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In this work we present an in-depth study of the how the composition of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend films influences their phase transitions using variable-temperature spectroscopic ellipsometry. We demonstrate that this non-destructive method is a very sensitive optical technique to investigate the phase transitions and to determine the glass transition temperatures and melting crystallization points of the P3HT:PCBM blend films. By analyzing the influence of the temperature T on the raw ellipsometric data, we have identified a high sensitivity of the ellipsometric angle Δ at a wavelength of 280 nm to temperature changes. Characteristic temperatures determined from the slope changes of the Δ(T) plot appeared to be very good guess values for the phase transition temperatures.

Optical and structural properties of a blend thin film of (1:1 wt.) of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) doped with iodine (I2) and then exposed to a stepwise heating were reported and compared with the properties of doped P3HT films. The UV-Vis(T) absorption measurements were performed in situ during annealing runs, at the precisely defined temperatures, in a range of 20–210 °C. It was demonstrated that this new method allows one to observe the changes of absorption spectra, connected with the iodine release and other structural processes upon annealing. In addition, the thermally-induced changes of the exciton bandwidth (W) and the absorption edge parameters, i.e., the energy gap (EG) and the Urbach energy (EU) were discussed in the context of different length of conjugation and the structural disorder in polymers and blends films. During annealing, several stages were distinguished and related to the following processes as: the iodine escape and an increase in P3HT crystallinity, the orderly stacking of polymer chains, the thermally inducted structural defects and the phase separation caused by an aggregation of PCBM in the polymer matrix. Moreover, the detailed X-ray diffraction studies, performed for P3HT and P3HT:PCBM films, before and after doping and then after their thermal treatment, allowed us to consider the structural changes of polymer and blend films. The effect of iodine content and the annealing process on the bulk heterojunction (BHJ) solar cells parameters was checked, by the impedance spectroscopy (IS) measurements and the J-V characteristics registration. All of the investigated P3HT:PCBM blend films showed the photovoltaic effect; the increase in power conversion efficiency (PCE) upon iodine doping was demonstrated.

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Optical properties of polymer/fullerene blend films upon thermal annealing are investigated by spectroscopic ellipsometry and described consistently within an optical model of the blend film. The optical model developed in this work treats both components, polymer and fullerene, as mixtures of reference materials, that is, their optically ordered and disordered phase. Then, the polymer/fullerene blend layer is also described as a mixture of these two components. In this manner, we extend an existing optical model, which accounts for the optical order within the polymer phases, on cases where the optically ordered PCBM phase occurs, too. Determination of the dielectric functions of all four reference materials allows for a unique quantitative characterization of a polymer/fullerene blend film by assigning to it thickness and the polymer to fullerene volume fraction of its layers as well as corresponding volume fractions of theirs optically ordered and disordered polymer and fullerene phases. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1094–1100