EFFECTS OF STRETCHING ON FLEXIBLE ORGANIC ELECTRONIC STRUCTURES

EFFECTS OF STRETCHING ON FLEXIBLE ORGANIC ELECTRONIC STRUCTURES O. K. Oyewole 1, J. Asare 1, M.G. Zebaze Kana 1, 2, A. A. Oberafo 2, W.O. Soboyejo 1, 3 1 Department of Physics, African University of Science and Technology, Abuja, Nigeria 2 Physics Advanced Laboratory, Sheda Science and Technology, Abuja, Nigeria 3 Department of Mechanical and Aerospace Engineering, Princeton University, USA ABSTRACT This work presents the responses of optical properties and microstructures of layers relevant flexible organic electronics under tension using both analytical modelling and experiments. The optical and surface roughness characterizations of each of the layers of both stretched and as-fabricated structures were carried out to fully understand their dependence on stretching. In order to calculate the interfacial adhesion energies, adhesion forces of interfaces were measured using atomic force microscopy and incorporated into developed models along with material properties of each of the layers. The deformation induced during stretching was also modelled. The finite element was used to simulate stress distributions of the layers and crack driving forces of each of the stretched layers were calculated. The implications of the results obtained are, therefore, summarized for the design of stretchable organic solar cell. Keywords: stretchable electronic, adhesion, structure, organic solar cell 1 Introduction In recent years, there has been increasing interest in the use of solar energy in several applications where stretchability, ductility, weight, cost and durability are priorities [1]. Some of these applications include solar panels which, in many cases, require stretching. There is also a vision of solar cells being integrated into electronic textiles [2] that are stretchable and energy harvesting devices, that are draped over the roof tops of houses [3, 4]. Since such solar cells can be subjected to stress, there is a need to develop a basic understanding of the effects of stress on their layers. In an effort to develop more robust solar cells that can deform by stretching or bending, significant efforts have been made to deposit solar cells on flexible substrates [5-8]. Stephenie et al [9] have studied the electrical properties of stretchable gold films and shown that gold films can function as interconnects for power and signal to a fully elastic thin film transistor inverter. The experimental investigations of the elastic-plastic deformation behaviour and cracking of nano/micro thick conducting metal on poly(dimethyl-siloxane) substrates have also been carried out in literature [10, 11]. In spite of the tremendous efforts on the study and fabrication of organic electronics, these authors have not discussed the effects of stretching on layers of the organic electronic in order to improve charge transport mechanism across the layers during service condition. In this paper, we study the effects of stretching on the layered structures of stretchable organic solar cells. 2 Modelling 2.1 Mechanical Stress Since organic electronic structures are being considered for application in stretchable electronic devices, they can deform under service conditions. However, there is an extent to which they

can be stretched without significant damage [12]. In this study, a simple (a) (b) Figure 1: Composite framework of (a) a stretched thin film on PDMS substrate (b) a quarter of the thin film composite framework (Figure 1) is used to model the deformation behaviour of stretchable solar cells. The stress on the composite is given by [11]: (1) where E, ε, and t are Young's modulus, strain, and thickness respectively; t total is the total thickness of the composite, s and f designate substrate and thin film layer, respectively. Equation (1) is used to estimate the stress-strain behaviour of each of the layers of stretchable organic electronics. 2.2 Adhesion Force Measurement The contact mode of atomic force microscopy (AFM) is generally used to measure the adhesion force between two surfaces. First, the tip is displaced towards the substrate of the bi-material pair. The tip then jumps to contact with the substrate when adhesive interactions are experienced. The tip is bent under elastic deformation as both surfaces remain in contact. The tip displacement is reversed but residual adhesion interactions still keep the tip in contact with the substrate even at zero loads until a negative force (pull off force), which eventually overcome the adhesion, is experienced. These various steps have been explained in literature [8, 13]. According to Hooke s law, the applied force is directly proportional to the displacement provided that the elastic limit is not exceeded. Hence, the adhesion force is related to the displacement as: (2) where k is the spring constant and x is the displacement. The negative sign accounts for the negative force experienced. 2.3 Adhesion Energy Derjaguin-Muller-Toporov (DMT) model is used to describe interactions between stiff materials [13]. Knowing the value of adhesion force, and effective radius, R, from AFM, we can estimate the adhesion energies of each of the interfaces of the layered organic electronic structures. The adhesion energy is related to adhesion force as [13]: (3a) (3b) where and are radii of AFM tip and root mean square roughness of the substrate, respectively. 3 Experimental Procedures 3.1 Fabrication of Layers First, PDMS substrate was fabricated by mixing Slygard 184 silicone elastomer base with Slygard 184 silicone elastomer curing agent in 10:1 weight

