supplementary information: ultrathin, highly flexible and ... · 1johannes kepler university, linz...

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Supplementary Information: Ultrathin, highly flexible and stretchable PLEDs

Matthew S. White1*, Martin Kaltenbrunner2,3,4, Eric D. Głowacki1, Kateryna Gutnichenko1, Gerald Kettlgruber4, Ingrid Graz4, Safae Aazou5,6, Christoph Ulbricht7, Daniel A. M. Egbe1, Matei C. Miron8, Zoltan Major8, Markus C. Scharber1, Tsuyoshi

Sekitani2,3, Takao Someya2,3, Siegfried Bauer4, and Niyazi Serdar Sariciftci1 1Johannes Kepler University, Linz Institute for Organic Solar Cells (LIOS), Dept. of Physical Chemistry, Altenbergerstrasse 69, 4040 Linz, Austria. 2The University of Tokyo, Electrical and Electronic Engineering and Information Systems, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan. 3Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST), 2-11-16, Yayoi, Bukyo-ku, Tokyo 113-0032, Japan. 4Johannes Kepler University, Soft Matter Physics, Altenbergerstrasse 69, 4040 Linz, Austria 5Laboratory for Solid State Physics, Interfaces and Nanostructures, Physics Department, University of Liege, Liege, Belgium. 6Laboratory of Instrumentation, Measure and Control, Physics Department, Chouaïb Doukkali University, El Jadida, Morocco. 7MEET Battery Research Center, Institute of Physical Chemistry, University of Muenster, Correnstr. 46, 48149 Muenster, Germany. 8Johannes Kepler University, Institute of Polymer Product Engineering, Altenbergerstrasse 69, 4040 Linz, Austria.

*Correspondence to: [email protected]

Supplementary Methods: Device fabrication

Glass slides were cut to the appropriate size (2.5 × 2.5 cm) and cleaned by ultrasonic bath in Hellmanex detergent, followed by an oxygen plasma treatment at 100 W for 5 min. A thin layer of PDMS (SYLGARD 184 Silicone Elastomer from Dow Corning) was applied to the glass. The solution of 10:1 weight ratio PDMS to hardener, diluted 1:1 in hexane was spin-coated at 8000 RPM for 60 s, and subsequently annealed at 150 °C for 30 minutes. The PET foil (Mylar 1.4 CW02) is then adhered to the surface by allowing the van der Waals forces to naturally form the flat film. Excess PET is trimmed from the surrounding area with shears. A dispersion of PEDOT:PSS was prepared from stock Clevios PH1000 with 5 vol% dimetyl sulfoxide and 0.5 vol% Zonyl FS-300 fluorosurfactant from Fluka. This formulation was spun onto the PET foil at 1000 RPM for 60s and annealed at 120 °C for 30 min. The light-emitting polymer AnE-PVstat (synthesis described in detail elsewhere17,19) was dissolved in toluene (20 mg/mL), stirring for 12 hours at 90 °C. Optimal film thickness of 300 nm was prepared by spin

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coating at 1000 RPM for 60 s with the solution and substrate both at 90 °C. The light-emitting polymer MDMO-PPV (purchased from Merck) was dissolved in toluene (8 mg/mL) and spin coated at 1500 RPM resulting in a 225 nm thick films. Finally, 0.5 nm of LiF and 100 nm of Al were thermally evaporated through a shadow mask to form the electron injecting back electrode.

It should be noted that there are film-thickness considerations related to the roughness of the ultrathin substrate. The PET foil is rough at the nm scale, with a listed RMS surface roughness of 26 nm and peak height of 700 nm. For this reason, a thick active layer (200 to 300 nm) is used in this work. With a thick active layer, substrates with 8 pixels, each 3 mm × 6 mm in size, show excellent reproducibility, uniformity of emission, and 3 or more orders of magnitude rectification at ±5 V driving bias. With thinner (50 nm to 100 nm) active layers the resulting diodes show poor emission uniformity and excessive leakage current indicating many pin-holes. Thicker layers (~450 nm) resulted in devices with slightly lower radiance and slightly higher operating voltage due to increased resistivity.

Device characterization The current-light-voltage characteristics were measured using an Agilent B1500

parameter analyzer with two source meter units, one driving the PLED device and one measuring the photocurrent in a Si photodiode. The quantitative electroluminescence spectra were measured using a Photo Research Spectrascan PR-655.

