At the moment, whilst solar cells are clean and environmentally friendly, the electricity they produce is still 3-4 times more expensive than electricity produced from fossil fuels such as coal. Part of the problem is their high captial cost of manufacture, but a less well known problem is the cost of installation. Because conventional solar cells are large and flat, they act like a great big sail in high winds, so they need to be securely bolted down. This is very expensive in terms of materials and time, especially if you are installing them on to an existing roof and in many cases this installation is more expensive than the solar cell itself.
A company called Solyndra is starting mass production of a solar cell which may solve this problem. They are using thin film copper indium gallium selenide cells which only use a very thin layer of semiconductor, using less materials than conventional cells which should make the cell itself cheaper. Instead of making the solar cells into flat plates, they then form them into tubes about 3cm across. This means that they can pick up light from any direction and if you paint the roof white they will also be able to pick up reflected light from the bottom too! More importantly it means that the wind can move between them, stopping the 'sail' effect of traditional sheets of cells. This means that you don't have to bolt them down on a flat industrial roof and makes the cost of their electricity competitive with that from a fossil fuel plant.