High-Performance Green Light-Emitting Diodes Based on MAPbBr3

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High performance green light-emitting diodes based on MAPbBrpolymer composite films prepared by gas-assisted crystallization Yun Cheol Kim, Yoann Porte, Sung-Doo Baek, Seong Rae Cho, and Jae-Min Myoung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14544 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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ACS Applied Materials & Interfaces

High performance green light-emitting diodes based on MAPbBr3-polymer composite films prepared by gas-assisted crystallization Yun Cheol Kim, Yoann Porte, Sung-Doo Baek, Seong Rae Cho, and Jae-Min Myoung*

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

*E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The morphology of perovskite films has significant impact on luminous characteristics of perovskite light-emitting

diodes

(PeLEDs).

In

order

to

obtain

a

highly

uniform

methylammonium lead tribromide (MAPbBr3) film, a gas-assisted crystallization (GAC) method is introduced with a mixed solution of MAPbBr3 precursor and polymer matrix. The ultra-fast evaporation of the solvent causes a high degree of supersaturation which expedites the generation of a large number of nuclei to form a MAPbBr3-polymer composite film with full surface coverage and nano-sized grains. The addition of the polymer matrix significantly affects the optical properties and morphology of MAPbBr3 films. The PeLED made of MAPbBr3polymer composite film exhibits an outstanding device performance of a maximum luminance of 6800 cd∙m-2 and a maximum current efficiency of 1.12 cd∙A-1. Furthermore, 1 cm2-area pixel of PeLED displays full coverage of a strong green electroluminescence, implying that the high quality perovskite film can be useful for large area application in perovskite-based optoelectronic devices.

KEYWORDS: organic-inorganic perovskite, gas-assisted crystallization, nano-sized grain, composite film, light-emitting diodes


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INTRODUCTION Organic-inorganic hybrid perovskite is an emerging semiconducting material which is applicable to light-emitting diodes (LEDs).1–3 This material provides a high carrier mobility and high photoluminescence quantum efficiency (PLQE) with narrow band emission.4 Moreover, the perovskite has an adjustable optical bandgap which can be tuned by changing halide anions in the entire visible region.5 In particular, a primary advantage of the perovskite is that it can be fabricated by solution process at low temperatures and this enables the perovskite to be useful in low-cost and large-area applications.6–8 However, the perovskite materials are easily crystallized at low temperatures, resulting in poor surface coverage with large-sized crystals.9–11 Consequently, non-uniform perovskite films cause low current efficiency and poor contact with the adjacent layers. Thus, producing uniform perovskite film is an important factor to achieve high performance perovskite LEDs (PeLEDs) with full coverage electroluminescence (EL). In order to improve the quality of perovskite film with high surface coverage, good uniformity, and being pinhole-free, various methods have been developed such as solvent engineering, addition of acid or additives, and inducing fast crystallization.12–14 Especially, these techniques have contributed to a dramatic increase in the power conversion efficiency of solar cells.15 In solar cells, the perovskite film should have large grain size to effectively diffuse generated excitons.16 On the other hand, for the use of perovskite film in LEDs, smaller grains should be formed to limit the diffusion length of excitons or charge carriers and to reduce dissociation of excitons into carriers.17 Therefore, it is necessary to form perovskite films with minimized grain size and full surface coverage for achieving high performance PeLEDs. Tan et al. achieved a maximum current efficiency of 0.3 cd∙A-1 with a MAPbBr3 PeLED fabricated by conventional spin coating.1 Yu et al. fabricated a uniform MAPbBr3 film by adding


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hydrobromic acid (HBr) in a precursor solution and the PeLED with an optimal concentration of HBr exhibited a maximum current efficiency of 0.43 cd∙A-1.13 Moreover, Cho et al. used solvent engineering to improve the formation of MAPbBr3 films and achieved a maximum current efficiency of 42.9 cd∙A-1.17 Such solvent engineering method is widely used and has been applied to achieve highly efficient perovskite solar cells and PeLEDs. However, the inhomogeneous growth of perovskite films by this method hinders the use of solvent engineering in scale-up production and large-area application. In this study, MAPbBr3 was used as a light-emitting material. In three dimensional organicinorganic lead halide (MAPbX3 with X: Cl, Br, I) perovskite materials, large MAPbBr3 crystals are easily formed during spin-coating process due to its high crystallinity.18 So, in order to effectively reduce crystal size of MAPbBr3, a MAPbBr3-polymer composite film was fabricated by using a gas-assisted crystallization (GAC) method. By controlling the compressed gas pressure during spin-coating process, morphological change of MAPbBr3-polymer composite films was demonstrated and the optimized film showed nanoscale grains with full surface coverage. It was found that this simple technique induced ultra-fast removal of the solvent resulting in a high degree of supersaturation and a formation of a large number of nuclei, which promoted the growth of nano-sized grains. Thus, the PeLED with MAPbBr3-polymer composite film deposited by GAC method exhibited the enhanced electroluminescent characteristics of a maximum luminance of 6800 cd∙m-2 and a maximum current efficiency of 1.12 cd∙A-1. Therefore, this work demonstrates that the morphology of perovskite film is the key to realizing high performance PeLEDs.


