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Electronic States and Exciton Dynamics in Dicyanovinyl-Sexithiophene on Au(111) Lea Bogner, Zechao Yang, Sebastian Baum, Martina Corso, Roland Fitzner, Peter Bäuerle, Katharina J Franke, Jose Ignacio Pascual, and Petra Tegeder J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07325 • Publication Date (Web): 04 Nov 2016 Downloaded from http://pubs.acs.org on November 6, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Electronic States and Exciton Dynamics in Dicyanovinyl-Sexithiophene on Au(111) Lea Bogner,† Zechao Yang,†,k Sebastian Baum,† Martina Corso,†,⊥ Roland Fitzner,‡ Peter Bäuerle,‡ Katharina J. Franke,† José Ignacio Pascual,¶ and Petra Tegeder∗,§ †Freie Universit¨ at Berlin, Fachbereich Physik, Arnimallee 14, D-14195 Berlin, Germany ‡Universit¨ at Ulm, Institut f¨ ur Organische Chemie II und Neue Materialien, Albert-Einstein-Allee 11, 89081 Ulm, Germany ¶CIC nanoGUNE and Ikerbaske, Basque Foundation for Science, Tolosa Hiribidea 76, 20018 Donostia San Sebastian, Spain §Ruprecht-Karls-Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany kCurrent address: Fritz-Haber-Institut der MPG, Berlin, Germany ⊥Current address: Centro de F´ısica de Materiales (CSIC-UPV/EHU) and Ikerbasque, Basque Foundation for Science, 20018 Donostia-San Sebastian E-mail: [email protected] Phone: +49 (0) 6221 54 8475

Abstract Dicyanovinyl (DCV)-substituted oligothiophenes are often used as donor materials in vacuum-processed small-molecule organic solar cells, which exhibit promising efficiencies up to 10%. We combine scanning tunneling mircroscopy/spectroscopy and

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two-photon photoemission (2PPE) to obtain a complete picture of the electronic structure of DCV-sexithiophene (DCV6T) adsorbed on a Au(111) surface. We thus show that the transport gap amounts to 1.4 eV. We also identified an excitonic state possessing a binding energy of 0.6 eV. Using femtosecond time-resolved 2PPE we followed the dynamics of optically excited electronic states at different molecular layer thicknesses. In the multilayer regime we resolved the decay dynamics of excitonic states involving processes ranging from femtoseconds to several tens of ps. The decay of the excitonic states is considerably slower than in DCV-dimethyl-pentathiophene (DCV5T-Me2 ). We ascribe this behaviour to weaker intermolecular couplings in the DCV6T film. Despite the faster exciton decay, DCV5T-Me2 is known for a better solar cell efficiency compared to DCV6T. We suggest that this is due to the concomitant better exciton and charge carriers transport in a well-coupled DCV5T-Me2 molecular structures.

Introduction For improvement and optimization of the performance of organic molecule-based devices a comprehensive knowledge about the physical and chemical properties of organic thin films is necessary. 1–4 This includes the adsorption behavior on metal electrodes and their electronic structure, i.e., energetic positions of occupied and unoccupied molecular electronic states or bands (transport levels) and excitonic states. The dynamics of electronically excited molecules after optical excitation 3,5–11 are particularly important for donor materials used in organic photovoltaic cells. Dicyanovinyl (DCV)-substituted oligothiophenes are low-band gap donor materials, which contain an electron-rich donor backbone and terminal electronpoor acceptor moieties. They have successfully been applied in vacuum-processed smallmolecule organic solar cells, 12–17 in which promising efficiencies up to 8.3 % for a single junction and up to 9.7 % for a triple junction cell have been observed. 18 Comparing the donor materials DCV-dimethyl-pentathiophene (DCV5T-Me2 ) and DCV-sexithiophene (DCV6T) with respect to the performance of the respective solar cell, it has been found that DCV5T-

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Me2 based cells exhibit higher efficiencies. In thin films both molecules have similar optical absorption properties, but the amount of intermolecular interactions are larger in DCV5TMe2 . This results in a higher exciton mobility which has been proposed as an explanation for the better device performance. 15,19,20 In contrast to non-substituted oligothiophens such as sexithiophene 21–27 or octithiophene 28–31 adsorbed on metallic substrates, for which detailed insights into the adsorption and electronic properties exist, little is known about DCV-substituted oligothiophenes. 32,33 Recently, we determined the electronic structure and electronically excited states dynamics of DCV5T-Me2 adsorbed on Au(111). 33 In order to get a hint on the reason for the different solar cell performance, we now determine the structure and electronic properties of the DCV6T/Au(111) system (see Fig. 1) using two-photon photoemission (2PPE) spectroscopy and low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). With this, we obtained a detailed picture on the energetic positions of affinity levels and ionization potentials originating from the lowest unoccupied molecular orbitals (LUMOs) and the highest occupied molecular orbitals (HOMOs) in DCV6T adsorbed on Au(111). We determined the transport gap to be 1.4 eV as well as an exciton binding energy of 0.6 eV. Additionally, fs time-resolved 2PPE spectroscopy enabled us to elucidate the exciton dynamics in DCV6T. While at low coverage (1 monolayer, ML) the excited states decay on an ultrafast timescale (< 10 fs), with increasing coverage the lifetimes rise due to the electronic decoupling from the metallic states. In comparison to DCV5T-Me2 the excited states dynamics in DCV6T is much slower indicating that intermolecular interactions are less developed and thus the exciton and charge carrier mobilities are lower.

