Impact of the solubility of organic semiconductors for solution-processable electronics on the structure formation: a real-time study of morphology and electrical properties

Abstract

The control of structure formation in the active layers of organic solar cells allows for improvement in their processability and enhancement of the efficiency of the final devices. In the present work, in situ studies of film formation from binary toluene solutions of an electron donor, poly(3-hexylthiophene) (P3HT), and an electron acceptor such as [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) or indene-C₆₀ bisadduct (ICBA) have been conducted. These experiments were carried out using GIWAXS with simultaneous electric current measurements. The comparative analysis of the intensity of the amorphous halo, and the 100 and 020 peaks of P3HT reveals the development of the semicrystalline morphology of the donor through a partly-ordered phase. The experiments show the impact of the chemical structure of the acceptor, as well as that of the donor : acceptor ratio on the kinetics of drying and crystallization. The optimal bulk heterojunction morphology was achieved for P3HT : ICBA 1 : 1, which exhibited the highest value of current. A more efficient phase separation in non-annealed P3HT:ICBA films as compared to P3HT:PCBM was accounted for by the differences in solubility of the components in toluene. The structure formation during solvent evaporation can be subdivided into three stages, including the ordering of the polymer in solution, phase separation during precipitation, and the perfectioning of P3HT crystals in the dry film.