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Flexibility of Semitransparent Perovskite Light-Emitting Diodes Investigated by Tensile Properties of Perovskite Layer Sang Yun Lee, Si-Hoon Kim, Yun Seok Nam, Jae Chul Yu, Seungjin Lee, Da Bin Kim, Eui Dae Jung, Jeong-Hyun Woo, Seung-min Ahn, Sukbin Lee, Kyoung Jin Choi, Ju-Young Kim, and Myoung Hoon Song Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04200 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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Flexibility of Semitransparent Perovskite Light-Emitting Diodes Investigated by Tensile Properties of Perovskite Layer Sang Yun Lee, † ,⁋,∥ Si-Hoon Kim, †,∥ Yun Seok Nam, †, ⁋ Jae Choul Yu, †, ⁋ Seungjin Lee, †, ⁋ Da Bin Kim, †, ⁋ Eui Dae Jung, †, ⁋ Jeong-Hyun Woo, † Seung-min Ahn, † Sukbin Lee, † Kyoung-Jin Choi, † Ju-Young Kim,†,* and Myoung Hoon Song†,⁋,*

†School

of Materials Science and Engineering, UNIST (Ulsan National Institute of Science and Technology), Ulsan 44919, Republic of Korea

⁋Low

Dimensional Carbon Center, UNIST (Ulsan National Institute of Science and Technology), Ulsan 44919, Republic of Korea

∥These

authors contributed to this work equally.

*Corresponding Authors: [email protected] (J.-Y. Kim), [email protected] (M.H. Song)

ACS Paragon Plus Environment

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ABSTRACT. Organic-inorganic hybrid perovskites have been investigated extensively for use in perovskite-based solar cells and light-emitting diodes (LEDs) owing to their excellent electrical and optical properties. Although the flexibility of perovskite LEDs has been studied through empirical methods such as cyclic bending tests, the flexibility of the perovskite layer has not been investigated systemically. Here, flexible and semitransparent perovskite LEDs are fabricated: PEDOT:PSS anode and Ag nanowire cathode allow for flexible and semitransparent devices, while the use of a conjugated polyelectrolyte as an interfacial layer reduces the electron injection barrier between the cathode and the electron transport layer (SPW-111), resulting in enhanced device efficiency. Cyclic bending tests performed on the electrodes and in-situ hole-nanoindentation tests performed on the constituent materials suggest that mechanical failure occurs in the perovskite MAPbBr3 layer during cyclic bending, leading to a decrease in the luminance. Tensile properties of MAPbBr3 layer explain critical bending radius (rb) of the perovskite LEDs of the order of 1 mm. KEYWORDS: perovskite light-emitting diodes, semitransparent, flexibility, mechanical analysis, in-situ microtensile tests


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The demand for next-generation flexible and stretchable displays has increased in recent years.1-9 For the realization of flexible and stretchable displays, every constituent material of the devices should be flexible and stretchable appropriately. As a flexible and stretchable display, organic light-emitting diodes (OLEDs) have been studied because of excellent

mechanical

flexibility

of

constituent

organic

materials.3-9

Organic

semiconductors have a few disadvantages such as poor color purity with a broad fullwidth at half maximum (FWHM) and low charge-carrier mobility. Meanwhile, organicinorganic hybrid perovskites have emerged as suitable materials for perovskite lightemitting diodes (PeLEDs) because of their hybrid properties, including high color purity with a narrow FWHM, high photoluminescence quantum yield (PLQY), and high chargecarrier mobility, which are characteristics of inorganic semiconductors, as well as their low-cost solution-based processing and tunable bandgap.10,11 So far, researches on flexible PeLED have focused mainly on usage of flexible electrodes. Bade et al. demonstrated flexible printed PeLEDs in which the perovskite and poly(ethylene oxide) composite layer was sandwiched between the carbon nanotube (CNT)/polymer substrate and the printed Ag nanowire (AgNW) cathode.12 Zhao et al. fabricated flexible organo-