Experimental section The Au(111) substrate was prepared by a standard procedure of ion bombardment followed by annealing at 800 K. DCV6T molecules were deposited from an effusion cell held at a tem-

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perature of 520 K while the surface was kept at 300 K. STM/STS and 2PPE measurements were conducted in two different ultrahigh vacuum chambers.

STM/STS experiments: A custom-built low temperature (4.8 K) scanning tunneling microscope was utilized for the STM/STS measurements. Tunneling spectroscopy was performed using a lock-in amplifier under open feedback loop conditions. The recorded differential conductance spectra reflect the local electronic density of states and enable us to determine the energetic position of electronic molecular states (resonances). The spacial distribution of the states are compared with Density Functional Theory (DFT) simulations of the wavefunction amplitude, which allows the assignment of the spectral resonances to specific molecular orbitals. For the DFT calculations we used the Gaussian package, 34 B3LYP exchange-correlation functional and the 6-31G basis set. Note that DFT was not employed to calculate energetic positions of molecular states since it produces imprecise results. Thus the orbital levels were determined on the basis of STS and 2PPE data.

2PPE experiments: In 2PPE, electrons are photoemitted from the sample in a twostep process by two laser pulses with photon energies hν1 and hν2 . The photon energies are smaller than the work function Φ in order to avoid direct photoemission. A photon of the first pulse, the pump pulse, excites an electron from an occupied electronic state below the Fermi energy (EF ) to an unoccupied intermediate state which lies below the vacuum level of the sample. The second pulse, the probe pulse, excites the electron above the vacuum level of the sample into a final state with an energy EF inal , which propagates toward the a time-of-fight (TOF) spectrometer (for details see Ref. 35 ). The pulse energy of both the visible (hν1 ) and UV (hν2 ) photons was in the order of 10−2 mJcm−2 . In order to assign the features seen in a 2PPE spectrum to occupied or unoccupied electronic states it is necessary to understand the excitation processes. Thereby the dependence of the kinetic energy Ekin of the photoemitted electrons on the photon energies is investi-

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gated: 35 In the case of photoemission from an occupied initial state (e.g. the HOMO) via a virtual intermediate state, the kinetic energy of the photoelectron depends on both pump and probe photon energies. Therefore increasing the photon energy by an amount of ∆hν (∆hν = hν −hν ′ ) leads to an increase of Ekin by 2∆hν, in a so-called one-color 2PPE scheme, in which the energy of the pump and probe photon is equal. Ekin of a photoelectron from a unoccupied intermediate state (e.g. LUMO) scales with the probe photon energy. In this case, varying the photon energy by ∆hν changes the kinetic energy of the emitted electron by 1∆hν. In the case of a two-color 2PPE experiment, i.e., the energies of the pump and probe pulses are different but the relation between the two photon energies is fixed: hν2 = 2hν1 ; hν2 lies in the UV regime, while hν1 is a visible photon. in this case, Ekin of the emitted electron scales with 3∆hν (with respect to hν1 ) for occupied states. For unoccupied electronic states Ekin varies with 2∆hν or 1∆hν. In the representation of the 2PPE spectra we choose an energy axis revealing the final state (EF inal ) of the photoemitted electrons with respect to EF (EF inal − EF = Ekin + Φ), thus the low-energy cutoff corresponds to the work function of the adsorbate/substrate system. We fitted the spectra in two parts, the low and high energy part, identical to the procedure which we applied to the 2PPE data of the DCV5T-Me2 /Au(111) system. 33 In the low energy range we used an exponential function in order to account for the secondary electron background and an appropriate number of Gaussians to describe the observed 2PPE features. Due to the flatness of the secondary electron background in the high energy part of the spectrum we only employed Gaussians. Time-resolved two-photon photoemission provides the possibility to measure the lifetimes of transiently populated intermediate states on time scales as low as tens of fs to hundreds of ps. This is realized by delaying the pump and probe pulses with respect to each other by changing the optical path length of one of the beams. The amount of adsorbed DCV6T was determined using thermal desorption spectroscopy and work function measurements.

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Results and discussion Electronic states in DCV6T/Au(111) The electronic properties of organic molecules on metal surfaces crucially depends on the electronic coupling of the molecular states with the metal states and is a result of the adsorption configuration of the adsorbate. Therefore we first analyze the structure submonolayers of DCV6T on Au(111) using STM.

a)

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a b

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Figure 1: a) An overview STM image (I = 72 pA, V = 0.82 V) of DCV6T molecules adsorbed on Au(111). The inset displays the most energetically favorable structure of an isolated DCV6T molecule, calculated using the Gaussian package, 34 with the B3LYP exchangecorrelation functional and the 6-31G basis set. b) A high resolution STM image (I = 47 pA, V = 0.85 V) of the island. Molecules within the island adopt a straight and antisymmetric configuration. The black rhombus depicts the unit cell of the island (a = 3.6 nm, b = 2.1 nm, α = 145◦). c) A high resolution STM image (I = 42 pA, V = 0.76 V) of the second layer. Deposition of DCV6T at room temperature allows for sufficient mobility of the molecules to arrange in densely packed molecular islands and some molecular chains 36 (see Fig. 1 a)). A high resolution STM image (Fig. 1b)) reveals that molecules within the islands adopt a straight thiophene backbone, i.e., with alternating orientation of the thiophene units and with the dicyano moieties pointing to opposite sides of the thiophene chain (transconfiguration). This configuration agrees well with the structure of the most energetically favored conformer, as calculated for an isolated molecule using DFT simulations (shown in the inset of Fig. 1a). DCV6T molecules connect to adjacent molecules ”end-to-end” 6


