Fabrication and assembly of ultrathin high-efficiency silicon solar microcells integrating electrical passivation and anti-reflection coatings
Intensive research into ultra-thin silicon solar cell applications has generated interesting approaches to reducing both critical materials usage and their associated device and module-level costs. Here we report an extension of previous work on micro-scale Si solar cells, devices fabricated using precise methods of micromachining Si(111) wafers in conjunction with wafer-scale processing. Here we describe a simple yet robust approach to fabricate the silicon microcells by re-designing the device structure, incorporating a thermally grown oxide layer to serve as a wet-etch and diffusion mask, and illustrate modes of light management and array assembly that can provide high-efficiency PV conversion of light to electrical energy. We report a best cell efficiency of 11.7% under an AM1.5D solar spectrum for an optically thin (30 µm thick) device, which is a substantial improvement over previously reported Si solar µ-cells. We show that the improvement attends to both the optimization made in the doping profiles of the device and the presence of the thermal oxide layer, which doubles as an effective electrical passivation and anti-reflection layer. External quantum efficiency measurements specifically show a marked improvement in the blue response that results from mitigating losses due to surface recombination in the high-surface-area, micro-scale devices. We highlight new strategies for integrating these devices into functional, interconnected arrays, using front and backside electrical bus contacts, with optimized spatial distributions on transparent glass substrates. This geometry, with optimized reflector and waveguiding planarization layers, creates a simple concentrator module by redirecting light via internal reflection to illuminate the sidewalls and bottom surfaces.