Dynamics of Charge Transfer Excitons Recombination in Polymer/Fullerene Solar Cells
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Dynamics of Charge Transfer Excitons Recombination in Polymer/Fullerene Solar Cells Markus Hallermann1, Felix Deschler1, Josef Berger1, Elizabeth von Hauff2 and Enrico Da Como1 1
Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-MaximiliansUniversity Munich, Munich 80799, Germany 2
Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky University Oldenburg, Oldenburg 26111, Germany ABSTRACT Among the different recombination mechanisms in organic solar cells the photoluminescence (PL) of charge transfer excitons (CTEs) has been identified has one of the most important, impacting both the open circuit voltage and the short circuit current. Here, we study their recombination dynamics, monitoring the decay of the PL on a time scale spanning three orders of magnitude from nanoseconds to microseconds. As a model system we investigate blends of the conjugated polymer poly(2-methoxy-5-(3´,7´-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We observe that the dynamics of recombination follows a power-law, which is independent of sample morphology. Upon application of a transient electric field, which is capable of separating the bound charge pairs, we observe different dynamics of recombination only for the separated pairs. Those also follow a power-law and show a strong dependence on the film morphology. INTRODUCTION Organic solar cells are a promising technology for thin-film based photovoltaics, because of the low costs of production and the potential for high efficiencies. Both solution processing and thermal evaporation are considered relevant with some of the best efficiencies scored by solution processed polymer/fullerene blends [1]. This type of cells relies on the photoinduced charge transfer occurring between the light harvesting conjugated polymer and the strong acceptor fullerene [2]. A critical parameter for boosting efficiencies is the control over morphology [3], which over the years have been extensively explored by using thermal annealing [4], solvent swelling [5], additives or templated nanostructures [6]. Optimization of morphology is a prerequisite for enabling efficient charge separation and percolation towards the electrodes. At the same time morphology is expected to have a role in recombination, though experiments probing charge carrier recombination on blends differing in morphology have reported very similar recombination time scales [7]. While these studies were performed using transient absorption spectroscopy, the recent observation of luminescence from CTEs or exciplexes at the polymer/fullerene interface allows for using a new probe to investigate recombination and its dynamics [8-10]. We expect that this unique fingerprint of interfacial recombination could shine light on the apparent enigma of better solar cell efficiencies as a consequence of controlled morphology, but with little apparent changes in the recombination dynamics.
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