Conductivity of solution-processed films of a metal-free chlorophyll derivative: drop casting vs. spin coating

Thin films of a chlorophyll-like compound methyl-pheophorbide a (Pheo) were prepared from a solution by both spin-coating and drop-casting methods. Despite the fundamental similarity of these two techniques, different self-assembly conditions of Pheo molecules determined the difference in the films morphology. Smoother films without detectable crystallinity were obtained by the spin-coating method, while the drop-cast films featured ordering of molecules and formation of clusters with a size of ~ 1 µm and more. As a result, the films have different photoelectrical properties and, hence, the conditions governing the aggregate growth should be controlled precisely. The Pheo films in any form are good photoconductors: photons from the red and near-infrared domain are the main contributors in the photoinduced current.