Compression and stretching Elastomeric adhesive tape (3M VHB 4905) was placed between two clamps mounted

to a compression jig as shown schematically in Supplementary Fig. S3. A compression zone is defined by placing two rigid delimiters (red tape in Fig. 2 and Supplementary Movie S2 and Movie S3). Using a manually adjustable screw, the clamps are moved further apart to pre-stretch the elastomer to a desired length. The ultra-thin PLED is then peeled away from the PDMS coated glass support and applied to the pre-stretched elastomer by lightly pressing with a finger. Electrical contact is made with wires and silver paste in the area above the rigid delimiter. The device is then operated continually while the compression clamps are moved closer together allowing the elastomer to relax to it’s un-strained position. The system can then be re-stretched to the same value of strain as originally specified, but further strain will result in disconnection or ripping of the ultrathin foil. At each stage of compression and stretching, the length of the PLED is used to determine the strain. As specified in Supplementary Fig. S3, the compression strain is defined as the change in length normalized to the fully extended (flat PLED) length, where as the tensile strain is normalized to the fully compressed (relaxed elastomer) state. The total attainable compression and tensile strain is determined by the pre-strain of the elastomer, here 100% tensile strain. The contact points, and the points where rigid areas meet the flexible/stretchable areas are points of high failure probability. In all test cases, contact was lost by either the wire or the ultrathin foil disconnecting at this junction before any device degradation was observed. The compression/stretching cycle can be repeated. Two such cycles are shown in Supplementary Movie S2, and up to 15 consecutive cycles were performed prior to losing wire contact during these trials. Supplementary Movie S3 shows an AnE-PVstat based ultrathin PLED undergoing 30%

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compression and 41% tensile strain while addressing the individual pixels via an external relay at 4 Hz. The driving frequency can be tuned up to 50 Hz using this system. Supplementary Movie S1 and S3 were filmed using a JVC GC-PX10E HD video camera.

Microstructure analysis Photographs of the PLEDs operating during crumpling and cyclic compression were

taken using a Canon Rebel XT with a Canon Compact-Macro Lens EF 50mm. The compression photographs were adjusted for exposure difference to render the brightness of the red delimiter tape roughly constant, and to show more detail. Brightness and color information should be taken from the spectroradiometer measurements and not from the photographs.

Digital image analysis was performed on each photograph, revealing information regarding the folding pattern of the PLEDs during compression. An example image and the intensity cross section in the direction of compression at the marked location (red line) are shown in Supplementary Fig. S5. Such cross sections were taken at 5 different locations on the images at each stage of compression. It is revealed that there are, on average, 25 folding peaks in the length of the 6.7mm long PLED. Upon compression to 50% and re-stretching, the number of peaks does not change. Under ideal compression conditions, no point on the film would be subjected to extremely sharp bending. We constructed a model for the “ideal” radius of curvature assuming that the peaks are connected by tangentially connected arc-segments of constant radius of curvature, bound by the condition that the peak-to-peak length of the film is 270 µm. This model is depicted in Supplementary Fig. S5. In reality, the film compresses through deformation somewhere between the “best case” and the “worst case” models.

Digital optical micrographs were taken using a Keyence VHX-2000 at 1000× magnification, scanning the focal plane to determine the height of each pixel. The resulting data are shown in Supplementary Fig. S6. From these height maps (f(x,y)), the radius of curvature at each point of the film can be calculated in the direction of compression by: r=(1+fy

2)3/2/fyy, or in both the x and y directions simultaneously by: r=(fx

2+fy2)3/2/(fxxfy

2-2fxyfxfy+fyyfx2). An example height and radius map pair are shown in

Fig. 3c, and the histogram of the radius of curvature data for 10%, 25%, 35%, and 45% compression is plotted in Fig 3d. The blue squares in Fig. 3e represent the radius of curvature measured by this technique. The symbol signifies that 20% of the total film area is bent to that radius or smaller (integrated fraction of film from histograms in Fig. 3d). The error bar is the length of the bin-width from 20% to 30% of the total film. Both comparatively (by looking at the micrographs in Fig. 3b and Supplementary Fig. S6), and quantitatively (from the radius map and the resulting histograms), we can describe the compression mechanism. Very sharp peaks (5 µm < r < 20 µm) are formed even under small degrees of compression. Further compression up to 45% puts marginally more of the film under that level of bending strain, but does not result in any significant quantity of extremely sharp peaks (r < 5µm).




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The surface bending strain was calculated based on the formula

where tL = 100 nm is the thickness of the aluminum electrode and tS = 1930 nm is the

combined thickness of the three polymer layers (PET, PEDOT:PSS, and AnE-PVstat). EL = 69 GPa is and ES = 2 GPa are the Young’s moduli for the Al and polymer layers respectively.

Quantitative mechanical analysis A mechanical model of the layer sequence shown in Fig. 1a is constructed using the following physical parameters. The Young’s modulous in GPa and density in g/cm3 for the materials involved were Aluminum: (70, 2.7), AnE-PVstat (6.79, 1.5), PEDOT:PSS (2.3, 1.011), and PET (4.3, 1.38). In the output reports, the Al layer is shown on the bottom of the stack, with the PET on top. The model was discretized using 2D 4-noded linear quadrilateral reduced integration elements. An initial static implicit analysis, including the elastomer tape gave preliminary results, yielding a set of boundary conditions that allow the elastomer to be removed in subsequent models. We present two resulting models.

