A Spanish-Finnish research team has fabricated an IBC solar cell with an ultrathin black silicon wafer with a thickness of 40 µm. The device is based on vanadium oxide and laser-processed phosphorus-doped silicon carbide stacks as hole and electron transport layers, respectively.
An international research team has sought to apply black silicon (bSi) to develop ultra-thin substrates for applications in interdigitated back-contact (IBC) solar cells.
This kind of silicon offers the advantage of enabling the production of ultrathin and flexible wafers with lower impurity levels, which result in exceptional light-absorbing properties and low production costs. “Black silicon is a random nanotexture that reduces surface reflectance from all directions to a minimum, so that Si becomes black to the naked eye as opposed to conventional micron-scale random pyramids,” the scientists explained.
They used deep reactive ion etching (DRIE) at cryogenic temperatures to produce Si substrates with different thicknesses down to 10 µm. This process is a plasma-based etching technique that yields deep holes and trenches with steep sides in silicon wafers.
The researchers utilized a Si substrate with a thickness of 40 µm to build a solar cell with IBC architecture, which they said can exploit the optical advantage of a bSi front surface in the material. “The IBC c-Si solar cell structure used is based on vanadium oxide (VOx) and laser processed phosphorus-doped silicon carbide stacks as hole and electron transport layers, respectively,” they explained.
The cells were also relying on a surface passivation layer made of aluminum oxide (Al2O3) and silicon carbide (SiC). “We chose a conformal Al2O3 layer deposited by atomic layer deposition (ALD) as it offers excellent surface passivation of bSi nanotexture, achieving average lifetimes above 250 µm in our bSi ultra-thin wafers,” the scientists said.
The PV device achieved a power conversion efficiency of 16.4%, an open-circuit voltage of 633 mV, a short-circuit current density of 35.4 mA cm−2 , and a fill factor of 73.4%. For comparison, a reference cell with a polished front surface and without the black silicon wafer achieved an efficiency of 11.5%, an open-circuit voltage of 600 mV, a short-circuit current density of 27.1 mA cm−2, and a fill factor of 70.7%.
“External Quantum Efficiency (EQE) results with bSi front surface improve the identical device with a polished front surface in all the measured spectrum ranges, from near-infrared (NIR) to ultraviolet (UV), demonstrating that the optical properties can be successfully transferred to electrical photovoltaic performance in a final, encapsulated, cell,” the scientists concluded, noting that further work is needed to improve light-trapping effectiveness of the front bSi texture.
They presented the new wafer and cell technology in the paper “Black Ultra-Thin Crystalline Silicon Wafers Reach the 4n2 Absorption Limit–Application to IBC Solar Cells,” published in Nano-Micro Small. The research group includes academics from the Universitat Politècnica de Catalunya in Spain and the Aalto University in Finland.
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