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EXPERIMENTAL SECTION Materials. Methylammonium bromide (MABr) (98%), lead(II) bromide (PbBr2) (99.999%), poly(ethylene oxide) (PEO) (average Mw ≈ 600,000), polyvinylpyrrolidone (PVP) (average Mw ≈ 40,000), poly(methyl methacrylate) (PMMA) (average Mw ≈ 120,000), and toluene (anhydrous, 99.8%) were purchased from Sigma-Aldrich. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was purchased from Heraeus Clevios. Silver nanowire (Ag NW) having an average diameter of 27 nm and length of 22 μm dispersion in isopropyl alcohol (IPA) was purchased from Nanopyxis. N,N-dimethylformamide (DMF) (anhydrous, 99.8%) was purchased from OCI Materials. All materials were used as received. Device fabrication. The methylammonium lead tribromide (MAPbBr3)-polymer composite solution was prepared by dissolving MAPbBr3 precursor (a concentration of 300 mg/mL with a 1:1.2 molar ratio of PbBr2 and MABr) and the polymer matrix (20 mg/mL of PEO, 20 mg/mL of PVP) in anhydrous DMF. The MAPbBr3-polymer precursor solution was stirred at 60 °C for 1 h before use. 1 wt% PMMA was dissolved in toluene and stirred overnight. For the fabrication of PeLEDs, indium tin oxide (ITO)/glass substrates (20 Ω/sq) were cleaned by consecutive ultra-sonication with acetone, IPA, and deionized (DI) water for 10 min each, then dried with nitrogen and treated with oxygen (O2) plasma at 150 W for 5 min to control the properties of ITO layer. PEDOT:PSS was spin-coated on the ITO/glass substrates at 3000 rpm for 60 s and annealed at 140 °C for 30 min. MAPbBr3-polymer composite solution of 50 μL was spin-coated onto the PEDOT:PSS-coated ITO/glass substrates. For the conventional spin-coating method, the solution was spun at 5000 rpm for 60 s, while for the GAC method, during the spin coating at the same conditions, nitrogen (N2) gas was blown over the film for 3 s after 5 s of


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coating process. The distance between the N2 gas nozzle (inner diameter of 3 mm) and the substrate was maintained at 6 cm. The MAPbBr3-polymer composite film was annealed at 100 °C for 10 min for further evaporation of the solvent. In the next step, a 40 nm-thick PMMA layer was spin-coated on the MAPbBr3-polymer composite film at 6000 rpm f or 60 s. Finally, the Ag NW film was deposited using a spray-coating method at 80 °C. The sheet resistance of the Ag NW film was about 4.03 Ω/sq. All the fabrication steps, solution and film preparation were processed in dehumidified atmosphere (∼25 °C and relative humidity < 25%). Film characterizations. Microstructural analyses of the perovskite films were conducted by a scanning electron microscopy (SEM, S-5000, Hitachi) and an atomic force microscopy (AFM, MFP-3D, Asylum). Structural characteristics of the perovskite films were investigated by X-ray diffractometer (XRD, Rigaku, SmartLab) with a Cu Kα radiation source (λ=1.5418 Å ). The contact angles of the MAPbBr3 precursor solution prepared with and without the polymer matrix on glass substrates were measured by a contact angle analyzer (Phoenix-300, SEO). Optical properties were measured by a micro-photoluminescence (μ-PL) measurement system (Dongwoo Optron) using a He–Cd laser (λ=325 nm) coupled with a MonoRa 320i monochromator and an Andor SOLIS simulation package. Sheet resistance of the ITO/glass substrate and Ag NW film was examined by a four-point probe measurement system (CMT-SR1000N, AiT). Work functions of the ITO, PEDOT:PSS, and Ag NW films were analyzed by a surface analyzer photoelectron spectrometer (AC-2, Riken Co. Ltd). To determine band positions of the perovskite films, ultraviolet photoelectron spectroscopy (UPS) and absorbance measurements were conducted by using a photoelectron spectrometer (AXIS-Ultra DLD, Kratos Inc.) with He I radiation (21.2 eV) and an ultraviolet-visible spectrophotometer (UV-vis-NIR, V-670, JASCO), respectively.


