Competition between recombination and extraction of free charges determines the fill-factor of organic solar cells

Irene Pérez^a, Jan Anton Koster^a, Davide Bartesaghi^a^,^b, Dieter Neher^c, Steffen Roland^c, Juliane Kniepert^c, Mathieu Turbiez^d

^aUniversity of Groningen, The Netherlands, Nijenborgh, 4, Groningen, Netherlands

^bDutch Polymer Institute, P.O. Box 902, Eindhoven, 5600 AX, Netherlands

^cUniversity of Potsdam, DE, Karl-Liebknecht-Straße, 24-25, Potsdam, Germany

^dBASF Schweiz AG, Schwarzwaldallee 215, Basel, CH-4002

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Davide Bartesaghi, presentation 028
Publication date: 5th February 2015

The fill-factor (FF) of a solar cell is the result of a complex interplay between charge generation, transport and recombination, and several factors, such as thickness or light intensity, contribute to determine the result of this interplay. A complete and systematic understanding of the fundamental mechanism governing the FF, which may translate into significant improvements of this parameter, is still lacking.Here, we show that the FF is governed by the competition between charge extraction and recombination, according to a relationship that holds for a wide variety of organic solar cells across the whole range of 0.26-0.74. We quantify this competition with a dimensionless parameter theta, proportional to the ratio between recombination and extraction rates. We empirically and theoretically demonstrate that there is a clear relationship between theta and the FF by experimental measurements as well as by means of drift-diffusion simulations. We simulate a large number of devices, varying charge carrier mobilities, recombination rate, light intensity, energy levels, and active layer thickness over a wide range to reproduce typical experimental conditions. Experimentally, we measure charge carrier mobilities and bimolecular recombination rates using a combination of steady-state and transient extraction techniques. The parameter theta is varied by producing devices based on 15 different donor:acceptor blends, by varying the processing and/or measuring conditions. Polymer:fullerene, polymer:polymer and small molecules blends are considered. For both simulated and experimental devices, when FF is plotted against theta the data collapse onto one universal curve, showing that all the fill-factors follow from the competition between recombination and extraction. Our results unify the FF of several very different donor:acceptor combinations, and explain why FFs change significantly with thickness, light intensity and material properties. In particular, we provide a guideline for the targeted improvement of the FF of organic solar cells with given thickness, generation rate of free charges and internal voltage. The possibility for targeted improvement of FF is a promising achievement towards the enhancement of the power conversion efficiency of organic solar cells.

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