Most of the time, doing something 28% efficiently is nothing to be proud of. But in photovoltaics, it is good enough to set a new world efficiency record—and earn the company that made the record-setting devices millions of dollars in venture capital.
In this case, the lucky company is Alta Devices, which until recently held a low profile in the crowded solar industry. For the last few years, Alta has been quietly developing a novel process for cheaply manufacturing ultra-efficient solar cells. Their hard work appears to be paying off, because last month the company reported that they can make cells that are 28.2% efficient at converting solar energy into electricity. This beats the previous record by more than one percent, a wide margin by the solar industry’s standards.
Alta’s solar cells are made of a semiconducting material called gallium arsenide. Because this is a relatively expensive material, most solar companies use cheaper, less efficient alternatives such as silicon. But Alta came up with a transfer process that allows them to lay gallium arsenide layers that are only one micron thick (about 100 times thinner than a sheet of paper) on each cell. The thin layers don’t use up much material so they don’t cost very much, but they still generate as much electricity as a thicker layer would. The company’s goal is to generate electricity at a cost of under 50 cents per watt, which is about twice as cost-effective as the current state-of-the-art.
Alta’s layer transfer process is tricky, and so far it has only been carried out in laboratories, not commercial-scale manufacturing facilities. The company’s next step is to demonstrate that they can mass-produce their solar cells in a way that fully realizes the potential cost savings. To finance this ambitious effort, they have raised $72 million in venture capital, which should be plenty to find out if their approach holds water outside the halls of academia.
Cal professor Eli Yablonovitch is one of the co-founders of Alta Devices, along with Caltech professor Harry Atwater. In a recent paper, Yablonovitch and one of his graduate students, Owen Miller, reveal one of the keys to the record-setting efficiency of Alta’s solar cells. According to their paper, when sunlight passes through the semiconducting layer of a solar cell, its energy is repeatedly transferred between photons (light) and electrons (electricity). In order to maximize electricity generation, it is important to prevent light from escaping from the semiconducting layer. That way, energy can only escape in the form of electricity. In Alta’s solar cells, a reflective layer is placed directly underneath the semiconducting layer to hold the light in. Other companies that do not use a layer transfer process can’t position a reflector in this manner, and as a result, their efficiency is lower.
With its advanced technology and potentially drastic cost savings, it is easy to see why Alta is getting so much attention. Should the rest of the solar industry be afraid? That all depends on the results of Alta’s attempts to scale up their manufacturing—so stay tuned!
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Owen D. Miller, Eli Yablonovitch, & Sarah R. Kurtz (2011). Intense Internal and External Fluorescence as Solar Cells Approach the
Shockley-Queisser Efficiency Limit arXiv arXiv: 1106.1603v2