Atomic and electronic structures of interfaces in dye-sensitized, nanostructured solar cells.

CHEMPHYSCHEM REVIEWS DOI: 10.1002/cphc.201301074

Atomic and Electronic Structures of Interfaces in Dye-Sensitized, Nanostructured Solar Cells Erik M. J. Johansson,*[a] Rebecka Lindblad,[b] Hans Siegbahn,[b] Anders Hagfeldt,[a] and Hkan Rensmo*[b] Key processes in nanostructured dye-sensitized solar cells occur at material interfaces containing, for example, oxides, dye molecules, and hole conductors. A detailed understanding of interfacial properties is therefore important for new developments and device optimization. The implementation of Xray-based spectroscopic methods for atomic-level understanding of such properties is reviewed. Specifically, the use of the chemical and element sensitivity of photoelectron spectrosco-

py, hard X-ray photoelectron spectroscopy, and resonant photoelectron spectroscopy for investigating interfacial molecular and electronic properties are described; examples include energy matching, binding configurations, and molecular orbital composition. Finally, results from the complete oxide/dye/holeconductor systems are shown and demonstrate how the assembly itself can affect the molecular and electronic structure of the materials.

1. Introduction During the past decades the energy demand and environmental issues connected to fossil fuels have intensified the quest for alternative sources of energy. Although the total electricity production from solar cells at the moment is very low, it has recently increased tremendously with a growth of about 30 % every year. Nanostructured dye-sensitized solar cells (DSCs) show promise as an alternative to conventional silicon-based solar cells.[1–3] In standard liquid-based DSCs, three different materials are responsible for the conversion of light to electrical energy: a mesoporous semiconducting inorganic material transparent to visible light, a monolayer of dye molecules, and a liquid electrolyte with a redox couple. Photons are generally absorbed at the interface by the dye molecules and the excited electron is injected into the semiconductor material. The electrons are thereafter transported through the semiconductor material to the contact and their potential energy can be utilized in an external load. The dye molecules are left in an oxidized state and are regenerated by the redox couple in the electrolyte, which transfers charges to the counter electrode (see Figures 1 and 2). Examples of efficient dye molecules are metal complexes, many based on ruthenium, and different kinds of organic dyes, including triphenylamine and indolines.[1–3] The redox couple is conventionally I /I3 , but recently other redox couples, such as cobalt complexes or ferrocene, have been shown to be effi[a] Dr. E. M. J. Johansson, Prof. A. Hagfeldt Physical Chemistry, Department Chemistry-ngstrçm Uppsala University, SE-751 20 Uppsala (Sweden) E-mail: [email protected] [b] R. Lindblad, Prof. H. Siegbahn, Prof. H. Rensmo Molecular and Condensed Matter Physics Department of Physics and Astronomy Uppsala University, SE-751 20 Uppsala (Sweden) E-mail: [email protected]

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cient in combination with organic dye molecules.[4–6] The liquid electrolyte can also be replaced with a solid material, such as 2,2’,7,7’-tetrakis (N,N-dimethoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD)[7–20] or other hole-transport materials, such as conducting polymers,[20–33] to make a solid-state DSCs (sDSCs). This may have practical advantages compared with liquid electrolyte based DSCs, although the highest energy conversion efficiency of sDSCs with organic dye molecules is currently lower than that in liquid electrolyte DSCs.[8] Moreover, the hybrid organic–inorganic perovskite was recently introduced as a promising replacement of the dye as a light absorber.[34–36] The final efficiency of DSCs is largely affected by electrontransfer processes occurring at the interface between the interacting materials (see Figure 1 b). The injection of electrons from the dye molecules into TiO2 needs to be efficient and this injection process competes with recombination of the excited state of the dye molecules. After electron injection, the rate of electron transfer from the electrolyte/hole conductor to the oxidized dye (regeneration of the dye) kinetically competes with recombination processes such as electron transfer from TiO2 to the oxidized dye or to the electrolyte/hole conductor. The transfer rates are closely connected to precise energy-level matching between the interacting materials, as well as the detailed atomic-scale interfacial structure (Figure 2). To optimize the efficiency of the solar cell, the energy levels and geometry of the interfaces should therefore be optimized to achieve advantageous electron transfer in the correct direction without large energy losses and to avoid recombination processes. Worldwide efforts to develop efficient systems have now reached the stage where an understanding of these fundamental processes on the atomic level is critical; this requires experimental tools for the investigation and control of matter on that scale. ChemPhysChem 0000, 00, 1 – 13

