Article pubs.acs.org/JPCC
Delayed Electron Transfer through Interface States in Hybrid ZnO/ Organic-Dye Nanostructures Christian Strothkam ̈ per,† Andreas Bartelt,† Philipp Sippel,† Thomas Hannappel,†,‡ Robert Schütz,† and Rainer Eichberger*,† †
Helmholtz Center Berlin for Materials and Energy, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany Ilmenau University of Technology, Gustav-Kirchhoff-Straße 5, D-98693 Ilmenau, Germany
‡
S Supporting Information *
ABSTRACT: Electron injection from photoexcited chemisorbed dyes into zinc oxide is known to proceed in a stepwise manner, yet the origin of the injection retardation remains controversial. Here we present a complementary time-resolved spectroscopy study on the electron injection dynamics from organic dyes into ZnO using model perylene derivatives with systematically lengthened bridge units to clarify the influence of the positively charged cation on the escape of the injected electron. The combination of transient absorption, opticalpump terahertz-probe, and time-resolved two-photon photoemission spectroscopy reveals that the delayed release of charges into ZnO is independent of Coulomb attraction between the dye cation and the injected electron. Rather, following dye photoexcitation the primary acceptor states of electron transfer into ZnO are interface states formed between the dye and the ZnO surface, which retard the formation of free charges.
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INTRODUCTION Dye-sensitized solar cells (DSCs) are a promising class of photovoltaic materials. Most studies in the past have focused on ruthenium-based inorganic coordination compounds and TiO2 colloidal electrodes.1 ZnO is considered a potential replacement for TiO2 because it possesses a variety of presumed advantages over TiO2. ZnO has a higher bulk mobility than TiO2 and can easily be grown as well-aligned and crystalline nanorods with direct contact to the back electrode. In principle, this should facilitate the collection of photogenerated carriers compared with the randomly organized percolation networks in colloidal TiO2 structures. A so-far unique occurrence in heterogeneous electron transfer (HET) is the delayed generation of free charges at ZnO electrodes occurring after electron injection.2−5 This may be a reason why ZnO-based DSCs have never reached the efficiencies of the TiO2-based counterparts. It was first proposed by Furube et al. that the electron transfer from the excited dye to the final ZnO bulk acceptor states proceeds via intermediate species on the ZnO surface.2 The intermediate state was explained as an exciplex formed between the dye excited state and a ZnO surface state.2 However, in another study on the same colloidal ZnO/N3 system,6 and on the organic dye rhodamine B attached to colloidal ZnO no intermediate products could be observed.7 The occurrence of a stepwise injection mechanism was confirmed in other studies with ZnO nanoparticles sensitized by coumarin derivatives3,4 and a porphyrine-based dye.5 In the latter case, the authors assigned the origin of the intermediate species to the Coulomb © 2013 American Chemical Society
attraction between positively charged dye cations at the surface and injected electrons. They also reported that the Coulomb attraction impedes electron transport by reducing the intraparticle mobility. Other suggestions for the nature of the intermediate state are injection into resonances or injection into dye-induced surface-localized states.4 Because the intermediate state is expected to be localized at the interface between ZnO surface and dye, the electron back reaction is potentially faster compared with a delocalized bulk acceptor state.8 Thus, the presence of intermediate states is of general importance for the functionality of dye-sensitized ZnO solar cells because it might jeopardize the injection process and allow for increased recombination of electrons with the oxidized dye. In this study we address the photoinduced electron transfer from chemisorbed model dyes into ZnO employing the three complementary methods transient absorption (TA), opticalpump terahertz-probe (OPTP), and two-photon photoemission (2PPE). TA is used to probe the state of the photoexcited molecules, while the arrival of free electrons in the ZnO is monitored by means of OPTP. As a surfacesensitive technique, 2PPE probes the transient energetics at the interface region. The study employs a series of perylene derivatives with systematically prolonged conjugated bridges and a carboxylic anchor (Figure 1). The increasing bridge length introduces a Received: February 27, 2013 Revised: July 31, 2013 Published: August 1, 2013 17901
dx.doi.org/10.1021/jp402042a | J. Phys. Chem. C 2013, 117, 17901−17908
The Journal of Physical Chemistry C
Article
solutions of the perylene derivatives while being gently shaken. Afterward, the samples were rinsed with spectroscopic grade chloroform for 5 min to remove any weakly bound dyes. For the 2PPE measurements, ZnO (101̅0) single crystals were purchased from Mateck. The surface was cleaned in a UHV chamber (10 −9 mbar) by several cycles of Ar + bombardment (650 eV, 10 min) and annealing at 400 °C for 10 min. The ZnO crystal was transferred under UHV conditions into another UHV chamber for in situ dyesensitization. This preparation chamber was flooded with ultrapure N2 to a pressure of ∼400 mbar. The 2 × 10−5 M degassed dye solution of C3 in chloroform was prepared using a Schlenk line and pumped from a glass flask connected to the preparation chamber into a cuvette inside the preparation chamber. The ZnO crystal was immersed into that cuvette for 60 min and then rinsed several times with pure, water-free chloroform to remove loosely bound dyes. As a last step, UHV conditions were restored inside the preparation chamber, and the crystal was transferred under UHV conditions back to the 2PPE chamber.
Figure 1. (a) Structure of the chromophoric unit based on a perylene molecule with two tert-butyl groups. (b−f) Chromophoric unit (abbreviated with Per) is attached to systematically prolonged bridge/anchor units to yield the complete sensitizer molecules. These are termed C3 (b), C5, (c), C7 (d), C9 (e), and C11 (f) according to the number of carbon atoms in the bridge/anchor unit.
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PUMP−PROBE SPECTROSCOPY The laser system consists of a 150 kHz amplifier (RegA, Coherent) seeded by 800 nm pulses of a Ti:sapphire 80 MHz oscillator (Mira, Coherent), yielding 7 μJ pulses with typical autocorrelation lengths of 70−80 fs (fwhm). Optical pump pulses around 425−450 nm (TA) and 500 nm (TA, THz, and 2PPE) were extracted from noncollinear optical parametric amplifiers (NOPAs).15 The white-light continuum (WLC) probe pulse for the TA was generated by focusing the 800 nm amplifier pulses into a 2 mm sapphire crystal. The measured TA signal is the pump-induced probe intensity transmission change ΔT. As long as the excitation is weak (ΔT/T ≪ 1), both the absorption and the transmission changes are proportional to the number of photoexcited species. The transmitted probe light and the differential absorption were measured with a monochromator, a photodiode, and a lock-in amplifier by chopping the probe and the pump light, respectively. For the acquisition of the differential absorption as a function of wavelength and pump−probe delay the whole WLC was used as the probe. In this case, the cross-correlation of the NOPA pump pulses and the WLC as measured on a rutile single crystal was generally