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LED measurement. The electrical characteristics and light output measurement of the PeLEDs were measured by a semiconductor parameter analyzer system (Agilent B1500A, Agilent Technologies) coupled with a chroma meter (CS-200, Minolta Co.). EL spectra and lightemission images were observed by using EL measurement system (Dongwoo Optron) combined with a cooled charged-coupled device (CCD, DV401A-BV, Andor) camera. All of the measurements were conducted in air at room temperature.


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RESULTS AND DISCUSSION

Figure 1. Schematic fabrication process of the PeLEDs by using a GAC method.

For the green light-emitting PeLEDs, MABr and PbBr2 were used as the perovskite precursors and DMF was used for solvent of the perovskite. This solvent of MAPbBr3 precursor has a high boiling temperature of 153 °C and low vapor pressure of 2.6 mmHg at 20 °C, which induce quite slow evaporation resulting in the poor surface coverage of perovskite films with large-sized crystals.19 The evaporation rate of the solvent can be increased by adjusting related parameters such as temperature, pressure, surface area, and flow rate of ambient gas. As the evaporation rate of the solvent increases, the precursor solution quickly reaches a supersaturation state and a large number of nuclei occurs. Therefore, the MAPbBr3 crystals grow in a short time and have a small grain size by inducing further evaporation of the solvent. In order to increase solvent evaporation rate in this study, a GAC method was carried out during spin-coating process. Figure 1 illustrates the fabrication process of PeLEDs with MAPbBr3-polymer composite films deposited by using a GAC method. Firstly, a cleaned ITO/glass substrate as an anode was treated by O2 plasma to increase work function and make the ITO surface hydrophilic.20 Then, PEDOT:PSS as a hole transport layer (HTL) was spin-coated onto the O2 plasma-treated ITO/glass substrate. Next, a


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mixed solution of MAPbBr3 precursor and polymer matrix in DMF solution was deposited onto the PEDOT:PSS-coated ITO/glass substrate. The main purpose of using the polymer matrix consisting of PEO and PVP is to increase the wettability of MAPbBr3 precursor solution with the PEDOT:PSS-coated ITO/glass substrate, which can form a thinner film with full surface coverage. Also, PEO is an ionic conductive polymer used as a binder for other phases with good electrochemical stability.21,22 PVP has been selected for the polymer matrix with the PEO because it is an amorphous polymer preventing the crystallization of PEO at low temperature.23 During the spin-coating process of the MAPbBr3-polymer composite solution, a compressed N2 gas was blown over the surface of the mixed solution to rapidly remove the solvent and accelerate the supersaturation of the solution. Then, the MAPbBr3-polymer composite film was annealed at 100 °C for 10 min for further evaporation of the solvent. In the next step, a 40 nmthick PMMA layer was deposited to prevent electrical shortage between the bottom ITO and top Ag NW electrode because it is an insulating material with an ultra-wide bandgap which provides large energy barrier. Moreover, the use of a thin PMMA layer has been previously reported to balance the charge injection and transport, improving the device performances.24,25 Finally, Ag NW as a cathode was deposited by using a spray-coating method.


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Figure 2. SEM images showing the surface morphology of MAPbBr3-polymer composite films deposited by GAC method on PEDOT:PSS-coated ITO/glass substrates as a function of N2 gas pressure: (a) 0.1, (b) 0.15, (c) 0.2, (d) 0.25, and (e) 0.3 MPa. The insets show corresponding cross-sectional SEM images of each film. (f) Average grain size and thickness of MAPbBr3polymer composite films at different N2 pressures.

