Ultrafast Electron Dynamics in Solar Energy Conversion - Chemical

Review pubs.acs.org/CR

Ultrafast Electron Dynamics in Solar Energy Conversion Carlito S. Ponseca, Jr.,† Pavel Chábera,† Jens Uhlig,† Petter Persson,‡ and Villy Sundström*,† †

Division of Chemical Physics, Chemical Center, and ‡Theoretical Chemistry Division, Chemical Center, Lund University, Box 124, Lund SE-221 00, Sweden ABSTRACT: Electrons are the workhorses of solar energy conversion. Conversion of the energy of light to electricity in photovoltaics, or to energy-rich molecules (solar fuel) through photocatalytic processes, invariably starts with photoinduced generation of energyrich electrons. The harvesting of these electrons in practical devices rests on a series of electron transfer processes whose dynamics and efficiencies determine the function of materials and devices. To capture the energy of a photogenerated electron−hole pair in a solar cell material, charges of opposite sign have to be separated against electrostatic attractions, prevented from recombining and being transported through the active material to electrodes where they can be extracted. In photocatalytic solar fuel production, these electron processes are coupled to chemical reactions leading to storage of the energy of light in chemical bonds. With the focus on the ultrafast time scale, we here discuss the lightinduced electron processes underlying the function of several molecular and hybrid materials currently under development for solar energy applications in dye or quantum dotsensitized solar cells, polymer−fullerene polymer solar cells, organometal halide perovskite solar cells, and finally some photocatalytic systems.

CONTENTS 1. Introduction 2. Experimental and Theoretical Methods 2.1. General Experimental Considerations 2.2. Time-Resolved Techniques 2.2.1. Time-Resolved Optical Spectroscopy 2.2.2. Time-Resolved Infrared Spectroscopy 2.2.3. Time-Resolved THz Spectroscopy: Transient Photoconductivity Measurements 2.2.4. Time-Resolved X-ray Spectroscopy and Scattering 2.3. Advanced and Emerging Experimental Characterization 2.3.1. Coherent Control 2.3.2. Multidimensional Spectroscopy 2.3.3. Time-Resolved Microscopy 2.3.4. Time-Resolved Spectroelectrochemistry 2.4. Theoretical and Computational Considerations 3. Solar Cell Technologies 3.1. Dye-Sensitized Solar Cells 3.1.1. Electron Injection from Sensitizer to Metal Oxide Acceptor 3.1.2. Electron−Cation Recombination 3.1.3. Formation of Mobile Charges 3.1.4. Earth Abundant Metal-Based Photosensitizers 3.1.5. Organic and Push−Pull Dyes 3.2. Quantum Dot-Sensitized Solar Cells 3.2.1. Photoinduced Electron Injection 3.2.2. Multiple Exciton Generation

© 2017 American Chemical Society

3.2.3. Charge Recombination in Quantum Dot−Metal Oxide Systems 3.3. Organic Solar Cells 3.3.1. Charge Generation 3.3.2. Charge Carrier Recombination 3.3.3. Carrier Photoconductivity and Mobility 3.4. Perovskite Solar Cells 3.4.1. From Carrier Generation to Recombination 3.4.2. Ion Dynamics Influencing Carrier Dynamics 3.4.3. Charge Carrier Transport and Mobility 3.4.4. Charge Carrier Dynamics of Perovskite/ Transport Layer Devices 3.4.5. Perovskite Single-Crystal Carrier Dynamics 3.4.6. Outlook for Organo-Metal Halide Perovskites 3.5. Advanced and Emerging Solar Energy Technologies 3.5.1. Solid-State Nanomaterials 3.5.2. Molecular Antennas 3.5.3. Singlet Fission 3.5.4. Triplet−Triplet Annihilation Photon Upconversion 4. Photocatalysis 4.1. Molecular Photocatalysis 4.1.1. Sensitizers

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Special Issue: Ultrafast Processes in Chemistry Received: December 1, 2016 Published: August 14, 2017 10940

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Chemical Reviews 4.1.2. Molecular Donor−Acceptor Systems 4.1.3. Proton Coupled Electron Transfer 4.2. Heterogeneous Photocatalysis 4.2.1. Quantum Dot and Nanomaterial Heterostructure Photocatalysts 4.2.2. Photocatalytic Surface Electron Transfer Processes 4.2.3. Hybrid Molecular-Semiconducting Photocatalysts 5. Summary and General Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

