Photovoltaics: solar cells on curtains.

Photovoltaic devices directly convert solar energy into electricity, and thus are highly attractive for a wide range of applications. Although the energy harvested by solar cell technology accounts for only about 0.04% of today’s global energy consumption1, it is one of the fastestgrowing energy sectors, in part because of the vast research and development efforts that have been undertaken in recent years for pursuing clean, renewable energy. On page 907 of this issue2, John Rogers and colleagues report on a technique that may contribute to this expansion of solar energy, as they demonstrate the integration of silicon solar cell modules on highly flexible, lightweight polymer substrates (Fig. 1), by using a simple transfer printing technique, previously pioneered by the same researchers. Solar energy is one of the most abundant natural sources of energy. For instance, the total solar energy that bombards the United States alone is about 20,000 times higher than the energy needed to power the entire country, and yet most of this energy is lost without ever being harvested. In recent years, considerable improvements have been made to the efficiency of inorganic and organic photovoltaics through materials innovation and device engineering. Silicon photovoltaic devices, however, remain the dominant technology in the market owing to the natural abundance of silicon (leading to relatively low costs), and its high reliability, ease of processing and high efficiency. For these reasons, there is great interest in exploring new Si-based structures in nonconventional approaches to broaden the application spectrum. In this area, Rogers and colleagues offer a versatile approach for achieving flexible, crystalline Si photovoltaics with user-defined, tunable properties. In their work, solar micro-cells are first fabricated on bulk silicon wafers by using conventional lithography and doping techniques. The fabricated micro-cells have thicknesses down to about 100 nm and widths down to a few micrometres, and are suspended on the source wafer with minimal anchoring by means of a wet etch process. Following the fabrication processing, these micro-cells are transferred to a soft, elastic stamp, and then printed on a foreign substrate, such as a plastic or glass. Given the right thickness of silicon combined with the use of backside reflectors, individual microcells demonstrate a respectable conversion efficiency of 4–13%. Uniquely, the surface coverage of the printed silicon micro-cells on a transparent substrate determines the total conversion efficiency, as well as the transparency of the final module. Specifically, the researchers demonstrate the modulation of transparency from 35% to 70% by varying the solar micro-cell spacing from 26 μm to 170 μm. This unique feature may have important implications for certain applications where some transparency is desirable, such as for windows of buildings. The concept of flexible solar cells has been around for many years. In the past, however, most efforts for flexible applications have focused on solutionprocessed organic and nanocrystal films. Although tremendous progress has been made in this field, and it remains an exciting and active area of research3, the relatively poor performance and stability of organics have limited their commercialization. But crystalline, inorganic semiconductors with high efficiency and robustness can also be made flexible if they are trimmed to small scales and transferred to flexible support substrates. In recent years, various approaches for printing crystalline thin films, micro-strips and nanowires on a wide range of foreign substrates, including plastics, have been successfully demonstrated4–7. As Rogers and colleagues have now shown, mechanically flexible and monocrystalline Si cells can indeed offer both high versatility and performance. An important feature is that most of the fabrication steps, including high-temperature doping and etching, are carried out on bulk silicon wafers using well-established Si processing. After the transfer of the micro-cells onto the Crystalline silicon solar cell arrays on flexible, transparent substrates may lead to unconventional new applications. PhotovoltAics