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teractions that alter their properties. The chemical structures, molecular orientations, and electronic properties may change due to the new conditions. For the solar cell function, it is therefore of great interest to know why and to what extent the interface with dye molecules in combination with the nanoparticles alters the geometry and energy levels. Understanding interactions between photons, electrons, and chemical bonds at an atomic level on their natural length scales requires X-ray-based methods. Such methods have, in recent years, shown a dramatic increase in performance, allowing for new methodologies to be applied to complex material systems. In particular, spectroscopic techniques based on synFigure 1. a) Illustration of the working principle of a DSC. b) Energy diagram indicating some basic electrical propchrotron radiation are constantly erties of the different materials. Energy diagram reprinted (adapted) with permission from ref. [2]. Copyright being developed for investiga(2010) American Chemical Society. tions into functional materials. These techniques were introduced in DSC research at an early stage and have extensively been used for the characterization of relevant materials in terms of interfacial molecular structures, valence molecular orbital composition, and energylevel matching in oxide/dye/ hole-conductor heterojunctions.[37–74] The aim of this review is to exemplify and summarize Figure 2. Illustration of a DSC structure with a solid-state molecular hole conductor, and a magnification of the inthe use of photoelectron specterfacial energy-level matching and the molecular structure at the TiO2/dye/hole-conductor interface. troscopy (PES), hard X-ray photoelectron spectroscopy (HAXPES), The material combinations in the nanostructured solar cells and resonant photoelectron spectroscopy (RPES) for atomicare often selected from knowledge of basic electrical properlevel understanding of the key interfaces in the DSC. To give ties of the different materials, such as band positions, absorpone example at this point, we mention the important contribution properties, and redox potentials (see Figure 1 b), as well as tion in delineating the molecular orbital structure and the suron simple chemical properties, for example, the position of face organization of the well-known dye N719. Electron specspecific linking groups and surface structures. The dye moletroscopy investigations have shown experimentally at an cules are also often designed with a molecular structure with atomic level how the dye molecules at the interface surface LUMO close to the nanoparticle surface, which favors electron layer allow for efficient charge transfer in the light absorption injection into the nanoparticles after excitation of the dye molprocess, in which the electron is transferred within the dye ecules. To reduce the recombination rate of the injected elecmolecule and towards the surface. Based on such knowledge, trons with holes on the dye molecules, the highest occupied current developments of inorganic and organic molecules are electronic structure of the dye molecule is designed to be lodominated by dyes with these charge-transfer properties. cated further away from the nanoparticle surface. In the solar cell, however, the combination of the materials (semiconductor, dye, and hole conductor) gives rise to new in 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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CHEMPHYSCHEM REVIEWS 2. PES on Interfaces Generally, X-ray-based tools have unique capabilities for studying the properties of interfaces at the atomic level. For example, in core-level PES, one can distinguish different chemical species of the same element through the chemical shift. Moreover, the chemical shift also allows for an atomic-level understanding of properties such as charge distribution, oxidation state, binding structure, and energy matching. A photoelectron spectrum is obtained by measuring the kinetic energy of the photoelectrons emitted during illumination of an interface with soft or hard X-rays. The technique is generally considered to be surface sensitive due to the short mean free path of the photoelectrons in the material. For example, for traditional photon energies using AlKa (1487 eV), the mean free path for most core levels is only 5–25 . Because the mean free path of the photoelectrons depends on the kinetic energy, the surface sensitivity can be varied by changing the X-ray energy. The surface sensitivity therefore implies constraints and advantages on the studies of interfaces. Also, the vacuum constraints limit measurements at high pressures, including measurements on the liquid phase. Therefore, to obtain useful information on the interfaces active for energy conversion and specifically for interfaces in molecular solar cells, the experiments discussed below are largely based on combining measurements on model systems with measurements on real systems. Depending on the properties investigated, the samples may be prepared differently, going from very clean conditions [ultrahigh vacuum (UHV)] to preparations very similar to those used in the device preparation. This has been realized by measurements on samples prepared under UHV conditions, samples prepared in glove boxes and transferred to the measurement chamber through a load lock, and samples prepared ex situ (see, for example, refs. [41, 42, 44, 46]). The preparation of samples by electrospray deposition of molecules in solution onto substrates under UHV may also be used to obtain relevant model systems.[67] Moreover, new technical developments in the source, detector, and sample environment allow for new type of measurements approaching the goal of measuring real functional devices. Currently, X-ray science is entering a new era in which powerful photon sources, for example, synchrotron radiation, provide radiation with finely tuned properties with extraordinary performance. This, in combination with new spectrometers for measurements at higher energies (HAXPES), allow for measurements of the electronic and molecular structures at much deeper probe depths (> 10 nm).[75] This technique enables analysis of the bulk electronic properties of the materials and of buried molecular interfaces. The surface sensitivity of PES may be changed by tuning the X-ray energy (as discussed above), which changes the kinetic energy of the emitted electrons.[76] The change in surface sensitivity can therefore be used to characterize the interfaces in the DSC at different surface depths, that is, by using different X-ray energies it may then be possible to understand the geometry of the different interfaces by following the intensities for different atoms at 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org the interfaces. Examples of these types of measurements are given in the sections below. For conducting condensed-phase samples, the Fermi level is often used as a reference level for the binding-energy scale. However, for samples based on thick films of semiconducting nanoparticles (e.g. TiO2), which have rather low conductivity, it is sometimes more difficult to refer the binding energy to the Fermi level due to a small and unknown contribution from charging of the samples when illuminated with high X-ray intensities. Internal calibration using a specific substrate core level is then an option. For example, in measurements on nanostructured TiO2, one of the Ti levels (e.g. Ti 2p) is usually used as a reference level to which all other levels are referenced. It should also be noticed that, upon changing the X-ray intensity, the substrate core-level binding energy might change slightly due to changes to the steady-state condition. Measuring the internal reference level before and after measurements of other core levels allow control over changes in the steady-state surface and is therefore a routine that is recommended in these types of measurements. At very low X-ray intensities on doped semiconducting films and/or for thin samples, it may still be possible to reference the spectra versus the Fermi level. Although the core-level spectrum has the advantage of directly giving atomic-level information, the valence-level spectra are often more difficult to interpret because the molecular orbitals or bands usually consist of contributions from many atoms. The valence electrons are however of great interest because they are directly involved in the formation of chemical bonds and govern many physical properties of the material as well as functional properties for the material combinations. Insight into energy matching between the substrate and the HOMO for different dye molecules on, for example, TiO2 may be obtained directly by comparing the spectra of different molecularly modified samples and using the TiO2 core levels as an energy reference. The position of the energy levels when the molecule is adsorbed at an interface is very useful to understand the device function, as described in the Introduction. In valence-level spectroscopy, quantum chemical calculations are an important tool to understand the spectra. Experimentally it may be possible to obtain increased atomic-level understanding of the orbital compositions from a combination of PES and RPES measurements. RPES implies PES with a photon energy that coincides (resonance) with the excitation energy of a core orbital to an unoccupied orbital in the system. The core-excited states can decay in different ways. In participator decay, the resonantly excited electron participates in the decay of the core hole to the final state with an electron in the continuum. This process results in the same final state as that for nonresonant photoelectron emission and may be observed as an increase in the cross section at a photon energy resonance with a specific core level. In short, RPES can be used to map out the atomic character in the outermost valence level with chemical sensitivity and in one example below we show how it may be used to measure the atomic character of the valence electronic structure of dye molecules.