Review

and processes under discussion to provide the more ambitious readers with ample opportunities to find further details, and more in-depth accounts of many interesting topics beyond the scope of this Review. By focusing on a more select set of results from several different systems often treated separately, however, it becomes possible to highlight both common features and intrinsic differences between several different approaches for solar energy conversion that together provide an opportunity for comparisons and cross-fertilization between different areas that hopefully will provide both a sense of the current state-ofthe-art for the field as a whole and an outlook for emerging opportunities for further research in many different directions. In photovoltaic materials, photons are converted to charges that are extracted from the active material into an external circuit where they perform work. For solar fuel production, the energy of light is stored in chemical bonds of energy-rich molecules. The overall energy converting events are comprised of a series of steps, where the characteristic time scale of the different steps varies from femtoseconds to microseconds, milliseconds, and sometimes even longer. In this Review, focus will lie on the ultrafast time scale of the solar energy converting processes, typically encompassing energy transfer, charge generation, charge migration and transport, charge recombination, change of electron spin, and molecular structure. Later steps in the conversion processes, like charge extraction from a solar cell device, or slow catalytic processes in a solar fuel device will not be discussed in detail. Several of the PV systems discussed here consist of a lightharvesting material with relatively high exciton binding energy, and exciton dissociation and charge generation is frequently accomplished with the help of an electron-accepting material. This is in contrast with the situation in traditional semiconductor photovoltaics (e.g., silicon), where electrons and holes generated by light absorption are separated by the electric field created by the depletion zone formed at the p−n junction between a p-doped and n-doped silicon layer. For polymer solar cells, a conjugated polymer is the light-harvesting (LH) material, and a modified fullerene generally acts as electron acceptor. In a dye (or quantum dot, QD)-sensitized cell, a dye molecule (or semiconductor QD) acts as light harvester, and the excitons are split at the interface between the dye (QD) and a metal oxide (e.g., TiO2 or ZnO) nanoparticle. In the recently appeared organometal halide perovskite (OMHP) solar cells, the perovskite originally had the function of a sensitizer, and a nanostructured metal oxide was used as electron acceptor. More recently, perovskite thin film solar cells, more similar to traditional semiconductor cells, have also appeared where the perovskite itself acts as both electron and hole transporter. As an introduction to the more detailed discussion below of the electron dynamics and the function of several different materials, we highlight some key processes and features for each material. The structures and organization of the discussed materials are illustrated in Figure 1, along with the organization of a typical photosynthetic light-harvesting-reaction center unit, which has served as a model for several of the synthetic systems. The lower part of Figure 1 serves to illustrate the lightinduced processes we will discuss below for several different materials. The figure visualizes the processes following light absorption using an organic solar cell as an example. Light is absorbed by the conjugated polymer network, and energy transfer brings it to the interface between polymer and the fullerene electron-accepting material where the exciton dissociates into an electron−hole pair. The charges of opposite

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1. INTRODUCTION For practical use, solar energy can be converted to heat, electricity, and energy-rich chemicals (fuel). Conversion to electricity and fuel is perhaps the most interesting and valuable because these forms of energy can be used to power most of the needs of our societies.1−3 Solar cells already exist for the production of electricity, and research level PV-powered catalytic systems for water oxidation and hydrogen production have been developed.4,5 There has also been a lot of recent progress on direct photocatalysis for solar fuel production, but this field is still in its infancy and large-scale solar fuel production is still lacking.6 Silicon solar cells are dominating the commercial solar cell market, but many novel materials are under development and investigation in research laboratories. In this Review, we will discuss several of these new organic and hybrid materials−organic polymer solar cells, dye- and quantum dot (QD)-sensitized solar cells, and organometal halide perovskite solar cells. Presently, there are extensive efforts to develop photocatalytic systems for production of energy-rich molecules (solar fuel) such as molecular hydrogen or simple alcohols by light-driven electron extraction from water and reduction of protons, or, for instance, carbon dioxide, to form the fuel. Here, we will discuss some efforts to study electron and structural dynamics in some photocatalytic systems. The development of new materials and technologies for solar energy conversion includes many areas of chemistry and physics, design and synthesis of materials, experimental and theoretical characterization of materials and processes, as well as device construction and characterization. The focus of our account will be on experimental results on ultrafast carrier dynamics in a few different solar energy conversion materials, but when appropriate we will also discuss theoretical and computational results relevant for understanding the experimental observations. The scope of the topic for this Review is still very broad, and in fact the ultrafast electron dynamics of several types of solar energy conversion materials/devices in many cases already have been subject to extensive reviews each by itself. The ambition with this Review is therefore not primarily to attempt to repeat the full content of several previous reviews in one place, but rather to highlight a number of key aspects judged to be of particular current interest or lasting impact for several different systems. In doing this, we greatly benefit from the opportunity to point to many excellent reviews available for several of the particular methods, systems, 10941

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sign are then transported through the two different material phases toward the external electrodes. The materials of the various solar cell technologies we will discuss are chemically quite different and have different morphology, but the lightinduced processes are often remarkably similar. The processes underlying the function of a photocatalytic system also have many similarities with those in a photovoltaic material. To conclude the photocatalytic processes, there are, in addition, chemical reactions storing the energy of light in chemical bonds. The vast success of natural photosynthesis (Figure 1, top left panel) to harvest and utilize solar energy continuously as the major source of energy for all life on earth over billions of years has served as an inspiration for the search for inexpensive and renewable solar energy conversion alternatives,4 where a full understanding of natural photosynthesis down to ultrafast time scales has for a long time remained a broad and fascinating research topic in itself,7 although not discussed further in this Review. Many different types of nanostructured thin film materials, and combinations of materials, are considered in the search for efficient, cheap, and environmentally benign solar cells. The introduction of dye-sensitized solar cells (DSC)8 in the early 1990s was a major step, and intense development work in many different directions has led to solar cells with power conversion efficiencies now approaching 15%.9 Here, we will briefly describe the development of the field and then give a more detailed account of some recent work aiming at describing dye−semiconductor sensitizer binding geometry and develop strategies to obtain better defined sensitizer/metal oxide interfaces that would lead to more predictable PV properties. We will also discuss recent work aiming at the development and characterization of sensitizers based on earth abundant elements. The ultimate aim of this work is to develop methods to produce inexpensive sensitizer/semiconductor materials with efficient and predictable light-harvesting and electron transfer properties suitable for large-scale applications. Recently, this field took an exciting new direction when organo-metal-halide perovskites (OMHP) were introduced as sensitizers,10−12 and after only a few years of development, solar cells based on such perovskites now have reached remarkable power conversion efficiencies of over 20%.13,14 Despite this very fast progress in material and device performance, many questions regarding the fundamental properties underlying this astonishing progress remain unanswered. Here, we will summarize recent results on ultrafast exciton and charge carrier dynamics in these novel materials. Yet another direction within the DSC field is to replace the often used Ru-sensitizers with earth abundant and benign materials. Fully organic dyes,15−17 or Zn-porphyrins9,18,19 are approaches that have been successfully explored. Another strategy to achieve this goal is to replace the Ru-based dyes with analogues based on iron (Fe),20,21 copper (Cu),22 and other earth abundant metals.23 Until very recently, this work was only rewarded with modest success. By replacing the conventionally used polypyridyl ligands in iron-based transition metal complexes with N-heterocyclic carbenes (NHCs), it has been possible to increase the lifetime of the triplet metal-to-ligand charge transfer (3MLCT) excited state by a factor of more than 100 as compared to conventional Fe-polypyridyl complexes.20 With this increase of the MLCT lifetime, Fe-based sensitizers are becoming of interest for DSC, and similar progress has occurred for Cu-based sensitizers.22,24

