The effects of processing parameters on the morphology of organic solar cells

Abstract

Organic solar cells (OSCs) are increasingly commercially relevant due to their ability to harvest renewable solar energy more economically than traditional inorganic solar cells; however, current OSCs fall short in terms of their efficiency and longevity. Studies have suggested that the performance of OSCs is strongly influenced by the molecular assembly within the active layer, and as a result, can be improved by manipulating the morphology through processing. In this study, we first investigated the effects of thermal annealing and compositions on the morphology of P3HT:PCBM films prepared from a good solvent. This is followed by an investigation of P3HT fibres which were fabricated by dissolving and precipitating different concentrations of polymer in a poor solvent. Optical microscopy, atomic force microscopy, x-ray diffraction and Fourier-transform infrared spectroscopy were used to analyse the films, while UV-Vis was used to analyse the solutions. We found that as-cast films, despite displaying minimal phase separation, exhibit some degree of crystallinity in the form of short P3HT ribbons. Thermally annealing the films resulted in increased crystallinity and the nucleation of PCBM aggregates at high PCBM loadings, signifying the partial miscibility of PCBM in P3HT. Although insensitive to PCBM loading in as-cast films, the crystallinity of P3HT in the annealed films was enhanced by the addition of PCBM up to a mass ratio of 1:1, at which the crystallinity significantly drops. It suggests the duality of PCBM behaviour in a P3HT:PCBM film: a nucleating agent at low loadings and a plasticiser at high loadings. Furthermore, the diffusion of PCBM to form aggregates was found to be temperature-dependent, with PCBM forming macroscopic ‘fans’ at high annealing temperature. Subsequently, the onset of nucleation and the rate of growth of P3HT fibres in solution were observed to be strongly influenced by the concentration of the polymer, hence allowing better control of P3HT crystal morphology. For the first time, we also observed the self-assembly of these fibres into fractal snowflake crystal aggregates upon annealing. Future work will examine how PCBM and doping affect the P3HT fibres in solution and in film.