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J. Phys. Chem. B 2006, 110, 25496-25503
Ultrafast Photoinduced Charge Separation Dynamics in Polythiophene/SnO2 Nanocomposites† Xin Ai,§,| Neil Anderson,#,| Jianchang Guo,| Janusz Kowalik,∧ Laren M. Tolbert,∧ and Tianquan Lian*,| Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322, and Department of Chemistry, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332-0400 ReceiVed: August 13, 2006; In Final Form: September 28, 2006
We present a study of photoinduced interfacial electron transfer (ET) dynamics of SnO2 nanocrystalline thin films sensitized by polythiophene derivatives (regioregular poly(3-hexylthiophene) (P3HT) and regiorandom poly(3-undecyl-2,2′-bithiophene) (P3UBT)). ET dynamics were measured by following the dynamics of injected electrons in SnO2 and polarons in the conjugated polymer using ultrafast mid-IR transient absorption spectroscopy. The rate of electron transfer from P3HT and P3UBT to SnO2 films was determined to occur on sub-picosecond time scale (120 ( 20 fs). In P3HT/SnO2 composite, interchain charge transfer was found to compete with and reduce the quantum efficiency of interfacial electron transfer at high polymer loading. This interchain charge separation processes can be reduced in non-regioregular polymer or at low polymer loading levels.
Introduction Conjugated polymer-based nanocomposite materials have received intense interest in recent years because of their potential application in efficient and low cost solar cells.1-3 Devices utilizing composites of conjugated polymers with a different conjugated polymer,4 fullerenes,5-11 and inorganic semiconductor nanoparticles12-16 have been demonstrated, and solar-toelectric power conversion efficiency as high as 4.4% has been reported.10 Composites of conjugated polymers and inorganic semiconductor nanostructures can potentially combine the unique properties of these materials.17,18 Conjugated polymers offer numerous advantages, such as low toxicity, adjustable electronic and mechanical properties through functionalization, and ease of fabrication. Inorganic semiconductors can readily form interconnected three-dimensional nanostructures of high surface area and high charge mobility. The high surface area enables a large effective concentration of conjugated polymer within exciton diffusion length (typically, 5-15 nm5,19) from the interface, which is crucial for achieving both high light harvesting and interfacial charge separation efficiency. In composites of conjugated polymer and wide band gap inorganic semiconductor nanoparticle, the solar photons are absorbed by the polymer to generate excitons. As shown in Figure 1, if the energy of the lowest unoccupied orbital (LUMO) of the polymer (such as poly[2-methoxy-5-(2-ethyl-hexyloxy)(phenylene vinylene)(MEH-PPV) or poly(3-hexylthiophene) (P3HT)) is higher than the conduction band edge of the oxide materials (such as TiO2 and SnO2), the excitons may dissociate at the polymer oxide interface, transferring an electron to the semiconductor nanoparticle and leaving a positive polaron on †
Part of the special issue “Arthur J. Nozik Festschrift”. * To whom correspondence should be addressed:
[email protected], phone: 404-7276649, fax: 404-7276586. | Emory University. ∧ Georgia Institute of Technology. § Current address: National Renewable Energy Laboratory, Golden, CO 80401. # Current address: Kapteyn-Murnane Labs Inc., Boulder, CO 80301.
Figure 1. (a) Schematic energy diagram of and photoinduced charge separation process in conjugated polymer/semiconductor nanoparticle composite materials. (b) Position (vs SCE) of the conduction band edge of ZrO2 and SnO233-35 and LUMO level in MEH-PPV and polythiophenes.36-38
the polymer.14-16 These charge carriers can recombine across the interface or be drawn to opposite electrodes to generate photocurrent. A high incident photon-to-current conversion efficiency requires efficient charge separation at the interface (fast charge separation and slow recombination rates) and charge transport through the composite materials. There have been several recent studies of the electron transfer in these types of composites,20-32 providing useful insight into interfacial processes in the composites and guidance for designing efficient photovoltaic devices. One factor ultimately affecting the efficiency of these composite photovoltaic materials is the efficiency of the initial charge separation. It is important that the photogenerated excitons diffuse to the polymer/semiconductor interface and dissociate within their limited lifetimes.39 Although charge transfer in conjugated polymer/semiconductor composites has been demonstrated for several systems, very little is known about the dynamics of the initial transfer.22,39 Its rapidity has been
10.1021/jp0652291 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/24/2006
Polythiophene/SnO2 Nanocomposites CHART 1: Structures of (a) P3HT and (b) P3UBT
inferred on the basis of efficient fluorescence quenching.12,22 For poly[2-methoxy-5-(2-ethylhexyloxy)-(phenylene vinylene)](MEH-PPV)/fullerene, the transfer occurs in ∼45 fs.40 Recently, our group demonstrated that electron transfer occurs in
