Thermal Annealing Effect on P3HT:PCBM Free Polarons Lifetime and Charge Transport
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Thermal Annealing Effect on P3HT:PCBM Free Polarons Lifetime and Charge Transport Kejia Li1, Yang Shen1, Lijun Li1, Petr Khlyabich2, Ellen S. Reifler1, Barry C. Thompson2 and Joe C. Campbell1 1 Department of Electrical and Computer Engineering, University of Virginia, 351 McCormick Road, Charlottesville, VA 22904, U.S.A 2 Department of Chemistry, University of Southern California, 837 Bloom Walk, Los Angeles, CA 90089, U.S.A ABSTRACT A transient response technique has been employed to investigate the lifetime of free polarons in bulk heterojunction blends of regioregular poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methylester (PCBM) at different annealing temperatures. Device efficiency and charge mobility were also measured. The longest lifetime, ~ 1.5 microseceonds, was achieved for an annealing temperature of 140ÛC; this represents a 2.5 x increase in lifetime relative to unannealed samples. The 140ÛC annealing temperature also yields the highest efficiency. These measurements provide an estimate of the mobility-lifetime product, a figure of merit for charge transport in organic bulk heterojunctions. INTRODUCTION Organic bulk heterojunction (BHJ) materials have been the focus of numerous studies owing to their potential for fabrication as thin, flexible and low-cost solar cells [1-3]. Over the past few years, the performance of BHJ organic solar cells have improved significantly to the point where power conversion efficiency as high as 8% has been achieved [4]. Thermal annealing plays an important part in the device fabrication and performance since it can improve nanoscale morphology of the interpenetrating donor-acceptor networks, crystallinity of the polymer, and transport across the interface between the bulk heterojuntion material and the aluminum electrode [5-6]. However, the mechanisms by which annealing affects charge transport are not fully understood. In BHJ devices, free charge transport has traditionally been characterized in terms of free polaron diffusion and drift length [3, 7, 8] (1) Ldrift = μτ E
kT μτ (2) q where μ is the free polaron mobility, τ is the free polaron lifetime, E is the internal electric field, D is the diffusion coefficient and k is the Boltzmann constant. Therefore, the product μτ is a very important characterization parameter. The mobility in organic semiconductors is typically quite low and can be measured by several techniques [9], such as time of flight (TOF) [10-12], space charge limited current (SCLC) [13-15] and source-drain current versus gate voltage in field effect transistors (FETs) [16-17]. The measured hole and electron mobility in annealed P3HT:PCBM solar cells are approximately 10–8m2V–1s–1 and 10–7m2V–1s–1 , respectively [17]. For the determination of lifetimes, however, the reported techniques have yielded a wide range Ldiffusion = Dτ =
127
of measured lifetimes. The approaches to lifetime measurement include time-resolved vibrational spectroscopy [18-19], impedance spectroscopy [20-21], transient photoconductivity [22] and transient res
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