title: Competition between recombination and extraction of free charges determines the fill factor of organic solar cells.
abstract: Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter θ, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor:acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor:acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.
authors.affiliation: 1] Department of Photophysics and Optoelectronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, NL-9747AG Groningen, The Netherlands [2] Dutch Polymer Institute, P. O. Box 902, 5600AX Eindhoven, The Netherlands.
authors.forename: Davide
authors.initials: D
authors.lastname: Bartesaghi
authors.affiliation: Department of Photophysics and Optoelectronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, NL-9747AG Groningen, The Netherlands.
authors.forename: Irene Del Carmen
authors.initials: Idel C
authors.lastname: Pérez
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: Juliane
authors.initials: J
authors.lastname: Kniepert
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: Steffen
authors.initials: S
authors.lastname: Roland
authors.affiliation: BASF Schweiz AG, Schwarzwaldallee 215, CH-4002 Basel, Switzerland.
authors.forename: Mathieu
authors.initials: M
authors.lastname: Turbiez
authors.affiliation: Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany.
authors.forename: Dieter
authors.initials: D
authors.lastname: Neher
authors.affiliation: Department of Photophysics and Optoelectronics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, NL-9747AG Groningen, The Netherlands.
authors.forename: L Jan Anton
authors.initials: LJ
authors.lastname: Koster