A new design of semiconductor arrays in nanoscale pillars can lower the cost of solar cell manufacture. Researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory and the University of California at Berkeley have demonstrated a way to fabricate efficient solar cells from low-cost and flexible materials by growing optically-active semiconductors in arrays of nanoscale pillars (nanopillars).

Each nanopillar consists of a single crystal with dimensions measured in a billionth of a meter. When the nanopillars are combined with a transparent, positive-type semiconductor that serves as a window, the resulting 3-D photovoltaic promises to be a low-cost, flexible yet efficient solar cell.

The efficiency of the test device was measured at 6%, which is less than the 10% to 18% range of mass-produced commercial cells but higher than most PV devices based on other nano structure materials.

Computer simulations have indicated that compared to flat surfaces, nanopillar semiconductor arrays should be more sensitive to light as they offer more surface for collecting light. This could give them a greater ability to convert light energy into charge-carrying electrons and be a more efficient collector to charge carriers.

The nanopillars were grown using a method called the “vapor-liquid-solid” process. Inside a quartz furnace, pillars of electron-rich cadmium sulfide on aluminium foil with geometrically distributed pores made by anodization as a template.

In the same furnace, they submerged the grown nanopillars in a thin layer of hole-rich cadmium telluride which acts as a window to collect light. The two materials in contact with each other form a solar cell in which electrons flow through the nanopillars to the aluminum contact below and the holes are conducted to thin copper-gold electrodes placed on the surface of the window above.

It is thought that the non-transparent copper-gold electrodes might be cutting the device’s effectivity. The researchers’ next step would be to improve top contact transparency.

Other factors that greatly affect the efficiency of a 3-D nanopillar-array solar cell include its density and the exposed length of the pillars in contact with the window material. These dimensions are easily optimized in future generations of the device.

The researchers made their design flexible as well. They etched away the aluminum substrate and substituted a thin layer of indium for the bottom electrode. They then sheathed the whole cell in a clear plastic known as polydimethylsiloxane to make a bendable device with only a marginal effect on performance.

The full results of the research are discussed in the paper “Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates,” by Zhiyong Fan, Haleh Razavi, Jae-won Do, Aimee Moriwaki, Onur Ergen, Yu-Lun Chueh, Paul W. Leu, Johhny C. Ho, Toshitake Takahashi, Lothar A. Reichertz, Steven Neale, Kyoungsik Yu, Ming Wu, Joel W. Ager, and Ali Javey, appears in the August issue of Nature Materials.

This work was supported in part by the Helios Solar Energy Research Center, which is supported by the US Department of Energy’s Office of Science, Office of Basic Energy Sciences.


- Katrice R. Jalbuena

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Sources:

¹ http://newscenter.lbl.gov/press-releases/2009/07/09/nanopillar-solar-cells/
² http://newscenter.lbl.gov/