Highly efficient tandem and semitransparent (ST) polymer solar cells utilizing the same donor polymer blended with [6,6]-phenyl-C61- butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C 71-butyric acid methyl ester (PC71BM) as active layers are demonstrated. A high power conversion efficiency (PCE) of 8.5% and a record high open-circuit voltage of 1.71 V are achieved for a tandem cell based on a medium bandgap polymer poly(indacenodithiophene-co-phananthrene-quinoxaline) (PIDT-phanQ). In addition, this approach can also be applied to a low bandgap polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′] dithiophene)-alt-4,7-(5-fluoro-2,1,3-benzothia-diazole)] (PCPDTFBT), and PCEs up to 7.9% are achieved. Due to the very thin total active layer thickness, a highly efficient ST tandem cell based on PIDT-phanQ exhibits a high PCE of 7.4%, which is the highest value reported to date for a ST solar cell. The ST device also possesses a desirable average visible transmittance (≈40%) and an excellent color rendering index (≈100), permitting its use in power-generating window applications.
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