A process under development at the Georgia Institute of Technology could potentially boost conventional c-Si cell efficiencies by as much as 2%. Researchers are using two different types of chemical etching to create surface features at both the micron and nanometre scale that increase light absorption, reducing reflection and keeps cells clear of stray particles.
“A normal silicon surface reflects a lot of the light that comes in, but by doing this texturing, the reflection is reduced to less than 5%,” said Dennis Hess, a professor in the Georgia Tech School of Chemical and Biomolecular Engineering. “As much as 10%of the light that hits the cells is scattered because of dust and dirt of the surface. If you can keep the cells clean, in principle you can increase the efficiency. Even if you only improve this by a few percent, that could make a big difference.”
The researchers use potassium hydroxide (KOH) solution to etch the silicon along crystalline planes, creating micron-scale pyramid structures in the surface. An e-beam process is then used to apply nanometre-scale gold particles to the pyramid structures. Using a solution of hydrogen fluoride (HF) and hydrogen peroxide (H2O2), the gold acts as the catalyst, producing controlled nanometre-scale features. The gold is removed via a potassium iodide (KI) solution and the surface coated with a fluorocarbon material, perfluorooctyl tricholosilane (PFOS).
Technical challenges remain before potential commercialisation is possible. The nano-scale structures are inherently fragile and prone to damage and destruction.
“Because the structures are so small, they are fairly fragile,” Hess noted. “Mechanical abrasion to the surface can destroy the superhydrophobicity. We have tried to address that here by creating a large superhydrophobic surface area so that small amounts of damage won’t affect the overall surface.”
Image shows silicon pyramid structures etched for one minute using a hydrogen fluoride/hydrogen peroxide/water solution. The resulting structure has roughness at the micron and nanometre scales.