Surface vs bulk phase transitions in semiconducting polymer films for OPV and OLED applications

Post deposition annealing treatments are considered one of the most important and effective ways to increase the performance of polymer-based thin films for organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). thermally induced morphological changes such as phase transitions are key phenomena which can have a determinant influence on the final properties and stability of the materials and devices based upon them. In this work, we have successfully proven that in-situ atomic force microscopy (AFM) and Raman spectroscopy can be used to measure the cold crystallization transition temperature of the widely studied blue-emitting polymer poly(9,9-dioctylfluorene) (PFO) and the workhorse system for photovoltaics based on mixtures of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Raman, as a bulk probe, evidences lower crystallization temperatures for PFO and P3HT films compared to those obtained at the surface with AFM which suggest the existence of morphological and/or molecular mobility differences between the bulk and the surface.