the second generation of photovoltaic materials is based on the use of thin-film deposits of semiconductors. these devices were initially designed to be high-efficiency, multiple junction photovoltaic cells. later, the advantage of using a thin-film of material was noted, reducing the mass of material required for cell design. this contributed to a prediction of greatly reduced costs for thin film solar cells. currently (2007) there are different technologies/semiconductor materials under investigation or in mass production, such as amorphous silicon, poly-crystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium selenide/sulfide. typically, the efficiencies of thin-film solar cells are lower compared with silicon (wafer-based) solar cells, but manufacturing costs are also lower, so that a lower price in terms of $/watt of electrical output can be achieved. another advantage of the reduced mass is that less support is needed when placing panels on rooftops and it allows fitting panels on light materials or flexible materials, even textiles. this allows for portable roll-up solar panels, which can fit in a backpack and be used to power cell phones or laptops in remote areas.