The Impact of Selective Solvents on the Structure and Function Evolution in Solvent Annealed Organic Photovoltaics

The role of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) solubility on the evolution of structure and function in solvent annealed organic photovoltaics is quantitatively investigated. ODCB solvent vapor distribution during the solvent uptake process is simulated on a 3-D profile. Determination of solvent uptake in the P3HT/PCBM mixture shows that the evolution of the morphology during solvent annealing (SVA) takes much longer than the diffusion of the solvent vapor into the sample. The evolution of P3HT crystallinity, as well as the growth of PCBM aggregates, in the solvent annealed thin films is observed by Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) and Atomic Force Microscopy (AFM) respectively. For the solvents that have a finite P3HT solubility, P3HT crystallinity initially increases then decreases with longer SVA time, where P3HT solubility of the annealing solvent plays a significant role in the variation of crystallinity. However, 2-Chlorophonel (2-CP) vapor, which has negligible P3HT solubility but high PCBM solubility, also exhibits an increase in P3HT crystallinity upon solvent annealing a P3HT/PCBM thin film. PCBM aggregates grow continuously with SVA, a process that is modulated by the PCBM solubility in the annealing solvent. Moderate PCBM aggregation correlates to improved power conversion efficiency (PCE) of the solvent annealed thin film. For the samples annealed using solvents that prefer P3HT, PCE benefits from further SVA after P3HT crystallinity peak time, which is ascribed to the additional PCBM aggregation; whereas the PCBM preferred solvents induce excess PCBM phase separation at further SVA times, which limits exciton dissociation hence PCE performance. Also, a flow SVA chamber is designed to realize in-situ light scattering experiments, where preliminary experiments using this apparatus are reported.