A Spanish-Finnish research team has produced an IBC solar cell with an ultra-thin black silicon wafer with a thickness of 40 µm. The device is based on vanadium oxide and laser-processed phosphorus-doped silicon carbide layers as hole and electron transport layers.
Such silicon offers an advantage enables the fabrication of ultra-thin and flexible wafers with low impurity content, resulting in exceptional light-absorbing properties and low production costs. “Black silicon is a random nanotexture that reduces surface reflectivity from all directions to a minimum, so that Si appears black to the naked eye unlike conventional microscale random pyramids,” the researchers explained.
Photo: Universitat Politècnica de Catalunya, Nano-Micro Small, Creative Commons License CC BY 4.0
They used deep reactive ion etching (DRIE) at cryogenic temperatures to produce Si substrates of various thicknesses down to 10 µm. This process is a plasma-based etching technique that produces deep holes and dimples with steep sides in silicon wafers.
The researchers used a Si substrate with a thickness of 40 µm to build a solar cell with an IBC architecture, which they said can take advantage of the optical advantage of the bSi front surface in the material. “The IBC c-Si solar cell structure used is based on vanadium oxide (VOx) and laser-processed phosphor-doped silicon carbide stacks as hole- and electron-transport layers, they explained.
The cells also relied on a surface passivation layer made of aluminum oxide (Al2O3) and silicon carbide (SiC). “We chose the conformal Al2O3 layer deposited by atomic layer deposition (ALD) because it provides excellent surface passivation of the bSi nanotexture, achieving an average lifetime of more than 250 µm in our bSi ultrathin wafers, the researchers 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%. In comparison, a reference cell with a polished front surface and no black silicon wafer achieved an efficiency of 11.5%, an open-circuit voltage of 600 mV, and a short-circuit current density of 27.1 mA cm.−2and a fill factor of 70.7%.
“External quantum efficiency (EQE) results on a bSi front surface improve upon an identical device with a polished front surface in all measured spectral regions, from the near-infrared (NIR) to the ultraviolet (UV), demonstrating that the optical properties can be successfully transferred to the electrical performance of the photovoltaic final, encapsulated, cell,” the researchers stated, that further work is needed to improve the light-stopping performance of the front bSi structure.
They presented the new wafer and cell technology in the paper “Black Ultra-Thin Crystalline Silicon Wafers Reach the 4of2 Absorption Limit – Application to IBC Solar Cells”, published in Nano-micro small. The research group includes researchers from e.g Universitat Politècnica de Catalunya in Spain and Aalto University in Finland.
