Using Bulk Heterojunction Field Effect Measurements to Understand Charge Transport in Solar Cell Materials
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Using bulk heterojunction field effect measurements to understand charge transport in solar cell materials Christopher J. Lombardo and Ananth Dodabalapur Microelectronics Research Center, The University of Texas at Austin, 10100 Burnet Rd., Bldg 160, Austin, TX 78758, U.S.A. ABSTRACT Ambipolar organic thin-film transistors (OTFTs) have been used to study the transport of charge carriers in bulk heterojunction (BHJ) organic photovoltaic devices. Active layers of phase separated blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), have been chosen due to their use in performance BHJ organic photovoltaic devices as well as ease of device fabrication. A method for determining recombination rate after exciton dissociation and measurement of excess carrier lifetime has been reported by studying drain current behavior which yields carrier mobility, conductivity, and carrier concentration both in dark and AM1.5g illumination. Channel-length dependent measurements of the photocurrent show that significant recombination of separated charge carriers begins to occur at lengths greater than 10 µm. A recombination rate of 2.6 × 1019 cm-3 s1 and a carrier lifetime of ≥ 8.8 ms has been calculated. INTRODUCTION Organic photovoltaic (OPV) cells have been actively studied for over 25 years and during that time, power conversion efficiency has increased from about 1% to about 8% [1, 2], yet much more needs to be known about the movement of charge carriers within OPV cells and the nature of recombination. Some researchers have developed transient methods like CELIV [3] and photo CELIV [4], but these methods are typically time consuming and require specialized equipment, making them not suitable for OPV manufacturers. We employ ambipolar OTFTs to study charge transport in BHJ PV cells as many researchers are familiar with fabricating and measuring FET parameters. Ambipolar OTFTs have been studied by multiple researchers in dark conditions [5-9] but to learn about charge transport during solar cell operation, these devices must also be studied under illumination. Although studies of these devices have been reported [10], recombination rates based on these devices have not been reported. We note that the morphology in a FET configuration is different from that in a solar cell configuration, and that this could place limits on the interpretation of these results with reference to charge transport in solar cells. Nevertheless, a wealth of information is available from FET-based measurements. To determine recombination rates and characterize other parameters of interest in this system, three basic measurements must be made: the output characteristic and the transfer characteristic (for both pFET and nFET modes), and a two-terminal measurement (an ungated source-drain I-V sweep). These parameters must be measured both in the dark and under illumination, preferably AM1.5g, to determine the necessary parameters. From the transfer characteristics, the dark electron and hole mobilities can be
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