In the past two or three decades, however, researchers have sought ways to exploit PV technology as part of a worldwide effort to develop and commercialize renewable energy sources and reduce the world's dependence on fossil and nuclear energy.  The main problem with PV technology over the years has been its relatively low efficiency (that is the percentage of solar energy that is converted into electricity), which, combined with the cost of producing solar PV panels, makes it too expensive to compete with conventional, centrally-generated electricity in most applications. Recent research, however, has yielded thin-film solar cells with efficiencies approaching 19 percent.  Many experts believe that, with additional R&D, cost reductions resulting from increased production, deregulation of the electric utility industry in many countries, and growing concern about global warming, PV technology will play a major role in our energy future.