Improving the power conversion efficiency of polymer-based bulk-heterojunction solar cells is a critical issue. Here, we show that high efficiencies of ∼10% can be obtained using the crystalline polymer PNTz4T in single-junction inverted cells with a thick active layer having a thickness of ∼300 nm. The improved performance is probably due to the large population of polymer crystallites with a face-on orientation and the ‘favourable’ distribution of edge-on and face-on crystallites along the film thickness (revealed by in-depth studies of the blend films using grazing-incidence wide-angle X-ray diffraction), which results in a reduction in charge recombination and efficient charge transport. These results underscore the great promise of polymer solar cells and raise the hope of achieving even higher efficiencies by means of materials development and control of molecular ordering.

Nature Photonics 9, 403–408 (2015) doi:10.1038/nphoton.2015.84