Influence of active layer processing on electrical properties and efficiency of polymer–fullerene organic solar cells

Abstract

New method of polymer-fullerene composite crystallization was examined as a possible way for efficiency enhancement of polymer organic solar cells. Since the structure of bulk-heterojunction is crucial for its efficiency, there is a strong need for new methods that can control a crystallization process. In this work we studied the crystallization process taking place under thermal annealing in ambient conditions, as well as annealing in a solvent atmosphere. Two polymer compounds, P3HT and PTB7, performing as donor materials were used. The acceptor material in the fabricated bulk heterojunction was [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The spin-coated layers were investigated by optical absorption and X-ray diffraction. We observed significant changes in the crystal structure of some of the annealed layers. The constructed solar cells were examined with use of current–voltage characterization method under AM1.5G sun irradiation, and by photocurrent spectroscopy. In the case of P3HT:PCBM cells significant increase of efficiency was observed, especially after annealing in solvent atmosphere.