Molecular Orientation and Site Dependent Charge Transfer Dynamics

Jan 29, 2014 - Molecular Orientation and Site Dependent Charge Transfer. Dynamics at PTCDA/TiO2(110) Interface Revealed by Resonant. Photoemission ...
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Molecular Orientation and Site Dependent Charge Transfer Dynamics at PTCDA/TiO2(110) Interface Revealed by Resonant Photoemission Spectroscopy Liang Cao,†,‡ Yu-Zhan Wang,‡ Jian-Qiang Zhong,‡ Yu-Yan Han,† Wen-Hua Zhang,† Xiao-Jiang Yu,∥ Fa-Qiang Xu,*,† Dong-Chen Qi,*,‡,§ and Andrew T. S. Wee*,‡ †

National Synchrotron Radiation Laboratory, School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui, P. R. China 230029 ‡ Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 ∥ Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603 § Department of Physics, La Trobe University, Bundoora, Victoria Australia 3086 S Supporting Information *

ABSTRACT: Charge transfer dynamics across the interface of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) organic molecules and the reduced rutile TiO2 (110) 1 × 1 surface has been investigated using core-hole clock implementation of resonant photoemission spectroscopy (RPES). It is found that ultrafast charge transfer from PTCDA molecules to TiO2 substrate takes place on the time scale of 8−20 fs due to their strong electronic coupling. Moreover, the charge transfer time scale at the PTCDA/TiO2 (110) interface shows evident orientational dependence which varies with the molecular site owing to different electronic coupling strengths.

I. INTRODUCTION In model studies and practical applications, organic dye sensitization for photovoltaic devices based on TiO2 electrodes has attracted considerable attention as one of the most promising low-cost replacements for the market-dominant silicon-based photovoltaics.1,2 The high power conversion efficiency of more than 11% has been achieved using ruthenium complexes as organic sensitizers.3 Increasing efforts have also been devoted to using metal-free organic sensitizers with better stability, lower cost, higher efficiency, and better environmental friendliness.4 The efficiency of around 10% has been achieved using conjugated spacer molecules such as indoline dye, dithienosilole, 3,4-ethylenedioxythiophene, and thienothiophene.5−8 Further improvement to the efficiency relies on better understanding of the critical interfaces between organic molecules and TiO2 substrates. It is well-known that the exciton dissociation through ultrafast electron transfer from the photoexcited state of sensitizer into the conduction band of large band gap semiconductors, mostly TiO2, is one of the most critical processes that determine the device efficiency. Charge transfer dynamics at the organic molecule/substrate interface is known to be determined by several factors, most noticeably the energy level alignment and the interfacial electronic coupling strength.9 In addition, the supramolecular packing and/or molecular orientation of organic molecules at interfaces are also found to © 2014 American Chemical Society

markedly affect the charge transfer dynamics both within the molecular assemblies and at the organic−inorganic interfaces.10 On one hand, for example, it was found for phthalocyanine (Pc) films that the delocalization of excited electrons in wellordered regions is faster than that in disordered regions.11 Similarly, improved charge delocalization was also observed in titanyl phthalocyanine (TiOPc) thin films subject to postannealing process which can lead to higher crystallinity.12 On the other hand, a much faster charge transfer process was observed for lying-down 4-fluorobenzenethiol SAMs on Au(111) as compared to the standing-up configuration of molecules due to the much enhanced d−π electronic coupling at the interface between the metal substrate and the molecules for the lying-down molecular configuration.13 Therefore, understanding the structural dependent charge transfer process has important implications for improving the performance of photovoltaic devices. It is worth noting that in practical applications photovoltaic devices such as dye-sensitized solar cells (DSSCs) or quantum dot-sensitized solar cells (QDSSCs)14 made from nanocrystalline TiO2 film with various highly crystalline morphologies; e.g., nanoparticles, nanotubes, nanowires, nanosheets, etc., are Received: October 19, 2013 Revised: January 4, 2014 Published: January 29, 2014 4160

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decay (resonant photoemission, Figure 1b) and spectator decay (resonant Auger, Figure 1c). Consequently, measurable photoelectron intensity is observed. However, if the molecular orbitals are strongly coupled to the substrate, the excited electrons can delocalize into conduction band of substrate (Figure 1d) within the core-hole lifetime. The autoionization process is quenched, and core holes decay via the normal Auger process (Figure 1e). By comparing the resonant photoemission intensity in coupled molecule−substrate system with that in isolated system following the analysis procedure outlined by Brühwiler,38 the change transfer time scale can be quantified. In this work, core-hole clock implementation of resonant photoemission spectroscopy (RPES) is used to study the charge transfer time scale at the PTCDA/TiO2(110) interface. Charge transfer dynamics at submonolayer (0.5 ML) and monolayer regimes are discussed in relation to the distinct molecular orientations and organization at different coverages.