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and to the neighboring rows ”end-to-center”. The structure is a consequence of CN-S and CN-H bonds to four neighboring molecules. The surface reconstruction underneath the islands appears intact, indicating a relatively weak interaction between the DCV6T and the metal surface. We also note that the packing of the molecular layer is not very dense. Since intermolecular interactions are important for the understanding of exciton and charge carrier mobilities in semiconducting organic materials 15,19,20 (see below), we compare the adsorption structure with the one of its derivative DCV5T-Me2 on Au(111). 33 DCV5T-Me2 adopts a cis-symmetric configuration which allows bonding to five adjacent molecules. 33 In the second DCV5T-Me2 layer the structure is the same as in the first layer, 33 however for DCV6T the situation is different. The second layer of DCV6T exhibits increased disorder (Fig. 1c)). This evidences significantly weaker molecular interactions both within and to the lower layer. Accordingly the interaction it is much weaker than in the second DCV5T-Me2 layer, which is highly ordered and densely packed. 33 We now characterize the electronic structure of the DCV6T molecules in the layer by STS. Fig. 2 a) shows dI/dV spectra recorded on top of a molecule within an island. Three resonances are observed with different distributions over the molecule. The two ends of the molecule possess states at 1.3 V and 1.6 V, while at the center a resonance at 1.3 V and a higher lying one at 2.3 V are observed. The bias-dependent STM images reflect the spatial distribution of these three resonances, as shown in Fig. 2 b). By comparing this shape with the calculated gas phase molecular orbital structure (Fig. 2 c)), we assign resonances located at 1.3 V, 1.6 V, and 2.3 V to the LUMO, LUMO+1, and LUMO+2, respectively. Note that at negative bias voltages no contributions of resonances originating from occupied molecular states are found. To gain further insight into the electronic structure, eventually observing moleculederived occupied electronic states or even excitonic features, which are expected for an electron donating material, we utilized 2PPE spectroscopy, including coverage dependent measurements. Figures 3 a) and b) depict 2PPE data of 1.4±0.4 ML DCV6T/Au(111)

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a)

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2.0

2.5

Bias (V)

Figure 2: a) dI/dV spectra recorded at constant height (set point: I = 70 pA and V = 3.0 V) at different locations at the molecule as indicated in the inset (I = 120 pA and V = 0.75 V, spectra offset for clarity). The grey colored dI/dV spectrum has been measured on the bare Au(111) surface. b) Constant current STM images of an island recorded at biases of 1.3 V, 1.6 V and 2.2 V (I = 50 pA), respectively, which reveal the distribution of the unoccupied molecular orbitals. c) LUMO, LUMO+1, and LUMO+2 orbital shapes of the free molecule calculated with DFT. recorded with one photon (one-color) and two different photon energies (two-color), respectively. We first note that the work function amounts to 5.1 eV, thus revealing a reduction of 350 meV with respect to the bare Au(111) surface. In the one-color 2PPE (hν = 4.6 eV) spectrum several peaks are observed, some of them are related to the d -band of the Au(111) substrate which are located around -2.0, -2.6, and -2.9 eV 37,38 below EF . In addition we detect number of peaks which we attribute to the DCV6T/Au(111) system. Due to the different origins that a peak in a 2PPE spectrum might have, variation of the photon energies allows an assignment of a spectral feature to an occupied or unoccupied electronic state (see experimental section and inset of Fig. 3a)). 39 The peaks named A and B shift with 1∆hν. Thus, these features can be assigned to unoccupied electronic states. Their energetic positions are 4.1 eV (state A) and 3.7 eV (state B) with respect to EF . At EF in − EF = 8.45 eV we obtain a peak which shifts with 2∆hν. This occupied electronic state possesses a binding energy of -0.6 eV. We relate this state to the HOMO-1, since we

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EXC hν1=2.3 eV hν2=4.6 eV

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2PPE Intensity

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2.4 2.5 2.6 Photon Energy (eV)

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Figure 3: a) One-color 2PPE data of 1.4±0.4 ML DCV6T/Au(111) measured with hν = 4.6 eV. b) Two-color 2PPE spectrum recorded with hν1 = 2.3 eV and hν2 = 4.6 eV. The insets in a) and b) show photon energy dependent peak positions to assign the features obtained in a 2PPE spectrum to occupied or unoccupied electronic states. c) and d) corresponding 2PPE spectra for a coverage of 4±1 ML. *refers to peaks arising from molecules in the second or higher layers (see text). observe at higher coverages an additional occupied state (see Fig. 3c) which possesses a lower binding energy, i.e., it is located closer to EF (see below). In the two-color spectrum (it is a correlated spectrum, which is obtained by subtracting the uncorrelated photoelectron signal produced by the pump and probe beam, respectively) shown in Fig. 3b) four additional features are found. All of them shift with 2∆hν1 (see experimental section). Hence they are attributed to unoccupied electronic states which have energetic positions of 0.7, 1.6, 2.0, and 2.3 eV above EF . Consulting the STS data, we allocate the peaks at 1.6 eV and 2.3 eV to the LUMO+1 and LUMO+2, respectively. Note that we do not observe a photoemission feature which can be assigned to the LUMO. This might be due to a weak wave function overlap (transition dipole moment). The 2PPE feature at 0.7 eV (not seen in STS) is most likely attributed to an excitonic state. The 2PPE intensity 9



