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

Highlights: Monitoring of crystallization transition of polymer films. Differences between bulk and surface transition temperatures are evidenced. In-situ AFM evidences transition as a roughness increase. Raman peaks suffer a blue-shift and intensity increase upon crystallization. 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). Hence, 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 C ₆₁-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.