In order to optimize a film deposition process by using a GAC method, N2 pressure was changed during the spin-coating process. The SEM images in Figure 2a–e show the morphology of MAPbBr3-polymer composite films deposited by GAC method on the PEDOT:PSS-coated ITO/glass substrates as a function of N2 gas pressure from 0.1 to 0.3 MPa in steps of 0.05 MPa. By increasing N2 gas pressure from 0.1 to 0.25 MPa, the average grain size of MAPbBr3polymer composite films was gradually reduced as shown in the Figure 2a–d. Also, the corresponding cross-sectional SEM images in the insets of Figure 2a–d show constant reduction of the thickness. However, when N2 gas pressure of 0.3 MPa is applied, the average grain size


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and thickness are abruptly increased as shown in Figure 2e. Figure 2f indicates the change in the average grain size and thickness of MAPbBr3-polymer composite films (Details about the average grain size distribution are shown in Figure S1, Supporting Information). As N2 gas pressure was increased from 0.1 to 0.25 MPa, the average grain size and the thickness of composite films were decreased from 409.8 to 79.3 nm and from 669.8 to 509.4 nm, respectively. However, when N2 gas pressure was increased to 0.3 MPa, average grain size and thickness of MAPbBr3-polymer composite films were suddenly increased to 247.3 and 623.1 nm, respectively. To investigate more surface characteristics, AFM images and root mean square (RMS) roughness of MAPbBr3-polymer composite films were observed as a function of N2 gas pressure (Details about the AFM images and RMS roughness are shown in Figure S2, Supporting Information). As N2 gas pressure was increased from 0.1 to 0.25 MPa, the grain size continued to decrease and films became smoother as shown in the Figure S2a–d. However, as explained above in the SEM images (Figure 2), the grain size of MAPbBr3-polymer composite film was suddenly increased at N2 gas pressure of 0.3 MPa. Figure S2f shows the RMS roughness of MAPbBr3-polymer composite films at different N2 gas pressures. When N2 gas pressure was increased from 0.1 to 0.25 MPa, RMS roughness of the composite films was reduced from 109.9 to 31.7 nm, whereas RMS roughness of the composite films deposited at N2 gas pressure of 0.3 MPa was increased to 70.4 nm. To deposit a uniform MAPbBr3-polymer composite film with small grain size, N2 gas was blown on the spinning film to evaporate the precursor solvent. At N2 gas pressure of 0.25 MPa, the MAPbBr3-polymer composite film showed the minimum grain size, thickness, and RMS roughness of 79.3 nm, 509.4 nm, and 31.7 nm, respectively. However, when N2 gas pressure was increased to 0.3 MPa, the average grain size, thickness, and RMS roughness of the composite films were greatly increased to 247.3, 623.1, and 70.4 nm,


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respectively. As N2 gas pressure increased from 0.1 to 0.25 MPa, the solvent evaporation rate increased as well, leading to the generation of multiple nuclei and, therefore, the growth of smaller grains. However, when the N2 gas pressure reached 0.3 MPa, an abrupt change in the morphology was observed. This sudden change in the morphological properties is believed to be caused by secondary nucleation being initiated on top of the pre-crystallized perovskite layer. When a very high degree of supersturation is reached, in this case when the N2 gas pressure reaches 0.3 MPa during the spin-coating process, the remaining solute is forced to crystalized very quickly as the solvent is removed, leading to the formation of larger crystallites.26,27 Therefore, the optimum N2 gas pressure was set at 0.25 MPa for the deposition of compact MAPbBr3-polymer composite films with small grain size, which is essential for the achievement of high performances in PeLEDs.

Figure 3. SEM images showing (a) MAPbBr3 crystals and (b) MAPbBr3-polymer composite prepared by conventional spin coating. (c) SEM images of MAPbBr3 film and (d) MAPbBr3-


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polymer composite film fabricated by using a GAC method with N2 gas pressure of 0.25 MPa. The insets show magnified view of SEM images for MAPbBr3 film and MAPbBr3-polymer composite film prepared by GAC method. (e) XRD patterns and (f) μ-PL spectra of MAPbBr3 crystals and MAPbBr3-polymer composites deposited on PEDOT:PSS-coated ITO/glass substrates by using conventional spin-coating and GAC methods.

The addition of the polymer matrix in the MAPbBr3 precursor solution can change the morphology of MAPbBr3 film. Thus, MAPbBr3 films were prepared with and without the polymer matrix by conventional spin coating. Figure 3a shows the SEM image of MAPbBr3 crystals prepared without the polymer matrix by conventional spin coating. Instead of film formation, many micro-sized crystals were formed with 30–40% surface coverage. In contrast, as shown in Figure 3b, MAPbBr3-polymer composite prepared by conventional spin coating showed a reduced size of crystals with a surface coverage greater than 80% by the addition of the polymer matrix within the MAPbBr3 precursor solution. Decrease of the crystal size can be attributed to the hindrance of crystal growth by the polymer matrix during the spin-coating process.28 In general, nucleation and growth of perovskite crystals are expected to occur easily when energy barrier for heterogeneous nucleation is low at the liquid/solid interface. High wettability between solution and substrate can reduce the nucleation energy barrier.29 As shown in Figure S3, the contact angles of the MAPbBr3 and MAPbBr3-polymer precursor solutions on glass substrates were measured to be 20.4° (Figure S3a) and 10.8° (Figure S3b), respectively (Details about contact angle results of MAPbBr3 precursor solutions prepared with and without the polymer matrix on glass substrates are shown in Figure S3, Supporting Information). Thus, it is believed that the high molecular weight of PEO/PVP increases the viscosity of the solution


