Mono-crystalline silicon single heterojunction solar cells on flexible, ultra-thin (∼25 μm) substrates have been developed based on a kerf-less exfoliation method. Optical and electrical measurements demonstrate maintained structural integrity of these flexible substrates. Among several single heterojunction ∼25 μm thick solar cells fabricated with un-optimized processes, the highest open circuit voltage of 603 mV, short circuit current of 34.4 mA/cm2, and conversion efficiency of 14.9% are achieved separately on three different cells. Preliminary reliability test results that include thermal shock and highly accelerated stress tests are also shown to demonstrate compatibility of this technology for use in photovoltaic modules.

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The optical absorption in 25-μm-thick, single-crystal Si foils fabricated using a novel exfoliation technique for solar cells is studied and improved in this work. Various light-trapping and optical absorption enhancement schemes implemented show that it is possible to substantially narrow the gap in optical absorption loss between the 25 μm Si foils and industry-standard 180-μm-thick Si wafer solar cells. An improvement of absorption by 58% in the near-infrared (740-1200 nm) range is observed for the 25 μm monocrystalline Si substrates with the use of antireflective coating and texturing. The back reflectance of the metal foil that provides mechanical support to the ultrathin Si semiconductor-on-metal foils is extracted to be ∼51.5%, based on the reflectance matching with the simulated escape reflectance in the sub-bandgap region. The back reflectance is enhanced to ∼58% by incorporating an intermediate silicon nitride layer on the back between the Si and the metal. The incorporation of Al as an improved metal reflector on top of the silicon nitride at the backside of the solar cell results in a 5.8 times enhancement in optical path length as a consequence of the improved effective back reflectance of ∼95%. A thin Si foil solar cell with an unoptimized amorphous Si/crystalline Si heterojunction with intrinsic-thin-layer design with implementation of such light-trapping schemes shows an efficiency of 13.28% with a short-circuit current density (JSC) of 35.97  mA/cm2, which approaches the JSC of industrial wafer-based Si solar cells.

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Here, we present work on epitaxial Ge films grown on a thin buffer layer of C doped Ge (Ge:C). The growth rate of Ge:C is found to slow over time and is thus unsuitable for thick (>20 nm) layers. We demonstrate Ge films from 10 nm to >150 nm are possible by growing pure Ge on a thin Ge:C buffer. It is shown that this stack yields exceedingly low roughness levels (comparable to bulk Si wafers) and contains fewer defects and higher Hall mobility compared to traditional heteroepitaxial Ge. The addition of C at the interface helps reduce strain by its smaller atomic radius and its ability to pin defects within the thin buffer layer that do not thread to the top Ge layer.

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The crystalline Si photovoltaic industry has been scaling down the Si wafer thickness in order to reduce costs and potentially attain higher efficiencies by minimizing bulk recombination. However, cell manufacturers are struggling to reduce the wafer thickness below 150l1m as there are no economically viable technologies for manufacturing very thin Si wafers and such thin silicon wafers impose stringent handling requirements as wafer breakage and yield loss impact final module cost. We have previously reported a novel kerfless exfoliation technology capable of producing ultra thin 2511m thin flexible mono c-Si foils from thick Si wafers. In this work, we report on scaling the technology to 8-inch diameter wafers. A 2511m thin exfoliated monocrystalline Si solar cell with a front heterojunction emitter and a diffused back surface field structure has been fabricated with a power conversion efficiency of 14.9%. Simulations show that with optimized texturing of the foil and better surface passivation, higher efficiencies (20%) can be attained. We have also fabricated dual heterojunction devices on 2511m thin exfoliated Si, which show high Voc of 680mV. Due to the kerfless exfoliation process and wafer reuse, a final cell cost of $0.30/Wp can be achieved.