Black ultra-thin crystalline silicon wafers reach the 4n2 absorption limit – application to IBC solar cells

Cutting costs by progressively decreasing substrate thickness has been a common theme in the crystalline silicon PV industry for the last decades, since drastically thinner wafers would significantly reduce the substrate-related costs. In addition to the technological challenges concerning wafering and handling of razor-thin flexible wafers, a major bottleneck is to maintain high absorption in those thin wafers. For the latter, advanced light-trapping techniques become of paramount importance. Here we demonstrate that by applying state-of-the-art black-Si nanotexture produced by DRIE on thin uncommitted wafers, the maximum theoretical absorption (Yablonovitch’s
4n2 absorption limit), i.e. ideal light trapping, is reached with wafer thicknesses as low as 40 µm, 20 µm and 10 µm when paired with a back reflector. Due to the achieved promising optical properties the results were implemented into an actual thin interdigitated back contacted solar cell. The proof-of-concept cell, encapsulated in glass, achieved a 16.4% efficiency with an JSC = 35 mA/cm², representing a 43% improvement in output power with respect to the reference polished cell. These results demonstrate the vast potential of black silicon nanotexture in future extremely-thin silicon photovoltaics.