Semiconductor nanocrystals, so-called quantum dots (QDs), have emerged as a viable alternative to replace the dye molecules in DSC to form QD-sensitized solar cells (QDSC).25 In QDs, a continuum of states in the conduction and valence bands of a semiconductor transforms into sets of discrete energy levels due to quantum confinement of carriers in the tiny volume of a “dot”. The level spacing qualitatively follows the particle-in-a-box principle. In this way, the absorption spectrum of the QD can be easily tuned by changing the size of the dots. The term “rainbow solar cell” was coined26 to visualize the possibility of covering the whole solar spectrum by this approach. Another interesting possibility is the ability of QDs to generate more than one electron−hole pair from a single high energy photon.27 Such multiple exciton generation (MEG) allows the Shockley−Queisser thermodynamic limit of single junction solar cells to be overcome.28 The power conversion efficiency of organic polymer:fullerene solar cells has been dramatically increased over the last 10−15 years, from ∼2%29−31 to now over 10%.32 This progress is a result of material development and improved understanding of how factors like material morphology and carrier dynamics correlate to device efficiency. Nevertheless, there are many gaps in our understanding of how light energy absorbed by the active solar cell material is converted into useful photocurrent. For an efficient solar cell, the energy of light has to be converted into free mobile charges that can be extracted from the active material into the electrodes of the solar cell. There is generally a good understanding and more or less consensus of what are the main steps of free charge formation, as illustrated in Figure 1. Polymer excitons are formed through light absorption, and they break up into a closely bound charge pair (also called CT state by many authors) on the ultrafast subpicosecond time scale.33 This charge pair then dissociates into eventually free noninteracting mobile charges. During this dissociation process, charges may recombine geminately, and free charges formed through separate light absorption events may also recombine in a nongeminate process, on a time scale determined by the carrier density. There is a great need for storage of solar energy. As mentioned above, this could be done by PV driven catalysis to generate a fuel,4,5,34 or through direct photocatalysis of water, mimicking the photosynthetic primary processes, to generate, for example, molecular hydrogen or a small-molecule carbonbased fuel. Here, we will briefly discuss a few different material options and with the help of a few examples highlight key lightinduced processes. In the case of molecular photocatalysis, we will show how a combination of ultrafast techniques, including optical and X-ray spectroscopies, as well as X-ray diffuse scattering, can give species and state-specific information on the reaction mechanisms.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. General Experimental Considerations

Time-resolved spectroscopy as we know it today was developed in the 1950s and early 1960s as flash photolysis and relaxation methods, a development that culminated with the Nobel Prize in chemistry 1967 to Porter, Norrish, and Eigen.35 This work gave us the millisecond and microsecond time scales. With the advent of lasers and Q-switching and mode-locking techniques, a giant leap into the nano- and picosecond domain occurred. Finally, mode-locking of dye lasers and Kerr lens mode-locking of the titanium-sapphire laser gave us the femtosecond time 10942

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Figure 1. Schematic illustrations of selected materials for emerging solar energy conversion (top). Top, from left to right: Photosynthetic antennareaction center protein complexes have often served as inspiration and models for synthetic systems, like the polymer−fullerene blends, or dye− semiconductor nanoparticle composites of polymer and dye-sensitized solar cells. In these materials, the polymer (dye) plays the role of the antenna, and the fullerene (metal oxide nanoparticle) has the electron-accepting reaction center function. In QD-sensitized and organo-metal-halide solar cells, the semiconductor QD and the perovskite have the role of light absorber; the perovskite distinguishes itself from the other materials in that it exhibits internal charge separation. In some more detail, the bottom panel illustrates the organization of a polymer−fullerene organic solar cell material and the flow of energy and charge through the material. We thank Eva Unger and Ivan Scheblykin for permission to use these figures.

Figure 2. Photon energies, frequencies, and wavelength from X-rays to THz related to typical molecular excitation processes. Mc.h.+, M+ and M* stand for the ionized molecule with a core hole, the ionized molecule with a hole in the valence region, and an excited molecule respectively.

kinetics are measured. Using a variable wavelength or a broadband probe pulse, a time-resolved (transient) spectrum can be measured. With the broad range of ultrashort pulse wavelengths available, virtually any molecule and/or material can be excited and process initiated, and most molecular and material absorptive or emissive responses monitored down to femtosecond time scales and beyond. This type of experiment is generally termed pump−probe measurements and is very versatile due to the flexibility in choice of pump and probe pulses.36 By probing coherences involving vibrational or electronic transitions in multipulse pump−probe measurements, the range of molecular and material responses and properties accessible to study is further broadened.37−40 Reviews and books can be found providing excellent descriptions of most ultrafast measurement techniques.41−44 We provide here a brief survey to illustrate the wide range of techniques that are now available for most wavelength regimes. Several techniques like time-resolved THz spectroscopy and ultrafast X-ray spectroscopy and scattering with sufficient

scale. A parallel development of pulse amplification and compression techniques and implementation of various nonlinear optics techniques broadened the wavelength range of ultrashort pulses to cover the spectral region from the infrared to the UV. Today, with the availability of large-scale facilities, specializing in the generation of ultrashort pulses in a wide spectral range, like free electron lasers, or high power laser facilities, femtosecond pulses can be generated from THz frequencies to hard X-rays. This enables the probing of light− matter interactions over a broad range of time-, energy-, and length-scales as illustrated in Figure 2. 2.2. Time-Resolved Techniques

