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1 Elastomeric polymer light-emitting devices and displays (Supplementary Information) Jiajie Liang, Lu Li, Xiaofan Niu, Zhibin Yu, Qibing Pei* Department of Materials Science and Engineering, Henry Samueli School of Engineering and Applied Science, University of California, Los Angeles, CA 90095, United States. Correspondence and requests for materials should be addressed to Q.B.P Here we discuss in more details and offer more data for some issues which did not find space in the main body of our contribution, as a source of extra clarification. NATURE PHOTONICS 1

2 Schematic illustration of the fabrication process for a stretchable PLEC (Scheme S1) Scheme S1. Schematic illustration of the fabrication process of an elastic transparent AgNW- PUA composite electrode, and an stretchable PLEC device based on the AgNW-PUA composite electrode. The processes are all-solution based. 2 2 NATURE PHOTONICS

3 SUPPLEMENTARY INFORMATION Properties of the PUA matrix (Figure S1) A siliconized urethane acrylate (UA) oligomer (CN990, from Sartomer) was chosen as the host resin due to the high transparency and stretchability of its homopolymer. However, Scotch tape adhesion/peeling tests shows a poor bonding between this polymer and AgNWs (Fig. S1c). To address this issue, we refer to our previous work 1, in which we found that the homopolymer based on ethoxylated bisphenol A dimethacrylate (EBA, SR540, from Sartomer) have high bonding force with AgNW, thanks to the high contents of polar functional groups in the EBA. The copolymers of UA and EBA could meet the required high transparency, stretchability, and strong bonding for the present study. As can be seen from Fig. S1a, the transmittance of the copolymers decreases with increasing contents of EBA. The transmittance of the copolymer with 5 (UA) : 1 (EBA) weight ratio, 150 µm thick, is greater than 92% in the wavelength range from 550 nm to 1,100 nm. This copolymer shows a tensile strain higher than 140% and Young s modulus about 38 MPa (Fig. S1b). The addition of EBA also can improve the bonding with AgNWs. In repeated adhesion and peeling test using 3M Scotch adhesive tape, the sheet resistance of the composite electrode based on this copolymer shows no change after 100 cycles (Fig. S1c). 3 NATURE PHOTONICS 3

4 Figure S1. Characterization of PUA matrix. Transmittance spectra (a), stress-strain responses (b), and sheet resistance after cycles of adhesion and peeling tests with Scotch tape (c) of copolymers containing different ratios of UA to EBA. 4 NATURE PHOTONICS

5 SUPPLEMENTARY INFORMATION Transmittance and sheet resistance values of AgNW-PUA composite electrodes (Table S1) Table S1. Typical transmittance and sheet resistance values of AgNW-PUA composite electrodes. The thickness of the composite electrodes is 150 µm. The transmittance data are inclusive of the PUA polymer matrix. Samples Transmittance nm Pure PUA polymer matrix 91% 92% 93% 10 ohm/sq 76% 80% 79% 15 ohm/sq 78% 83% 83% 25 ohm/sq 80% 84% 85% NATURE PHOTONICS 5

6 Elastic modulus and mechanical loss tangent of PUA matrix (Figure S2) Figure S2. Mechanical characterization of PUA matrix. Elastic modulus and mechanical loss tangent vs frequency for the PUA matrix. A 5% offset strain to prevent buckling and an oscillatory strain of 0.1% was applied. 6 NATURE PHOTONICS

7 SUPPLEMENTARY INFORMATION Composite electrodes with (Figure S3) and without (Figure S4) a strong polymer-agnws interfacial bonding When stretching the composite electrode, the embedded AgNWs are under tension. AgNWs could move in different modes depending on the bonding between AgNWs and the polymer host. If the bonding is weak, delamination and phase separation between AgNWs and polymer can occur. The AgNWs can also slide past one another. Therefore the contact between the nanowires would be weakened or even lost, resulting in a large loss of conductivity. In this case, the AgNW network is irreversibly damaged and its original state cannot be restored when releasing the applied strain 2. Fig. S4 shows a SEM image of an AgNW/poly(tert-butylacrylate) (PtBA) composite electrodes under 50% elongation. The PtBA matrix has a weak bonding force with the AgNWs 2. It can be clearly seen that delamination and phase separation between AgNWs and polymer indeed occur. In contrast, if the interfacial bonding can be strengthened, the tensile stress would be transferred from the polymer matrix to AgNWs 3,4. The AgNWs would orient along the stretching direction. This oriented AgNW network could return back to a state similar to its initial unstretched state when the external force is removed, and no phase separation or delamination occurs. These are observed in the AgNW-PUA composites (see Fig. 1h and Fig. S3). The large aspect ratio of the AgNWs is beneficial such that: 1) the AgNW percolation network will not be broken even at strains up to 100% (Fig. 1e); 2) the highly conductive AgNW network can be easily formed at a low AgNW density so that the composite electrode has fairly high transparency. The interpenetrating networks in the AgNW-PUA composite suppress the formation of wavy patterns on the surface, which can thus retain its smoothness even after stretch/relaxation cycles at large strain. NATURE PHOTONICS 7