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from this state rises significantly with increasing DCV6T coverage as can already be seen in the spectrum of 4±1ML (see Fig. 3d)). At even higher coverages the spectrum is dominated by photoemission from this state (see Fig. 5a)). Note that this state exhibits a considerable lifetime as we will demonstrate below. The feature observed at 2.0 eV, which is also not seen in the STS data of the DVC6T molecule located in the molecular island (sub-monolayer regime), could possibly be the result of photoemission from molecules in the second layer. In the case of DCV5T-Me2 /Au(111) we found for coverages above one ML pronounced shifts in energy up to 0.5 eV for several molecular states. This is the result of a reduced electronic coupling between molecular and metal states. Thus, assuming similar energetic shifts here, the peak at 2.0 eV could originate from the LUMO+1 of molecules in the second monolayer. In fact, at higher coverages of 4±1 ML (Fig. 3d)) the peak at 2.0 eV is also observed. Figure 3c) and d) display the 2PPE results obtained from 4±1 ML. In the one-color spectrum the two electronic states labeled as A and B are clearly seen, while the two-color spectrum is dominated by photoemission from the excitonic state as mentioned before. Furthermore, a feature close to EF at EF in − EF = 9.1 eV is observed in the one color spectrum of Fig. 3c). Photon energy dependent measurements show that this state is occupied and, thus, it lies -0.1 eV below EF . In the two-color spectrum is the same state found at EF in − EF = 6.85 eV. We attribute it to the HOMO of DCV6T. Note that photoemission from the HOMO is also noticeable for a DCV6T coverage of 1.4±0.4 ML but its intensity is very weak. Merging the results from both the STS and 2PPE measurements, we gain comprehensive information on the energetic positions of unoccupied as well as occupied molecular electronic states and their assignment. In Fig. 4 we summarize the levels alignment with respect to the vacuum level. As discussed in our previous work on the electronic structure of the DCV5TMe2 /Au(111) system, 33 both STS and 2PPE measures transport levels and gaps. In STS, resonant electron tunneling into unoccupied states takes place via a negative ion resonance (NIR) as a transition state, while a tunneling process out of an occupied states leads to the formation of a positive ion resonance (PIR). A similar mechanism holds for 2PPE.

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vacuum level [eV] A

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Ionization potential

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EF

HOMO-1

Figure 4: Energy level diagram of DCV6T/Au(111). The blue levels are the ionization potentials (left axis) and the red ones are the electron affinities (right axis), EF is the Au(111) Fermi level. A NIR is formed via an optically induced electron transfer from the metal substrate into an empty molecular state. A PIR is simply created by photoionization of an occupied state. Thus, the energy of the obtained molecular states corresponds to the ionization potential (IP) or the electron affinity (EA) of the respective HOMO or LUMO levels. However, 2PPE can also detect an intramolecular, i.e., for instance a HOMO to LUMO excitation (the molecule remains neutral), which is required for exciton formation. The energy needed for this process is the optical gap (Eopt ). The difference between IP and EA is the transport gap (Etransp. ), Etransp. = IP − EA = EB + Eopt with EB the exciton binding energy. 40 In 2PPE the ionization potential is measured for the quasi-particle (the exciton). For the DCV6T/Au(111) system we found a transport gap of 1.40 eV (EA(LUMO) = 3.8 eV, IP(HOMO) = 5.2 eV). Using cyclic voltammetry it is possible to determine the transport gap of DCV6T in solution via the oxidation and reduction potentials. Thereby a value 1.56 eV has been obtained, 14 which is 0.16 eV larger than the gap of the surface-bound DCV6T. The excitonic state exhibits a binding energy (EB ) of 0.6 eV, thus the optical gap measured for DCV6T/Au(111) is 0.8 eV. For comparison, the optical gap obtained from a DCV6T film is 1.68 eV, 14 hence a reduction of more than half of the gap size is found for the molecules in direct contact with the metal substrate due to electronic coupling. 41 Very 11



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similar effects have been observed for the DCV5T-Me2 adsorbed on Au(111) (see below). 33 The higher lying unoccupied states labeled as A and B possess the same properties as found for electronic states at the DCV5T-Me2 /Au(111) interface. 33 As discussed in detail in Ref. 33 they can be assigned either to charge-transfer excitons or to higher lying molecule-derived states. Comparing the electronic structure of DCV6T/Au(111) with the one of DCV5T-Me2 adsorbed on Au(111), 33 we indeed found that they are nearly identical. The values for transport gaps (1.4 eV) as well as the exciton binding energies (0.6 eV) are the same. Although a smaller gap due to the additional thiophene ring in DCV6T would have been expected. The transport gap of DCV5T-Me2 on Au(111) is reduced by 0.5 eV compared to the value measured in solution. In DCV6T/Au(111) we found a decrease of 0.16 eV (see above). Thus the electronic coupling between the DCV5T-Me2 and the metal substrate as well as the intermolecular coupling seems to be stronger than for DCV6T.

