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Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends

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Author: Veldman, D. · Ipek, Ö. · Meskers, S.C.J. · Sweelssen, J. · Koetse, M.M. · Veenstra, S.C. · Kroon, J.M. · Bavel, S.S. van · Loos, J. · Janssen, R.A.J.
Type:article
Date:2008
Institution: TNO Industrie en Techniek
Source:Journal of the American Chemical Society, 24, 130, 7721-7735
Identifier: 280041
Keywords: Materials · Absorption · Absorption spectroscopy · Astrophysics · Charge transfer · Charged particles · Dissociation · Electric field effects · Electric field measurement · Electric fields · Electric instrument transformers · Electroacupuncture · Electromagnetic field theory · Electromagnetic fields · Electromagnetism · Electron microscopes · Electron microscopy · Electron optics · Electrons · Emission spectroscopy · Fullerenes · Imaging techniques · Ion exchange · Light emission · Luminescence · Magnetism · Mass transfer · Microscopic examination · Optical properties · Polymers · Semiconducting cadmium telluride · Thick films · Thin films · Transmission electron microscopy · (ethylene vinyl alcohol) copolymers · Alternating polyfluorene (APFO) · Applied (CO) · Charge transfer excitons · Charge-transfer (CT) transitions · Concentration (composition) · Donor-acceptor interfaces · Electric field dependences · Electro optical properties · Electron donor acceptor (EDA) · emission shifts · External electric fields · Fluorene (FI) · fullerene derivatives · Lower energies · Nano crystalline · Open-circuit voltage (OCV) · photovoltaic devices · red shifting · Electric charge · cation · copolymer · fluorene derivative · fullerene derivative · nanocrystal · organic compound · article · chemical composition · chemical model · chemical structure · concentration (parameters) · device · dissociation · electric field · electric potential · electricity · electron · energy · film · intensity modulated radiation therapy · quantitative analysis · transmission electron microscopy · Industrial Innovation

Abstract

The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor-acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (V<sub>OC</sub>) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (ε<sub>r</sub> = 4.0) compared to that of the polymer (ε<sub>r</sub> = 3.4), stabilizing the energy (ECT) of CT states and of the free charge carriers in blends with higher PCBM concentration. We show that the CT state has a short decay time (τ = ca. 4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field-induced quenching can be explained quantitatively with the Onsager-Braun model for the dissociation of the CT states when including a high electron mobility in nanocrystalline PCBM clusters. Furthermore, photoinduced absorption spectroscopy shows that increasing the PCBM concentration reduces the yield of neutral triplet excitons forming via electron-hole recombination, and increases the lifetime of radical cations. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers. © 2008 American Chemical Society.