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metal halide perovskite quantum dot-based LEDs using a AgNW-polymer composite electrode that could be bent to a radius of 4 mm without discernible performance degradation even after 1,000 bending cycles.13 Seo et al. reported flexible PeLEDs based on an graphene anode that could be bent to a radius of 7.5 mm for 1,200 cycles.14 Recently, Zhao et al. reported flexible PeLEDs using a AgNW embedded polyimide substrate that could be bent to a radius of 2 mm for 10,000 cycles.15 Various types of flexible and transparent electrodes have been applied for flexible PeLEDs; metal NWs, 3,9,16,17

metal meshes, 16 conducting polymers, 18-20 CNTs,21,22 and graphene.23 Electrodes

compatible with PeLEDs are important because appropriate solvent and processing temperature should be considered to coat flexible and transparent top electrode without damage of perovskite film and other component layer. While electrodes are selectable in the flexible PeLEDs, perovskite has critical advantages as an active material for flexible LEDs. The crystallinity of perovskite is so high that it is expected to have a lower elastic deformability than those of other organic constituent materials used in PeLEDs as well as a lower ductility than those of the other metallic

constituent

materials.11,24,25

The

elastic

modulus

of

single-crystalline


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methylammonium lead tribromide (MAPbBr3) was determined to be 29.1 GPa based on density functional theory (DFT) calculations and 28.3 and 30.2 GPa using a laser ultrasonic technique.

26-28

Using nanoindentations, elastic modulus of single-crystalline

MAPbBr3 was determined to be 19.6 GPa and 17.7 GPa and hardness was determined to be 0.36 GPa and 0.31 GPa.29,30 Mechanical properties of single-crystalline perovskite may be different from those of the perovskite used in thin-film form in PeLEDs, since the microstructure of perovskite thin films is highly dependent on the chemical composition and the fabrication process.31,32 Thus, it is difficult to analyze flexibility of perovskite layer in the flexible PeLEDs based on theoretical mechanical properties for single-crystalline perovskite materials. Even though nanoindentations allow one to determine elastic modulus and hardness, the nanoindentation depth should be limited considering the thickness of the perovskite layer. This means that elastic modulus and hardness as measured by nanoindentation would only be reflective of the local volume and would not represent the actual mechanical properties of the entire perovskite layer. The best way to evaluate the flexibility might be to perform uniaxial tensile tests on free-standing perovskite layers prepared using the same procedure as those used for the device itself.


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This would allow the stress and strain values corresponding to the onset of plasticity and fracture in the uniaxial stress to be determined and also allow the deformation and fracture behavior in the complex stress state to be evaluated based on continuum mechanics. Here, we demonstrate the flexible and semitransparent perovskite LEDs with highly conductive PEDOT:PSS as an anode and spray-coated AgNW as a cathode. Poly [(9,9bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluornene)]

(PFN),

which is a conjugated polyelectrolyte, was introduced between an electron transport layer (ETL) and a cathode to enhance the electron injection, resulting in improvement of device efficiency. Cyclic bending tests for top and bottom electrodes and hole-nanoindentations on suspended electrode materials, MAPbBr3, and SPW-111 layers were performed, by which incipient mechanical failure is found to be occurred in the MAPbBr3 layer. The elastic modulus and fracture strength of the perovskite MAPbBr3 layer were measured through in-situ microtensile tests. Based on the tensile properties, the critical bending radius of the perovskite LEDs is discussed. Figure 1a and b show a schematic of the fabricated devices and a cross-sectional scanning

electron

microscopy

(SEM)

image

of

an

entire

device


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(PEDOT:PSS/MAPbBr3/SPW-111/PFN/AgNW).