Excited states dynamics The efficiency of solar cells is largely influenced by the exciton and charge carrier dynamics. We used femtosecond time-resolved 2PPE to study how the electronic coupling between the DCV6T molecules and the metal substrate influences the dynamics of optically excited states such as excitons in DCV6T. For low coverages (1–2 ML) none of the electronically excited states, e.g. the LUMO+1 or excitonic states, exhibit a detectable lifetime, thus τ < 10 fs. The reason for these ultrashort lifetimes is a strong electron coupling enabling an efficient electron transfer from the molecule back to the metal. Such short lifetime in the order of a few femtoseconds have been found also for other adsorbates on metal surfaces. 42–45 In contrast, for higher coverages longer lifetimes are usually found. This is the case of our findings for 20±5 ML DCV6T on Au(111) shown in Fig. 5b). In this two-dimensional representation, we plot the 2PPE yield at an energy of the intermediate state, EInt. , with respect to EF as a function of pump-probe delay in a two-color scheme (hν1 = 2.4 eV and hν2 = 4.8 eV). For 12


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positive delays the UV pulse (probe) reaches the sample after the visible pulse (pump). At a coverage of 20 ML, i.e., in a high coverage regime, the optical excitation process in 2PPE is intramolecular (e.g. an HOMO-LUMO transition), which is needed to observe exciton formation. The optical gap measured in the present study for the monolayer DCV6T is 0.8 eV. For higher coverages (decoupled from the metallic substrate) the optical gap should increase. We note that the optical gap of a condensed DCV6T films measured by UV/Vis absorption spectroscopy is 1.68 eV. 14 In fact, these experiments found a broad adsorption band between 730 and ≈ 450 nm, which is due to the S0 → S1 (HOMO → LUMO) as well as the S0 → S2 -transition (HOMO → LUMO+1). In our experiment we used a photon energy of 2.4 eV (corresponding to 520 nm) to optically excite DCV6T, thus we are able to stimulate both transitions. Based on this, we attribute the broad photoemission signal (see Fig. 5 a)) to excitonic states related to both the LUMO and LUMO+1. As can be seen in Fig. 5 a) the 2PPE spectrum is dominated by this feature and it gains intensity with increasing coverage (see Figs. 3d) and 5a)). In Fig. 5(c) we show the cross correlation (XC) curve for the energy region EInt. − EF = 0.5–0.8 eV. In order to fit the XC curve we used a Gaussian function which represents the laser pulse duration convoluted with a response function of the intermediate state. Since the dynamics of two different excitonic states are involved the response function is not single exponential. A superposition of three exponential decays with different time constants τ1 , τ2 , and, τ3 describes the time-resolved photoemission data well (see solid line in Fig. 5(c)). The time constants are τ1 = 200±20 fs, τ2 = 1.4±0.2 ps, and τ3 = 70±7 ps at a coverage of 20±5 ML. Note that the third time constant exceeds the measured time range and a further slowing down of the decay can be assumed (at 10 ps only 32 % of the excited state population has been decayed). Coverage-dependent measurements showed that the lifetimes increase with rising coverage (data not shown here). As it has been shown before, 27,43 decay times of excited states which depend on the adsorbate coverage can be explained by the existence of two relaxation channels. One channel is an intrinsic one due to the decay in the bulk

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Figure 5: Time-resolved 2PPE measurement of a 20±5 ML DCV6T film on Au(111). a) 2PPE spectrum at no time delay between pump and probe pulse (∆t = 0 ps). b) Twodimensional spectrum of time-resolved 2PPE measurements recorded with hν1 = 2.4 eV and hν2 = 4.8 eV. Positive values of pump-probe delay denote a delayed UV pulse, hν2 . (c) Cross correlation (XC) trace of the 2PPE intensity integrated over the peak intensities of the excited states (XC energy range in b)). The XC trace is fitted with an exponential decay using three different time constants τ1 − τ3 (red solid line). material. The second decay pathway is distance-dependent (external) and is the quenching by the metal substrate (transfer of the electron to metal). Unfortunately in literature no timedependent measurements on the DCV6T excited states dynamics are available. However, by comparing with our previous time-resolved 2PPE results obtained from DCV5T-Me2 33 we consider the following processes: Hot and delocalized excitons in the first two excitonic bands relax and localize on an ultrafast time-scale (τ1 = 200±20 fs). A very fast relaxation of hot excitons has also been found in other organic semiconductors. 46–49 Strong electron-phonon coupling within the organic film can result in an efficient energy dissipation on a time-scale of a few picoseconds (τ2 = 1.4±0.2 ps). The longer lived component (τ3 = 70±7 ps) may be related to the decay of the lowest lying excitonic state. However, it could also be connected to the lifetime of polarons as well as electrons bound at defect sites. 14


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In the following, we compare the excited states dynamics of DCV6T with the one in DCV5T-Me2 , in order to find out whether the obtained results reflect the better solar cell performance of DCV5T-Me2 in comparison to DCV6T. The XC-traces of 20±5 ML thick _ 5 ML DCV6T XC 20 + triexponential fit

norm. 2PPE Intensity

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_ 5 ML DCV5T-Me2 XC 20 + triexponential fit

0

2

4 6 Pump-Probe Delay (ps)

8

t1 = 200 +_ 20 fs t2= 1.4 +_ 0.2 ps t3= 70 +_ 7 ps

t1 = 125 +_ 13 fs t2= 0.9 +_ 0.1 ps t3= 11 +_ 1 ps 10

Figure 6: Comparison of the XC-traces of 20±5 ML DCV6T and DCV5T-Me2 adsorbed on Au(111). Both can be fitted with the same triexponential decay function. The dynamics in DCV6T is significantly slower than in DCV5T-Me2 . DCV6T and DCV5T-Me2 films are strikingly different (see Fig. 6). While for DCV5T-Me2 the intensity drops by more the 65% within the first two ps only a slight decease in intensity is observed in DCV6T. This indicates that ultrafast decay processes are much more efficient in DCV5T-Me2 which might be due to stronger coupling to vibrational states. 33 A comparison of the time constants reveals that especially the third component (τ3 ) is significant slower in DCV6T. As mentioned before, τ3 might be related to the exciton or polaron lifetime, which is considerably lower in DCV5T-Me2 . This may have several reasons such as a faster recombination rate, a faster exciton diffusion or in the case of polarons a faster charge carrier transport away from the surface towards the metal interface where charge transfer to the metallic substrate occurs. A faster electron-hole recombination rate seems unlikely due to the very similar optical properties of the two compounds. However in a DCV5T-Me2 bilayer film (see above) as well as DCV5T-Me2 crystals a larger number of intermolecular interactions have been found, which are known to promote the exciton mobility. This fact has been consulted as an explanation for the superior performance of solar cells containing 15



