title: Nanomorphology of P3HT:PCBM-based absorber layers of organic solar cells after different processing conditions analyzed by low-energy scanning transmission electron microscopy.
abstract: In this study the nanomorphology of P3HT:PC61BM absorber layers of organic solar cells was studied as a function of the processing parameters and for P3HT with different molecular weight. For this purpose we apply scanning transmission electron microscopy (STEM) at low electron energies in a scanning electron microscope. This method exhibits sensitive material contrast in the high-angle annular dark-field (HAADF) mode, which is well suited to distinguish materials with similar densities and mean atomic numbers. The images taken with low-energy HAADF STEM are compared with conventional transmission electron microscopy and atomic force microscopy images to illustrate the capabilities of the different techniques. For the interpretation of the low-energy HAADF STEM images, a semiempirical equation is used to calculate the image intensities. The experiments show that the nanomorphology of the P3HT:PC61BM blends depends strongly on the molecular weight of the P3HT. Low-molecular-weight P3HT forms rod-like domains during annealing. In contrast, only small globular features are visible in samples containing high-molecular-weight P3HT, which do not change significantly after annealing at 150°C up to 30 min.
authors.affiliation: Laboratory for Electron Microscopy (LEM), Karlsruhe Institute of Technology (KIT), Engesserstrasse 7, 76131 Karlsruhe, Germany. marina.pfaff@kit.edu
authors.forename: Marina
authors.initials: M
authors.lastname: Pfaff
authors.forename: Michael F G
authors.initials: MF
authors.lastname: Klein
authors.forename: Erich
authors.initials: E
authors.lastname: Müller
authors.forename: Philipp
authors.initials: P
authors.lastname: Müller
authors.forename: Alexander
authors.initials: A
authors.lastname: Colsmann
authors.forename: Uli
authors.initials: U
authors.lastname: Lemmer
authors.forename: Dagmar
authors.initials: D
authors.lastname: Gerthsen