Time-resolved experiments to monitor the temporal evolution of chemical reactions still rely on the principles already introduced through flash photolysis, an excitation pulse that initiates the reaction and a delayed probe pulse to monitor the progress of the reaction. With the help of a variable time delay between the pump and probe pulse, the excitation-induced 10943

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Figure 3. Modified Jablonski diagram for transient absorption (TA) spectroscopy showing initial (pump) excitation (absorption) as well as transient absorption (TA) together with typical excited-state relaxation processes including fluorescence, phosphorescence, intramolecular vibrational relaxation (IVR), internal conversion (IC), and intersystem crossing (ISC).

concentrations after each laser pulse, it is sufficient to lie at pulse energies below the limit of nonlinear intensity dependence of the different processes. Typically, experiments can be easily conducted below the thresholds of two-photon absorption, exciton annihilation, and nongeminate charge recombination. However, it is important to remember that the onset of these nonlinear processes is material dependent, so in each case it is important to establish the “safe” intensity region where they do not contribute to the measured dynamics. On the other hand, systematic exploitation of these effects may be a useful way to obtain additional dynamic information. The excitation pulse repetition rate is another important parameter and must be low enough that the system relaxes to the starting ground state before the next pulse arrives to the sample unless the sample is replaced between excitation pulses. Ideally, the same background concentration of intermediates as under solar conditions should be aimed at because this controls, for instance, effects such as trap filling or diffusion limited processes, an important aspect for e.g., solar cell function.46−49 2.2.2. Time-Resolved Infrared Spectroscopy. Ultrafast time-resolved infrared vibrational spectroscopy (TR-IR) is a widely useful technique to investigate molecular and material dynamics.50 Progress in use of TR-IR and related techniques to investigate connections between photoinduced charge transfer and structural/vibrational processes has been used extensively to study a wide range of materials and interfaces,51−53 and has recently been highlighted by Weinstein and co-workers,54 as well as Vauthey and co-workers.55 The application of TR-IR to investigations of solar energy conversion involving metal oxides and interfaces, with several examples for DSC type systems, was also recently discussed by Furube,56 and Lian and coworkers.52,57,58 Femtosecond stimulated Raman spectroscopy (FSRS) and closely related techniques such as femtosecond inverse Raman spectroscopy (FIRS) also provide capabilities to investigate photoinduced structural dynamics in a wide variety of materials and molecular systems, including such processes as charge and electron transfer in push−pull chromophores and charge transfer states in metal complexes relevant for solar energy conversion applications.59 Advanced method extensions of considerable interest include recent development of both

temporal resolution have only recently emerged as powerful tools of broad significance for monitoring electron dynamics in photovoltaic materials and catalytic systems. They are therefore presented a bit more comprehensively. We also provide a brief account of some theoretical and computational approaches often used in the study of solar energy converting materials, again largely as an opportunity to provide pointers to many excellent reviews for further reading, rather than for a complete coverage that is well beyond the scope of this Review. 2.2.1. Time-Resolved Optical Spectroscopy. The development of time-resolved laser spectroscopy operating in the UV−visible part of the spectrum provided a natural entry point to investigate dynamics of photoinduced solar energy conversion processes down to ultrafast time scales, to the extent that for a long time ultrafast dynamics was largely synonymous with transient absorption and emission laser spectroscopy with typical photoinduced processes illustrated in Figure 3 by a modified Jablonski diagram for photoexcitation, transient absorption, and luminescence.45 Early time-resolved laser spectroscopy measurements were often performed with low repetition rate and high energy (many photons) pulses, which often led to nonlinear effects and dynamics not reflecting the processes aimed at. An example is exciton−exciton annihilation occurring at high excitation densities in coupled molecular systems like molecular aggregates or photosynthetic antennas. At sufficiently high intensities, the process is manifested as a fast decay of excited-state population, which distorts the energy transfer dynamics in the system. For studies of coupled chromophore systems for solar energy harvesting, a common misconception in this regard is to compare the fluxes of sunlight and a single laser pulse and consider this as the relevant excitation density. A full sun delivers on the order of 100 mW cm−2; with the same flux, a 100 fs laser pulse, typically focused on approximately 10−3 cm2, would contain only about 100 photons. In reality, optical pump−probe experiments typically use on the order of 1011−1014 photons/cm2 per pulse. However, this simple comparison is misleading, because the laser only fires at typically 1 kHz repetition frequency. Thus, the number of photons/cm2/s can still be similar to solar irradiation conditions, and the steady-state concentrations of intermediates are also similar. For the transiently higher 10944

