Black polymers in bulk-heterojunction solar cells.

The active materials in polymer solar cells have a decisive role on the performance of the cells. Polymers with extended absorption, i.e., black polymers with absorption covering the whole visible region are desired in order to capture the important parts of the solar irradiation. Different ways of achieving black active materials are discussed and two new alternating polyfluorene (APFO) copolymers with broad absorption, APFO-Black 1 and APFO-Black 2, using two different design strategies are described. The UV-Vis absorption spectra of the polymers extend to approximately 850 nm, and the polymers were used as donors and 6,6-phenyl-C $-{61}$-butyric acid methyl ester (PCBM)60 or PCBM70 as acceptors in solar cell devices in various mixing ratios. The best combinations yielded an overall power conversion efficiency of 1.2 for APFO-Black 1 and 1.5 for APFO-Black 2.