In several articles in the past, we featured black silicon– a technology that can be used to tweak the current solar cell production process and come up with more efficient solar cells. Although the silicon isn’t really black, it looks dark because very, very little light gets reflected – less than 5%, in fact.
This allows solar cell efficiencies to jump from 15% using conventional processes to 18.7%. Now if that isn’t enough good news, things just get even better, the process for making black silicon is bound to become even cheaper.
Before we go further, let’s review why the material works. Black silicon is composed of extremely tiny needles that rise above the silicon surface. These micro sized needles are smaller than the wavelength of light, forming a so-called “effective medium” that reduces reflection of light. So, instead of light being reflected and hence wasted, absorption is increased. Furthermore, it becomes efficient to collect light at any time of the day – from dawn to dusk, hereby increasing electric power production. This is all made possible because of the micro sized spikes.
Now the problem becomes how to produce these extremely small spikes on a silicon wafer surface. The current production process is a two-step process that involved metal deposition and electroless chemical etching. The metal used to etch the surface and produce the spikes is either gold or silver and the work is done at cryogenic temperatures. Although this still comes out cheaper than the way solar cells are currently produced, the use of these precious metals and the energy costs to produce the low working temperature add to the cost.
To address this, researchers at Rice University found a way to reduce the production process to single step that works at room temperature and uses copper. The silicon wafer is treated with a cocktail of copper nitrate, phosphorous acid, hydrogen fluoride and water. When the mix hits the silicon surface, copper nanoparticles are formed.
These copper nanoparticles, in turn, attract electrons from the silicon wafer surface and the silicon gets oxidized. The hydrogen fluoride then comes into play, burning nanopores into the silicon in the shape of inverted pyramids 590 nanometers in size. The reduction in size results in increased light absorption. Black silicon solar cells produced with this process uses more than 99 percent of the sunlight, hereby producing more electricity.
Work is underway to further improve the process. One of these efforts is to reduce the 8-hour etching process time. Another is to find a way to protect the nano scale spikes produced. Just the same, the use of a cheaper etching metal and being able to work at room temperature represents breakthroughs that will make solar cell production even cheaper.