In Situ Study of Sputtering Nanometer-Thick Gold Films onto 100-nm-Thick Spiro-OMeTAD Films: Implications for Perovskite Solar Cells

The performance of many perovskite solar cells is closely related to the spiro-OMeTAD/gold interface since gold is used as top contacts, which renders the detailed understanding of the interface formation very important. In this work, sputter deposition as an industry-relevant, high-rate, large-scale, and well-controllable deposition technique is used to prepare gold electrodes on top of a 100-nm-thick spiro-OMeTAD film. In situ grazing-incidence small-angle X-ray scattering (GISAXS) is used to study the nanostructure-growth kinetics of the gold contact on top of the spiro-OMeTAD film during the sputter process. The results show that the gold grows in nanoscale clusters, which then coalesce into a complete yet still nanogranular layer forming the top contact with a thickness of 90 nm. Based on simulations of the two-dimensional GISAXS patterns, additional information about the shape of the nanosized gold cluster is gained at the different cluster growth stages. Furthermore, the diffusion of gold into the spiro-OMeTAD film occurs during the sputter process as verified with X-ray reflectivity. In a depth of 3.5 nm below the gold contact, the gold doping level of the spiro-OMeTAD film is 6.3% irrespective of the final gold contact thickness. Thus, the interface between the spiro-OMeTAD film and the Au contact is not sharp as commonly sketched and the contact is grainy, which will be both of importance for the performance of devices such as perovskite solar cells.