Ali Javey and Zhiyong Fan at Berkeley Lab have invented a method for growing highly regular, single-crystalline nanopillar arrays of optically active semiconductors to produce efficient, 3D solar cells. The 3D configuration allows for less stringent requirements in terms of the quality and purity of the input materials, providing for a reduction in cost compared to other solar cell configurations.
The Berkeley Lab invention uses a "vapor-liquid-solid" process that produces large-scale modules of dense, ordered arrays of nanopillars. Researchers tested their method by producing a solar cell composed of electron-rich CdS nanopillars embedded in a polycrystalline thin film of hole-rich CdTe. The efficiency of this prototype was 6%, which may be readily improved with concentrators, more transparent top contacts, and optimization of the nanopillar dimensions. The technology was also used to produce solar modules on flexible substrates that offer more efficient light absorption and carrier collection than rigid arrays. These flexible arrays could be bent repeatedly without damage or loss of cell performance.
The ability to deposit single-crystalline semiconductors on support substrates is crucial in the development of efficient photovoltaics. However, the process, usually performed with epitaxial crystal growth, has been expensive and inefficient. In addition, when amorphous substrates have been used to grow single-crystalline nanowires non-epitaxially, at less expense, the nanowires have varied in size, alignment, and density giving the resulting arrays a limited efficiency of approximately 0.5%. The Berkeley Lab technology offers a significant improvement in efficiency and manufacturability.