An international team of university researchers, led by Dr. Taylor, reports solving a major fabrication challenge for perovskite cells — the intriguing potential challengers to silicon-based solar cells. These crystalline structures show great promise because they can absorb almost all wavelengths of light. Perovskite solar cells are already commercialized on a small scale, but recent vast improvements in their power conversion efficiency are driving interest in using them as low-cost alternatives for solar panels.
Tech Briefs:What drew you to this project?
Dr. André D. Taylor: My group has worked on a lot of different aspects of solar technologies including small-molecule polymer solar cells. The attraction of these cells is that they are very cheap — cheaper than silicon. But the problem is that right now the efficiency of these polymer small molecules is not high. The power conversion efficiency of silicon solar panels is between 18 and 24%, while polymers have been in the single digits.
Just recently, however, perovskite cells have been considered as a photovoltaic material. At first, their efficiency was down in the single digits, and then they started rapidly progressing upwards. The present record for perovskites is 22.7% power conversion efficiency. Perovskites can be made with a solution-based process, as opposed to silicon wafers that require batch manufacturing, high temperatures, and vacuum deposition processes. Since perovskites are solution-based, you can conceive of them being made in a roll-to-roll setup where you deposit different layers and then your substrate — a flexible substrate with a fully functional solar cell — comes out the other end.
Tech Briefs: How does the cell work?
Dr. Taylor: We have a P-I-N sandwich structure, where you can think of the P as a positive layer — a hole transport layer. The I is the intrinsic layer and the N-type is the electron transport layer. Light shining into the intrinsic layer generates the electron-hole pairs that provide the electrical current. Perovskites are very good hole and electron transporters and are desirable as the intrinsic layer because they absorb energy across a wide spectrum of light, which means more energy is captured. Our unique contribution is to show that you can use a spraying technique to apply the top electron transporting layer (ETL).
Tech Briefs: How long before the cells are available commercially, and what about other applications?
Dr. Taylor: A big issue right now is scalability. First, these cells are not very stable. Since any exposure to moisture could degrade them, they have to be encapsulated. Second, you want large-area devices but right now, our solar cells are extremely small — 2 to 5 millimeters square. That's a far cry from a 3 × 4-foot panel. And the third hurdle is durability. You want them to last for a significant amount of time under all weather conditions.
There are many possible applications for integrated flexible photovoltaics; for example, you could put one on your automobile and it would conform to the surface. Then, instead of having to find the next charging station, the integrated solar panel enables you to just drive, park, and while your car is parked and idling, charge the batteries.
Tech Briefs: What's most exciting about this project?
Dr. Taylor: We're showing there's a pathway to making perovskites scalable, which could make them cheaper than conventional solar cell materials. Cheap solar cells would be a game-changer across all economies, in developed as well as developing countries.
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