Coating ZnO nanoparticle films with DNA nanolayers for enhancing the electron extracting properties and performance of polymer solar cells

Here we present for the first time polymer solar cells that incorporate biological material that show state of the art efficiencies in excess of 8%. The performance of inverted polymer solar cells was improved significantly after deposition of ZnO nanoparticles (ZnO-NPs) together with a thin deoxyribonucleic acid nanolayer and used as an electron extraction layer (EEL). The ZnO-NPs/DNA double layer improved the rectifying ratio, shunt resistance of the cells as well as lowering the work function of the electron-collecting contact. Importantly, the ZnO-NPs/DNA bilayer enhanced the power conversion efficiency of cells considerably compared to cells with EELs made of only DNA (improvement of 56% in relative terms) or only ZnO-NPs (improvement of 19% in relative terms) reaching a best power conversion efficiency of 8.5%. The ZnO-NPs/DNA double layer cells also outperformed ones made with one of the most efficient previous synthetic composite EELs (i.e. ZnO/PEIE(poly(ethyleneimine)-ethoxylated)). Since all fabrication procedures were carried out at low (<150 degrees C) or room temperature, we have applied the findings to flexible substrates as well as on glass obtaining a high PCE of 7.2%. The solar cells with the biological/metal-oxide composite EELs also delivered an improvement in the stability (similar to 20% in relative term) compared to that with ZnO-NPs only. All these findings show that natural materials, in this case DNA, the premium biological material, can be incorporated in organic semiconductor devices in tandem with inorganic devices delivering uncompromising levels of performance as well as significant improvements.