The price of photovoltaic hardware has dropped so dramatically in recent years that, according to some projections, a well-sited panel may become competitive with fossil fuels before the decade is out. To reach that point, which comes when panels cost below $2 per Watt, prices will have to continue their steep decline. During our visit to IBM’s Watson research center, we talked to two people who are working on ways to drive the cost down—but they are taking radically different approaches.