Characterization of Highly Inhomogeneous Bulk Heterojunctions by Optical Methods: Morphology, Carrier Mobility, and Charge Separation Pathways

Ramunas Augulis^a, Domantas Peckus^a, Vidmantas Gulbinas^a, Andrius Devizis^a, Dirk Hertel^b, Klaus Meerholz^b

^aCenter for Physical Sciences and Technology, LT, Savanorių prospektas, 231, Vilnius, Lithuania

^bUniversity of Cologne, Luxemburger Straße, 116, Köln, Germany

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

Poster, Ramunas Augulis, 300
Publication date: 5th February 2015

One of the currently dominant organic solar cell concepts – bulk heterojunction enables relatively simple fabrication of cells with efficiencies of up to about 10%. Despite the progress made in this field, the mechanisms of carrier generation in these cells and factors determining their efficiency are still debated. It has been found that film morphology parameters, such as phase segregation, formation of domain structure, and domain sizes have a huge influence on the performance of the cells. Unfortunately, none of the direct imaging methods can provide thorough and reliable information about the microscopic structure of bulk-heterojunctions in solar cells. In this work we aim to extract structural information about bulk-heterojunctions by combining a range of readily available or relatively simply implementable optical methods. As model systems we chose solar cells based on vacuum deposited merocyanine:fullerene and solution processed merocyanine:PCBM blends with highest efficiencies of about 5%. Charge carrier generation and drift dynamics has been investigated in by combining electric field-induced fluorescence quenching, electroluminescence, and ultrafast time-resolved carrier drift measurements. On the basis of the obtained results we draw conclusions about fullerene percolation and its domain sizes, map the pathways of charge separation/recombination and estimate their efficiencies. We demonstrate that interfacial charge transfer (CT) states are strongly heterogeneous with energies dependent on the acceptor material and its domain sizes. We distinguish two interfacial CT state dissociation pathways: ultrafast, weakly dependent on the electric field, and slow field-assisted dissociation during entire CT state lifetime. We attribute the first process to low energy, weakly bound CT states formed on large acceptor domains, and the second one to strongly bound CT states formed on small domains or single acceptor molecules. Electron mobility in the blends with 50% fullerene is several times higher than in the films with PCBM and orders of magnitude higher than the hole mobility. We conclude that efficient carrier generation at low electric fields typical for operating solar cells relies on unperturbed motion of highly mobile electrons, thus fast motion and extraction of electrons is crucial for efficient solar cells. The combination of techniques and the findings of this study could be generalized for other types of blend heterojunction or even other types of solar cells.

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