8 Figure S3. Surface characterization of composite electrode. SEM micrograph of a 15 ohm/sq AgNW-PUA composite electrode after being stretched to 80% strain and released to 0% strain. Figure S4. Surface characterization of AgNW-PtBA composite electrode. SEM micrograph of a AgNW-PtBA composite electrode under 50% elongation. The black arrow indicates the stretching direction. Alignment of the AgNWs along the stretching direction is observed. The red arrows indicate locations of delamination and phase separation between AgNWs and PtBA matrix. 8 NATURE PHOTONICS

9 SUPPLEMENTARY INFORMATION Lifetime test and transient light emission response for the PLEC (Figure S5 and Figure S6) Figure S5. Lifetime test of stretchable PLECS. tress test of a fresh PLEC device driven at constant 9 ma/cm 2 for 600 min with 16 V voltage compliance. Figure S6. Time response of PLEC. Transient light emission response under voltage pulse between 0 and 12 V with 50% duty cycle for an initially charged PLEC (50% duty cycle at 0.25 Hz). NATURE PHOTONICS 9

10 Uniform light emission around the emission threshold (Table S2) Table S2. Optical photographs of an unstretched PLEC at various applied current densities and brightness. The emissive area is 3 5 mm 2. The PLEC device exhibits fairly uniform light emission, even at very low brightness around the emission threshold, such as 0.34, 1,2 and 5.4 cd/m 2. Current density (ma/cm 2 ) Optical images Luminance (cd/m 2 ) NATURE PHOTONICS

11 SUPPLEMENTARY INFORMATION Transmittance spectra of composite electrode with and without PEDOT:PSS (Figure S7) Figure S7. Transparent property of composite electrode with and without PEDOT:PSS. Transmittance spectra of a 15 ohm/sq AgNW-PUA composite electrode, 15 ohm/sq PEDOT:PSS/AgNW-PUA composite anode (All measurements are inclusive of polymer matrix). The transmittance of the 15 ohm/sq AgNW-PUA composite and 15 ohm/sq PEDOT:PSS/AgNW-PUA composite anode at 550 nm is ~83% and ~81%, respectively. NATURE PHOTONICS 11

12 Device performance of a PLEC measured from anode and cathode sides (Figure S8) Figure S8. PLEC measured from different sides. (a) Current density-luminance-driving voltage characteristics, and (b) current efficiency-luminance characteristics of a stretchable PLECs measured from the anode side. (c) Current density-luminance-driving voltage characteristics, and (d) current efficiency-luminance characteristics of the PLECs measured from the cathode side. 12 NATURE PHOTONICS

13 SUPPLEMENTARY INFORMATION Charge carrier transporting characteristic of the emissive layer under different strain (Figure S9) The charge carrier transport characteristics of the emissive material are displayed in Fig. S11. Fig. S11a is for a hole-dominated device. The device structure is AgNW-PUA composite electrode/pedot:pss/emissive material/agnw-pua composite electrode. The LUMO (Lowest Unoccupied Molecular Orbit) energy level of the emissive materials is around 2.7 ev and the work function of AgNW is around 4.2 ev. The electron-injection barrier is 1.5 ev. The current density is dominated by hole injection. Apparently, the hole injection current decreases with strain from 0% to 100% strain. The device structure for Fig. S11b is AgNW-PUA composite electrode/emissive material/pei/agnw-pua composite electrode. Herein, electron injection is assisted by polyethylenimine (PEI) 5. The HOMO (Highest Occupied Molecular Orbits) energy level of the emissive material is 5.2 ev. The hole injection barrier is 1 ev. Therefore, the current density of the device is dominated by or significantly dependent on electron injection. With the strain increasing, the electron injection is enhanced from 0% to 20% strain, and then turns to decrease at higher strains. To fabricate the hole-dominated stretchable device, PEDOT:PSS was first spin-coated on 15 ohm/sq composite electrode at 4,500 rpm for 60 s. A solution of SuperYellow in THF (5 mg/ml) was spin-coated onto the composite electrode at 3,000 rpm for 60 s. The films were then dried at room temperature under vacuum. The SuperYellow layer was 150 nm thick. A second AgNW- PUA composite electrode was stacked, face down, onto the emissive polymer layer, and the stack was heated to 90 o C to enhance adhesion among the layers. The stack was then fed through a hot-press setup at 150 o C. NATURE PHOTONICS 13

