Years after attempting to decrease the reflectivity of solar cells even further, Natcore has finally came up with its ‘blackest’ solar cell to date, reflecting only 0.3% of incident of sunlight, without the use of an antireflective coating, but rather, using a cheaper wet chemistry route.
This new accomplishment is so good and economical that it may render the application of antireflective coatings an outdated practice.
Antireflective coating has been an essential component of a solar cell in as they trap incident light and diminish the amount of escaped sunlight; but not for long as Natcore scientists have perfected their black solar cell technology, which involves two main steps: chemical etching and liquid phase deposition, which are both claimed to be cheap and nontoxic.
Natcore’s black solar cell technology is based on the fact that solar cells reflect light due to the difference of refractive indices of silicon, glass, and air. Hence, the research firm’s solution is to make this difference lower, not by adding a layer of different material that would act as intermediate between the two media, but by boring very small pillars into the silicon as shown in the figure and SEM micrograph above.
How does this structure indulge a very small difference between silicon and air’s refractive indices? When silicon has a flat surface, the light travels from air and then, immediately to silicon’s surface. If narrow and deep holes are incorporated into the silicon’s surface, air packets increases, such that light travels from air to silicon and air packets, thus, reducing difference in refractive indices. As these pillars are made narrower (nano-scale) and deeper, density of air packets increases such that the light now travels as if from air to air!
The dimensions of these pillars are easily customized by controlling chemical etching parameters, and it is of utmost importance that their depth are at least twice the wavelength of incident light. How these nano-pillars are made is just as amazing as how the optical properties of silicon wafers are changed.
With silver nanoparticles acting as catalysts to oxidation of silicon in an acidic medium (HF), which removes or etches oxidized silicon, the silver nanoparticles drill the silicon surface to a desired depth, as shown in the schematic diagram above.
These silicon surfaces are then rendered chemically inert by NatCore’s passivation technique, liquid phase deposition, which oxidizes the silicon surface and also utilizes the waste materials from the initial production of the silicon wafers.
The next step for these black solar wafers is solar cell assembly at the National Renewable Energy Laboratory (NREL). Natcore has been collaborating with NREL in producing and testing solar cells. Recently, they have collaboratively produced 18.2% efficient solar cell using a different passivation technique. With the latest black silicon wafers and Natcore’s liquid phase deposition technique, this efficiency is expected to further increase with the productions cost also expected to decline.
NREL Research Scientist Dr. Hao-Chih Yuan says, “We have a good synergy with Natcore on black silicon technology. A silicon surface, without proper coating, is detrimental to the energy conversion efficiency of the solar cell. It is not unusual to grow silicon dioxide coatings on black silicon surfaces for this purpose, but the growth is typically at very high temperatures. Natcore’s coating uses chemistry. They are the ones who can passivate a black silicon surface cheaply.”
Natcore President and CEO Chuck Provini says, “NREL holds the efficiency record with black silicon, but they used a passivation technology that requires expensive thermal oxidation. We will replace that cumbersome step with our LPD oxide process. We believe the combination of the two technologies could significantly exceed NREL’s record cell efficiency.”