Phase separation in vacuum codeposited pentacene/6,13-pentacenequinone thin films

Pentacene (P) and 6,13-pentacenequinone (PQ) have been vacuum codeposited onto SiO2 in order to control phase separation in thin films for the application as bulk heterojunctions in organic photovoltaic devices. Structural investigations by means of scanning electron microscopy (SEM) and atomic force microscopy revealed pronounced phase separation of the two materials at length scales that turned out to be tunable by the variation of the deposition rate. X-ray diffraction provided evidence for polymorphism in pure films of P and PQ on SiO2. While pure films exhibited both the bulk and thin-film phase, the bulk phase is mainly suppressed within the co-deposited films (P+PQ). This was corroborated by Fourier-transform infrared spectroscopy results. SEM investigations of pure and codeposited films indicated that PQ bulk crystallites of up to 200 nm height form continuous paths to the substrate and grow within a matrix formed of P and PQ thin-film phases. The obtained heterojunction morphologies thus appear interesting for the application in organic-based photovoltaic cells.