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and offers better wettability at the liquid/solid interface. Therefore, MAPbBr3 with polymer matrix produced smaller-sized crystals with higher surface coverage than that of MAPbBr3 without polymer matrix. During the spin-coating process, solvent evaporation occurs and the evaporation rate can be modulated by spin speeds. Rapid solvent removal promotes a high degree of supersaturation, which affects the crystallization rate of the perovskite. In both cases of the MAPbBr3 crystals and MAPbBr3-polymer composites, the crystal size decreased when the spin-coating speed was increased from 1000 rpm to 6000 rpm (Details about the crystal size of the MAPbBr3 crystals and MAPbBr3-polymer composites prepared by conventional spin coating as a function of spin speed are shown in Figures S4 and S5, Supporting Information). These results demonstrate that the fast removal of the solvent influences the morphology of the perovskite. Also, the MAPbBr3polymer composites showed much higher surface coverage than that of the MAPbBr3 crystals due to the increased viscosity of the polymer matrix. In order to induce ultra-fast evaporation of the solvent, the optimized N2 gas pressure of 0.25 MPa was used for both MAPbBr3 crystal and MAPbBr3-polymer composite precursors. It is confirmed that there is a significant change in the morphology and decrease of the crystal size when blowing N2 gas with a pressure of 0.25 MPa. As shown in Figure 3c, the morphology of MAPbBr3 film prepared without the polymer matrix by GAC method shows a surface coverage greater than 90%. The inset of Figure 3c shows sub-micro grains of MAPbBr3 crystals connected with some surrounding ones. Moreover, when N2 gas was sprayed onto the surface of MAPbBr3polymer composite precursors, the spacing between the MAPbBr3 crystals was completely filled and full coverage was observed as shown in Figure 3d. In addition, as shown in the inset of Figure 3d, pinhole-free surface of MAPbBr3-polymer composite film was confirmed. Therefore,


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in order to reduce the grain size and induce fast crystallization of perovskite films, the addition of a polymer matrix to MAPbBr3 precursor and the use of N2 gas during the spin-coating process are necessary. To examine the crystal structures of MAPbBr3 samples, XRD patterns were measured as shown in Figure 3e. By using the Bragg’s law, diffraction peaks were converted and all MAPbBr3 samples exhibited the XRD patterns corresponding to the (100), (110), (200), (210), (211), (220), and (300) planes. The lattice constants of MAPbBr3 samples were extracted as shown in Table S1 (Details about extracted lattice constant of MAPbBr3 samples are shown in Table S1, Supporting Information). Along with the [100] direction, the lattice constants of MAPbBr3 crystals and MAPbBr3-polymer composite prepared by conventional spin coating, and MAPbBr3 film and MAPbBr3-polymer composite film deposited by GAC method were confirmed to be 5.937 and 5.938, and 5.935 and 5.931 Å , respectively, which are in agreement _

with the reported cubic Pm3m phase (space group no. 221).17,30 Moreover, the crystallite sizes of MAPbBr3 samples were calculated from the average of four largest peaks ((100), (110), (200), and (210)) by using Scherrer equation (𝐿 = 𝐾𝜆⁄𝐵𝑐𝑜𝑠𝜃), where 𝐿 is the crystallite size (nm). 𝐾, 𝜆, 𝐵, and 𝜃 are the Scherrer constant (dimensionless), X-ray wavelength (nm), full width at half maximum (rad) of a XRD peak, and X-ray angle (rad), respectively.17 The crystal sizes of the MAPbBr3 crystals and MAPbBr3-polymer composite prepared by conventional spin coating, and MAPbBr3 film and MAPbBr3-polymer composite film deposited by GAC method were calculated to be 31.1 ± 6.9 and 27.5 ± 4.4, and 28.1 ± 2.9 and 22.7 ± 1.2 nm, respectively. The extracted crystallite sizes were smaller than the apparent grain sizes (SEM images in Figure 3a–d) because grains of MAPbBr3 samples might consist of many crystallites. The variation in the crystallite sizes extracted from XRD are consistent with the apparent grain sizes observed from


