Ultra-Thin Si Solar Cells with Hyperuniform Disordered Light Trapping

Mono- and multicrystalline silicon as absorbing materials in solar cells comprise about 95% of the global photovoltaics market and a total worldwide installed PV capacity of close to 3TW was reached in 2020. The benchmark efficiency of 19% for industrial modules reached in 2018 was enabled by the widespread adoption of Passivated Emitter and Rear Cell (PERC) device architecture. Typically, these cells feature 160µm-thick absorbing layers produced via the Czoralski method, which expunges up to 40% high-quality silicon as kerf loss. It has been inferred that the PERC baseline is not able to achieve the cumulative installed PV capacity target set by the IPCC of 7-10 TW by 2030 and further reduction of capital expenditure (capex) is needed. Reducing the thickness of Si-wafers for solar cells to 50µm could potentially reduce the manufacturing capex by 48%, module cost by 28%, and LCOE by 24% [1]. Thin c-silicon suffers from poor light absorption already at 100µm thickness necessitating light trapping to reach competitive efficiencies. High-performance light-trapping structures with sub-micron features, however, usually involve slow and complex manufacturing processes hindering their adoption by industry.<br/>We present hyperuniform disordered (HUD) light-trapping structures applied to ultra-thin solar cells via substrate-conformal imprint lithography. This approach enables rapid patterning of large areas (m²) at the nanoscale and can be performed on virtually any substrate and for any type of functional layer, such as the absorber, back-reflector, anti-reflection coating or carrier transporting layers. Correlated-disorder structures were shown to outperform periodic as well as random light trapping approaches [2] and the hyperuniform platform in addition offers engineered light scattering [3]. We developed a coupled-mode theory (CMT) approach for estimating absorption with HUD patterns, which reduces the parameter space at low computational effort and is used as a starting point for numerical optimizations. Furthermore, we show experimentally measured absorption in ultra-thin, free-standing, hyperuniform-patterned silicon slabs of thicknesses ranging from 1-30µm and compare with simulations. Though silicon is our main candidate, also other PV materials can benefit from our approach and some of these examples will be presented.<br/>Finally, we show the performance of ultra-thin Si solar cells before and after the application of HUD light trapping structures.<br/>In conclusion, our work aims to further expand the success of earth-abundant silicon to ultra-thin, flexible and semi-transparent PV devices, which can be produced with kerfless bottom-up technologies at significantly reduced capex and accompanying CO₂ emissions while maintaining high power conversion efficiencies – A type of device that could literally pave all roads, roofs, walls, and windows by 2050!<br/><br/><b>Publication</b><br/>N. Tavakoli et al., “Over 65% Sunlight Absorption in a 1 μm Si Slab with Hyperuniform Texture”<i>, ACS Photonics 2022, 9, 4, 1206–1217</i>, DOI: 10.1021/acsphotonics.1c01668<br/><br/><b>References</b><br/>[1] Liu, Z. et al (2020). Revisiting thin silicon for photovoltaics (..). Energy & Environmental Science, 13(1), 12-23.<br/>[2] Bozzola et al. (2014). Broadband light trapping with disordered photonic (..). Prog. Photovolt: Res. Appl., 22, 1237– 1245.<br/>[3] Florescu et al. (2009). Designer disordered materials with large, complete photonic band gaps. PNAS, 106(49), 20658-20663.