The solar market has been on fire. In the U.S., it's grown by 600 percent over the past five years, culminating in 3,313 installed megawatts in 2012. This past March, seven solar projects added the only new utility power of any kind to the U.S. grid. But solar energy isn't quite cost-competitive yet. Bridging the final gap requires breakthroughs that increase efficiency while cutting costs.