根据以下材料，回答题Better Solar Energy Systems: More Heat， More LightSolar photovoltaic th

根据以下材料，回答题

Better Solar Energy Systems: More Heat， More Light

Solar photovoltaic thermal energy systems， or PVTs， generate both heat and electricity， but__________ （51） now they haven"t been very good at the heat-generating part compared to a stand-alone solar thermal collector. That"s because they operate at low temperatures to cool crystalline silicon solar cells，__________ （52） lets the silicon generate more electricity but isn"t a very efficient way to gather heat.

That"s a problem of economics. Good solar hot-water systems can harvest much more energy than a solar-electric system at a substantially lower__________（53）. And it"s also a space problem: photovoltaic cells can__________ （54） up all the space on the roof， leaving little room for thermal applications.

In a pair of studies， Joshua Pearce， an associate professor of materials science and engineering，has devised a__________ （55） in the form. of a better PVT made with a different kind of silicon.

His research collaborators are Kunal Girotra from Thin Silicon in California and Michael Pathak and Stephen Harrison from Queen"s University， Canada.

Most solar panels are made with crystalline silicon， but you can also make solar cells out of amorphous silicon， commonly known__________ （56） thin-film silicon. They don"t create as much electricity， but they are lighter， flexible， and cheaper. And， because they__________ （57） much less silicon， they have a greener footprint. __________ （58）， thin-film silicon solar cells are vulnerable to some bad-news physics in the form. of the Staebler-Wronski effect.

"That means that their efficiency drops when you __________（59） them to light——pretty much the worst possible effect for a solar cell，" Pearce explains， which is one of the__________ （60） thin-film solar panels make up only a small fraction of the market.

However， Pearce and his team found a __________（61） to engineer around the Staebler-Wronski effect by incorporating thin-film silicon in a new __________（62） of PVT. You don"t have to cool down thin-film silicon to make it work. In fact， Pearce"s group discovered that by heating it to solar-thermal operating temperatures， near the boiling__________ （63） of water， they could make thicker cells that largely __________（64） the Staebler-Wronski effect. When they applied the thin-film silicon directly to a solar thermal energy collector， they also found that by baking the cell once a day， they__________ （65） the solar cell"s electrical efficiency by over 10 percent.

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A.until

B.unless

C.when

D.if