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CHEMPHYSCHEM REVIEWS 3. PES for Understanding the DSC 3.1. Interfacial Molecular Structures

www.chemphyschem.org were deposited onto TiO2 and the O 1s spectra are shown in Figure 4 (left). In the O 1s spectrum of the multilayer of diisonicotinic acid on TiO2, two different peaks appear corresponding to the two oxygen atoms in the carboxylic acid. When a monolayer of diisonicotinic acid is deposited onto TiO2, only one peak from the carboxylic group and one peak from the TiO2 substrate are observed. This shows that the oxygen atoms in the carboxylic group are in a very similar chemical state, which suggests that the molecule binds with both oxygen atoms in the carboxylic group to the TiO2 surface in a bidentate mode, as shown in the model in Figure 4 (left).[37] The core-level spectra of the complete ruthenium dyes adsorbed on TiO2 have also been studied by using PES. Based on a comparison of findings for the model molecule, diisonicotinic acid (see above), results showed that a similar binding configuration was also possible for the ruthenium dyes.[42] Moreover, for these dyes, the S 2p spectra originating from the NCS group in the dye molecule were analyzed (see Figure 5). The S 2p spectra showed an extra feature (Figure 5) in the spectra

As discussed in the Introduction, the molecular and electronic structure of the interfaces between the materials strongly affects the charge-transfer properties, and therefore, also the solar cell efficiency. In this section, we discuss how electron spectroscopy can be used to study the interface molecular structure of dye-sensitized electrodes. The combination of chemical stability and tunability of the electronic properties of the ground and excited state makes metal–polypyridyls interesting compounds for use in the conversion of light to energy-rich compounds or electricity. Specifically, ruthenium-based dyes are among the most studied and efficient dye molecules used in DSCs. Recently, organic dye molecules with a donor–acceptor structure containing triphenylamine derivatives, as well as the very stable rylene dyes, have also been developed and show promising results as absorbers in DSCs.[2] Examples of a range [N719, Black dye (BD), D5, N3, D35, and ID28] of dyes are shown in Figure 3. PES can be used to investigate the molecular surface structure of these dyes when adsorbed to the TiO2 surface both regarding the specific chemical bond and the general geometrical orientation of the molecule with respect to the surface. Many of these dye molecules are attached to the TiO2 surface through carboxylic acid units. To model the chemical binding of the dyes to the TiO2 surface, the binding of different anchor units adsorbed on TiO2 has been studied in detail by depositing model molecules under UHV conditions.[37, 66] These very clean experimental conditions are necessary to determine the binding of the anchor group to the surface, partly because the main binding is between the oxygen atoms in the anchor group and the TiO2 surface, and contaminants containing oxygen would otherwise disturb the interpretation of the results. In an early study, the chemical bonding of the ruthenium polypyridyls were modeled by investigating diisonicotinic acid on single-crystal TiO2.[37] Both monoand multilayers of the ligand Figure 3. Molecular structures of some of the dye molecules investigated for DSCs. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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of the monolayers of the dyes on TiO2 relative to the spectra of the dye multilayers. A model was suggested in which a fraction of the NCS groups interacted with the TiO2 surface through the sulfur atoms. This mixing in binding configurations is important for understanding the mechanism of the energy conversion in the solar cell because differences in binding configurations are expected to affect the charge-transfer rates at the dye/TiO2 interface. By replacing two protons on the carboxylic groups in the N3 dye with tetrabutylammonium (TBA + ) ions, the N719 dye is obtained; this results in a higher energy conversion efficiency.[77, 78] In a study of N3 and N719, the amount of TBA + counterions relative to the amount of adsorbed dye molecules on the surface was determined by measuring the PES N 1s core level.