title: Moving through the phase diagram: morphology formation in solution cast polymer-fullerene blend films for organic solar cells.
abstract: The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.
authors: Thermal Process Engineering, Thin Film Technology, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, D-76131 Karlsruhe, Germany. schmidt-hansberg@kit.edu
authors: Benjamin
authors: B
authors: Schmidt-Hansberg
authors: Monamie
authors: M
authors: Sanyal
authors: Michael F G
authors: MF
authors: Klein
authors: Marina
authors: M
authors: Pfaff
authors: Natalie
authors: N
authors: Schnabel
authors: Stefan
authors: S
authors: Jaiser
authors: Alexei
authors: A
authors: Vorobiev
authors: Erich
authors: E
authors: Müller
authors: Alexander
authors: A
authors: Colsmann
authors: Philip
authors: P
authors: Scharfer
authors: Dagmar
authors: D
authors: Gerthsen
authors: Uli
authors: U
authors: Lemmer
authors: Esther
authors: E
authors: Barrena
authors: Wilhelm
authors: W
authors: Schabel