br (0.16 mmol) and PbBr

₂

(0.2 mmol) dissolved in 5 mL of DMF that contains 20 μL of n-octylamine and 0.5 mL of oleic acid as a surfactant. PMMA-CH

₃

NH

₃

PbBr

₃

nanocomposite solution was fabricated by the precipitation method in the presence of PMMA ( M

W

= 120,000 g/mol).

⁶⁾

Precursor solution (2 mL) was dropwise added into the PMMA (200 mg) in toluene (10 mL) under vigorous stirring. For the reference sample, the NPs were synthesized in toluene, and PMMA (200 mg) was subsequently added. PMMA- perovskite composite was obtained by centrifugation at 7000 rpm for 30 min and dispersing in toluene (0.5 mL).

Thin film is fabricated by spin-coating on an ozone- treated slide glass. The phase formation is measured by X-ray diffraction using a Bruker AXS D8 diffractometer with Cu-K α radiation at λ = 1.54 Å. The scanning electron microscope(SEM) and the transmission electron micros- cope(TEM) was performed on a Hitachi S4800 and JEM 2100, respectively. UV-vis absorption spectra(OA) were obtained using Shimadzu UV-2600 spectrophotometer. Pho- toluminescence(PL) spectra were recorded using Hitachi F-7000 fluorescence spectrometer at room temperature.

3. Results and Discussion

Fig. 2a shows the typical appearance of as-prepared PMMA-CH

₃

NH

₃

PbBr

₃

perovskite nanocomposite solution under daylight that emits bright blue-green light under ultra violet(UV) illumination (365 nm). It should be noted that the reference sample shows the identical appearance.

XRD pattern(Fig. 2b) confirms a formation of CH

₃

NH

₃

- PbBr

₃

nanocrystals belonging to a space group of Pm-3m with a lattice spacing, a = 5.9896 Å. Electron micrographs show that the solution of the CH

₃

NH

₃

PbBr

₃

perovskite formed in the presence of PMMA consists of ~5 nm perovskite NPs(Fig. 2(c-e)).

Fig. 3 shows a comparison of UV-vis absorbance and photoluminescence emission spectra of PMMA - CH

₃

NH

₃

-

PbBr

₃

nanocomposites. The UV-vis absorption spectra of all samples exhibit a band edge at 492 nm (~2.52 eV).

The band edge value is blue-shifted by ~55 nm (250 meV) from that of the bulk CH

₃

NH

₃

PbBr

₃^26-27)

due to quantum confinement effect.

^6,24,25)

In the solution, PL emission spectra are peaked at 496 nm (2.54 eV), and the full width at half maximum(FWHM) value of the com- posite and the reference is 22 nm, respectively. PL emis- sion is also blue-shifted by ~40 nm compared to the bulk CH

₃

NH

₃

PbBr

₃

.

^26-27)

Small Stokes shift (~10 meV) implies that the PL emission originates from excitonic recom- bination.

²⁷⁾

In the film, the PL emission of the PMMA - CH

₃

NH

₃

PbBr

₃

is red-shifted by 10~15 nm compared to that of the solution while the band edge on absorbance spectra does not show a significant change. The red-shift of PL emission for film could be originated from the re- absorption of the shorter wavelength PL emission bet- ween CH

₃

NH

₃

PbBr

₃

particles in the proximity of the CH

₃

NH

₃

PbBr

₃

nanoparticles is dominant in the film case.

In contrast, the absorption edge of film remained the same suggesting that there is no particle growth and thus the particles size remains the same. In addition, we calculated the ratio(IPL/OA) of the integral PL emission (underline area of the PL emission spectra, IPL) to optical absorbance(OA) at 365 nm. This parameter rela- tively characterizes the power conversion efficiency. In the solution, the IPL/OA

_reference

is 1.4 times higher than IPL/OA

_composite

, whereas the IPL/ OA

_composite

is 1.8 times higher than IPL/ OA

_reference

in the film.

To understand the origin of enhanced PL performance in the film, we further examined the surface morphology of the film for the reference and the composite(Fig. 4).

The composite sample shows larger coverage compared

to the reference sample where micro-scale aggregates are

observed in both cases(Fig. 4(a-b)). Furthermore, higher

magnification images(Fig. 4(c-d)) exhibit large amounts

of such aggregates in the reference sample compared to

the composite sample where the individual CH

₃

NH

₃

Pb-

Br

₃

perovskite particles form a uniform film. We per-

formed energy disperse x-ray spectroscopy(EDS) to reveal

the compositional variation in Fig. 4e. In spot 1, a large-

Fig. 1. Schematic illustration of the experimental flow to prepare PMMA-CH₃NH₃PbBr₃ nanoparticle composites.

size particle shows about 1:3 ratio of Pb to Br contents, confirming the atomic ratio of CH

₃

NH

₃

PbBr

₃

perovskite materials. Similarly, the uniform film region in spot 2 also shows the same atomic ratio, indicating the presence of CH

₃

NH

₃

PbBr

₃

perovskite nanoparticles in PMMA. In contrast, negligible amounts of Pb is observed in the uncovered area (spot 3), indicating the absence of CH

₃

- NH

₃

PbBr

₃

perovskite nanoparticles. Br/C ratio in spot 3

results from the presence of PMMA along with methyl- ammonium bromide and octylammonium bromide. Syn- thesis of perovskite particles in the presence of PMMA is shown to suppress their aggregation into coarse clusters, and thus form a uniform film with homogeneous particle distribution, smooth coverage, and enhanced PL perfor- mance compared to the reference.

Fig. 2. (a) As-prepared PMMA-CH3NH₃PbBr₃ perovskite nanocomposite solution under day light and under UV illumination (365 nm) and (b) corresponding XRD pattern. (c-e) Low and high magnification of TEM images of the PMMA-CH₃NH₃PbBr₃ perovskite nano- composites prepared by drop-casting at low concentration where the squares denote the magnified area.

Fig. 3. (a) UV-vis absorption and (b) PL emission spectra of the PMMA-CH₃NH₃PbBr₃ perovskite nanocomposite and reference. The solid and dashed lines denote PMMA-CH₃NH₃PbBr₃ perovskite nanocomposite solution and film, respectively. PL was recorded with an excitation source (365 nm).

4. Conclusions

To summarize, we report PMMA-CH

₃

NH

₃

PbBr

₃

perovskite nanoparticle-based composite film. We intro- duced PMMA during perovskite particle synthesis process that prevents the particle aggregation, thus enables uniform film formation. PMMA-CH

₃

NH

₃

PbBr

₃

perovskite nano- composite showed improved PL performance and higher PL emission to absorption ratio over a PMMA-CH

₃

NH

₃

- PbBr

₃

perovskite mixture. This approach opens doors toward to CH

₃

NH

₃

PbBr

₃

perovskite NP-polymer composite with controlled NP concentration.

Acknowledgement

This study was financially supported by research fund

of Chungnam National University.

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