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Figure 4. Schematic transient photoconductivity setup used in probing charge carrier dynamics in solar cell materials. Two delay lines are used. Delay 1 is to map to THz electric field probe (gate delay), while delay 2 is to monitor the change in photoconductivity as a function of time (pump− probe delay). A more detailed description of the setup can be found in the literature.60,64,65

photoconductivity kinetics. On one hand, a rise in photoconductivity kinetics reflects generation of charged species and/ or increase in mobility of the charges. On the other hand, a decay represents a decrease of the mobility (maybe due to relaxation) and/or disappearance of charge carriers either by recombination or by injection to a low mobility acceptor material. Pulsed THz radiation can be generated through an optical rectification process by pumping a ZnTe crystal with 800 nm, ∼100 fs, and 100 μJ pulses.63 Another ZnTe crystal can be used for detection by spatially and temporally overlapping the pulsed THz radiation with 800 nm gating pulses in a process known as electro-optical sampling. The transient THz photoconductivity kinetics is collected by fixing the delay of the 800 nm gating pulses (delay line 1 in Figure 4) at the peak of the THz electric field and scanning the pump−probe delay (delay line 2) within a desired time interval, typically up to 1 ns. The schematic diagram of a typical time-resolved THz setup is shown in Figure 4. To obtain the THz photoconductivity spectra, delay 2 is fixed at a desired pump−probe delay, while delay 1 is swept to map the THz electric field. It should be noted that at the earliest time scale, the photon to charge conversion yield φ is often assumed to be unity, while at longer times this represents the change in charge population at a particular time. φ near 1 means that all absorbed photons are converted to mobile charges. However, because accurate measurement of φ is generally difficult, this assumption means that reported mobility values are lower limits and can in reality be considerably higher. 2.2.4. Time-Resolved X-ray Spectroscopy and Scattering. A range of time-resolved X-ray spectroscopic and scattering techniques have recently emerged as powerful tools to investigate structural dynamics.66−72 In addition to the structural information obtained from scattering techniques, the atomic selectivity of spectroscopic techniques, often probing the local environment of critical atoms, such as transition metal

multidimensional spectroscopy and femtosecond stimulated Raman microscopy.59 2.2.3. Time-Resolved THz Spectroscopy: Transient Photoconductivity Measurements. To directly access the behavior of charge carriers on the ultrafast time scale, timeresolved terahertz (THz) spectroscopy is a powerful tool.60 Upon pulsed (∼80−100 fs) light excitation with a photon energy above the band gap, charged species, either loosely or tightly bound, are generated in solar cell materials. This results in a change of photoconductivity (Δσ) in the material, which will modulate the characteristics of pulsed THz radiation used as probe. This change in the Δσ can be calculated using the following relation:61 ΔE(ω) ε0c Δσ 1 = φ ·(μe + μ h ) = − · nexce0 Egs(ω) Fe0 1 − e−αL

(1)

where nexc is charge density, e0 is the elementary charge, φ is photon-to-charge conversion quantum yield, μe and μh are the electron and hole mobilities, respectively, ΔE is the difference between Eexc (transmitted THz electric field for excited sample) and Egs (transmitted THz electric field for sample in its ground state), respectively, ε0 is permittivity of vacuum, c is velocity of light, F is the fluence in ph/cm2, α is the absorption coefficient, and L is the thickness of the sample. The quantity that can be obtained from this equation is mobility in cm2/(V s). As shown, Δσ is a product of both quantum yield and mobility, meaning that to obtain Δσ, the photo generated species should be both charged and mobile. A tightly bound molecular exciton, which may be created by light excitation, will not be detected because by definition it has no charge (neutral). However, excitons can be detected through interband transitions if they occur in the THz spectral region.62 In the same manner, if the pump pulse creates ions, whose mobility is very low, this may also not be seen by this technique. The temporal evolution of the charge population and mobility defines the shape of the THz transient 10945

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Figure 5. Overview of X-ray transitions and signals commonly used for ultrafast tracing of electron dynamics. The data used here contributed to the investigation of the RuCo photoinduced dynamics described in a later section. The schematic energy level diagram in the lower right corner and the inset illustrate the transitions giving rise to the observed spectral features A−F. To the X-ray absorption near edge structure (XANES) belong transitions from the core 1s level into empty states starting with the LUMO’s A, the absorption edge originating in the increasing density of receiving bound transition states B, as well as the features in C originating from a combination of transitions into electronic states and modulation due to interfering, scattered free electrons. The latter develops into the extended X-ray absorption fine structure (EXAFS) with increasing photon energy. The core hole created during the single step absorption process is filled in a second step, giving rise to the emission of the valence to core (VTC) transitions D, the Kβ transition E and E1, and the Kα transition F. The radial wide-angle X-ray scattering (WAXS) difference pattern G of pumped versus unpumped collected from the solute molecules (and solvent response) in a liquid jet gives rise to the radially integrated differential X-ray diffuse scattering (XDS) signal shown in H (see text for further details).

VTC).88−90 As can be seen in Figure 5D, the VTCs originate from transitions between the HOMO and core states, which have been shown to be very sensitive to oxidation state, coordination, and bonding structure.74,90−94 The low efficiency of the emission process, however, poses a challenge to measure VTC features time-resolved. The significantly higher efficiency of the Kβ1,3 (M−K shell, Figure 5E,E1) or Kα (Figure 5F) emissions has enabled a number of studies using the relative shape and position of fingerprinted features influenced by, for example, the Zeeman effect and spin−orbit coupling (shape of Kβ1,3), the total charge and spin density (width and intensity of Kα), and charge distribution with high time-resolution.71,83,84,95−98 Time-resolved scattering of noncrystalline media (X-ray diffuse scattering, XDS) to study large structural changes in a solute molecule, or solvent cage dynamics, is now feasible,95,99−104 as is work with photon-in, electron-out techniques like X-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy.105,106 Time-resolved X-ray scattering and spectroscopy is a currently rapidly developing field with an increasing number of experimental stations and setups available to the experimentator. Recent reviews give a historic and current account of the development of the field.70,82,107,108 A brief summary of the most often applied measurement schemes and energy ranges, with emphasis on sources capable of producing radiation in and higher than the water window (carbon K-edge ∼280 eV), is listed in Table 1. The localized nature and high energy of the core-hole allows in situ, near background free spectroscopic transient probing of electron energies around a single atomic sensor in a complex

