Field effect measurements on charge carrier mobilities in various polymer-fullerene blend compositions Elizabeth von Hauff a, * , Ju ¨ rgen Parisi a , Vladimir Dyakonov b a Institute of Physics, Energy and Semiconductor Research Laboratory, Carl von Ossietzky University of Oldenburg, 26111 Oldenburg, Germany b Experimental Physics VI, Faculty of Physics and Astronomy, University of Wu ¨rzburg, 97074 Wu ¨rzburg, Germany Available online 18 January 2006 Abstract In this study we investigated materials typically used in polymer photovoltaics. Field effect measurements were performed in order to determine the hole mobilities in the conjugated polymer poly(3-hexylthiophene) (P3HT) and the electron mobilities in the methanofullerene[6,6]- phenyl C 61 -butyric acid methyl ester (PCBM), and, particularly, in the polymer-fullerene composite blends. Regarding the pure films, electron mobilities in PCBM were found to be in the 10  2 cm 2 /Vs range, and hole mobilities in P3HT were found to be in the 10  3 cm2/Vs range. In the PCBM:P3HT blends, it was found that varying the PCBM content in PCBM:P3HT blends led to a steep increase in electron mobility with increasing PCBM content, while the hole mobility was found to slightly decrease with the increasing PCBM concentration. In 2:1 PCBM:P3HT tempered blends, the charge carrier mobilities were found to be roughly balanced, at 10  3 cm 2 /Vs. For improved electron transport in the blends, tempering was found to be crucial. D 2005 Elsevier B.V. All rights reserved. Keywords: Polymer solar cells; OFETs; Charge carrier mobility 1. Introduction Organic photovoltaics offer a potentially cost effective technology for large scale or flexible photovoltaic applications, for which conventional photovoltaic materials are too expen- sive or too brittle. Power conversion efficiencies have been steadily increasing over the last few years [1–4]. Electrical losses between the metal contacts and the semiconductor [5,6], or within the bulk of the device [7–9] negatively affect device performance, and therefore also device efficiency. Investiga- tions of the electrical and optical properties of organic semiconducting materials are necessary to isolate potential losses, and to generally improve device output. Organic field effect transistors (OFETs) offer the possibility of investigating the transport properties of charge carriers in organic semi- conductors in both pure semiconducting films as well as in blends of semiconducting materials. It should be mentioned that, generally, studies of the charge transport in an OFET may not be fully representative of the charge transport in a polymer solar cell, as the device structure and operation are quite different. In this study, however, we are interested in monitoring changes in the semiconducting film quality that arise due to changes in blend composition and tempering. These changes directly affect charge transport through the blend. For such a study, field effect measurements can be quite useful, as the field effect mobility is a parameter that can be directly determined and compared between the various blend compositions. It is noted, however, that the field effect mobility measured from an OFET yields a higher value for charge carrier mobility than values determined via other methods, such as space charge limited current (SCLC) measurements, as OFETs operate with considerably higher charge carrier densities than diodes, and the charge carrier mobility in organic semiconductors is known to be dependent on the charge carrier density [10]. In this study, we investigated the field effect mobilities of charge carriers in materials typically used in polymer photo- voltaics. The charge carrier mobilities in the electron acceptor C 61 -butyric acid methyl ester (PCBM) and in the electron donor polymer poly(3-hexylthiophene) (P3HT) were deter- mined. The chemical structures of PCBM and P3HT are shown in Fig. 1. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.025 * Corresponding author. E-mail address: elizabeth.von.hauff@mail.uni-oldenburg.de (E. von Hauff). Thin Solid Films 511 – 512 (2006) 506 – 511 www.elsevier.com/locate/tsf