14 To fabricate the electron-dominated stretchable device, PEI (80% ethoxylated solution (35-40 wt% in H 2 O, average M w ~70,000)) was first dilute by 2-Methoxyethanol to 0.4wt%. Then, the PEI solution was spin-coated on the 15 ohm/sq composite electrode at 5,000 rpm for 60 s. The remaining of the device fabrication was the same as the hole-dominated device above. Figure S9. Charge carrier transport characteristics of the emissive material under strain. (a) Current density versus bias voltage responses of a hole-dominated stretchable device under various strains. (b) Current density versus bias voltage responses of an electron-dominated stretchable device under various strains. 14 NATURE PHOTONICS

15 SUPPLEMENTARY INFORMATION Brightness distribution of PLEC under specified strain (Figure S10 and Figure S11) Figure S10. Brightness distribution of PLEC under specified strain. Brightness distribution of a PLEC device at 0% strain (original emission area: 5.0 mm 4.5 mm), 80% and 120% strains, biased at 14 V. The black circles represent the measurement spots of the Photoresearch PR-655. Brightness of the circled spots is indicated in cd/m 2. The stretched device displays a fairly uniform emission across the entire luminous area. Figure S11. PLEC with low brightness under different strain. Optical photographs of a PLEC (original emission area: 3 mm 5 mm) biased at 8 V at specified strains. The luminance for the PLEC under 0%, 20%, 40% and 60% strain is 16.1, , and 0.5 cd/m 2, respectively. NATURE PHOTONICS 15

16 Investigation of PLEC subjecting to continuous cycles of stretching-relaxing (Figure S12, Figure S13 and Figure S14) Fig. S12a shows the luminance and current efficiency characteristics of a PLEC device during initial 5 stretch-relaxation cycles between 0% and 10% strains. In each stretching cycle, it can be seen that the brightness (original value: 104 cd/m 2 ) and current efficiency (original value: 1.5 cd/a) of the stretched device were both higher than the corresponding values of the unstretched state. Fig. S12b further plots the luminance and current efficiency of the device at 0% strain during 1,000 stretch-relaxation cycles with strains between 0% and 10%. The brightness increased slightly after the first cycle, but then decreased steeply in the following 50 cycles to 60 cd/m 2 ; after that, the decrease subdues, and remains a fairly high value of 35 cd/m 2 after 1,000 cycles. The current efficiency sharply increases from 1.5 cd/a to 2.75 cd/a during the first 4 cycles, and then turns to decrease, to 2.22 cd/a in 50 cycles. The current efficiency is fairly stable in the subsequent 950 cycles. Repeated stretch and relaxation between 0 and 30% strains showed somewhat similar trends, as is shown in Fig. S12c and S12d. These results demonstrate a fairly high elasticity of the PLECs at small strains (30%) at room temperature. However, when the strain is 40% or larger, the electroluminescent performance of the devices rapidly deteriorated (Fig. S13). SEM imaging was used to study the microstructural changes of the emissive layer after the PLECs had been subjected to 0, 1, 10, 100, and 200 strain cycles between 0 and 30%. The topview SEM images in Fig. S14 show that the emissive layer has a fairly smooth surface texture before and after 1 strain cycle. After 10 strain cycles, pinholes are observed at a density of about 30 pinholes/µm 2, increased to 70 and 100 pinholes/µm 2 after 100 and 200 strain cycles, respectively. The size of the pinholes also increases with strain cycles. The formation of pinholes, 16 NATURE PHOTONICS