Highly

conductive

PEDOT:PSS

(PH1000, Clevios) containing dimethyl sulfoxide (DMSO) and Zonyl FS-300 (Zonyl) (the mixture is hereafter referred to as PDZ), was used to form the flexible and transparent anode, MAPbBr3 was used as the green emissive layer, SPW-111 (White Polymer, Merck Co.) was used as the ETL, and PFN/AgNW were used to form the flexible and transparent cathode, in order to realize the flexible and semitransparent devices. Details of the fabrication of the perovskite LEDs are given in the Supporting Information (Materials and Device fabrication); the entire fabrication process of the perovskite LEDs is also described in Figure S1. Figure 1c shows the energy band diagram of the flexible and semitransparent perovskite LEDs. PFN was used as an interfacial layer to reduce the electron injection barrier between the SPW-111 ETL and the AgNW cathode. In general, it is difficult to ensure high efficiency in optoelectronic devices with a top AgNW cathode because its work function is high (4.82 eV),33 and the efficiency of electron injection is low. Thus, an interlayer is required to inject electrons efficiently; this subsequently enhances device efficiency. The conjugated polyelectrolyte PFN has a π-conjugated primary backbone and


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ionic functional group and can be used as an effective ETL material for reducing the energy barrier between the contact layers because of the spontaneous dipolar polarization.34 During the formation of the PFN/AgNW layer, the hydrophobic polymer backbone is preferentially located on the side of the organic SPW-111 layer while the negative counterions are on the side of the hydrophilic AgNW layer; this results in spontaneous dipolar polarization. To confirm the formation of a uniform and dense MAPbBr3 film on the PDZ layer, X-ray diffraction (XRD) and SEM imaging analyses were performed, as shown in Figure S2a and b. Three distinct diffraction peaks were observed corresponding to the (100) (14.97°), (200) (30.16°), and (300) (45.92°) planes; these were in keeping with the previously reported crystalline peaks of MAPbBr3 (Figure S2a).35 Moreover, the MAPbBr3 film deposited on the PDZ layer showed good coverage, as can be seen from the top-view SEM image in Figure S2b. In this study, PDZ was used as the anode material for the perovskite LEDs instead of indium tin oxide (ITO). The conductivity of the PEDOT:PSS film was increased by doping it with 5 wt.% DMSO and 0.5 wt.% Zonyl.18 In particular, the fluorosurfactant Zonyl was added to the PEDOT:PSS precursor solution to ensure that


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the resulting PEDOT:PSS film coated the hydrophobic poly(ethylene terephthalate) (PET) substrate well.18 The flexible and transparent PDZ layer, which had a thickness of 200 nm, showed good electrical and optical properties, with its sheet resistance being 74 Ω sq-1 and transmittance being 85.7% at 550 nm (see Figure S4 and Table S1). The spraycoated AgNW film formed on the PFN layer, which was used as the cathode in the perovskite LEDs, exhibited a low sheet resistance (20.5 Ω sq-1) and a transmittance of 81.3% at 550 nm (see Figure S5 and Table S2). To confirm the transparency of the fabricated devices, they were photographed (see Figure S3a), and their transmittance spectrum was measured (see Figure S3b). The transmittance of the fabricated perovskite LEDs was 48.1% at 550 nm. Flexibility of the PDZ anode and the PFN/AgNW cathode were evaluated by monitoring the changes in the sheet resistances of the anode (see Figure S6a) and the cathode (see Figure S6b) during 1,000 bending cycles under ambient conditions; a bending radius (rb) of 1.0 mm was used. The maximum change in the sheet resistance per unit initial sheet resistance (△R/R0) was 0.135 for the anode and 0.127 for the cathode. These values confirmed that both the anode and the cathode were highly flexible, as shown in Figure


