A versatile fluoro-containing low-bandgap polymer for efficient semitransparent and tandem polymer solar cells

The versatility of a fluoro-containing low band-gap 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) in organic photovoltaics (OPVs) applications is demonstrated. High boiling point 1,3,5-trichlorobenzene (TCB) is used as a solvent to manipulate PCPDTFBT:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) active layer morphology to obtain high-performance single-junction devices. It promotes the crystallization of PCPDTFBT polymer, thus improving the charge-transport properties of the active layer. By combining the morphological manipulation with interfacial optimization and device engineering, the single-junction device exhibits both good air stability and high power-conversion efficiency (PCE, of 6.6%). This represents one of the highest PCE values for cyclopenta[2,1-b;3,4-b']dithiophene (CPDT)-based OPVs. This polymer is also utilized for constructing semitransparent solar cells and double-junction tandem solar cells to demonstrate high PCEs of 5.0% and 8.2%, respectively. The versatility of a fluoro-containing polymer, PCPDTFBT, in diverse organic photovoltaic applications is demonstrated. By combining morphological, interface, and device engineering, the single-junction cell shows both a high power conversion efficiency (PCE) of 6.6% and good air stability. In addition, the applications of PCPDTFBT in highly efficient semitransparent cells (PCE = 5.0%, average visible transmittance (AVT) = 47.3%) and double-junction tandem cells (PCE = 8.2%) are also demonstrated.