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DCV5T-Me2 . 15 Thus a more efficient exciton and charge carrier diffusion in DCV5T-Me2 compared to DCV6T would also explain the faster decay dynamics in DCV5T-Me2 . In general the photovoltaic performance of organic solar cells depends on various parameters such as molecular properties (optoelectronical) and bulk properties responsible for e.g. the charge carrier mobility or thermal stability. On the other hand the bulk characteristics depend on intermolecular interactions leading to a particular molecular order, packing and morphology in the solid state. 15,19,50 For the DCV5T-Me2 it has been demonstrated that the packing of the molecules in single crystal is similar to the packing in the blended film, i.e. in the solar cell. 15 Assuming a crystal-like structure of DCV5T-Me2 in the multilayer film 51 studied here we can correlate the observed fast exction dynamics with an efficient exciton diffusion and charge transport in the organic solar cell.

Conclusions In conclusion, we have determined the energetic position of affinity levels and ionization potentials as well as the exciton binding energy of DCV6T adsorbed on a Au(111) by means of scanning tunneling spectroscopy (STS) and two-photon photoemission (2PPE). A comparison with the DVC-derivative DCV5T-Me2 exhibits very similar optoelectronic properties for both oligothiophenes. With increasing DCV6T coverage, we observed excitonic states which gain 2PPE intensity with rising film thickness due to a decoupling of the molecular states from metallic states. Using femtosecond (fs) time-resolved 2PPE enabled us to elucidate the dynamics of the two lowest exciton bands after optical excitation. Thereby three decay processes have been observed possessing different time scales ranging from fs to tens of picoseconds. The excited state dynamics in DCV6T is slower compared to the dynamics in DCV5T-Me2 . The existence of stronger intermolecular interactions in DCV5T-Me2 promoting exciton and charge carrier transport may be a possible reason. The faster exciton decay and charge carrier transport in DCV5T-Me2 found in this study may explain the better solar

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cell performance of DCV5T-Me2 in comparison to DCV6T.

Acknowledgement Funding by the Deutsche Forschungsgemeinschaft (DFG) through the priority program SPP 1355 and the collaborative research center Sfb 658 is gratefully acknowledged. Z.Y. acknowledges the Chinese CSC program for his grant. JIP also acknowledge financial support from Spanish MINECO (Grant No. MAT2013-46593-C6-01) and Basque Government (PI2015042).

References (1) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 1999, 11, 605–625. (2) Braun, S.; Salaneck, W. R.; Fahlman, M. Energy-Level Alignment at Organic/Metal and Organic/ Organic Interfaces. Adv. Mater. 2009, 21, 1450–1472. (3) Koch, N., Ueno, N., Wee, A., Eds. The Molecule-Metal Interface; Wiley-VCH, Weinheim, 2013. (4) Oehzelt, M.; Koch, N.; Heimel, G. Organic Semiconductor Density of States Controls the Energy Level Alignment at Electrode Interfaces. Nat. Commun. 2014, 5, 4174. (5) Gruenewald, M.; Wachter, K.; Meissner, M.; Kozlik, M.; Forker, R.; Fritz, T. Optical and Electronic Interaction at Metal–Organic and Organic–Organic Interfaces of UltraThin Layers of PTCDA and SnPc on Noble Metal Surfaces. Org. Electron. 2013, 14, 2177–2183.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

(6) Forker, R.; Gruenewald, M.; Fritz, T. Optical Differential Reflectance Spectroscopy on Thin Molecular Films. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 2012, 108, 3468. (7) Bommel, S.; Kleppmann, N.; Weber, C.; Spranger, H.; P. Schäfer, J. N.; Roth, S.; Schreiber, F.; Klapp, S. H.; Kowarik, S. Unravelling the Multilayer Growth of the Fullerene C60 in Real-Time. Nature Comm. 2014, 5, 5388. (8) Yamane, H.; Gerlach, A.; Duhm, S.; Tanaka, Y.; Hosokai, T.; Mi, Y.; Zegenhagen, J.; Koch, N.; Seki, K.; Schreiber, F. Site-Specific Geometric and Electronic Relaxations at Organic-Metal Interfaces. Phys. Rev. Lett. 2010, 105, 046103. (9) Breuer, T.; Klues, M.; Witte, G. Characterization of Orientational Order in πConjugated Molecular Thin Films by NEXAFS. J. Elect. Spec. Rel. Phenom. 2015, 204, 102–115. (10) Casu, M. B. Growth, Structure, and Electronic Properties in Organic Thin Films Deposited on Metal Surfaces Investigated by Low Energy Electron Microscopy and Photoelectron Emission Microscopy. J. Elect. Spec. Rel. Phenom. 2015, 204, 39–48. (11) Jones, A. F.; Chattopadhyay, B.; Geerts, Y. H.; Resel, R. Substrate-Induced and ThinFilm Phases: Polymorphism of Organic Materials on Surfaces. Adv. Funct. Mater. 2016, 26, 2233–2255. (12) Mishra, A.; Uhrich, C.; Reinold, E.; Pfeiffer, M.; Bäuerle, P. Synthesis and Characterization of Acceptor-substituted Oligothiophenes for Solar Cell Applications. Adv. Energy Mater. 2011, 1, 265–273. (13) Ziehlke, H.; Fitzner, R.; Körner, C.; Gresser, R.; Reinold, E.; Bäuerle, P.; Leo, K.; Riede, M. Side Chain Variations on a Series of Dicyanovinyl-Terthiophenes: A Photoinduced Absorption Study. J. Phys. Chem. A 2011, 115, 8437–8446.