The first, presented in Supplementary Figure S7a, is a static implicit simulation with 0.1 µm element size. The model was constrained to bend between two perfectly rigid sinusoidal shapes designed in such a way that the model’s median section will form a curvature with radius of 30 µm. Frictionless contact constraints were modeled between both rigid structures and the outer surfaces of the PLED film. The aim of this simulation is to quantify the bending strain in various layers under a representative wrinkled geometry. The maximum observed tensile and compressive strain is 4.3%, localized in the PET film on the side away from the PLED itself. Maximum strain in the PLED layers (PEDOT:PSS, AnE-PVstat, and Al) is found to be 2.1%, in the Aluminum electrode.

The second model is presented in Supplementary Fig S7b. This represents a dynamic explicit analysis of the compression of a 3 mm long film segment undergoing 11% compression over a 0.5 s time span. The buckling pattern shows sinusoidal development from the edges inward, where the compression waves interfere at later times causing the degeneration to random buckling.




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Figure S1 | Performance of PLEDs on ITO coated glass. Luminance-voltage and current-voltage traces of MDMO-PPV (blue) and AnE-PVstat (red) PLEDs on ITO coated glass substrates. The MDMO-PPV film is slightly thicker (roughly 15 %) than the one used in the ultrathin layers and consequently has lower current and luminance at the same bias.

Figure S2 | Color chromaticity calculations for the ultrathin PLEDs. a, A plot of the CIE color matching functions, the radiance of the ultrathin PLED (arbitrary scale), and the calculations for the XYZ tristimulus values. b, The CIE x-y chromaticity diagram, with the value pair measured for the ultrathin PLED, overlaid with what are considered useful RGB colors for display technology, and the calculations for x and y, from XYZ.




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Figure S3 | Compression and stretching apparatus. a, The initial placement of the flat ultrathin PLED onto the pre-stretched elastomer. The length at this pre-stretch condition is defined as L0. b, The elastomer in a relaxed state, with the PLED now wrinkled, with L<L0. When the elastomer is fully relaxed and in its unstrained state, the PLED cannot be compressed further. This length is defined as Lmin. L0 and Lmin are used to normalize in calculating the compression and tensile strain.

Figure S4 | Performance of flat and compressed PLEDs. a, Current-voltage curve of ultrathin PLED in the flat state and under 30% compression. The curves are virtually indistinguishable. b, Radiance measured at 5V driving voltage in the flat and compressed




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states. The flat spectrum shows interference features that are dependent on the layer thicknesses of each component. These features are “washed-out” under compression where the measurement technique samples a larger emitting surface area and a wide range of emission angles.

Figure S5 | Modeling of “ideal” compression under constant radius of curvature. a, A black and white image of one stage of the compression cycle with one vertical cross section marked by the red line. b, The signal intensity profile of the above vertical cross section. This signal does not give height information, but does give an average peak-to-peak spacing. We find that the number of peaks per unit of flat PLED length does not change during compression. c, Model of compression under constant radius of curvature, subject to the constraint that the total peak-to-peak film length does not change. Other forms of compression would result in some bends of smaller radius and higher strain. This is illustrated by the “worst case” schematic.




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Figure S6 | 3D digital micrographs of the film. Note that each image has a different scan length. a, 10% compression. b, 25% compression. c, 35% compression. d, 45% compression.




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Figure S7 | Quantitative mechanical analysis. Finite-element models of the ultrathin PLED system under various conditions. a, Static implicit model of the film bent to periodic radius of 30 µm. b, Dynamic explicit compression of 3 mm film segment compressing by 11% over 0.5 s timespan. Times 0.02 s, 0.075 s, 0.25 s, and 0.5 s are shown, from top to bottom, with the zoomed in images of the left edge for the first three cases.




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Movie S1 | Free-standing ultrathin PLED undergoing various crumpling motions. This video shows an MDMO-PPV based ultrathin PLED in two configurations. The first configuration suspends the film between two flexible (roughly 100 µm thick) plastic supports. The PLEDs are driven at 7 mA/cm2 current density while the film is crumpled between the two supports. In the second configuration, only one plastic support is used. The PLEDs are driven at 14 mA/cm2 current density while the film is crumpled and smoothed by hand.

Movie S2 | Compressing and stretching PLEDs. Stop motion video of the ultrathin PLED under continuous operation during 2 compression and stretching cycles. The color of the PLEDs in the image is saturated because it is much brighter than the surrounding area, and does not represent the deep red output detected by the eye, and shown in Fig. 1c.

Movie S3 | Pixel addressed compressing PLED. A video of the ultrathin PLED, with individual pixels addressed via switching relay, during compression and stretching. The driving frequency shown here is 4 Hz, but can be up to 50 Hz in this configuration. Quasi-linear compression and stretching up to 30% and 41% is demonstrated. The extreme compression (around time 22 and 47 s) exceeds the neutral position of the elastomer and results in a macro-scale buckling.




supplementary information: ultrathin, highly flexible and ... · 1johannes kepler university, linz...

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