Monday, June 12, 2023

Black Ultra-Thin Crystalline Silicon Wafers Achieve Maximum Absorption Limit for Improved Solar Cell Efficiency

State-of-the-art black silicon nanotexture enables ultra-thin silicon photovoltaics with enhanced light trapping and improved performance.

Finnish and Spanish researchers have made a breakthrough in the development of ultra-thin crystalline silicon wafers for solar cells by reaching the maximum theoretical absorption limit using advanced black silicon nanotexture. The achievement not only addresses the challenge of maintaining high absorption in thin wafers but also offers significant cost reductions in the photovoltaic industry. The study demonstrates that wafer thicknesses as low as 10 µm can achieve ideal light trapping.

a) Measured absorption of thin silicon wafers (10, 20, and 40 µm nominal thickness) with polished surfaces (orange) and with black silicon texture etched on the front side (blue). Solid and dashed lines represent absorption with and without back a reflector, respectively. The dotted line corresponds to Yablonovitch's 4n2 absorption limit. b) Scanning electron microscope (SEM) image, bird's eye view, of the black silicon nanotexture obtained by DRIE. The scale bar represents 1 µm. c) A free-standing 10µm-thick black silicon wafer, where its high flexibility can be appreciated. d,e) Top view of two 10 µm wafers: d) textured with black silicon and e) out-of-the-box with polished surfaces.

Reducing wafer thickness is a key strategy for cutting costs in the crystalline silicon photovoltaic industry. Thinner wafers significantly reduce substrate-related expenses. However, the weak absorption of silicon at long wavelengths poses a challenge when reducing wafer thickness. To overcome this, the researchers employed black silicon nanotexture, generated through deep reactive ion etching (DRIE) at cryogenic temperatures. The nanotexture allows for better light management and extends the optical path through internal dispersion and scattering, thus improving photon absorption.

The study also includes the implementation of black silicon nanotexture in an interdigitated back-contacted (IBC) solar cell. The proof-of-concept cell, encapsulated in glass, achieved an impressive 16.4% efficiency, representing a 43% increase in output power compared to a reference polished cell. The results highlight the potential of black silicon nanotexture for future ultra-thin silicon photovoltaics, offering both economic savings and improved cell efficiency.

Conventional techniques like chemical texturization through random pyramids and advanced nanopatterning methods have limitations in terms of material consumption, surface damage, and cost. Black silicon nanotexture produced through cryogenic DRIE offers several advantages, including minimal silicon consumption, low surface recombination, and compatibility with high-efficiency IBC solar cell structures. The researchers successfully applied black silicon nanotexture to ultra-thin monocrystalline substrates, demonstrating its potential for mass-produced ultra-thin crystalline silicon photovoltaics.

This study contributes to the ongoing efforts to make solar energy more cost-effective and efficient. The use of black silicon nanotexture in ultra-thin silicon wafers opens up new possibilities for next-generation solar cell technologies, paving the way for widespread adoption of renewable energy solutions.

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