title: Organic solar cells with graded absorber layers processed from nanoparticle dispersions.
abstract: The fabrication of organic solar cells with advanced multi-layer architectures from solution is often limited by the choice of solvents since most organic semiconductors dissolve in the same aromatic agents. In this work, we investigate multi-pass deposition of organic semiconductors from eco-friendly ethanol dispersion. Once applied, the nanoparticles are insoluble in the deposition agent, allowing for the application of further nanoparticulate layers and hence for building poly(3-hexylthiophene-2,5-diyl):indene-C60 bisadduct absorber layers with vertically graded polymer and conversely graded fullerene concentration. Upon thermal annealing, we observe some degrees of polymer/fullerene interdiffusion by means of X-ray photoelectron spectroscopy and Kelvin probe force microscopy. Replacing the common bulk-heterojunction by such a graded photo-active layer yields an enhanced fill factor of the solar cell due to an improved charge carrier extraction, and consequently an overall power conversion efficiency beyond 4%. Wet processing of such advanced device architectures paves the way for a versatile, eco-friendly and industrially feasible future fabrication of organic solar cells with advanced multi-layer architectures.
authors.affiliation: Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstrasse 13, 76131 Karlsruhe, Germany. alexander.colsmann@kit.edu.
authors.forename: Stefan
authors.initials: S
authors.lastname: Gärtner
authors.affiliation: Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstrasse 13, 76131 Karlsruhe, Germany. alexander.colsmann@kit.edu.
authors.forename: Stefan
authors.initials: S
authors.lastname: Reich
authors.affiliation: Institute of Applied Materials and Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Bruns
authors.affiliation: Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstrasse 13, 76131 Karlsruhe, Germany. alexander.colsmann@kit.edu.
authors.forename: Jens
authors.initials: J
authors.lastname: Czolk
authors.affiliation: Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstrasse 13, 76131 Karlsruhe, Germany. alexander.colsmann@kit.edu.
authors.forename: Alexander
authors.initials: A
authors.lastname: Colsmann