Optimizing solar-cell technology can be a complex job, requiring expertise in material science, physics, and optics to convert as much sunlight as possible into electricity. But despite this complexity, a simple fact is key to making a high-performance solar cell: any sunlight reflected off the cell can’t possibly be converted into electricity.
Manufacturers have tried to minimize the reflection of sunlight off of solar cells by first chemically etching micrometers-deep structures into the surface of solar cells and then depositing one or more thin anti-reflection layers. Unfortunately, the equip- ment and processes for these conventional methods add significant cost to the solar cell, and the cells still absorb only 93%–97% of the sunlight.
To address this problem, scientists at the National Renewable Energy Laboratory (NREL) have invented the “black silicon” nanocatalytic wet-chemical etch, an inexpen- sive, one-step process that literally turns the solar cells black, allowing them to absorb more than 98% of incident sunlight. The process costs just a few cents per watt of solar-cell power-producing capacity.
To etch the silicon, a wafer is immersed in a solution that contains chloroauric acid, which is composed of hydrogen, chlorine, and gold. Tiny nanoparticles of gold instant- ly form and act as a catalyst for chemical reactions, producing a nanometer-scale porous surface on the cell wafer. The nanoscale pores—on the order of a billionthof a meter in diameter—are much smaller than the wavelength of the incident light, so they suppress reflection across the full spectrum of sunlight. As the tiny holes deepen, they make the metallic gray silicon appear increasingly dark until it becomes almost pure black, absorbing nearly all frequencies of sunlight. The surface becomes riddled with minute pores of varying depths with no sharp interfaces that would reflect light, creating a highly absorbent silicon wafer.
Using a closely-related process that employs less-expensive silver nanoparticles, NREL has made a black silicon cell with a validated 18.2% conversion efficiency—about the same efficiency as a typical crystalline silicon solar cell with a more costly antireflec- tive coating.
At 100°F, NREL’s black silicon etching process takes less than a minute. In contrast, the etching process that prepares silicon wafers for conventional antireflective coatings takes 8–30 minutes, and applying the coatings adds even more processing time.
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