Theoretically, renewable energy sources seem to be the most sustainable alternative route to concurrently address energy security and climate change problems. Increasing advances in solar cell efficiency and cost-reduction endeavors, along with clean energy initiatives are the primary factors for the impetus in solar energy production to date. At least 40 GW of PV systems were installed globally in 2014, up from 37 GW in 2013, setting a new record for the solar PV sector. China, Japan and the United States were the three top markets in 2014. The rest of the world is also rapidly increasing development capabilities to manufacture solar cells in mass quantities.
Gallium arsenide (GaAs)-based tandem solar cells hold the record of highest efficiency among all kinds of solar cells made to date. However, it is prohibitively expensive to manufacture compared to a conventional silicon solar cell manufacturing has prevented its widespread usage. However, there are many ways to reduce the overall cost of solar cells such as reducing the materials used, increasing light trapping, decreasing power losses, etc. As a part of meeting this goal, companies like Nanoflex and Alta devices have worked extensively and achieved efficiency of approximately 28 percent using microns thick layer. This new technology might change the way we see solar cells today. With these solar cells in the market, solar cells will be flexible, thin and more efficient.
Conventional solar cells industry uses wafer to fabricate solar cells but with thin film technology in place, significant cost savings can be realized without affecting the overall efficiency. In general, there are two kinds of material used in solar cells fabrication; e.g., direct and indirect band gap material. Direct band gap materials need a very thin layer to absorb whole solar spectrum, whereas indirect band gap material needs to be thick enough to absorb most of the solar spectrum. Even one micron of direct band gap material is more than enough to achieve a substantial amount of solar light absorption. For comparison, silicon is an indirect band gap material, whereas GaAs is a direct band gap material. So, GaAs requires substantially less material than silicon to absorb same amount of light.
Any electronic device begins with the raw material that is called a wafer. These are flat, circular platters of ultra-purified (for solar cells 99.9 percent purity) material. The material required for GaAs solars itself is very costly. It can cost about $5,000 to make an 8-inch diameter wafer of gallium arsenide, versus $5 for a silicon wafer. In the new fabrication process of solar cells, high purity wafers are used for nucleation of a thin film that is just one or two microns thick. The same wafer can be used many times as seed, which will ultimately lower the cost by almost hundred times (the material required has gone from 350 microns to 1–2 microns). Once the thin film is grown , it s transferred onto a flexible substrate to complete the fabrication process. Unlike conventional solar cells, these cells are joined in series using laser processing.
Further cost reduction has been achieved by light trapping. Light trapping is the capturing of as much light as possible from impinging electromagnetic waves with the objective of generating free carrier (electrons and holes) that constitute the current. The active layer of some semiconducting material is used to capture the solar radiation. Light trapping can be achieved in many ways such as tapering of the top surface or using nanostructures. However, GaAs in itself can act as a very efficient light trapping medium. In a video on YouTube, Eli Yablonovitch, director of the NSF Center for Energy Efficient Electronics Science and co-founder of Altadevices, explains that the key to an efficient photovoltaic material is minimizing the wasteful recombination of electrons and holes. He further explains that a good photovoltaic material should also be a good LED. In these materials, photons get absorbed and re-emitted a hundred times before being productively harvested, and the inside of the material is a sea of photons. A photon can be understood as a constituting unit of light (like atom constitute matter). However, some of these photons escape back out of the material. To restrict that, the GaAs can be put on a reflective backing that sends the stray photons right back into the chip.
For a solar cell to work efficiently, it’s very important that it absorbs maximum light. Also, it’s necessary that minimum light is reflected back from the surface. The later is achieved by texturing the top surface. A textured solar cell can achieve a higher voltage than plane parallel solar cells. Overall, external light extraction increases the internal luminescence and photon density, thereby increasing the carrier density and open-circuit voltage. A complete physics behind the working of low-cost high-efficiency GaAs solar cells has been described by Eli Yablonovitch.
Gallium arsenide has long been used for solar space application, but its application for common uses is very restricted because of it high cost. However, researchers from different universities have been successful in obtaining high-efficiency GaAs solar cells (almost double to market grade silicon solar cells) by depositing very thin layers that will minimize the cost to the large extent. While there is still room for improvement, I believe GaAs will become an alternative to silicon solar cells in coming years.
Vidur Raj is a PhD student at Australian National University. He works part time with Auvisa.org, a professional Australian visa agency. He specializes in investment visa in the energy sector.
