Quantifying Charge Extraction in Organic Solar Cells: The Case of Fluorinated PCPDTBT.
We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.
†Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany.
Steve
S
Albrecht
‡Department of Physics, North Carolina State University, 421 Riddick Hall, Raleigh, North Carolina 27695-8202, United States.
John R
JR
Tumbleston
§Fraunhofer Institut für Angewandte Polymerforschung, Geiselbergstr. 69, 14476 Potsdam, Germany.
Silvia
S
Janietz
∥Bergische Universität Wuppertal, Institut für Polymertechnologie, Gauss-Str. 20, 42097 Wuppertal, Germany.
Ines
I
Dumsch
∥Bergische Universität Wuppertal, Institut für Polymertechnologie, Gauss-Str. 20, 42097 Wuppertal, Germany.
Sybille
S
Allard
∥Bergische Universität Wuppertal, Institut für Polymertechnologie, Gauss-Str. 20, 42097 Wuppertal, Germany.
Ullrich
U
Scherf
‡Department of Physics, North Carolina State University, 421 Riddick Hall, Raleigh, North Carolina 27695-8202, United States.
Harald
H
Ade
†Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany.
Dieter
D
Neher
PCPDTBT; bimolecular recombination; charge extraction; field-dependent charge generation; fluorination; low-band-gap polymers; mobility