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SEM. Furthermore, it should be noted that the MAPbBr3-polymer composite film by GAC method showed the smallest crystallite size with the smallest deviation. These result implies that a uniform MAPbBr3-polymer composite film is produced and the polymer matrix does not affect the crystal structure of MAPbBr3. The optical properties of the MAPbBr3 samples were investigated by using a μ-PL at room temperature. Figure 3f shows the μ-PL spectra of the MAPbBr3 samples and it can be seen that there are blue-shifts upon addition of the polymer matrix in the MAPbBr3 regardless of deposition method. The μ-PL spectra centered at 530 nm corresponding to the photon energy of 2.34 eV were observed with the MAPbBr3 crystals and film prepared without the polymer matrix by both methods. However, the spectra of the MAPbBr3-polymer composites deposited by both methods were shifted to lower wavelengths centered at 521 nm corresponding to the photon energy of 2.38 eV which is higher than those of the MAPbBr3 crystals without the polymer matrix. The blue-shifts observed in the PL spectra of the MAPbBr3-polymer composites are due to the constrained growth of MAPbBr3 crystals by the polymer matrix (Schematic illustrations of crystal growth in MAPbBr3 crystals and MAPbBr3-polymer composite are shown in Figure S6 Supporting Information). The polymer chains help the MAPbBr3 precursor to spread out smoothly, which induces nucleation between the polymer chains and partially suppresses the crystal growth by constraining it. Therefore, the small nanocrystals formed in the polymer matrix may cause quantum confinement effect, resulting in blue-shifts in PL spectra of the MAPbBr3polymer composites.31


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Figure

4.

(a)

Cross-sectional

SEM

image

of

the

full

device

structure

(Ag

NWs/PMMA/MAPbBr3-polymer (GAC)/PEDOT:PSS/ITO/glass) with the inset of top Ag NWs electrode image and (b) corresponding energy-level diagram under applied external bias of Vbias. (c) J–V and L–V characteristics, (d) current efficiency, and (e) normalized EL intensity of PeLEDs with different MAPbBr3 emission layers at 3 V. (f) Light-emitting images (1×1 cm2) of PeLEDs at 3 V under the dark condition.


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In order to investigate the impact of improved morphology of the perovskite materials on the light-emitting performance, PeLEDs were fabricated with different MAPbBr3 samples. Figure 4a shows a cross-sectional SEM image of the full device structure (Ag NWs/PMMA/MAPbBr3polymer (GAC)/PEDOT:PSS/ITO/glass). The MAPbBr3-polymer composite film was prepared by optimized condition using a GAC method. The inset in Figure 4a shows the top Ag NWs electrode image and this electrode exhibited a transmittance of 82.8% in the visible range (380– 750 nm) and a sheet resistance of 4.03 Ω/sq (Detail about the transmittance spectrum of the Ag NWs electrode prepared on a glass substrate is shown in Figure S7, Supporting Information). Figure 4b presents the corresponding energy-level diagram of the PeLED under applied external bias of Vbias. Work functions of the ITO, PEDOT:PSS, and Ag NWs were measured to be 5.0, 5.2, and 4.5 eV, respectively, by using the quantum yield of the photoelectrons (Details about photoelectron spectroscopy results are shown in Figure S8, Supporting Information). The valence band maximum (VBM) and conduction band minimum (CBM) values of MAPbBr3polymer composite film were found to be 5.56 and 3.18 eV, respectively. In order to confirm the VBM, UPS measurements were conducted for the MAPbBr3 samples. Work functions of MAPbBr3 samples were obtained by subtracting the energies at secondary cut-offs of the UPS spectra from the source light energy of 21.2 eV (Details about UPS spectra, Tauc plots, and energy-level diagrams of MAPbBr3 samples are shown in Figure S9, Supporting Information). As shown Figure S9a, work functions of the MAPbBr3 crystals and MAPbBr3-polymer composite prepared by conventional spin coating, and MAPbBr3 film and MAPbBr3-polymer composite film deposited by GAC method were found to be 5.00 and 4.53, and 5.06 and 4.60 eV, respectively. The VBMs were calculated by adding the work function to the energy offset of the VBM region. To determine the CBM, the optical bandgap was extracted from absorbance


