Dye-sensitized solar cells (DSSCs) are cheap, thin, flexible, and easy to manufacture. As a result, many researchers work with them instead of with other, higher-efficiency solar cell technologies that use solid-state semiconductors such as silicon or gallium arsenide. Scientists have been interested in building DSSCs out of the semiconductor zinc oxide because the material holds attractive properties such as transparency and a simple synthesis process. But the efficiencies of such solar cells have been low.
Now chemist Yulin Deng of Georgia Institute of Technology and his colleagues have found a way to densely pack zinc oxide nanowires in a solar cell that significantly improves its efficiency (Nano Lett., DOI: 10.1021/nl301407b).
Deng’s team grew the nanowires in caterpillar-like arrays, with multiple nanowires branching from a central, linear core. The key to their method lay in sequentially seeding the growth of dense nanowire structures. They used a technique called electrospinning to deposit fine strands of zinc oxide on top of an initially deposited nanowire array. The strands serve as seeds for the growth of the nanowires, which then branch out in all directions.
Previously, researchers had used zinc oxide nanoparticles as seeds, which resulted in less growth because they tended to fall into the empty spaces between the nanowires instead of perching on top. With the new method, Deng says, “you can increase the number of layers and easily control the total thickness until the light is 100% absorbed.”
With a five-layer solar cell, the researchers achieved 5.20% light-to-electricity efficiency, improving upon the previously reported 0.1 to 3% efficiencies for DSSCs made of zinc oxide nanowires.
In comparison, DSSCs made from more commonly used titanium dioxide have achieved 12.3% efficiency (Science, DOI: 10.1126/science.1209688). Deng thinks that, with more-densely packed zinc oxide nanowires, his DSSCs can perform as well as titanium dioxide devices. “But,” he admits, “we have a long way to go.”