17 SUPPLEMENTARY INFORMATION as well as the gradually increase of the sheet resistance of the composite electrodes, are considered being responsible for the performance degradation of the PLECs over continuous strain cycles. SEM top-view image of the emissive layer after 10 strain cycles between 0 and 40% also displayed in Fig. S14f shows the formation of large numbers of pinholes and even cracks after 10 stretching cycles. These results put an upper limit of ~30% strain for the PLECs in cyclic deformation. Additionally, the microstructural change of the PEDOT:PSS layer may also play a role in the degradation (see Fig. S15). Figure 12. Stretchability characterization of PLEC under continuous stretching-relaxing cycles. Luminance and current efficiency characteristics of elastomeric PLECs during 5 stretch- NATURE PHOTONICS 17

18 relaxation cycles (a) between 0 and 10% strains and (c) between 0 and 30% strains. Plots of the luminance and current efficiency at 0% strain during 1,000 continuous cycles of stretchingrelaxing (b) between 0% and 10% strains and (d) between 0 and 30% strains. The bias voltage is 12 V. Figure S13. Stretchability characterization of PLEC under stretching-relaxing cycles with 40% strain. Luminance characteristics of a PLEC device during 5 stretch-relaxation cycles between 0 and 40% strains, driven at constant 12 V. 18 NATURE PHOTONICS

19 SUPPLEMENTARY INFORMATION Figure S14. Microstructure change of emissive layer after specific stretching cycles. SEM images of (a) the top-view of unstretched emissive layer of an composite electrode, and the topview of emissive layer on composite electrode after (b) 1 strain cycles, (c) 10 strain cycles, (d) 100 strain cycles and (e) 200 strain cycles between 0 and 30%. (e) SEM images of the top-view of emissive layer on composite electrode after 10 strain cycles between 0 and 40%. The samples were prepared by carefully peeling off the top AgNW-PUA composite electrode (cathode) from the PLEC device after continuous stretching relaxation operation. The emissive layer sticks to the PEDOT layer on the composite electrode anode. NATURE PHOTONICS 19

20 Morphology change of PEDOT:PSS layer under strain (Figure S15) The main purpose of using the PEDOT:PSS layer inserted between the composite electrode and active layer is to protect the composite electrode from solvent attack when spin-coating the active layer. Fig. S15 shows the SEM images of five samples of the same PEDOT:PSS film coated on PUA surface after stretching to and releasing from strains of 20%, 40%, 60%, 80% and 100%. After stretching the PEDOT:PSS/PUA substrates to and releasing them from a strain of 20%, the PEDOT:PSS films exhibited nonperiodic buckles. The samples stretched to and released from 40% exhibited regular, periodic buckles, and some cracks also appear. These observed buckles and cracks after stretching indicates irreversible (plastic) deformation of the PEDOT:PSS film on PUA substrate. Similar phenomenon was also observed by Bao et al. 6. This may also be one of the causes for the lower reversibility of stretching of the PLECs at strains higher than 30%. 20 NATURE PHOTONICS

21 SUPPLEMENTARY INFORMATION Figure S15. Microstructure change of PEDOT:PSS layer after specific strain. SEM images of top-view for a PEDOT:PSS layer on PUA after stretching and releasing the films from strains of (a) 20%, (b) 40%, (c) 60%, (d) 80% and (e) 100% along the horizontal axis. NATURE PHOTONICS 21

22 Mechanical properties of PLECs (Figure S16) Figure S16. Mechanical properties of stretchable PLEC. (a) The representative stress-strain curves for AgNW-PUA composite electrode and PLEC device. (b) Elastic modulus and mechanical loss tangent vs frequency for the PLEC (A 5% offset strain was applied to prevent buckling and the oscillatory strain applied is 0.1%). 22 NATURE PHOTONICS