receiving a lot of attention because such structures may improve the device performance.15−19 Specifically, nanocrystalline anatase TiO2 would be more desirable in DSSCs because of high photoactivity, high surface area for dye adsorption, high light harvesting efficiency, and efficient charge transport properties.20−22 However, the less availability of synthetic techniques for single crystalline anatase TiO2 strongly influences its application as substrate for model studies. In this regard, the rutile TiO2(110) with advantages including chemical stability, commercial availability, well-defined electronic structure, and easy surface cleaning serves as a model surface for fundamental research.23−26 On the other hand, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) has been widely used in organic electronics and also serves as a model planar organic semiconductor molecule for fundamental research.27−29 It has been studied on a wide range of substrates focusing on morphologies, optical properties, and electronic properties.30−33 In a recent study,34 we have systematically investigated the electronic structure, chemical interactions, molecular orientations, and energy level alignment at the PTCDA/rutile TiO2(110) interface using synchrotron-based photoemission spectroscopy (PES) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that PTCDA molecules adopt distinct molecular orientations within the first monolayer (ML) region: slightly tilted at 0.5 ML and disordered at 1 ML due to strong interfacial interactions between PTCDA molecules and TiO2(110) surface.34 Furthermore, the strong covalent bonds formed between PTCDA molecules and the TiO2(110) surface give rise to robust electronic coupling and favorable energy alignment at the interface that are critical for efficient photosensitization. Core-hole clock spectroscopy serves as a powerful tool that has been successfully applied to investigate the ultrafast charge transfer dynamics of various organic molecules/TiO2 systems with unprecedented temporal resolution (60

a

The errors are estimated from the uncertainties in the intensities of both integrated RPES and NEXAFS spectra.58

IV. CONCLUSIONS In summary, the charge transfer dynamics of PTCDA on rutile TiO2(110) have been investigated in situ using synchrotronbased core-hole clock spectroscopy. The charge transfer time scales at the PTCDA/TiO2(110) interface are on the order of 8−20 fs due to strong interfacial electronic coupling and favorable energy level alignment. It is revealed that the charge transfer time scale at different molecular sites (i.e., perylene core vs anhydride end groups) is affected differently by the molecular orientation (and thus coupling strength) in the PTCDA/TiO2 system. Our results thus have important implications for the understanding of the charge transfer dynamics at PTCDA/TiO2 interfaces and pave the way for the design and realization of PTCDA-based organic electronic devices.

comparable with that of monolayer and submonolayer IRPES/ INEXAFS ratio within the error, corresponding well with conclusion that electron transfer from LUMO to the substrate does not occur due to the energy mismatch. Substituting these values into eq 1, the charge transfer time scales can be estimated. For submonolayer, τsubmono = 20 ± 7 fs at Cp2 Cp2 resonance. At Ca2 resonance, τsubmono is around 8 fs. The range Ca2 of 3 fs < τsubmono < 40 fs is obtained when taking into account Ca2 the error of intensity. For monolayer, τmono Cp2 = 19 ± 4 fs and τmono Ca2 > 60 fs; that is, the charge transfer time scale is beyond the limitation (∼10× core-hole lifetime) of the core-hole clock technique. It is worth noting that for the monolayer coverage with tilted PTCDA orientation the molecule−substrate coupling strength is asymmetric with one anhydride group close to substrate surface (strong coupling) and the other one tilted away from the substrate (weak coupling).34 The weak coupling anhydride group is proposed to contribute to the significantly larger charge transfer time scale observed. It is likely that the core hole created at the tilted anhydride group may localize the LUMOs, thereby preventing efficient charge transfer of the photoexcited electrons through the other strongcoupling anhydride group. LUMO localization upon core-hole creation has previously been reported as the limiting factor in fast electron transport through partially fluorinated 1,1′;4′,1″terphenyl-4″-thiol (BBB) counterpart (p-thiophenylnonafluorobiphenyl, BFF) on Au(111), whereas BBB shows much shorter charge transfer time of ∼6 fs.56 A similar observation has been made for 1,4-benzenediamine molecules on Au in which significantly longer charge transfer time on the nitrogen site occurs for the tilted phase.57 In this essence, the charge transfer time τmono Ca2 determined above is an averaged value due to the distinct charge transfer times of the two anhydride terminal groups that are coupled differently to the substrate. If we assume the charge transfer time scale is same as τsubmono for Ca2 strongly coupling anhydride groups within monolayer coverage, the charge transfer time scale for tilt anhydride group pointing toward vacuum must be even larger than τmono Ca2 , which is already beyond the limitation of core-hole clock technique. Intriguingly, the charge transfer time scale associated with the Cp2 transitions for monolayer PTCDA is comparable with that for submonolayer, in despite of the drastically different molecular orientations and order at these two thickness regimes. In contrast, the charge transfer time scale of Ca2 resonance is much shorter in submonolayer. Apparently, the change of molecular orientation has a stronger impact on the

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ASSOCIATED CONTENT

S Supporting Information *

Original RPES spectra series at valence band regime for PTCDA molecules on TiO2(110). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Ph (86)-551-63602127; Fax (86)551-65141078 (F.-Q.X.). *E-mail [email protected]; Ph (61)-3-9479-2128; Fax (61)(3)-9479-1552 (D.-C.Q.). *E-mail [email protected]; Ph (65)-6516-6362; Fax (65)6777-6126 (A.T.S.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support by National Natural Science Foundation of China (Grants 10975138 and 11175172), Singapore ARF (Grant R398-000-056-112), and NUS Core Support (Grant C-380-003-002-001).

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