18


Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Fitzner, R.; Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Körner, C.; Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; et al., Dicyanovinyl-Substituted Oligothiophenes: Structure-Property Relationships and Application in Vacuum-Processed Small-Molecule Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 897910. (15) Fitzner, R.; Mena-Osteritz, E.; Mishra, A.; Schulz, G.; Reinold, E.; Weil, M.; Körner, C.; Ziehlke, H.; Elschner, C.; Leo, K.; et al., Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance. J. Am. Chem. Soc. 2012, 134, 11064–11067. (16) Baumeier, B.; Andrienko, D.; Ma, Y.; Rohlfing, M. Excited States of DicyanovinylSubstituted Oligothiophenes from Many-Body Greens Functions Theory. J. Chem. Theory Comput. 2012, 8, 997–1002. (17) Bader, M. M.; Pham, P.-T. T.; Elandaloussi, E. H. Dicyanovinyl-Substituted Oligothiophenes. Cryst. Growth Des. 2010, 10, 5027–5030. (18) Meerheim, R.; Körner, C.; Leo, K. Highly Efficient Organic Multi-Junction Solar Cells with a Thiophene Based Donor Material. Appl. Phys. Lett. 2014, 105, 063306. (19) Fitzner, R.; Elschner, C.; Weil, M.; Uhrich, C.; Körner, C.; Riede, M.; Leo, K.; Pfeiffer, M.; Reinhold, E.; Mena-Osteritz, E.; Bäuerle, P. Interrelation between Crystal Packing and Small - Molecule Organic Solar Cell Performance. Adv. Mater. 2012, 24, 675–680. (20) Schrader, M.; Fitzner, R.; Hein, M.; Elschner, C.; Baumeier, B.; Leo, K.; Riede, M.; Bäuerle, P.; Andrienko, D. Comparative Study of Microscopic Charge Dynamics in Crystalline Acceptor-Substituted Oligothiophenes. J. Am. Chem. Soc. 2012, 134, 6052– 6056. (21) Hill, I.; Kahn, A.; Soosb, Z.; R.A. Pascal, J. Charge-Separation Energy in Films of π-Conjugated Organic Molecules. Chem. Phys. Lett. 2000, 327, 181–188. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(22) Kiguchi, M.; Entani, S.; Saiki1, K.; Yoshikawa, G. One-dimensional Ordered Structure of α-Sexithienyl on Cu(110). Appl. Phys. Lett. 2004, 84, 3444–3446. (23) Koch, N.; Heimel, G.; Wu, J.; Zojer, E.; Johnson, R. L.; Brédas, J.-L.; M¨ ullen, K.; Rabe, J. P. Influence of Molecular Conformation on Organic/Metal Interface Energetics. Chem Phys. Lett. 2005, 413, 390–395. (24) Kiel, M.; Duncker, K.; Hagendorf, C.; Widdra, W. Molecular Structure and Chiral Separation in α-Sexithiophene Ultrathin Films on Au(111): Low-energy Electron Diffraction and Scanning Tunneling Microscopy. Phys. Rev. B 2007, 75, 195439. (25) Grobosch, M.; Knupfer, M. Charge-Injection Barriers at Realistic Metal/Organic Interfaces: Metals Become Faceless. Adv. Mater. 2007, 19, 754. (26) Varene, E.; Martin, I.; Tegeder, P. Optically Induced Inter- and Intrafacial Electron Transfer Probed by Two-Photon Photoemission: Electronic States of Sexithiophene on Au(111). J. Phys. Chem. Lett. 2011, 2, 252–256. (27) Varene, E.; Bogner, L.; Bronner, C.; Tegeder, P. Ultrafast Exciton Population, Relaxation, and Decay Dynamics in Thin Oligothiophene Films. Phys. Rev. Lett. 2012, 109, 7601–1. (28) Yokoyama, T.; Kurata, S.; Tanaka, S. Direct Identification of Conformational Isomers of Adsorbed Oligothiophene on Cu(100). J. Phys. Chem. B 2006, 110, 18130–18133. (29) Kakudate, T.; Tsukamoto, S.; Nakaya, M.; Nakayama, T. Initial Stage of Adsorption of Octithiophene Molecules on Cu(111). Surf. Sci. 2006, 605, 1021–1026. (30) Varene, E.; Pennec, Y.; Tegeder, P. Assembly and Electronic Structure of Octithiophene on Au(111). Chem. Phys. Lett. 2011, 515, 141–145.