sites in photocatalysts, allows a spatial localization of specific transitions.73 Figure 5 shows a combination of hard X-ray-based techniques used to trace the ultrafast electron dynamics in transition metal complexes. The schematic indicates the transitions used to probe the photoinduced dynamics in a RuCo model catalytic complex exhibiting ultrafast Ru → Co electron transfer, as discussed in detail in section 4.1.2. During the X-ray absorption process, an electron from a highly localized core orbital is elevated into either a vacant, bound electronic state, or gains enough energy to escape the complex. The energy and intensity distribution measured in an X-ray absorption fine structure spectroscopy (XAFS) experiment thus allows conclusions on oxidation state, coordination, and local structure around the absorber atom and was one of the first X-ray-based techniques reaching subpicosecond resolution.74−80 Features analyzed are the existence and position of pre-edge absorption (Figure 5A) typical for transitions into LUMO and related bound states. The manifold of empty bound states gives rise to the edge and near edge structure (XANES) (Figure 5B,C), which can be modeled and/ or compared via fingerprinting. The structural component giving rise to the extended X-ray absorption fine structure (EXAFS) modulations observed after the edge (Figure 5C) is useful to follow the structural dynamics of the complexes. Recent developments in pulse brilliance,70,81,82 high repetition rate systems,83,84 and detector technology85−87 have advanced the use of pulsed X-ray radiation in the study of electron dynamics. Photon hungry techniques like X-ray emission spectroscopy (XES) can now use weaker, but more sensitive transitions (e.g., Kβ emission line, or valence to core transitions 10946

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2.3. Advanced and Emerging Experimental Characterization

Table 1. Overview of Some Common Time-Resolved X-ray Measurement Schemes and Their Typical Current Time Scales time scale >10−6 s >60 × 10

−12

Time-resolved optical absorption and emission spectroscopy techniques in the “standard” UV/vis spectral region have gradually reached a level of maturity over the last several decades that today makes them well-established workhorses to study dynamic processes in a wide range of solar energy conversion materials all the way down to the few-femtosecond time-range. Only some initial attosecond responses associated with the photoexcitation itself remain largely out of reach for ultrafast photochemistry, where attosecond spectroscopy is being actively developed but still largely remains limited to fundamentally interesting investigations of simple prototype atomic, solid state, and molecular systems rather than functioning solar energy conversion materials.151 The interpretation of time-resolved optical spectroscopy, however, remains an important and frequently far from trivial scientific task in many cases, where the focus for the time-resolved UV/ vis pump−probe spectroscopy is often on the more applied side of understanding and interpreting physical phenomena in a wide class of complex systems that naturally includes most forms of solar energy conversion materials and devices. A remarkable benefit of the broad range of available experimental and computational techniques that has emerged in recent years to complement traditional ultrafast optical laser spectroscopy in different frequency regimes, however, is that many solar energy conversion processes can now be studied much more comprehensively to provide full perspectives of the electron dynamics at different energy and time-scales. Particularly promising is the usefulness of several techniques, including, for example, time-resolved infrared and X-ray techniques, to complement the information about electronic excitation dynamics obtained with traditional transient absorption and emission studies with information also about structural dynamics. Many such combinations of techniques therefore provide excellent opportunities to understand the combined structural and electronic evolution that is necessary to paint a full picture of the ultrafast dynamics in many solar energy conversion systems, providing a promising path for further research. The overall energy-conversion processes and capabilities of many working solar energy conversion devices are markedly different as compared to what could be expected from basic investigations of the electron dynamics in individual material components. It is therefore important to develop strategies to investigate electro-optical dynamics under in situ and in operando conditions to elucidate detailed electron dynamics under realistic working conditions. The recent review by Douhal and co-workers illustrates how this can be done for dyesensitized solar cells.152 We conclude the discussion of time-resolved spectroscopic techniques by briefly highlighting some emerging or “advanced” techniques that look set to have a broad impact on our understanding of solar energy conversion dynamics by going decisively beyond the traditional “pump−probe” spectroscopy approach in different respects. 2.3.1. Coherent Control. Coherent control, achievable, for example, through pulse shaping, constitutes an interesting area of continued development of optical pump−probe laser spectroscopy with demonstrated possibilities to steer the outcome of a variety of chemical reactions such as selective bond breaking and photoisomerization on ultrafast time-scales typically in the range of ∼100 fs.153,154 A versatile setup for

method

s

>10−12 s >25 × 10−15 s

pump−flow probe, quick-XAFS, electronic delay (e.g., pump sequential probe)66,75,77,108−119 pump−probe with single or multiple synchrotron pulses66,69,77,83,108,120−126 laser-based sources66,77,86,127−141 pulse slicing or other bunch manipulation (very low flux), linear accelerator (e.g., FEL)66,74,76,77,108,142−144

molecular structure. Changes around this atom lead to recognizable features that, as compared to measured reference spectra71,145,146 or simulations, allow the identification and conclusions on the dynamics around the absorbing/emitting atomic sensor. Experiments then combine optical excitation with one or more of the complementary X-ray-based techniques. The combination of XAS and XES, for example, can overcome the limitation of energy resolution due to the lifetime of the excited states.147−149 XES and XDS can be performed simultaneously (as shown in Figure 6) with the

Figure 6. Scheme of simultaneous XES and XDS pump−probe spectroscopy. The cylindrical bent crystal in the von-Hamos geometry focuses the full spectrum for each X-ray pulse onto an area detector. A second segmented area detector behind the fast flowing liquid sample jet collects simultaneously the diffuse X-ray scattering.

