Here's a seemingly simple solar power factoid: the sun bathes the Earth with enough energy in one hour (4.3 x 10 20 joules) to more than fill all of humanity's present energy use in a year (4.1 x 10 20 joules). So how to convert it? In the world of solar energy harvesting, there's a constant battle between cost and efficiency. On the one hand, complex and expensive triple-junction photovoltaic cells can turn more than 40 percent of the (specially concentrated) sunlight that falls on them into electricity. On the other, cheap, plastic solar cells under development convert less than 5 percent. [More]