[42] The results showed that, for N719 on TiO2, there were TBA + ions on the surface, which might have Figure 4. Left: The O 1s PES spectra of mono- and multilayer diisonicotinic acid on (110) rutile TiO2. The spectrum influenced the surface molecular of the multilayer shows two peaks corresponding to the two different oxygen atoms in the carboxylic acid. The configuration of the dye, commonolayer shows only one peak from diisonicotinic acid and one peak from oxygen in the TiO2 substrate; this pared with the N3 dye without suggests that the carboxylic acid binds to the TiO2 in a bidentate configuration.[37] Right: The O 1s PES spectra of multi- and monolayer of maleic anhydride (MA) on (101), (001), and (100) anatase TiO2 are shown together with TBA + ions. The number of TBA + the suggested molecular structure of MA on (101) anatase TiO2 (in the monolayer anatase spectra, the TiO2 subions on dye-sensitized TiO2 is strate peak is subtracted by using the spectra from the clean TiO2 anatase surfaces). Partly reprinted (adapted) however lower than that expectwith permission from ref. [55]. Copyright (2010) American Chemical Society. ed from the concentration in the dye solution, which is important to consider when comparing dyes with different counterions. The effect of water on the N719 dye molecules on the surface has also been studied and the results from PES measurements show that the TBA + counterions are removed from the surface when small amounts of water are added. From the S 2p spectra, it was also possible to conclude that the binding configuration changed when water was added, which therefore also affected the performance of the solar cell.[59] When replacing TiO2 with ZnO, the ruthenium dyes have a strong tendency to aggregate into dye multilayers on the ZnO surface. However, from the PES results, it was found that water in the dye-sensitization process decreased this aggregation, which increased the performance of the solar cells.[60] In an UHV experimental procedure similar to that discussed above, an anhydride anchoring moiety modeling many of the rylene-based dyes for DSC, was studied by investigating MA on the (101), (100), and (001) anatase TiO2 single-crystal surfaFigure 5. The S 2p spectra of dyes on TiO2. Spectra partly reprinted (adapted) ces.[55] Similarly to the experiments discussed above, a compariwith permission from ref. [42]. Copyright (2005) American Chemical Society. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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CHEMPHYSCHEM REVIEWS son between the multi- and monolayer spectra indicates that the oxygen atoms are chemically equivalent when MA is bound to the (101) or (100) anatase surfaces and together with the angle-resolved C 1s spectra the results indicate that the molecule is in a standing configuration on the surface. This suggests that the MA molecule ring opens and forms two bidentate carboxylic bonds to the surface (see Figure 4). One extra oxygen atom originating from the TiO2 surface is required for this mechanism because there are only three oxygen atoms in the MA molecule itself. From these experiments, atomiclevel information on the anchor group of the rylene dyes could therefore be obtained, which is of importance to understand the electron-transfer mechanisms between the dye and the surface. Electron-transfer mechanisms are dependent on both the geometrical relationship between the dye and the surface as well as the bond between the dye and the surface, especially affecting the electronic coupling. Moreover, binding configurations on different facets of the anatase crystal could be investigated in detail and interestingly the (101) and (100) surfaces could accommodate the anhydride anchor group in a similar manner. Information of the general molecular orientation at the surface has also been obtained by PES, although such an interface may be very complex.[47, 52] These investigations were performed on surfaces sensitized with dye molecules from solution. A core level that resolved two peaks originating from different parts of the molecule was measured with different surface sensitivities through variation of the X-ray photon energy (Figure 6). Specifically, this example uses the relative intensity of two nitrogen states. In this way, normalization in terms of Xray intensity is avoided. Recently, such measurements on simple gas-phase molecules have shown large modulations in relative intensities. This was explained by photoelectron backscattering against neighboring sites manifested in variations of the absorption cross section.[79] Clear modulation effects could not be detected for the D5 dye molecules adsorbed at the