23 SUPPLEMENTARY INFORMATION PLECs using composite electrodes with different sheet resistances (Figure S17, Figure S18, and Figure S19) The PLEC using 25 ohm/sq AgNW-PUA composite electrodes both as anode and cathode (named as 25 ohm/sq PLEC ) shows a maximum efficiency of 5.4 cd/a at 2,000 cd/m 2 (Fig. S17a and S17b), which is slightly lower than the PLEC using 15 ohm/sq AgNW-PUA composite electrodes. The stretchability of the 25 ohm/sq PLEC is much lower than that of the 15 ohm/sq PLEC, as shown in Fig. S17c and S17d. The luminance of the 25 ohm/sq PLEC dropped from an initial 101 cd/m 2 (0% strain) to 20 cd/m 2 when being stretched to 40% strain. This should be mainly due to the relative low stretchability of the 25 ohm/sq AgNW-PUA composite electrode. As can be seen from Fig. S17e, the sheet resistance of the 25 ohm/sq AgNW-PUA composite electrode increases to about 1,000 ohm/sq at 40% strain. As to the PLEC based on 10 ohm/sq AgNW-PUA composite electrodes (named as 10 ohm/sq PLEC ), the device exhibits a maximum current efficiency of 5.7 cd/a at the maximum luminance of 2,080 cd/m 2 (Fig. S18a and S18b), which is almost the same with the 15 ohm/sq PLEC. The device performance under uniaxial strain is shown in Fig. S18c and S18d. It can be seen that the current efficiency for the 10 ohm/sq PLEC increases from about 1.0 cd/a before stretching, to around 3.0 cd/a at 40% strain. The current efficiency shows a lower tendency to deceasing with strain compared to the 15 ohm/sq PLECs, which could be explained by the higher stretchability of the 10 ohm/sq AgNW-PUA composite electrode as shown in Fig. 1e. However, the stability of the 10 ohm/sq PLEC is much lower than that of the 15 ohm/sq PLEC. As shown in Fig. S18e, the emission intensity of a 10 ohm/sq PLEC during stress test reaches a peak value of 86 cd/m 2 in 9 min and then declined gradually to 45 cd/m 2 in 5 h (at constant 6 ma/cm 2 ). The AFM images in Fig. S19 show that the conductive surface of the 10 and 15 ohm/sq composite NATURE PHOTONICS 23

24 electrodes. The roughness about is 11 nm for the 10 ohm/sq composite electrode, which is much larger than that of 15 ohm/sq composite electrode (about 3.4 nm). It is suggested that the relatively large roughness for the 10 ohm/sq AgNW-PUA composite electrodes is the main cause for the relatively short lifetime of the 10 ohm/sq PLEC. Thus, taking into account all these different behaviors, the 15 ohm/sq PLECs were selected for extensive investigation in this work. 24 NATURE PHOTONICS

25 SUPPLEMENTARY INFORMATION Figure S17. Device characterization of PLEC using 25 ohm/sq AgNW-PUA composite electrodes. (a) Current density-luminance-driving voltage and (b) current efficiency-luminance characteristics of an elastomeric PLEC device using 25 ohm/sq AgNW-PUA composite electrode as anode and cathode. (c) Current density and luminance characteristics and (d) current efficiency characteristics of the device with increasing strains. The device is driven at 12 V. (e) Evolution of sheet resistance of 25 ohm/sq AgNW-PUA composite electrodes with increasing strain. NATURE PHOTONICS 25

26 Figure S18. Device characterization of PLEC using 10 ohm/sq AgNW-PUA composite electrodes. (a) Current density-luminance-driving voltage and (b) current efficiency-luminance characteristics of an elastomeric PLEC device using 10 ohm/sq AgNW-PUA composite electrode as anode and cathode. (c) Current density and luminance characteristics and (d) current efficiency characteristics of the device with increasing strains. The device is driven at 12 V. (e) Stress test of a fresh PLEC device based on 10 ohm/sq AgNW-PUA composite electrode driven at constant 6 ma/cm 2 for 300 min. Figure S19. Surface roughness of composite electrodes. AFM images for (a) 15 ohm/sq AgNW-PUA composite electrode, and (b) 10 ohm/sq AgNW-PUA composite electrode. The peak to valley value (Ra) between two red arrows is 3.4 nm and 11 nm for the 15 ohm/sq and 10 ohm/sq AgNW-PUA composite electrodes, respectively. 26 NATURE PHOTONICS

