Ultrafast relaxation dynamics of photoexcitations in poly(3-hexylthiophene) for the determination of the defect concentration Yu Hsien Lee a,⇑ , Atsushi Yabushita a , Chain Shu Hsu b , Sheng Hsiung Yang c , Izumi Iwakura d , Chih Wei Luo a , Kaung Hsiung Wu a , Takayoshi Kobayashi a,e,f,g a Department of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan b Department of Applied Chemistry, National Chiao-Tung University, Hsinchu 300, Taiwan c Institute of Lighting and Energy Photonics, National Chiao-Tung University, Hsinchu 300, Taiwan d Innovative Use of Light and Materials/Life, PREST, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan e Department of Applied Physics and Chemistry and Institute for Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan f ICORP, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan g Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0971, Japan article info Article history: Received 28 June 2010 In final form 17 August 2010 Available online 20 August 2010 abstract Ultrafast pump–probe spectroscopy of poly(3-hexylthiophene) (P3HT) films was performed using a 9-fs laser and a broadband lock-in detection system. The fast geometrical relaxation (GR) time was attrib- uted to the transition from a free exciton (FE) to form a bound polaron pair (BPP), and the time con- stant was estimated to be s GR = 90 ± 2 fs. The relaxation time constant of BPP was determined as s BPP = 710 ± 40 fs. The measurement of pump-power dependence enabled us to distinguish the defect trapping process of BPPs from other parallel decay processes and to determine the defect concentration of a P3HT thin film. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Conjugated polymers have been studied extensively because of their characteristic properties, including plasticity and semicon- ductivity, that make them attractive for use in large-area devices for electro-optical and optoelectronic applications [1–8]. For in- stance, a considerable progress has been made in developing elec- troluminescent devices based on poly(arylenevinylene) [9,10] for an optoelectronic application, and organic solar cells that use con- jugated-polymer-based compounds have recently been studied widely as a potential low-cost replacement for conventional sili- con photovoltaics. Polymer-based solar cells have many advanta- ges over conventional ones, including low toxicity, adjustable electronic and mechanical properties, and ease of fabrication. The energy conversion efficiency of a solar cell based on a bulk heterojunction consisting of a polymer donor of regioregular poly(3-hexylthiophene) (P3HT) and an acceptor of [6,6]-phenyl- C61-butyric acid methyl ester was reported to be 4.4% [11]. The dynamics of charge carriers contribution in P3HT was recently studied by Ellingson et al. [12] using THz pulses to probe the picosecond relaxation of P3HT, and the basic scheme of interchain transfer was also illustrated on the basis of previous works [13– 15] with the formation and relaxation of bound polaron pairs. However, ultrafast formation and relaxation of BPP could not be time-resolved by the THz system due to the limited time-resolu- tion of their measurement. In the present study, we have studied the ultrafast dynamics of P3HT using a sub-10-fs visible laser pulse. The ultrafast time-reso- lution has enabled us to determine the time constants of 90 ± 2 and 710 ± 40 fs, which correspond to the formation and relaxation, respectively, of a bound polaron pair (BPP). Defect concentration on P3HT has also been estimated by observation of the pump intensity dependence of the signal. 2. Experiment 2.1. Non-collinear optical parametric amplifier We have constructed a non-collinear optical parametric ampli- fier (NOPA) to generate visible laser pulses, whose spectral width is sufficiently broad to support sub-10 fs visible pulses useful for ultrafast time-resolved spectroscopy [16–18]. The smooth spectral shape of the NOPA output is suitable for time-resolved spectral measurements. A regenerative chirped pulse amplifier (Legend- USP-HE; Coherent) seeded with a Ti:sapphire laser oscillator (Micra 10; Coherent) is used to pump and seed the NOPA. The amplifier generates 40-fs pulses with a central wavelength of 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.08.036 ⇑ Corresponding author. Fax: +886 3 572 5230. E-mail addresses: raymond.ep95g@nctu.edu.tw (Y.H. Lee), yabushita@mail. nctu.edu.tw (A. Yabushita). Chemical Physics Letters 498 (2010) 71–76 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett