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Photoinduced Carrier Generation and Recombination Dynamics of a Trilayer Cascade Heterojunction Composed of Poly(3-hexylthiophene), Titanyl Phthalocyanine, and C 60
Jaehong Park, Obadiah George Reid, and Garry Rumbles J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b00110 • Publication Date (Web): 20 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015
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The Journal of Physical Chemistry
Photoinduced Carrier Generation and Recombination Dynamics of a Trilayer Cascade Heterojunction Composed of Poly(3hexylthiophene), Titanyl Phthalocyanine, and C60 Jaehong Park,† Obadiah G. Reid,†,‡ Garry Rumbles*,†,‡ †
Chemical and Materials Science Center, National Renewable Energy Laboratory, 15013 Denver
West Parkway, Golden, CO 80401, United States ‡
Department of Chemistry and Biochemistry, and Renewable and Sustainable Energy Institute,
University of Colorado at Boulder, Boulder, CO 80309, United States
ABSTRACT. We use flash-photolysis time-resolved microwave conductivity experiments (fpTRMC) and fs-ns pump-probe transient absorption spectroscopy to investigate photoinduced carrier generation and recombination dynamics of a trilayer cascade heterojunction (P3HT/TiOPc/C60) composed of poly(3-hexylthiophene) (P3HT), titanyl phthalocyanine (TiOPc), and fullerene (C60). Carrier generation following selective photoexcitation of TiOPc is independently observed at both the P3HT/TiOPc and TiOPc/C60 interfaces. The transient
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absorption results indicate that following initial charge generation processes to produce P3HT•+/TiOPc•— and TiOPc•+/C60•— at each interface from (P3HT/TiOPc*/C60), the final charge separated product of (P3HT•+/TiOPc/C60•—) is responsible for the long-lived photoconductance signals in fp-TRMC. And in both P3HT/TiOPc and P3HT/TiOPc/C60 cases, the electron transfer appears to occur only with the crystalline (weakly coupled H-aggregate) phase of the P3HT.
KEYWORDS. Transient microwave conductivity, transient absorption spectroscopy, excitedstate dynamics, carrier dynamics, and photoinduced electron transfer.
Introduction Our fundamental understanding of photoinduced electron transfer in organic solids has progressed substantially in the last three decades.1-18 Concurrently certified efficiencies of organic photovoltaic (OPV) devices have increased from ~1% in 1984 to 12% in 2014.19 Nevertheless, new device architectures that go beyond binary mixtures and bilayers favored in the past will likely be needed to push the efficiency of OPV devices higher. Ternary (or higher order) blend solar cells are a promising alternative to binary composites because of the potential for better absorption of the solar spectrum, and thus higher JSC.20-32 In spite of this advantage, it is yet to be elucidated how additional components impact photoinduced carrier generation and recombination processes. For example, one might expect that planar ternary junctions would exhibit slower charge recombination kinetics than bilayers because the central layer acts as a barrier to recombination, which could allow enhanced VOC and/or fill-factor in devices. Such hypotheses have not been tested. Therefore, more fundamental work is clearly required to fully
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The Journal of Physical Chemistry
understand the operating mechanism of these ternary systems, and their potential advantages over binary ones. In this work we study the photoinduced carrier generation and recombination dynamics between constituents of a model ternary composite, and the subsequent dynamics in ternary systems where a second charge transfer step may occur. Our model system is a trilayer cascaded heterojunction composed of titanyl phthalocyanine (TiOPc) sandwiched between regioregular poly(3-hexylthiophene) (RR-P3HT) and C60. Due to the simplicity of its morphology and welldefined interface, a trilayer system can be advantageous to interrogate photoinduced carrier generation and recombination dynamics over popular bulk heterojunction structures that may exhibit more complicated excited-state dynamics due to their complex poorly understood microstructure. Since phthalocyanines (Pc) have high extinction coefficients (>105 M-1cm-1) in the near-infrared (NIR), Pc derivatives are attractive as a photon-capturing layer. Recently, Placencia et. al. reported AM 1.5G efficiencies of ~1.4% of the as-deposited TiOPc/C60 bilayer, and ~4% when the TiOPc is crystallized.33 Thus, the trilayer structure P3HT/TiOPc/C60 serves as an interesting model, possessing both structural simplicity and efficient photoinduced carrier generation. TiOPc has broad Q-band absorption in the NIR (Figure 1B) whereas the RR-P3HT and C60 exhibits absorption in the visible (Figure 1AC), providing continuous strong absorption from 300-800 nm. (Figure 2) In addition, the electron affinity and ionization potentials of the TiOPc are situated between those of the P3HT and those of C60: TiOPc* is an electron acceptor relative to P3HT, and an electron donor relative to C60; likewise it is a hole donor relative to P3HT, and a hole acceptor relative to C60. These energy levels are shown approximately in Figure 1D.
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To explore excited-state dynamics, we use flash-photolysis time-resolved microwave conductivity (fp-TRMC) and femtosecond-nanosecond pump-probe transient absorption spectroscopy (fsTA and nsTA). Fp-TRMC is useful for investigating charge carrier dynamics in the nanosecond-microsecond time domain;8,9,34-36 it provides a contactless probe of photoconductivity sensitive enough for use at very low excitation fluence conditions (
