An effective way to improve polymer solar cell efficiency is to use a tandem structure, as abroader part of the spectrum of solar radiation is used and the thermalization loss of photonenergy is minimized. In the past, the lack of high-performance low-bandgap polymers was themajor limiting factor for achieving high-performance tandem solar cell. Here we reportthe development of a high-performance low bandgap polymer (bandgap o1.4 eV), poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:20,30-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothiadiazole)] with a bandgap of 1.38 eV, high mobility, deep highest occupied molecularorbital. As a result, a single-junction device shows high external quantum efficiency of460%and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%.The polymer enables a solution processed tandem solar cell with certified 10.6%power conversion efficiency under standard reporting conditions (25 1C, 1,000Wm2,IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.
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