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S6. Figure 1d, Movie S1, and Movie S2 shows photographs and movies of the perovskite LED under a bending and twisting stress. It can be seen that the perovskite LED is semitransparent and very flexible. Device performance parameter curves for the perovskite LEDs are given in Supporting Information. The device performance of the perovskite LED without PFN was significantly lower than that of the perovskite LEDs with PFN. On the other hand, the perovskite LEDs with PFN exhibited markedly better device performance, owing to the presence of the interfacial layer of PFN, showing a maximum luminance of 1,260 cd m-2 (at 5.5 V), luminous efficiency of 0.79 cd A-1 (at 5.0 V), and maximum external quantum efficiency of 0.17% (at 3.0 V). Moreover, the perovskite LEDs with PFN showed a lower operating voltage as well as a lower turn-on voltage, as shown in Figure S7b. Table S3 lists the device performance parameters of the perovskite LEDs with and without PFN. To analyze the limits to the mechanical robustness of the perovskite LEDs, the luminance changes of the perovskite LEDs with different rb were measured under cyclic bending in ambient conditions, as shown in Figure 2b. The luminance of perovskite LED with rb of 1 mm falled down completely after 320 bending cycles, while the luminance of


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perovskite LEDs with rb of ∞ (flat) and 2.5 mm retained 80% of their initial luminance (L0) (L0 = 160.64 cd m-2, and 170.78 cd m-2, respectively) after 400 bending cycles. This could be caused by mechanical failure such crack in constituent materials and delamination at interface for cyclic bending with rb of 1 mm. We performed cyclic bending tests for flexible electrodes used in the perovskite LEDs, PDZ and PFN/AgNW, the results are presented in Figure S6. The electrodes are placed between substrate and encapsulation materials used in cyclic bending tests for the perovskite LEDs in order that similar stress state is applied in the flexible electrodes during cyclic bending tests. Figure S6 shows decreases in sheet resistance by cyclic bending for rb of 1 mm are negligible up to 1,000 bending cycles, implying that one or more among other constituent materials MAPbBr3, and SPW111 does not sustain applied stress by cyclic bending for rb of 1 mm resulting in decrease in luminance change of the perovskite LED. The measurement time intervals of the perovskite LEDs with rb of 2.5 mm and 1 mm were the same as that of the perovskite LEDs with rb of ∞ (flat). Moreover, perovskite LEDs were encapsulated by 50 μm-thick Norland optical adhesive 63 (NOA63), which was not perfectly good for encapsulation. Figure S8 shows change in normalized luminance for bending radius of 1.0 mm by cyclic bending


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deformation under ambient and dry N2 environments. Significant difference between two environments was not observed which means that decrease in luminance for bending radius of 1.0 mm by cyclic bending deformation was not accelerated by moisture and oxygen in ambient environment. Therefore, we believed that the reduction of luminance of the perovskite LEDs with rb of ∞ (flat) and 2.5 mm did not stem from mechanical breakdown, but rather came from a degrade by moisture and oxygen due to incomplete encapsulation compared to glass. Figure S9 shows normalized EL spectra before and after cyclic bending deformations. No changes in EL spectra by cyclic bending deformation were observed, which indicates that microstructural changes affecting energy bandgap of perovskite materials were not carried out by the cyclic bending deformations. Figure 2a shows photographs of perovskite LEDs under various rb of 10 mm, 5 mm, 2.5 mm, and 1mm to confirm experimental condition of cyclic bending test. Figure 3 shows SEM images for hole-nanoindentation and typical force-depth curves for PDZ, MAPbBr3, and SPW-111. It is important to note that these materials were prepared by same procedures as those employed for fabricating the perovskite LEDs that they, PDZ, MAPbBr3, and SPW-111 suspended on the holes, are expected to have identical microstructure and thickness in the device. The relationship between the force-


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depth curves and the properties of the suspended film being tested by holenanoindentation measurements can be described by the following equations: 𝛿

𝛿 3

𝐹 = 𝜎0(𝜋𝑎)(𝑎) + 𝐸(𝑞3𝑎)(𝑎) ,

𝜎𝑚 =

(1)

1 2

( ) , (2) 𝐹𝐸 4𝜋𝑅

where F is the applied force, 𝛿 is the deflection at center position, 𝜎0 is the pretension,

