| Flexible electronic technology is a new technology that integrates electronic components on flexible substrates,resulting in a greater stretchability of the electronics as a whole,and has been rapidly evolving over the last decade.Stretchable photovoltaics gets much improved stretchability by using this technology,which has the brittle GaAs cells bonded onto the flexible substrate PDMS.A new design scheme is proposed in order to further increase the stretchability of the photovoltaics without increasing the overall thickness.The scheme is simply that,a cube is set between the basement and island of the structured substrate(named as "single-island"structure),which thus makes the substrate become a "basement-cube-island" structure,a so-called "compound island" structure.Geometry parameters,like the width and thickness of cube,and the thicknesses of island and of basement,should have effects on the stretchability of whole structure.Finite element analyses show that the width of cube has particularly significant influences on the deformation of GaAs cell and the interface stresses.The numerical simulation results also indicate that the structure above the basement will tend to be tilted due to the instability of the cube when its width is less than 300μm.In comparison,as the deformation of GaAs cells and interface stresses are concerned,the thicknesses of the cube and the island are much less influential than the width of the cube.The influence from the thickness of the basement is negligible compared to that of the width of the cube.In addition,the greater thickness of the encapsulation layer,the more evident deformation in GaAs cells and the higher interface stresses can be observed.In practice photovaltaics will normally undergo cyclic loading,and thus GaAs cells can be possibly debonded from the substrate.This interface debonding can lead to the failure of entire structure.However,so far,few research reports or publications can be found on this issue.The reason is that the interface failure mechanism is very complicated due to the stiffnesses of two bonded materials are dramatically different,up to 4 orders.Therefore,here we focus on the differences of interface failure manner between the "single island" scheme and the optimized one,i.e.the so-called "compound island" design under exactly the same interface failure model.Cohesive force models,including shear and tension,are considered and the analyses on interface failure are carried out numerically by employing the cohesive element.It is found that the speed of interface failure in "single island" design is much faster than that in"compound island" design under the same cyclic loading conditions.What is more,the initiation location of interface damage is significantly different between the two designs.For example,under cyclic loading of tension,interface failure starts from the four corners and then gradually evolve inward in the "single island" design.However,in the "compound island" design,the interface failure starts approximately from the perpendicular projections on interface of the four cube corners.Furthermore,the amplitude of cyclic loadings has strong effects on interface failure life in the "single island" design.In addition,the influence on interface failure from interconnections can be neglected in both designs.In the end,the proposed "compound island" design for substrate in stretchable has shown much more improved deformation isolating effect than the "single island" design.Accordingly a longer fatigue life can be expected in photovoltaics based on the new design if the interface failure is the dominant one.All these numerical investigation results are anticipated to be of theoretical help in guiding the design of substrate of photovaltaics. |