27 SUPPLEMENTARY INFORMATION Packaging the stretchable PLEC (Figure S20 and Figure S21) The elastomeric PLEC devices, like OLEDs in general, are vulnerable to attack from moisture and oxygen, and thus require hermetic sealing. The aforementioned device fabrication and testing were all carried out in a glovebox protected with dry nitrogen. To take the devices out of the box and test in air, a thermally cross-linked polyurethane (TCPU, Clear Flex 50) was selected to seal the PLECs (Fig. S20). To encapsulate the stretchable PLEC, the device was laminated between a pair of pre-crosslinked TCPU, which was left to fully cross-link overnight at room temperature. All stacking and lamination operations were carried out in glove-box with oxygen and moisture levels both below 0.5 ppm. TCPU and PUA are both polyurethane-based elastomers, and thus can form strong adhesion bonding between each other. TCPU also has a low Young s modulus (~1 MPa), 300% elongation at break (see Fig. S21a), and a high transmittance (>91% over the range from 400-1,100 nm; see Fig. S1b). Fig. S20a illustrates the architecture of a PLEC device sealed with two layers of TCPU each 150 µm thick. The encapsulated device, with quite uniformly lighting area (4.0 mm 7.0 mm), can be stretched repeatedly (Fig. S20b and S20c), bent (Fig. S20d), and twisted (Fig. S20e) at a 12 V bias in air. Supplementary Movie S3 shows the device being twisted and stretched repeatedly while being lit at 12 V. The storage lifetime of the TCPU-sealed devices in air was only about one week. We note that elastomeric materials capable of hermetic sealing are not currently available. However, as the demand for stretchable sealing mounts, such materials could become available in the near future. Some encouraging recent developments include low permeability atomic layer deposited alumina 7, elastomer incorporated with graphene 8 and layered silicate 9 with very large aspect ratios. Alternatively, with proper interface engineering, OLEDs can be made more resistance against ambient air 10. NATURE PHOTONICS 27

28 Figure 20. Encapsulating the stretchable PLEC. (a) Schematic illustration of an elastomeric PLEC device sealed between two layers of a polyurethane packaging material. Images of an encapsulated PLEC device at (b) 0% and (c) 25% strain. Images of an encapsulated PLEC device (d) bent and wrapped around a finger, and (e) in twisted conformation. All the PLEC devices are operated at 12 V, in air and at room temperature. The lit area of the unstretched device is 4.0 mm 7.0 mm area. 28 NATURE PHOTONICS

29 SUPPLEMENTARY INFORMATION Figure S21. Properties of sealing material TCPU. (a) The representative stress-strain behavior for packing materials TCPU. (b) Transmittance spectra of the packing materials TCPU. NATURE PHOTONICS 29

30 Supplementary Movies: Supplementary Movie S1: PLEC undergoing large uniaxial strain. A stretchable PLEC device is stretched up to 120% linear strain in glove-box at room temperature. The driven voltage is 12 V. Supplementary Movie S2: PLEC undergoing repeatedly stretching-relaxing cycles. A stretchable PLEC device is stretched repeatedly at various strains in glove-box at room temperature. The driven voltage is 12 V. Supplementary Movie S3: Encapsulated PLEC undergoing repeatedly stretching-relaxing cycles. An encapsulated stretchable PLEC device is stretched repeatedly to various strains in air at room temperature. The driven voltage is 12 V. 30 NATURE PHOTONICS

31 SUPPLEMENTARY INFORMATION References: 1. Yu, Z. et al. Highly flexible silver nanowire electrodes for shape-memory polymer lightemitting diodes. Adv. Mater. 23, (2011). 2. Yun, S. et al. Compliant silver nanowire-polymer composite electrodes for bistable large strain actuation. Adv. Mater. 24, (2012). 3. Gao, J. B. et al. Continuous spinning of a single-walled carbon nanotube-nylon composite fiber. J. Am. Chem. Soc. 127, (2005). 4. Qian, D., Dickey, E. C., Andrews, R. & Rantell, T. Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites. Appl. Phys. Lett. 76, (2000). 5. Xiong, T., Wang, F. X., Qiao, X. F. & Ma, D. G. A soluble nonionic surfactant as electron injection material for high-efficiency inverted bottom-emission organic light emitting diodes. Appl. Phys. Lett. 93, (2008). 6. Lipomi, D. J. et al. Electronic Properties of Transparent Conductive Films of PEDOT:PSS on Stretchable Substrates. Chem. Mater. 24, (2012). 7. Kaariainen, T. O. et al. Atomic layer deposition on polymer based flexible packaging materials: Growth characteristics and diffusion barrier properties. Thin Solid Films 519, (2011). 8. Lee, S., Hong, J. Y. & Jang, J. Multifunctional graphene sheets embedded in silicone encapsulant for superior performance of light-emitting diodes. Acs Nano 2013, DOI: /nn NATURE PHOTONICS 31

32 9. Kunz, D. A. et al. Clay-Based Nanocomposite coating for flexible optoelectronics applying commercial polymers. Acs Nano 7, (2013). 10. Zhou, Y. H. et al. A universal method to produce low work function electrodes for organic electronics. Science 336, (2012). 32 NATURE PHOTONICS