same, fixed excitation energy at sources with limited excitation bandwidth (like XFELs) to obtain electronic and structural information from the same system, elucidating, for example, electron/energy transfer and solvent dynamics.67,84,95,150 Examples later in this Review emphasize the strength of combining spectroscopies in multiple wavelength regions to fully understand the dynamics of complex systems. However, the mismatch between optical absorption and X-ray interaction length poses a challenge for the experimenter. The required compromises often include the need for fast flowing thin jets, high excitation yields, and high sample concentrations, which often lead to the need of a large amount of sample material to perform the experiments and challenging experimental condition. With increasing stability and maturity of the setups, these challenges become easier but are currently still not solved. 10947

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such experiments are illustrated in Figure 7. This type of advanced pulse-shaping is arguably in itself somewhat remote

from large-scale solar energy applications striving for the efficient harvesting of natural sunlight. Coherent control techniques can, nevertheless, be useful as an advanced probe of fundamental photoinduced processes in solar energy conversion systems.155 For example, Bruggemann et al. used shaped laser pulses to enhance heterogeneous electron injection yields from dye molecules to TiO2 substrates in model DSCs, thus identifying key vibrational signatures for promoting efficient charge separation.156 Yartsev and coworkers have recently utilized coherent control to probe photoinduced excited-state dynamics in transition metal complexes,157 and Cao and co-workers have shown how photocurrents in strongly scattering DSCs can be influenced by shaping of the incident wavefront of a laser beam.158 Piatkowski et al. have reviewed recent progress in the area of pulse control and coherent excitation of individual molecules and nanoantennas.159 A further example of emerging advanced coherent control applications, Mukamel and co-workers have also recently outlined how coherent control can be extended to stimulated X-ray Raman processes to probe and influence longrange photoinduced electron transfer processes in donorbridge-acceptor (D-B-A) systems of fundamental relevance for ultrafast energy transfer dynamics.160 A range of time-resolved multidimensional spectroscopy methods outlined in the

Figure 7. Prototype experimental scheme for adaptive quantum control using a femtosecond pulse shaper where feedback can be obtained optically (SHG is second-harmonic generation, detected in a photodiode, PD), from gas-phase time-of-flight (TOF) mass spectrometry, or from liquid-phase emission (using photomultiplier tubes, PMT). Reproduced with permission from ref 154. Copyright 2003 John Wiley and Sons.

Figure 8. Illustration of multidimensional spectroscopic investigation of the vibronic origin of long-lived coherences in artificial molecular lightharvesting systems. Panel a shows the orientationally ordered experimental conditions, panels b and c show optical and vibrational signals, respectively, and panel d illustrates the pulses used. Reproduced with permission from ref 169. Copyright 2015 Nature Publishing Group. 10948

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disorder and/or morphological complexity. To achieve a complete understanding of the light-induced dynamics arising in such complex systems calls for the development of techniques that can combine high spatial and temporal resolution, ideally probing dynamics down to the femtosecond regime with molecular or single-nanoparticle resolution in a combined experimental setup that combines the advantages of temporal resolution from ultrafast spectroscopy with spatial resolution offered by single molecule optical spectroscopy or electron microscopy techniques.173,174 Electron transfer dynamics and related phenomena have with increasing success been investigated by several groups for relevant single object light harvesters of several kinds including polycyclic aromatics,175 semiconductor and metal nanostructures,176 CdSSe nanobelts,177 porphyrin-sensitized nano-TiO2 and related interfacial electron transfer systems,178−182 conjugated polymers,183,184 and perovskites.185−188 Ultrafast time-resolved photoemission spectroscopy (TR-PEEM) provides appealing opportunities to study propagating charge density waves and surface plasmons in nanomaterials,189 and was, for example, recently used to study photoinduced electron motion in III−V semiconductor heterostructure photovoltaics with high simultaneous spatial and temporal resolution.190,191 There are also efforts in monitoring changes in ultrafast photoconductivity of semiconductor materials at the length scales of a few nanometers. The group of Cocker et al. demonstrated the first scanning tunneling microscope (STM) whose tip is gated by a THz pulse.192 Using this technique, they were able to monitor the hole capture time in an InAs nanodot on the 1 ps time scale. This could be a very important step in realizing a spectroscopic technique where ultrafast electron transfer can be studied with nanometer length scale spatial resolution. 2.3.4. Time-Resolved Spectroelectrochemistry. A special feature of emerging photoelectrochemical approaches for solar fuel generation is the close coupling of photoinduced dynamics with extensive single- and multielectron reduction and oxidation (redox) processes in supramolecular systems and at interfaces. Traditional steady-state spectroelectrochemistry (SPECH) provides a very useful standard technique to assess different spectral features associated with different oxidation states, but does not automatically provide a dynamical perspective of fast photoelectrochemical processes under different redox conditions. Progress in this direction can, however, be achieved by combining time-resolved spectroscopy with an electrochemical setup capable of altering the redox state of the investigated material, to achieve time-resolved spectroelectrochemical measurements. Optical pump−probe setups offer the most obvious approach to investigate excitedstate electron dynamics for different oxidation states, but also time-resolved techniques in different wavelength regimes (IR, X-ray, etc.) provide unique opportunities to probe dynamical responses in different oxidation states. For example, different types of time-resolved nanosecond photoluminescence blinking of individual semiconductor nanoparticles could recently be demonstrated via electrochemically controlled charging.193

