Scanning electron microscope image of endview of Solar3D prototype

Solar3D Inc, a small research company founded in 2010, has recently fabricated a working prototype of a three-dimensional (3D) silicon solar cell [1]. Conventional solar cells employ a 2D geometry for capturing light. That is, light enters the cell through a 2D plane and is absorbed inside. A 3D solar cell uses 3D structures (the ridge-like structures pictured above) to capture light. An obvious advantage of this geometry is that there is a greater surface area for light absorption, and thus more light can enter a cell. The concept of a 3D cell is not new; cells that use thin nanowire structures to capture light have been known and studied for some time now. However, to the author’s knowledge, Solar 3D’s cell is the first of its kind to employ the ridge-like structures pictured above. Solar3D claims that their cell design offers many advantages over conventional 2D cells [2].

Firstly, conventional cells suffer from unavoidable reflection losses. Techniques exist to reduce these losses, but inevitably some light is always reflected away. A 3D cell is not immune to these losses either, however by virtue of the ridge-structures in such a cell, reflected light may be bounced back and forth between adjacent ridges, increasing the chances of it being absorbed. As well many 2D cells have metal contacts on the front to harvest the electrical current created by sunlight; these contacts are not transparent, so they block some of the sunlight from reaching the cell. In Solar 3D’s cell, the metal contacts run below the light-capturing ridges so that light-absorption is not compromised.

Another advantage offered by Solar 3D’s cell are thin absorbing regions. In a conventional silicon cell, the cell is made relatively thick to promote light-absorption. However, if a cell is too thick the electrons generated by sunlight may not make it out of the cell before being reabsorbed. Solar 3D’s cell overcomes this by using many thin absorbing regions. The thinness allows electrons to be efficiently collected, and the fact that there are many of these regions allows light to be efficiently absorbed.

Finally, Solar3D claims their cell has superior light-collection throughout the day (pictured above). Conventional cells operate most efficiently when the sun is directly overhead. This means that these cells will only perform optimally for a small part of the day. Many solar cell installations mount cells on trackers so that they can follow the sun throughout the day and overcome this disadvantage. According to Solar3D, their cell can collect light effectively over a range of angles, obviating the need for tracking systems, and allowing their cells to operate efficiently throughout most of the day.

The company fabricated their cell in July, 2012 and so far have not released any concrete details about the prototype’s performance. According to Solar 3D’s Director of Technology, Dr. Changwan Son, “When measured relative to a conventional solar cell design, our working prototype produces electricity beyond our previous expectations. First, we fabricated our working prototype. Then we created a simple cell based on the conventional design, using the same fabrication environment, to serve as a control sample. By measuring the side-by-side power output of both cells, we were able to determine the relative performance under a number of conditions, ranging from bright sunlight to lower, diffuse light. In each test, our 3D Solar Cell consistently outperformed the control cell and produced at least 2½ times the amount of electricity under the same conditions.” Unfortunately, the company does not provide any information on what particular “conventional solar cell design” they are comparing their 3D cell to. In the context of silicon, a “conventional cell” can refer to a myriad of devices covering a broad range of performance, and so these claims are rather ambiguous. The company has stated that simulations they have run on their cell design show that it should have an efficiency of 25.47%. This is a rather ambitious claim, considering that the record efficiency achieved in practice for a silicon cell is 24.7% [3]. The top 10 commercially available silicon cells have efficiencies ranging from 19.1%-22.5% [4]. Solar3D has not yet quoted an efficiency for their working prototype.

According to Dr. Son, Solar 3D’s next step is to improve the fabrication process so that they can drive down the cost of manufacturing their 3D devices. Son says “We believe that the result will be a 50% reduction in the cost of solar electricity. Perhaps the installed system cost savings will be even greater.” Again, this is a rather ambitious claim, considering how remarkable a 50% reduction of cost would be for solar energy. Solar 3D’s cell is certainly an interesting design, and the purported advantages it offers (mentioned above) are quite plausible. However, until the company releases detailed performance characteristics for their device, it is difficult not to remain skeptical regarding the company’s more ambitious claims.

-Kevin Boyd (Year 1 of M.A.Sc in Engineering Physics at McMaster University)