ratio. The mixture was degassed in a vacuum oven of pressure 15inHg for 60min for all bubbles to clear off. The degassed PDMS was then poured in a glass mold of dimension 25x25x1mm 3. This was cured at different conditions (70 o C for 60min, 100 o C for 45min, 125 o C for 25min, and at room temperature for 3days).The optical transmittance of PDMS substrates was measured using Avantes UV-VIS Spectrometer while their stress-strain was measured using universal tensile machine (TIRA test 2810). 100nm thick of ITO layer was subsequently sputtered on PDMS substrates that were cured at room temperature for 3 days using the process parameters in the previous work [14] but at room temperature. Furthermore, a 90nm thick of PEDOT:PSS thin film was spin coated on ITO-coated PDMS at 1000rpm. The spin coated PEDOT:PSS was baked at 70 o C for 5min after which an 100nm thick of P3HT:PCBM thin film was deposited at a speed of 1500rpm. A 150nm thick of a 99.99% pure Aluminium was thermally evaporated on the P3HT:PCBM/PEDOT: PSS/ITO-coated PDMS substrates. root mean square surface roughness of the layers. The thicknesses of the layered structures were measured using surface profiler. 4 Results and Discussion 4.1 Finite Element Simulation of Stretching and Crack Driving Forces. To understand the behaviour of layers of organic electronics better, ABAQUS software package was used to create and implement a 2D model of the system under tensile force. The results of the simulations are shown in Figure 2. The results of the stress distribution show that layers with high Young s modulus are liable to fracture first as more stress are being distributed across them. The material properties of the layers used are shown in Table 1. From the results, more stress is distributed on ITO layer, followed by Aluminium while the least stress is being distributed on the active layer. Furthermore, energy release rate was calculated by varying the length of propagated crack across each layer. The results presented in Table 2 show that 3.2 Characterization of the Layers energy release rate increases with increase in length of the crack propagated. The optical transmittance of asdeposited and stretched samples (PDMS, 4.2 Stress-Strain Measurement ITO/PDMS, PEDOT:PSS/ITO/PDMS, P3HT:PCBM/PEDOT:PSS/ITO/PDMS, The stress-strain curves of various and Al/P3HT:PCBM/PEDOT:PSS/ITO/ PDMS substrates cured at different PDMS) were measured using Avantes UV- VIS-NIR spectrometer. Mechanical tensile test was carried out on all the layers in conditions are shown in Figure 6. The results show that the curing time and temperature contribute to flexibility of order to estimate their stress-strain PDMS substrates. In other words, the behaviour. The applied strain was varied from 0-150%. The grain structures of the as-deposited and stretched layers were Young's modulus of PDMS decreases with decrease in curing temperature and time. The results obtained from the tensile test observed under optical microscope. of the composites were incorporated into Scanning electron microscopy of the equation (1) for estimation of the stressstrain relaxed stretched layers was also behaviour of each of the layers. The measured using Carl Zeiss EVO MA/10 SEM. Atomic force microscopy technique was used to measure the interfacial adhesion forces of the structures and the stress- strain curve of ITO layer coated on PDMS substrate is shown in Figure 4. At points A, B, C, and D, applied strains are 0, 20, 50, and 70%, respectively. Table 1: Material Properties of the Layers Used for Simulations

Figure 2: Distribution of Stress across the Layer of a Typical Stretchable Organic Electronics Table 2: Energy Release Rate (G) for Cracks of Different Lengths 4.3 Change in Optical Transmittance of the Layers of the Structures Figure 3 is the change in the measured optical transmittance of ITO layer due to change in the applied strain. From the results, increasing applied strain reduces the optical transmittance of the layer. The optical properties of other layers that are relevant to stretchable organic electronic structures have similar behaviour under monotonic loading. 4.4 Change in Microscopic Structures of Layers of the Structures Optical microscopy images of grain structures of the ITO layer at different applied strains are shown in Figure 5. The grain reduces as the applied strain increases. The same behaviour was observed in other layers of the organic electronic structure. In other words, the grains of the as-fabricated layers of organic electronic structures are more than the stretched layers. Each of the layers starts to fracture at different applied strains. 4.5 Surface Roughness and Interfacial Adhesion of the Structure Values of the measured adhesion forces of the interfaces were used to calculate interfacial adhesion energies of the layered structures. The results show that P3HT:PCBM/PEDOT:PSS interface has the highest adhesion force and energy of 187nm and 40J/m 2 respectively. The

