Photo: Padture Lab/Brown University
鈣鈦礦(Perovskite)太陽能電池可以是矽晶太陽能電池的廉價有效替代品。一種新技術有潛力在室溫大量生產更薄的鈣鈦礦薄膜而不損品質。薄膜鈣鈦礦太陽能電池可用來製造可發電的彩色玻璃窗。由布朗大學(Brown University)一名華裔博士生所領導的研究，找出新方法來製造太陽能電池所用的吸光鈣鈦礦薄膜。
這新方法利用室溫溶劑浸泡來製造鈣鈦礦晶體，取代現時結晶所用的加熱法。這項刊登在皇家化學學會《物料化學期刊》的研究顯示，這技術可以在大面積以精準控制生產出高質量結晶薄膜，並可能是大量生產鈣鈦礦太陽能電池日後的路向。
鈣鈦礦薄膜是很好的吸光材料，生產成本比一般太陽能電池所用的矽芯片更加低。在短短數年間，鈣鈦礦電池的光電效率突飛猛進。在2009年出現的首片鈣鈦礦電池只能有約4%的效率，遠比標準矽晶電池的25%為低。但到去年，鈣鈦礦電池已證明到有超過20%的效率。在表現上的快速改善有巨大潛質，研究人員爭相開始在商業成品上採用鈣鈦礦電池。
製造這薄膜有不同方法，但幾乎大部分是要使用熱力。鈣鈦礦的前驅物質化學品溶解在一種液體裡，之後塗在一片基質上。加熱把溶液移除後，留在基質上的鈣鈦礦晶體便形成一片薄膜。
布朗大學Padture實驗室的Yuanyuan Zhou要找不必加熱的方法來製造鈣鈦礦薄膜，想出使用溶液-溶液抽除法。
利用Zhou的方法，先把鈣鈦礦前驅物質溶在一種名為NMP的溶劑並塗在基質上，之後取代加熱的是把基質浸泡在第二種溶劑乙醚(diethyl ether, DEE)來選擇性地抽走NMP溶劑。
A new low-temperature method for making perovskite solar cells paves the way for high-efficiency, colorful, see-through photovoltaic films that could be laminated on windows or plastered on walls.
Perovskite solar cells have become darlings of the photovoltaic world in the past five years. Their efficiencies in that time have soared from a meager 4% to over 20%, quickly catching up to silicon’s 25%.
Another appeal of these solar cells is that the light-absorbing perovskite layers are easy and much cheaper to make than silicon wafers. Mix some precursor chemical solutions, coat it on a substrate and then heat it to evaporate the solvent, and you have yourself a film of perovskite crystals.
But that heat can cause problems. The crystals often form unevenly during heat-treatment, which leaves tiny holes in the film, says Nitin Padture, a professor of engineering at Brown University. The typical temperatures of 100–150°C that are used to make the crystals also limits them from being made on plastic surfaces, which would melt at those temperatures.
So Padture and his colleagues first made a perovskite precursor solution in a solvent called NMP that is used in the petrochemical and plastics industries. They coated the solution on a substrate. And then, instead of heating, they used a second, different solvent to wash the surface. This second solvent attaches to the NMP and whisks it away.
The researchers were able to make smooth, high-quality crystalline films and could precisely control the film’s thickness across large areas. Initial tests at the US National Renewable Energy Laboratory shows that the films have light-to-electricity conversion efficiency more than 15%.
Standard perovskite films are around 300nm thick, and are typically black and opaque. By comparison, the new method gave films as thin as 20 nanometers that were nearly transparent. Tweaking the precursor chemicals led to films of different colors.
The process is also much faster. It took two minutes as opposed to an hour or more for heat-treating, which makes it more suitable for mass production, the researchers say.
"We think this could be a significant step toward a variety of commercially available perovskite cell products," Padture said in a press release. The researchers presented their findings in the Royal Society of Chemistry’s Journal of Materials Chemistry A.
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