Design optimization could help maximize the power conversion efficiency of thin-film silicon solar cells

The best performing thin-film silicon solar cells at present have efficiencies that are about half that of conventional bulk silicon solar cells. “By investigating a variety of appropriate vertical nanopillar designs we can enhance the light-trapping and -collection efficiency of thin films to compensate for the efficiency loss caused by reduced material quality and quantity,” says Singh.

The researchers investigated various factors that might affect the performance of a thin-film solar cell. These factors include the diameter and length of the nanopillar, as well as the spacing between nanopillars (see image). Similarly important is the design of the positively and negatively charged layers in the solar cells that are needed to separate the electrical carriers created by the absorbed light.

The researchers’ simulations showed that the thickness of the negatively charged layer on the outer side of the pillars should be as thin as possible in order to reduce ‘parasitic’ absorption—the annihilation of light-generated carriers before they cross the junction between layers where they would contribute to electrical power generation. They also found that an axial junction design in which the junction between positive and negative layers is confined to the very top of the pillars leads to a higher open-circuit voltage compared with more conventional radial junction structures in which the negative layer wraps around the entire pillars. Yet they found the converse to be true for the open-circuit current.

Singh and his co-workers therefore show that a balance of these factors is needed in order to optimize designs for light-to-power conversion efficiency in surface-textured thin-film structures, which could eventually lead to thin-film silicon solar cells that are able to match the efficiency of the more expensive single-crystalline silicon solar cells.

More information: Wong, S. M. et al. Nanopillar array surface-textured thin-film solar cell with radial p-n junction. IEEE Electron Device Letters 32, 176–178 (2011). http://ieeexplore. … mber=5671473

Provided by Agency for Science, Technology and Research (A*STAR)