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measurements by using the Tauc plot ((𝛼ℎ𝜈)1⁄𝑛 𝑣𝑠. ℎ𝜈) model, where 𝛼 and ℎ𝜈 are absorption coefficient (cm-1) and photon energy (eV), respectively31. Because the perovskite is a direct bandgap material, 𝑛 is set at 1/2.32 As shown Figure S9b, the bandgap of 2.32 eV was observed from the MAPbBr3 crystals and film prepared without the polymer matrix by both methods. However, the Tauc plots of the MAPbBr3-polymer composites deposited by both methods showed the bandgap of 2.38 eV. Thus, energy-level diagram of the MAPbBr3 samples were illustrated as shown in Figure S9c. The VBM values of MAPbBr3 crystals and film prepared without the polymer matrix by both methods were found to be 5.70 and 5.69 eV, respectively. However, with the addition of the polymer matrix to MAPbBr3 precursor, the VBM values of the MAPbBr3-polymer composites deposited by both methods moved to 5.55 and 5.56 eV, respectively. These changes in VBM values can reduce hole-injection barriers from HTL to the emitting layers in PeLEDs.17 In the structure of the PeLED, PMMA layer, which is an insulating material with an ultra-wide bandgap, is expected to fill pinholes of perovskite layer. Moreover, the energy barrier of PMMA layer could limit the diffusion of holes to the Ag NWs electrode and regulate the flow of electrons to avoid the perovskite layer from charging. Therefore, the PMMA layer modulates the charge transport in the PeLEDs, increasing the probabilities for charge recombination, and thus, potentially improving the device performances.24 Figure 4c shows the current density–voltage (J–V) and the luminance–voltage (L–V) characteristics of PeLEDs. The PeLED with MAPbBr3 prepared by conventional spin coating showed the lowest luminance, despite the fact that the device exhibited the highest current level. The poor luminous characteristics are mainly due to high leakage current from the non-uniform and low surface coverage.33,34 However, the PeLED with MAPbBr3-polymer composite prepared by conventional spin coating exhibited lower current density and higher luminance than those of


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the PeLED fabricated from the MAPbBr3 without polymer matrix. When the polymer matrix was added to the MAPbBr3, the luminance of PeLED was significantly increased from 803.1 to 5368.7 cd∙m-2 at 6 V. It has been demonstrated that perovskite layer becomes p-doped at the anode side and n-doped at the cathode side due to the ionic migration of a “poling” effect, inducing the formation of a homogeneous p–i–n junction under an applied bias.35 Li et al. demonstrated that p–i–n junction formation by mixing MAPbBr3 with PEO/PVP reduced the contact resistance at the junctions between electrodes and MAPbBr3 layer and lowered the effective barrier height.36 This p–i–n junction formation resulted in efficient electron and hole injection into the perovskite emitting layer.36,37 In comparison to the PeLED with MAPbBr3 prepared by conventional spin coating, the PeLED with MAPbBr3 by GAC method exhibited an enhanced luminance of 1327.3 cd∙m-2 at 6 V. Moreover, the PeLED with MAPbBr3-polymer composite film deposited by GAC method exhibited the lowest current density and the highest luminance of 6800 cd∙m-2 at 6 V. It is noteworthy that the PeLED with MAPbBr3-polymer composite film deposited by GAC method showed a lower current density than the PeLED with MAPbBr3-polymer composite film deposited by conventional spin coating. The higher current density of the PeLED with MAPbBr3-polymer composite film prepared by conventional spin coating is caused by large-sized grains with pinholes and voids of the film which result in leakage current and imbalanced charge injection.38 Therefore, the lower current density of the PeLED with MAPbBr3-polymer composite film deposited by GAC method indicates that the leakage current and charge injection imbalance have been suppressed. Moreover, the presence of small-sized grains in the MAPbBr3-polymer composite film deposited by GAC method implies more grain boundaries which limit the mobility of the charge carriers.39