E is the elastic modulus; 𝑞 = 1/(1.05 ― 0.15𝜐 ―0.16𝜐2) is the dimensionless constant where 𝜐 is the Poisson’s ratio, a is the radius of hole, 𝜎𝑚 is the maximum stress at center position, and R is the radius of tip.36 The elastic modulus and yield strength of the suspended films, which were indicative of the maximum strength in the elastic deformation range, were determined by fitting Equations (1) and (2) to the nanoindentation force-depth curves, as shown in Figure 3b. The elastic modulus and yield strength were 5.47 (±0.25) GPa and 606 (±23) MPa for PDZ, 28.15 (±2.94) GPa and 506 (±58) MPa for MAPbBr3, and 7.53 (±0.25) GPa and 792 (±82) MPa for SPW-111, respectively. The values of the elastic modulus and yield strength are also listed in Table S4. If constituent material shows linear elastic deformation in uni-axial tensile loading,


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yield strain is calculated as 11.1 (±0.4)% for PDZ, 1.81 (±0.19)% for MAPbBr3, and 10.5 (±0.8)% for SPW-111 with elastic modulus and yield strength measured by holenanoindentation. Based on the results of the cyclic bending tests for electrodes and holenanoindentations, it can be concluded that MAPbBr3 is the weakest of all the constituent materials of perovskite LEDs. The yield strain of a material, which is an elastic deformation limit, and a critical parameter affecting the mechanical robustness of the material under bending and stretching, cannot be measured directly by holenanoindentation measurements but is determined through uniaxial tensile tests. Nanoindentation is a useful way of measuring the mechanical properties of thin films and two-dimensional materials. However, the yield strain as determined by holenanoindentation could be inaccurate compared to that determined through uniaxial tensile testing, as several experimental issues have been reported with regards to holenanoindentation measurements, such as the concentration of stress around the contact region with the nanoindenter tip, determining the effective tip radius with accuracy, and friction between the suspended film and the tip/substrate hole edge. 37-40


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We performed in-situ uniaxial microtensile tests on MAPbBr3; a typical stress-strain curve is presented in Figure 4a. MAPbBr3 exhibited linear elasticity and negligibly low plasticity and fractured at a strain of 1.1 (±0.14)%. Its elastic modulus was 4.38 (±0.23) GPa and fracture strength was 45.4 (±7.5) MPa. As shown in Figure 4b and Movie S3, failure occurred by intergranular fracture owing to rapid crack propagation along the grain boundaries. Reported values for elastic modulus of MAPbBr3 range from 17.7 to 30.2 GPa, and those for yield strength is between 103 and 120 MPa that are calculated by one third of hardness.41 Because these values were determined through DFT calculations and nanoindentation measurements performed on single-crystalline MAPbBr3, it is difficult to evaluate the properties of the MAPbBr3 layer in LEDs, which contains numerous grain boundaries and defects. On the other hand, with respect to in-situ microtensile testing, the gauge section in the uniaxial stress state during the tests contains enough grain boundaries such that it can be considered to represent the actual MAPbBr3 layer.42,43 Based on a fracture strain of 1.1% for MAPbBr3, we calculated the critical bending radius, rc, of the perovskite LEDs. It was assumed that the devices consisted of a 70-m-


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thick PET substrate and a 50-m-thick NOA63 top encapsulation layer. The critical bending radius, rc, is given by 𝑟𝑐 =

𝑑𝑓 + 𝑑𝑛 𝜀

,

(3)

where df is the thickness of the film (i.e., the thickness of the MAPbBr3 layer) and dn is the distance from the neutral plane. 44 The critical bending radius, rc, of the MAPbBr3 layer in the perovskite LEDs was calculated to be 0.96 mm. As described above, the luminance of perovskite LEDs decreased gradually by cyclic bending tests for rb of 1 mm, but not for

rb of 2.5 mm. Bending radius of 1 mm is close to calculated critical bending radius, rc of 0.96 mm, which suggests that the observed decrease in the luminance of the perovskite LEDs for bending radius of 1 mm is most likely caused by the gradual introduction of local fractures in the MAPbBr3 layer. In summary, we fabricated flexible and semitransparent perovskite LEDs using transparent and flexible electrodes. The conjugated polyelectrolyte PFN was used to reduce the electron injection barrier between the AgNW and SPW-111 layers. Both electrodes exhibited high flexibility and transparency, as confirmed by cyclic bending tests