www.chemphyschem.org TiO2 surface (Figure 6). This may be expected from a large variation in short-range ordering around the two nitrogen atoms in the molecular layer. However, we observe a clear trend showing a larger difference in relative intensity for high surface-sensitivity measurements, that is, for measurements at lower photon energies. Based on the difference in surface sensitivity, the results for the organic dye molecule (D5) at the TiO2 surface therefore indicate a dominating orientation of the dye with the TAA moiety pointing out from the surface and the cyano group close to the TiO2 (Figure 6).[47] The results given in Figure 6 thus exemplify how changing the surface sensitivity by variation in photon energy can be used to understand the surface molecular structure. Comparing different organic dye molecules based on similar molecular structures, we found some small differences, but the results suggested that the dominating orientation was similar.[52] The configuration of the dye molecules on the TiO2 surface may also be changed by coadsorbent molecules. Coadsorbents are sometimes used to decrease dye aggregation on the surface to improve the performance of the solar cells and PES may be used to investigate the influence of coadsorbents on the surface molecular structure and specifically measure the relative concentration in the mixed molecular layers.[54] For example, in a recent investigation on mixed self-assembled monolayers, hydrophobic coadsorbents were combined with the dye sensitizer. The possibility of translating the peak intensities into molecular surface concentrations was used to show that a particular coadsorbent (NHOOP) replaced a dye molecule in a mixed layer and that each dye molecule was replaced by three NHOOP molecules.[64] 3.2. Valence Molecular Orbital Composition The valence energy levels of the dye molecule are responsible for the light absorption properties and for the electron-transfer reactions occurring during and after light absorption in the

Figure 6. Left: The N 1s core-level spectra of an organic dye (D5) adsorbed onto nanostructured TiO2 measured at different photon energies, see ref. [47]. Spectra partly reprinted (adapted) with permission from ref. [47]. Copyright (2007) American Chemical Society. The variation in relative intensity between the CN peak at 398.5 eV and the triarylamine (TAA) peak at 399.8 eV shows that the TAA unit is pointing out from the surface. Right: The intensity ratio between the CN peak and the TAA peak obtained for different photon energies. The trend, as represented by the dotted line, shows a general decrease in the ratio of measurements with lower photon energies, that is, for measurements with higher surface sensitivity.



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solar cell. Structures observed in the valence spectra of the dyes show large variations in relative intensity when varying the photon energy from soft to hard X-ray energies. By using HAXPES, the relative ruthenium contribution in the valence spectra of the ruthenium dyes is enhanced due to variations in cross section with photon energy. By using this property, we could map out the Ru 4d character in the outermost valence level in terms of energy and density[50, 53] (Figure 7). It was also possible to observe the influence of different ligands on the molecular levels with a ruthenium contribution to the frontier electronic structure.

Figure 8. Valence electronic structure of N719 measured at different photon energies around the N 1s resonance. This resonance experimentally shows that the HOMO level is localized on the NCS ligand and, together with results showing that the LUMO is located on the bipyridine ligands, the results show that light absorption is accompanied with an intermolecular chargetransfer process in which the electron is moving towards the surface. Spectrum partly reprinted with permission from ref. [48]. Copyright (2007) AIP Publishing LLC.

Figure 7. Valence electronic structure of BD. The spectra were measured by using photon energies of 100 and 2800 eV.[50] The two outermost peaks in the spectrum are attributed to the Ru 4d contribution in the molecular orbitals in the valence electronic structure. Spectra partly reprinted (adapted) with permission from ref. [50]. Copyright (2008) Elsevier.

Complementary studies to determine contributions to the electronic structures from the different ligands were also investigated by using RPES.[48] RPES implies PES with a photon energy that coincides with the excitation energy of a core orbital to an unoccupied orbital in the system. The core excited states can decay in different ways. In participator decay, the resonantly excited electron participates in the decay of the core hole to the final state with an electron in the continuum. This process results in the same final state as that for nonresonant photoelectron emission. Thus, for example, for the N719 molecules, this implies that the N 2p contribution to the valence band from a specific ligand (either N 2p in NCS or N 2p in the bipyridine) will be enhanced when exciting resonantly from the N 1s core level of the same ligand (see Figure 8). In short, RPES can be used to map out the atomic character in the outermost valence level in terms of energy and density. For the ruthenium dyes N719, N3, and BD, there are two valence peaks observed at low binding energy (Figure 7). By using RPES, the HOMO was determined to be a mix of the center ruthenium and the NCS ligands. The experimental results suggested that the character of the outermost valence structures contained a mixture of Ru and NCS ligand contributions in a ratio of 2:1, whereas the second valence peak contained a mixture of Ru and ligand contributions (mainly NCS) in a ratio of 1:2.[48, 50] Thus, the highest occupied states have an 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

important NCS contribution and, since the lowest unoccupied states are centered at the bipyridine ligands, the light absorption in these ruthenium dyes therefore have a large chargetransfer character. The contribution from the metal in the outermost electronic structure of metal–organic complexes may also be determined by using resonant HAXPES. Figure 9 shows the valence spectra of [Ru(bpy)3]2 + (bpy = 2,2’-bipyridine) measured by using three different photon energies.[53] At a photon energy of 2841 eV, there is a clear resonance corresponding to excitation from Ru 2p to unoccupied energy levels, resulting in an enhanced emission of electrons from the Ru 4d contribution in the valence electronic structure.

Figure 9. Valence electronic spectra of [Ru(bpy)3]2 + measured by using photon energies of 2800, 2841, and 2850 eV. Spectra partly reprinted (adapted) with permission from ref. [53]. Copyright (2010) American Chemical Society.


