Focus issue: thin-film photovoltaic materials and devices.

The recent tragic event of the massive oil spill in the Gulf of Mexico reminds us of the challenges of developing sources of energy that are not only abundant and economical but, more importantly, are also as harmless as possible to the environment. Among renewable energy sources, solar photovoltaic (PV) technologies show great potential, as they convert directly sun light into electricity with little impact to the environment, if one excludes those caused during manufacturing, installation, maintenance, and disposal at the end of the product life cycle. Despite these advantages, the share of PV technologies in the energy market has remained small, largely because of the higher cost of energy produced by solar panels compared with that produced by burning fossil fuels such as coal, oil, or natural gas. Hence, the PV industry, which is currently dominated by crystalline and high-purity polycrystalline silicon solar technologies, is making continuous advances to further increase the power conversion efficiency and lower the cost of solar cells and modules.