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Page 20 of 24

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Varene, E.; Bogner, L.; Meyer, S.; Pennec, Y.; Tegeder, P. Coverage-Dependent Adsorption Geometry of Octithiophene on Au(111). Phys. Chem. Chem. Phys. 1012, 14, 691–696. (32) Yang, Z.; Corso, M.; Robles, R.; Lotze, C.; Fitzner, R.; Mena-Osteritz, E.; Bäuerle, P.; Franke, K.; Pascual, J. Orbital Redistribution in Molecular Nanostructures Mediated by MetalOrganic Bonds. ACS Nano 2014, 8, 10715–10722. (33) Bogner, L.; Yang, Z.; Corso, M.; Fitzner, R.; Bäuerle, P.; Franke, K. J.; Pascual, J.; Tegeder, P. Electronic Structure and Excited States Dynamics in a DicyanovinylSubstituted Oligothiophene on Au(111). Phys. Chem. Chem. Phys. 2015, 17, 27118– 27126. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; J. A., J. M.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al., Gaussian 03, Revision C.02. Gaussian, Inc.: Wallingford, CT 2004, (35) Tegeder, P. Optically and Thermally Induced Molecular Switching Processes at Metal Surfaces. J. Phys.: Condens. Matter 2012, 24, 394001 (34pp). (36) Note that, molecular chains are found to be of two types. Some chains, shorter and with multiple shapes, are stabilized by coordination bonds to Au adatoms, as in our previous publication [32]. Other chains, longer and more periodic, are bonded by intermolecular non-covalent interaction, probably involving terminal CN moieties and H atoms. (37) Eckardt, H.; Fritsche, L.; Noffke, J. Self-Consistent Relativistic Band Structure of the Noble Metals. J. Phys. F: Met. Phys. 1984, 14, 97–112. (38) Courths, R.; Zimmer, H. G.; Goldmann, A.; Saalfeld, H. Electronic Structure of Gold: An Angle-Resolved Photoemission Study along the Γ line. Phys. Rev B 1986, 34, 3577.

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(39) Bronner, C.; Schulze, G.; Franke, K.; Pascual, J. I.; Tegeder, P. Switching Ability of Nitro-Spiropyran on Au(111): Electronic Structure Changes as a Sensitive Probe During a Ring-Opening Reaction. J. Phys.: Condens. Matter 2011, 23, 484005. (40) Muntwiler, M.; Zhu, X.-Y. In Dynamics at Solid State Surfaces and Interfaces; Bovensiepen, U., Petek, H., Wolf, M., Eds.; Wiley-VCH, 2010. (41) Torrente, I. F.; Franke, K. J.; Pascual, J. I. Spectroscopy of C60 Single Molecules: The Role of Screening on Energy Level Alignment. J. Phys.: Condens. Matter 2008, 20, 184001. (42) Yang, A.; Shipman, S. T.; Garrett-Roe, S.; Johns, J.; Strader, M.; Szymanski, P.; Muller, E.; Harris, C. B. Two-Photon Photoemission of Ultrathin Film PTCDA Morphologies on Ag(111). J. Phys. Chem. C 2008, 112, 2506. (43) Dutton, G.; Quinn, D. P.; Lindstrom, C. D.; Zhu, X.-Y. Exciton Dynamics at MoleculeMetal Interfaces: C60 /Au(111). Phys. Rev. B 2005, 72, 045441. (44) Hagen, S.; Luo, Y.; Haag, R.; Wolf, M.; Tegeder, P. Electronic Structure and Electron Dynamics at an Organic Molecule/Metal Interface: Interface States of tetra-tert-butylImine/Au(111). New J. Phys. 2010, 12, 125022 (17pp). (45) Bronner, C.; Schulze, M.; Hagen, S.; Tegeder, P. The Influence of the Electronic Structure of Adsorbate-Substrate Complexes on the Photoisomerization Ability. New J. Phys. 2012, 14, 043032. (46) Marks, M.; Sachs, S.; Schwalb, C.; Schöll, A.; Höfer, U. Electronic Structure and Excited State Dynamics in Optcally Excited PTCDA Films Investigated with Two-Photon Photoemission. J. Chem. Phys. 2013, 139, 124701. (47) Engel, E.; Koschorreck, M.; Leo, K.; Hoffmann, M. Ultrafast Relaxation in Quasi-OneDimensional Organic Molecular Crystals. Phys. Rev. Lett. 2005, 95, 157403. 22


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(48) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Near-IR Femtosecond Transient Absorption Spectroscopy of Ultrafast Polaron and Triplet Exciton Formation in Polythiophene Films with Different Regioregularities. J. Am. Chem. Soc. 2009, 131, 16869–16880. (49) Chen, K.; Barker, A.; Reish, M.; Gordon, K.; Hodgkiss, J. M. Broadband Ultrafast Photoluminescence Spectroscopy Resolves Charge Photogeneration via Delocalized Hot Excitons in Polymer: Fullerene Photovoltaic Blends. J. Am. Chem. Soc. 2013, 135, 18502–18512. (50) Ojala, A.; Petersen, A.; Fuchs, A.; Lovrincic, R.; Poelking, C.; Trollmann, J.; Hwang, J.; Lennartz, C.; Reichelt, H.; et al., H. W. H. Merocyanine/C60 Planar Heterojunction Solar Cells: Effect of Dye Orientation on Exciton Dissociation and Solar Cell Performance. Adv. Funct. Mater. 2012, 22, 86–96. (51) It is known for many organic molecules grown on metal substrates that they adopt in the multilayer regime a crystal-like structure [11, L. Hahn et al., Chem. Eur. J. 21 (2015) 17691.].

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DCV6T

+ hn2

e- -

hn1

+

Au(111)

Figure : Table of Contents Graphic.

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Electronic States and Exciton Dynamics in ... - ACS Publications

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