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CHEMPHYSCHEM REVIEWS Combining HAXPES measurements with RPES and X-ray absorption spectroscopy, we could obtain a diagram of the frontier electronic structure of metal organic dyes, and determine the composition of the different orbitals.[53] This knowledge of the molecular orbital composition and relative energies is especially important to understand light absorption and electron-transfer reactions in these molecules, which is useful in the design of new dye molecules. In general, the results on valence molecular orbital composition, together with information on interfacial molecular structures, give unique experimental evidence of the notion that the most efficient dye molecules contain a charge-transfer absorption with an acceptor state close to the oxide surface and a donor state further out from the surface. This molecular structure of the dye therefore results in efficient electron injection, since the excited electron is close to the oxide surface. The hole is instead located further away from the oxide surface, which decreases the rate of recombination of the electron in the oxide with the hole on the dye. Also, the hole will be close to the redox couple in the electrolyte, which increases the rate of regeneration of the dye (electron transfer from the redox couple to the dye), which is advantageous for the efficiency of the solar cell. 3.3. TiO2/Dye Energy-Level Matching

www.chemphyschem.org between the HOMO level and the substrate. The outermost valence structure of D5 shows a peak that is shifted about 0.2 eV towards higher binding energy in comparison with the peak for N3. This shift is different from that expected from electrochemical measurements of the redox potential in solution, which is more similar for the dyes. The different results from the electrochemical and PES measurements may occur for several reasons. One reason is that the electrochemical measurements are performed in solution, whereas the PES measurements are performed in vacuum, which would certainly affect the results. Another difference is that the electrochemical measurement contains structural changes for atoms in the molecule and the solvent molecules, whereas during the photoemission of the electron in PES, the molecular structure is approximately constant. Comparing a series of different TAA dye molecules, a shift of the highest occupied electronic structure can be observed for different substitutions on the TAA group.[47, 52] These shifts determine energy-level matching between TiO2 and the dye, and to understand and characterize these shifts is therefore important in the optimization of the solar cell. As explained above, precise energy matching between the dye and TiO2 will affect the injection of excited electrons from the dye to TiO2 and also partly determine energy loss in the solar cell. The electronic structure of the TiO2 nanoparticles contains band-gap states, which may be important for the function of the solar cell. PES has been used to study these band-gap states both on nanoporous TiO2[41] and on single-crystal (101), (100), and (001) anatase TiO2 surfaces.[55] It was found that these band-gap states could be found over the entire band gap and that they were filled with electrons up to an energy very close to the energy position we expect for the conduction band edge. It is therefore suggested that the Fermi level is pinned to the conduction band edge in the nanoparticles under these conditions, which means that there are electrons throughout the band gap up to the conduction band edge, at

