Electronic Properties of Polymer-Fullerene Solar Cells StudiedWith Light-Induced Electron Spin Resonance and Admittance
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Electronic Properties of Polymer-Fullerene Solar Cells Studied With Light-Induced Electron Spin Resonance and Admittance Spectroscopy V. Dyakonov, I. Riedel, Z. Chiguvare, C. Deibel, J. Parisi, C.J. Brabec1, N.S. Sariciftci1 and J.C. Hummelen2 Dept. of Energy and Semiconductor Research, University of Oldenburg, D-26111 Oldenburg, Germany 1 Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes-KeplerUniversity of Linz, A-4040 Linz, Austria 2 Stratingh Institute and Materials Research Centre (MSC), University of Groningen, NL-9747 AG Groningen, The Netherlands ABSTRACT Within recent years, the development of polymer-fullerene plastic solar cells has made significant progress. In such devices, an efficient charge generation takes place via photoinduced charge transfer between the photoexcited conjugated polymer and acceptor-type fullerene molecules. Due to the paramagnetic nature of the radical species, the photoinduced charge transfer can be studied by means of light induced electron spin resonance (LESR) techniques. We carried out W-band (95 GHz) LESR at high magnetic field strengths. Two well separated line groups with a strong anisotropic structure were detected for the composite MDMOPPV:PCBM. From the line shape analysis, we obtained an environmental axial symmetry for the positive polaron P+ and a lower, rhomboedric symmetry for the fullerene anion. The signals were found to be independent of each other with different spin-lattice relaxation times; hence, the radical species can be investigated separately. In order to study the bulk transport properties, we carried out admittance spectroscopy on the ITO/PEDOT:PSS/MDMO-PPV:PCBM/Al device. Two frequency-dependent contributions to the device capacitance with the activation energies 9 meV and 177 meV were found. For the very shallow trap state, we assume a bulk impurity, whereas the latter one is assigned to an interfacial defect state, located at the composite-aluminium interface.
INTRODUCTION Conjugated polymers are well known as intrinsic, quasi-one-dimensional semiconductors with band gap energies covering the whole visible range of photon wavelengths [1]. Their application in opto-electronic devices like organic light emitting diodes (OLEDs) has already reached the state of an industrial low-cost fabrication. However, the performance of photovoltaic devices, with a light absorber consisting only of a single-type conjugated polymer, is comparably poor, due to the low photo-generation yield of charge carriers, strong radiative recombination and low mobility of carriers [2]. In 1992 Sariciftci et al. demonstrated that doping of several conjugated polymers with fullerenes, C60, results in a strong quenching of photoluminescence [3]. Furthermore, it has been observed that the photocurrent of photovoltaic devices which have an absorber of a conjugated polymer increases by nearly two orders of magnitude, when fullerenes are embedded in the photoactive film . The underlying photophysics may be described by a light-induced, ultrafast forma
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