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The current efficiency was extracted from the J–V and L–V characteristics of PeLEDs as shown in Figure 4d. The PeLED with MAPbBr3 prepared without polymer matrix by conventional spin coating showed poor current efficiency (maximum current efficiency of 0.05 cd∙A-1) mainly due to high leakage current. In contrast, maximum current efficiency of 0.38 cd∙A-1 was measured from the PeLED with MAPbBr3-polymer composite prepared by conventional spin coating. The PeLED with MAPbBr3 prepared by GAC method showed a slightly increased current efficiency (maximum current efficiency of 0.09 cd∙A-1) when compared to the PeLED with MAPbBr3 prepared by conventional spin coating. The maximum current efficiency was measured to be 1.12 cd∙A-1 from the PeLED with MAPbBr3-polymer composite film deposited by GAC method. Therefore, it is believed that uniform perovskite film with nano-sized crystals, which can be obtained by both an addition of the polymer matrix and an introduction of N2 gas during the spin-coating process, is a key factor to achieve the better luminous properties from the PeLEDs. The normalized EL spectra of the PeLEDs shown in Figure 4e, exhibit the same blue-shifts observed upon addition of the polymer matrix in the MAPbBr3 during the PL measurements. The EL spectra centered at 530 nm were observed with the MAPbBr3 samples without the polymer matrix, whereas the peaks of the MAPbBr3-polymer composites were shifted to lower wavelength centered at 528 nm. In the PL spectra, 9 nm blue-shifts were observed when the polymer matrix was added to the MAPbBr3, but only 2 nm blue-shifts were observed in the EL spectra. It revealed that polymer can act as a template that supports nucleation of nano-sized perovskite crystals. Thus, these nano-sized perovskite crystals in MAPbBr3-polymer composites can cause blue-shifts due to quantum confinement effect.31 The difference between the PL and EL spectra of MAPbBr3-polymer composites is assumed to be caused by the nano-sized


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MAPbBr3 crystals present in the insulating polymer matrix which require an external bias more than 3 V to be activated.40 By increasing the external bias up to 6 V, blue-shifts of EL spectra were observed from the PeLED with MAPbBr3-polymer composite film deposited by GAC method (Detail about the EL spectra of the PeLED with MAPbBr3-polymer composite film deposited by GAC method is shown in Figure S10, Supporting Information). Figure 4f shows the green light-emitting images from the PeLEDs with 1 cm2-area pixel at a forward bias of 3 V. The PeLED with MAPbBr3 prepared by conventional spin coating exhibited inhomogeneous emission (MAPbBr3 (spin coating)), whereas the PeLED with MAPbBr3 prepared by GAC method displayed relatively homogeneous emission (MAPbBr3 (GAC)). As described in Figure 4d, compared to the PeLED without polymer matrix, the PeLED with polymer matrix resulted in the enhanced luminous characteristics (MAPbBr3-polymer (spin coating)). Moreover, the PeLED with MAPbBr3-polymer composite film deposited by GAC exhibited the brightest and most homogenous green emission (MAPbBr3-polymer (GAC)). Therefore, it is believed that uniform perovskite films deposited by GAC method could satisfy high performance and large area application in perovskite-based optoelectronic devices.


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CONCLUSIONS Highly uniform MAPbBr3-polymer composite films with nano-sized grains were developed by using a GAC method. Compressed N2 gas blowing during the spin-coating process promoted ultra-fast removal of the solvent, resulting in a high degree of supersaturation. Moreover, an addition of a polymer matrix in the MAPbBr3 precursor lowered the nucleation energy barrier, and that helped formation of a large number of nuclei. Thus, a uniform MAPbBr3-polymer composite film with full surface coverage and nano-sized grains was obtained. The optimized MAPbBr3-polymer composite film deposited by GAC method showed the minimum grain size, thickness, and RMS roughness of 79.3 nm, 509.4 nm, and 31.7 nm, respectively. Upon using the MAPbBr3-polymer composite film for PeLEDs, a maximum luminance of 6800 cd∙m-2 and a maximum current efficiency of 1.12 cd∙A-1 were confirmed and the device with 1 cm2-area pixel exhibited a strong homogenous green emission centered at 528 nm. Therefore, this study demonstrates that the uniform MAPbBr3-polymer composite film deposited by GAC method can satisfy high performance and large area application in perovskite-based optoelectronic devices.


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ASSOCIATED CONTENT Supporting Information Size distribution and RMS roughness of MAPbBr3-polymer composite films on PEDOT:PSScoated ITO/glass substrates as a function of N2 gas pressure. Contact angles of MAPbBr3 and MAPbBr3-polymer precursor solutions. SEM images of the MAPbBr3 samples deposited onto PEDOT:PSS-coated ITO/glass substrates by using conventional spin-coating and GAC methods at different spin speeds. Extracted lattice constants of MAPbBr3 samples. Schematic illustration of crystal growth in MAPbBr3 crystals and MAPbBr3-polymer composite. Transmittance spectrum of Ag NWs on a glass substrate. Photoelectron spectroscopy results of ITO, PEDOT:PSS, and Ag NWs on glass substrates. UPS spectra, Tauc plots, and energy-level diagrams of MAPbBr3 samples. EL spectra of the PeLED with MAPbBr3-polymer composite film.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1301-07.


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High-Performance Green Light-Emitting Diodes Based on MAPbBr3

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