Author:

Abstract:

Molecular-based solar cells have the potential to provide low-cost energy production on lightweight and flexible substrates. Organic solar cells are either produced by solution processing of polymers or evaporation of small molecules. In the first case, spin coating is considered the most reliable and reproducible method, but is limited to small areas. To realize large-area coverage, various deposition techniques, such as ink-jet and screen or gravure printing, have been proposed and demonstrated. Here, we present a simple method for the deposition of large area devices based upon spray coating, and show that this method is a valid alternative to other techniques. Spray coating is a technique that is well established in graphic arts, industrial coatings, and painting. This high-rate, large-area deposition technique ensures an ideal coating on a variety of surfaces with different morphologies and topographies, and is often used in inline productions. Moreover, the fluid waste is reduced to minimal quantities. To justify the usefulness of this technique, we compared a standard spin coated solar cell based on a mixture of poly(3-hexyl thiophene) (P3HT) and the C60-derivative (6,6)-phenyl C61-butyric acid methyl ester (PCBM) with a spray coated one, where the P3HT:PCBM blend was sprayed by a N2-powered airbrush. Spray coated solar cells were found to have power conversion efficiencies above 2%, a performance which is comparable to that of the spin coated devices. We also performed atomic force microscopy, ellipsometry and absorption measurements to compare the film quality of the two techniques. Significantly, we found that the spray coating technique allows for the demonstration of polymer solar cells with distinct layers, owing to the differences in the kinetics of the solvent evaporation process when compared to other solution based techniques. The small femto-liter scale droplet size intrinsic to the spray-coating technique means that thin films can dry very rapidly, and subsequent films can be deposited despite a common solvent. In the case of techniques that use at least nano-liter scale solutions, (i.e. spin coating and inkjet printing) this process would have the undesired effect of dissolving the underlying layers. This finding also has important consequences for polymer-based light emitting devices, particularly for white light emission, where having multilayered structures is advantageous for high-efficiency devices. We investigated different methods of spray coating, from single-pass to multiple-passes, and varied the concentration of the solutions, in order to optimize the technique, and looked into the effects of thermal annealing to the layers. Then we analyzed the relation between the performance of the devices and the characteristics of the films obtained from the two different depositions techniques to show that spray coating is an excellent alternative to spin coating for the fabrication of large area polymer-based devices.