root mean square (RMS) roughness values of the layers range from 0.73nm to 9nm. A=0 % Strain B=20 % Strain C=50 % Strain D=70 % Strain Figure 3: Change in Transmittance of ITO Layer on PDMS Substrate under Stretching Figure 4: Stress-Strain Curve of ITO Layer on PDMS Substrate A=0 % Strain B=20 % Strain C=50 % Strain D=70 % Strain Figure 5: Change in Microscopic Structure of ITO Layer on PDMS Substrate under Stretching

Figure 6: Stress-Strain Curves of PDMS substrates Cured at Different Conditions 5 Conclusion In this work, we have presented the effects of stretching on optical property and microscopic structure of layers relevant to organic electronics. The optical transmittance of the layers decreases with increase in the applied strain. We have also found that energy release rate increases with increase in crack length. Fracture begins to occur in the studied layered structures when strained to about 50%. The interfacial adhesion energy between layers of P3HT:PCBM and PEDOT:PSS is the highest in the studied layered structures. Acknowledgements This research is supported by World Bank Step-B Project. The authors would also like to extend their appreciation to all staff of Physics Advanced Laboratory, Sheda Science and Technology Complex, Abuja and Tifany Tong, Princeton University, USA for their immense contributions to the success of this work. References [1] M. Pagliaro, C. Palmisano, and R. Ciriminna, Flexible Solar Cells, (2008), 120 [2] A. Rohatgi, Crystalline Silicon Solar Cells and Modules, NREL 15 th Workshop, (2005) 11-12 [3] J. Weiss, P. Kukuri, J. Chiguma, J. Gendon, B. Arfaei, P. Borgesen, and W. Jones, Fabrication of low cost flexible solar using solution-based coating techniques, Global Solar Technology (2011) 10-15 [4] G. Dennler, C. Lungenschimied, H. Neugebauer, and N. S. Sariciftci, J. Matter. Res.,Vol. 20, No. 12, (2005) [5] F. C. Krebs: Sol. Energy Mater Solar cells 93 (2009) 1968-1977 [6] A.G. Evans and J.W. Hutchinson: The Thermomechanical Integrity of Thin Films and Multilayers, Acta metal. Mater. Vol. 43. No 7, pp. 2507-2530, 1995. [7] L. Blankenburg, K-Schulthsis, H. Schach, S. Sensfuss, M. Schroder, Sol. Energy Mater. Sol. Cell 93(2009) 476-483 [8] D. R. Hodges, V. Palekis, S. Bhandaru, K. Singh, D. Morel, E. K. Stefenakos, and C. S. Ferekides: Mater. Res. Soc. Symp. Proc. Vol. 1165 (2009) [9] Stephanie P. Lacour, Joyelle Jones, Sigurd Wegner:Elastomeric Interconnects, International Journal of High Speed Electronics and System, Vol. 16, No 1 (2006) 397-407 [10] S. Midturi: Stress-Strain Behaviour of Nano/Micro Thin Film Material, ARPN Journal of Engineering and Applied Science, Vol. 5, No. 3, 2010 [11] O. Akogwu, D. Kwabi, S. Midturi, M. Eleruja, B. Babatope, W. O. Soboyejo: Large Strain Deformation and Cracking of Nano-Scale Gold Films on PDMS Substrate, Materials Science and Engineering B 170 (2010) 32-40 [12] Tang, C. W.: Two-layer Organic Photovoltaic Cell. Appl. Phys. Lett. 48, 183-185 (1986) [13] T. Tong, B. Babatope, S. Admassie, J. Meng, O. Akwogu, W. Akande and W. O. Soboyejo: Adhesion in Organic Structures, Journal of Applied Physics 106, 083708 (2009). [14] M. G. Zebaze Kana, E. Centurioni, D. Iencinella, C. Summonte, Thin Solid Films 500 (2006) 203 208