Study of polymer/ZnO nanostructure interfaces by Kelvin probe force microscopy

ZnO nanostructured films including nano-ridge and nanorod morphologies were used as electron acceptors/transporters and poly(3-hexylthiophene) (P3HT) as light absorber and electron donor in organic-inorganic hybrid solar cells. Bilayer solar cells with ZnO nano-ridges and ordered heterojunction solar cells with ZnO nanorods were each fabricated and characterized. ZnO nanorod/P3HT ordered heterojunction devices exhibited higher efficiency (0.31%) than the bilayer cells (0.094%) because the photocurrent was increased, attributed to the larger P3HT/ZnO interface area. Surface potential (SP) of P3HT/ZnO nano-ridges and P3HT/ZnO nanorods was studied by Kelvin probe force microscopy (KPFM). The SP difference between P3HT and ZnO nano-ridges was ∼0.10 V in the dark, but increased to ∼0.25 V under illumination. In the ZnO nanorod/P3HT films, ZnO nanorods showed lower SP with darker color, while P3HT exhibited higher SP with lighter color. In both nano-ridge/P3HT and nanorod/P3HT films, the SP difference demonstrated that electron transfer is energetically favorable from P3HT to ZnO.