Energy matching between the energy levels of the dye molecules and the energy levels of the TiO2 substrate is crucial for electron-transfer processes from the dye to the TiO2 surface that occur after light absorption. In an efficient solar cell, the conduction band edge of TiO2 should have an energy that is lower than the energy of the excited electron in the dye molecule to enable transfer of the electron from the dye to TiO2. On the other hand, the energy difference between the TiO2 conduction band edge and the excited electron in the dye should not be too large, since this energy difference will be lost as heat in the solar cell. An example of using PES to measure this energy-level matching is shown in Figure 10, in which the organic D5 dye and different ruthenium dyes on TiO2 are compared.[42, 47] The valence PES shown in Figure 10 shows that the binding energy of the highest occupied orbitals of N3 and N719 are higher than the binding energy of the highest occupied orbitals of BD when adsorbed onto TiO2, which may be expected from the redox potentials of the dye molecules. Interestingly, the relative energylevel matching of N3 versus TiO2 and N719 versus TiO2 are similar; this indicates that the presence of TBA + on the TiO2 surface Figure 10. A sketch of the valence electronic structure of a dye-sensitized TiO2 surface, together with PES spectra does not affect energy matching of the valence electronic structure of TiO2 sensitized with the dyes D5, N719, N3, and BD.[42, 47] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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which the density of states increases rapidly. It was also found that, when a lithium salt was added to the surface of the nanoparticles, the intensity of the valence electrons in the band gap decreased. This represents a direct measurement that reveals the removal of surface-state electrons.[57] Such states influence the electron-transfer reactions between TiO2 and the dye, and may explain changes in photoconversion efficiency when a lithium salt is added. The injection of electrons from excited N3 dye molecules was investigated by RPES in a model system.[65] In the model system, the ligand diisonicotinic acid was deposited on a TiO2 single crystal in UHV, and electron injection from the ligand to TiO2 was studied by using the “core-hole clock”. In this method, RPES is used and the injection time of the electron is compared with the decay of core hole in RPES. By using this model method, an extremely fast electron injection of only a few femtoseconds was obtained. The quick injection of electrons limits the loss of energy in excited-state recombination for this dye, which may be a problem for some dyes. 3.4. Complete TiO2/Dye/Hole-Conductor Heterojunctions The TiO2/dye/hole-conductor interfacial regime is the key functional entity in the solar cell. The photovoltaic properties of the solar cell largely depend on the interfacial electronic and molecular structure between the different materials in this interfacial regime. This interface is difficult to measure with PES because TiO2 and the dye are buried under the holeconductor material and PES is a very surface sensitive technique. One way to follow effects in a complete interface is to evaporate very thin layers of the hole conductor on the dye-sensitized TiO2. This procedure Figure 11. Illustration of the molecular interface structure of TiO2/dye/hole-conductor inmakes it possible to observe the structure and follow terfaces with three different hole conductors, as suggested from PES measurements.[46] In changes in the layers beneath the hole conduc- A) a small hole-conductor molecule is located between the dye molecules ; in B) and tor.[44, 46] For example, hole conductors based on TAA C) the hole-conductor molecules are located further away from the TiO2 surface, and parunits were evaporated onto the dye-sensitized sur- ticularly in (C) the larger hole-conductor molecules are on top of the dye molecules, as suggested by PES measurements. face and the energy levels and the molecular structure of the complete system could be investigated. As an example of the results from such a preparation, it was Fermi level, it was possible to show that the energy levels of concluded that the hole-conductor molecules affected the dye the hole conductor close to the TiO2 surface were shifted (simigeometry on the surface and that small hole-conductor molelar to band bending in inorganic materials).[49] The shift of the cules could squeeze in between the dye molecules, whereas energy levels of the hole conductor was represented by the larger hole-conductor molecules were located over the dye “energy-level bending” at the TiO2 surface, which could act as molecules (see Figure 11).[46] In solar-cell devices with smaller a hole trap, holding the photogenerated holes at the hole-conductor/TiO2 interface. These holes may then recombine with hole-conductor molecules substantial losses were observed. This can be a result of enhanced recombination due to the the electrons in TiO2. Such an energy structure may further exshort distance between the small hole conductor and the TiO2 plain the loss in photoconversion efficiency. The use of HAXPES also represents a procedure to obtain surface. photoemission electronic structural information from complete Further support that energy matching facilitated recombinainterfaces.[61, 75] As one example, energy-level matching in tion between hole conductors and TiO2 was obtained from UHV measurements on the pure TiO2/hole-conductor interface. a system containing TiO2 and the hole-conductor poly(3-hexBy following work functions and core-level shifts versus the ylthiophene-2,5-diyl) (P3HT) was studied.[61] The change in 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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www.chemphyschem.org the system without the dipole molecules (see Figure 12).[61] The results from the HAXPES measurements are consistent with a simple theoretical model (see Figure 13). In the model, the dipole molecules shift the levels of the polymer hole conductor relative to the TiO2 substrate and, by changing the dipole moment from positive to negative, the polymer holeconductor levels are shifted in the opposite direction. By using the effect of inserting dipole molecules at the interface, it may be possible to control the energy levels in the complete TiO2/dye/hole-conductor system to maximize the photovoltaic performance of the solar cell. For example, by controlling the dipole moment of the dye molecule, or by using coadsorbents with a dipole moment, electron injection may be efficient without a large loss in energy.

4. Summary and Outlook

Figure 12. The S 2p and Ti 2p HAXPES spectra for samples with different dipole molecules between TiO2 and P3HT: a) 4-nitrobenzoic acid, b) without dipole molecule, and c) benzoic acid. Spectra partly reprinted (adapted) with permission from ref. [61]. Copyright (2011) Elsevier.

In the quest for higher performance solar cells, there is a need for a better understanding on the atomic level to be able to find routes for improvement. To achieve this, X-ray-based techniques will be of key importance. We have described examples of how PES can be used to investigate DSCs, including binding configurations, molecular orbital composition, and energy matching at an atomic level. Specifically, we have demonstrated how the PES results can be used to determine how dye molecules bind to the surface of TiO2, the geometry of the dye on the TiO2 surface, and how the coverage of dye molecules compares to coadsorbents in mixed layers. The molecular orbital composition was also investigated by using HAXPES and RPES; these results are important to understand light absorption in the dye and to understand the energies of the orbitals from different parts of the dye molecule. Specifically, the results show that efficient dye molecules have a charge-transfer absorption with an acceptor state close to the oxide surface and a donor state further out. This structure of the dye on the surface is preferable for efficient electron injection and also reduces the recombination rate of the injected electrons with the holes on the dye molecules further out from the surface.

