Stretchable and Flexible Crystalline Silicon Photovoltaic Modules Embodying an Auxetic Rotating-Square Structure for Adjustable Transmittance

This work describes the segmentation of commercial crystalline silicon solar cells into smaller sections and their subsequent restructuring into interconnected arrays, based on an auxetic rotating-square architecture, to produce a lightweight, flexible and stretchable solar module. As expected, the sectioning of the solar cells reduces their power conversion efficiency due to increased carrier recombination at the sawn edges. However, average cell section efficiencies are shown to be less than 1.8% lower than the original cells. Output voltage and current can be tailored according to the combination of series or parallel connections between solar cell sections in the design. Due to the negative Poisson's ratio of the auxetic structure, bidirectional expansion with uniaxial stretching is achieved, opening gaps in the module, which allows the light transmittance to be adjusted. Mechanical tests reveal that the structures are robust to repeated cycles of expansion and relaxation, aided by the joint rotation mechanism of expansion that avoids excessive strain on the joint material. The modules are fully expanded when each cell section is rotated by 45°. In this expanded state, modules made of 31.75 mm × 31.75 mm solar cell sections have a strain of 67% and transmittance of 41.9%. Modules incorporating the smaller 20 mm × 20 mm cell sections have a maximum strain of 60%, with a corresponding transmittance of 49.5%. A geometric model is used to show that by varying the design parameters, the transmittance maximum, minimum and range can be tuned, opening up various potential applications that include BIPV (e.g., partially shaded windows), AgriPV (e.g., greenhouse roofs), portable PV devices and wearables.