Organic solar cells and organic LEDs are typically made of conductive and semi-conductive thin films. The uniformity requirement for these films is exceptionally high. In the case of multi-layer structures, surface characterization based methods (e.g. profilometer, atomic force microscope, scanning electron microscope) encounter certain challenges when attempting to detect the defects inside the structure. One way to overcome this drawback is by using synchronized thermography (ST). In this work ST is used to study multi-layered thin film structures. Indium Tin Oxide (ITO) was used as an example of conductive thin film and poly(3,4-ethylenedioxy-thiopene):poly(styrene-sulfonate) (PEDOT:PSS) was used as an example of a hole transporting layer. Uniformity differences were generated in these layers and ST was used to detect them. The results show that ST is capable of localizing small defects in the stack using a single infrared (IR) image. It can often be deduced from the same image in which layer the defect is located. This shows that ST is capable of profiling the structures of multi-layer thin films.
Uniformity of conductive materials is an important property which is measured during manufacturing and in finished
products, especially in electronics applications such as organic solar cells. Differences in uniformity are often very small,
invisible or below the surface of the sample. Therefore, they are not always detectable even by high-resolution imaging
systems. Respectively, electrical conductivity measurements are limited to those mainly between the measuring probes.
Uniformity difference measurements are time-consuming in the case of a large area characterization. To bypass the
described limitations, a simple heating and IR-imaging based system was designed and demonstrated with conductive
materials. Samples with different defects were used to investigate the correlation of conductance and defect positioning.
By making punched holes in the samples, it was possible to demonstrate how the local resistances of thin films have
functions to each other and how this may be observed on an IR-figure. Thermographs of punched thin films confirm that
those areas where the holes prevented the current flow have lower heat emissions. Therefore, it can also be concluded
that, generally, the temperature is highest at the areas where current density is highest. When comparing the defects of
bent samples to these punctured ones, the correlations of resistance and breakage areas were comparable. The applied
system is capable of localizing small defects in large-area samples using a single IR-image. This is a significant
advantage from the manufacturing process measurement point of view.