energy matching was followed when incorporating different dipole molecules (benzoic acid or 4-nitrobenzoic acid) between TiO2 and P3HT. By using HAXPES it was possible to detect photoelectron emission from TiO2 through the P3HT layer, and therefore, to measure energylevel alignment in systems with different dipole molecules. Energy-level alignment was followed by measuring the core levels corresponding to TiO2 (Ti 2p) and P3HT (S 2p). By changing the dipole moment of the dipole molecule between TiO2 and P3HT, it was possible to change the energy-level alignFigure 13. Illustration of a simple model of energy-level alignment in the TiO2/dipole/P3HT system. Left: The ment, so that the energy levels system without a dipole molecule between TiO2 and P3HT. Middle and right: Two different dipole molecules inof P3HT were shifted both posi- serted between TiO2 and P3HT, resulting in different energy-level alignments at the interface. Figure partly reprinttively and negatively relative to ed (adapted) with permission from ref. [61]. Copyright (2011) Elsevier. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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CHEMPHYSCHEM REVIEWS During solar cell operation, the charge-transfer processes are essential for light-to-electricity conversion and these chargetransfer processes are closely related to the energy levels of the materials at the interface. Measurements of energy-level alignment between the materials may therefore lead to new insights into how to improve the charge-transfer processes for photocurrent generation and how to reduce the recombination currents. The results of energy-level mapping of the dyesensitized TiO2 electrodes, and also for the complete photoactive TiO2/dye/hole-conductor interfacial regime, were also described. Energy-level alignment for different dye molecules adsorbed on the TiO2 surface can be used to understand differences in charge injection from the dye to TiO2 and charge regeneration of the dye. The complete TiO2/dye/hole-conductor interface could also be measured by using different techniques. The molecular structure of the hole-conductor molecule influenced the total interface molecular structure, which also had important effects on the performance of the solar cell. By using HAXPES, it was also possible to measure the effect of dipole molecules between TiO2 and the hole conductor. By using different dipole molecules, the relative energy of the core levels of TiO2 and P3HT were controlled; this may be useful, for example, to maximize the photovoltage in the solar cell. Further development of X-ray spectroscopic techniques will further widen the scope to obtain atomic-level information in real functional devices. Time-resolved studies using pulsed Xray sources, such as high harmonic generation (HHG) of X-rays or free electron lasers (FELs), makes it possible to obtain sitespecific (atomic) information on processes on a femtosecond timescale; this is of interest for the detailed fundamental understanding of the charge-transfer processes in the solar cell. PES of liquids and PES under high-pressure conditions are techniques that are now developing and will be useful to study the key interfaces under realistic conditions. In summary, it is to be expected that X-ray-based spectroscopy methods will be increasingly important for the future development and understanding of solar-cell materials.

Acknowledgements This work has been supported by the Carl Trygger Foundation, the Swedish Research Council (VR), the Gçran Gustafsson foundation, the Swedish foundation for Strategic Research (SSF), the Swedish Energy Agency, and the Knut and Alice Wallenberg Foundation. Keywords: dyes/pigments · interfaces · nanostructures · photoelectron spectroscopy · solar cells [1] B. O’Regan, M. Grtzel, Nature 1991, 353, 737. [2] A. Hagfeldt, G. Boschloo, L. C. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595. [3] H. J. Snaith, L. Schmidt-Mende, Adv. Mater. 2007, 19, 3187. [4] A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Gratzel, Science 2011, 334, 629 – 634.


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Received: November 15, 2013 Published online on && &&, 2014


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REVIEWS E. M. J. Johansson,* R. Lindblad, H. Siegbahn, A. Hagfeldt, H. Rensmo* && – && Atomic and Electronic Structures of Interfaces inDye-Sensitized, Nanostructured Solar Cells Core of the matter: Key processes in nanostructured dye-sensitized solar cells (DSC) occur at material interfaces containing, for example, oxides, dye molecules, and hole conductors (see picture). The implementation of X-ray-based

spectroscopic methods for atomic-level understanding of such properties is reviewed. Examples include energy matching, binding configurations, and molecular orbital composition.



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Nanostructured Solar Cells.

Interfaces in perovskite solar cells.

organic interfaces in organic solar cells.

Atomic and electronic structures of an extremely fragile liquid.

Atomic layer deposition in nanostructured photovoltaics: tuning optical, electronic and surface properties.

Metal-organic frameworks at interfaces in dye-sensitized solar cells.

Effect of interface atomic structure on the electronic properties of nano-sized metal-oxide interfaces.

The generalized Shockley-Queisser limit for nanostructured solar cells.

Understanding the electronic structures and absorption properties of porphyrin sensitizers YD2 and YD2-o-C8 for dye-sensitized solar cells.

graphite interfaces by electrochemical frequency modulation atomic force microscopy.

Electronic and atomic structures of the Sr3Ir4Sn13 single crystal: A possible charge density wave material.

Energetic stability, atomic and electronic structures of extended γ-graphyne: A density functional study.

DFT and TD-DFT study on geometries, electronic structures and electronic absorption of some metal free dye sensitizers for dye sensitized solar cells.

Unified Electromagnetic-Electronic Design of Light Trapping Silicon Solar Cells.

Acceptor Interfaces in Organic Solar Cells.

Conditions for diffusion-limited and reaction-limited recombination in nanostructured solar cells.

Materials and interfaces in quantum dot sensitized solar cells: challenges, advances and prospects.

Role of surface recombination in affecting the efficiency of nanostructured thin-film solar cells.

Investigation of size-dependent cell adhesion on nanostructured interfaces.

Interaction of Sensitizing Dyes with Nanostructured TiO2 Film in Dye-Sensitized Solar Cells Using Terahertz Spectroscopy.

Progress and Design Concerns of Nanostructured Solar Energy Harvesting Devices.

Inverted Silicon Nanopencil Array Solar Cells with Enhanced Contact Structures.

Development of Efficient and Stable Inverted Bulk Heterojunction (BHJ) Solar Cells Using Different Metal Oxide Interfaces.

Design and realization of transparent solar modules based on luminescent solar concentrators integrating nanostructured photonic crystals.