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and transmittance spectral measurements. As a result, the fabricated perovskite LEDs were also highly flexible and maintained 80% of their initial luminance even after being bent for 400 cycles at an rb of 2.5 mm. However, the luminance of the perovskite LEDs for an rb of 1 mm decreased sharply with an increase in the number of bending cycles. Based on the cyclic bending tests performed on the top and bottom electrodes and the nanoindentation measurements performed on suspended PDZ, MAPbBr3, and SPW-111 films, MAPbBr3 was found to be the weakest constituent material under mechanical bending. In-situ microtensile tests were performed on MAPbBr3, and its elastic modulus and fracture strength were determined to be 4.38 (±0.23) GPa and 45.4 (±7.5) MPa, respectively. These results imply that the MAPbBr3 layer can bear a tensile strain of up to 1.1% under uniaxial stretching and exhibits a bending radius of 0.96 mm during cyclic bending when the layer is located between a flexible 70-μm-thick substrate and a 50-μmthick top encapsulation layer. These results explain the decrease in the luminance of the perovskite LEDs for a bending radius of 1 mm. Recently, enhanced mechanical properties of perovskite materials as light emitter have been reported such as 2 dimensional perovskites and perovskite/polymer composites.45,46 Comparing to MAPbBr3 light emitters in this study, 2


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dimensional perovskites can have low defect density and high crystallinity and polymer matrix in the perovskite/polymer composites can serve as elastic connectors. These perovskite materials are expected to have enhanced elastic deformation limits and achieve better mechanical flexibility of perovskite LEDs.

ASSOCIATED CONTENT

Supporting Information

The following files are available free of charge. Supplementary figures and tables (PDF)

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (J.-Y. Kim), [email protected] (M.H. Song)

Author Contributions


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S.Y.L. and S.-H.K. contributed equally to this work.

∥

ACKNOWLEDGMENT This work is financially supported by the Mid-Career Research Program (2018R1A2B2006198), by the UNIST-Samsung Display Co. OLED center, by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2012M3A6A7054855), and by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2015R1A5A1037627).


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For Table of Contents Only


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Figure 1. Schematic diagram, energy band diagram, and photographs of flexible and semitransparent perovskite LEDs. (a) Schematic showing structure of fabricated perovskite LEDs (PDZ/MAPbBr3/SPW-111/PFN/AgNW). (b) Cross-sectional scanning electron microscopy image and (c) energy band diagram of the perovskite LEDs. (d) Photographs of the perovskite LED twisted under bending stress; it can be seen that both sides of the device exhibit luminescence.


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Figure 2. Cyclic bending test of the perovskite LEDs. (a) Side-view images of emissive the perovskite LEDs with different rb. (white dashed circles are visual aids. inset: diagonal views of emissive the perovskite LEDs wrapped around cylindrical objects of described

rb). (b) Change in luminance under cyclic bending of the perovskite LEDs with rb of ∞ (flat), 2.5 mm, and 1.0 mm under ambient conditions when constant current of 21 mA is applied.


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Figure 3. Hole-nanoindentation tests performed on PDZ, MAPbBr3, and SPW-111 layers suspended on hole-patterned substrates. (a) SEM images of nanoindentation made on suspended perovskite MAPbBr3 layer, and (b) typical indentation force-depth curves for PDZ, MAPbBr3, and SPW-111 as measured by nanoindentation tests; the elastic modulus and yield strength values were determined based on these curves.


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Figure 4. In-situ microtensile test of perovskite MAPbBr3. (a) Typical stress-strain curve for perovskite MAPbBr3 layer obtained by in-situ microtensile testing and still SEM images of gauge section used for measuring tensile strain. (b) SEM images of fracture surface showing that failure occurs by intergranular fractures owing to rapid crack propagation along the grain boundaries.


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Flexibility of Semitransparent Perovskite Light ... - ACS Publications

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