following section constitute a further advanced approach to investigate ultrafast dynamics that goes beyond the traditional pump−probe approach. 2.3.2. Multidimensional Spectroscopy. Building on the general point about the coevolution of structural and electronic properties of photoinduced dynamics above are fundamental questions about the existence and usefulness of coherent electron dynamics,161 and coupling of electron dynamics with structural evolution in, for example, vibronic coupling.162 These processes are often essential to understand the ultrafast dynamics in nonequilibrated systems on the very early timescales following photoexcitation, typically on a femtosecond to few-picosecond time-scale.161,163 The role of long-lived coherences as a potential contributing factor for the impressive efficiency of the initial light-harvesting steps in natural photosynthesis has also been subject to many intriguing, and often hotly debated, interpretations of complex observations.161,164,165 To the extent that such coherences can indeed be utilized, this also has potentially important implications for the development and design of what might be called superefficient artificial solar energy harvesting systems, that is, that outperform the efficiency limitations of conventional lightharvesting approaches relying on traditional incoherent energy conversion.163 Emerging techniques for two-dimensional, or more generally multidimensional, spectroscopy, together with nonlinear optical theory166 and advanced quantum simulations, are starting to provide advanced capabilities to explore many such fundamental aspects in great detail.167 Emerging opportunities to explore and utilize coherent phenomena also for advanced solar energy materials are illustrated by, for example, a recent demonstration of coherent charge transfer in organic photovoltaics by Falke et al.,168 and an elucidation of the vibronic origin of long-lived coherences in artificial molecular light-harvesting systems by Lim et al.,169 as illustrated in Figure 8. Performing dual-pump experiments that combine an optical and IR pump pulse for electronic and vibrational excitations, respectively, has further proven to be a very useful technique to investigate vibrational influence and control of photoinduced electron transfer reactions in the fast coherent transfer regime for donor-bridge-acceptor systems, illustrated in Figure 9, which is relevant for understanding both natural and artificial solar energy conversion systems.170−172 2.3.3. Time-Resolved Microscopy. Several promising solar energy conversion technologies, such as dye- and QDsensitized semiconductor solar cells, OPVs, and hybrid organometal perovskites, as well as heterogeneous and supramolecular solar fuels systems, are characterized by high degrees of intrinsic

2.4. Theoretical and Computational Considerations

Theoretical and computational modeling has emerged as a vital tool to understand many fundamental properties for ultrafast photoinduced processes in solar energy conversion systems such as light-harvesting and charge separation, and can increasingly also be used as predictive tools to guide ongoing

Figure 9. Illustration of dual optical and IR pump experiments to investigate vibrational influence on the charge separation electron transfer rate (kCS) in a molecular donor−acceptor system. Reproduced with permission from ref 171. Copyright 2009 American Chemical Society. 10949

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experimental investigations of complex processes in functional materials. A comprehensive theoretical description of electron dynamics in various solar energy conversion applications typically requires combined descriptions of both traditional excited-state aspects in photophysics and photochemistry, on the one hand,194 and charge and energy transfer phenomena, on the other hand.195,196 Modern investigations of solar energy conversion processes can thus draw on a wealth of theoretical background from both areas, but often need to go beyond the traditional boundaries of either area, which has also provided an important inspiration for theoretical developments in recent years. General theoretical aspects of photochemistry as well as energy and electron transfer have both been reviewed extensively for many relevant types of systems.197 The focus here will be on such aspects that are of particular relevance for modern solar energy conversion applications such as lightharvesting, charge separation, and surface electron transfer.197−200 From a broader perspective, it is noteworthy that there are many similarities on a fundamental level in terms of the basic processes such as light-harvesting, excited-state relaxation, initial charge separation, and long-range charge transport in several emerging types of solar energy conversion approaches. Computational capabilities to model complex solar energy conversion materials and processes have improved significantly in recent years, with a rapidly growing number of dedicated research publications and reviews. That only a small selection can realistically be provided as meaningful examples in this broad context is a testament to a significant surge in theoretical and computational work on essentially all different types of emerging solar energy conversion systems including dye-sensitized, quantum dot, organometal halide perovskite solar cells,198,201−204 organic photovoltaics,205,206 as well as various aspects of the rapidly growing field of photocatalysis that currently variably goes under headings such as artificial photosynthesis and solar fuels.207,208 Comparative analysis of the relevant fundamental processes in this Review provide significant opportunities for cross-fertilization even when the materials involved can be chemically very different such as photoinduced electron transfer from light-harvesting components to organic and nanocrystalline semiconductor electron acceptors in OPV and DSC devices, to mention just one example. A common challenge for advanced simulations is that many of the most promising and efficient solar energy conversion approaches involve complex nanoscale materials and heterogeneous interfaces, and the dynamics often span from ultrafast femtosecond processes during the initial excitation to much slower processes, for example, millisecond or slower time scales, for long-range charge transport. In addition to this time-size computational challenge that is commonly addressed in multiscale approaches, theoretical models often also need to address issues of manifolds of electronic states and complex multidimensional excited-state energy surfaces that provide further challenges for accurate first-principles and multiscale modeling as illustrated in Figure 10.209 Investigations of both well-characterized model systems that can be used to elucidate basic principles and complex champion materials that can be used to identify the most promising conversion strategies provide fertile grounds for theoretical investigations. Solar energy conversion is generally initiated by lightabsorption by a light-harvesting unit that can be either molecular, such as dyes in dye-sensitized solar cells, or a solid material such as a quantum dot or a low-bandgap semi-

Figure 10. Multiscale modeling challenges for solar energy conversion dynamics and devices from ultrafast processes at the molecular, or corresponding atomistic material, scale to functioning solar energy conversion devices.

conductor material. This is generally accompanied by rapid excited-state relaxations such as internal conversion in organic molecules or electron and hole relaxation in semiconductor materials. These photophysical processes are well understood from a basic theoretical perspective for several important classes of light-harvesting materials such as organic chromophores and transition metal complexes, bulk semiconductors, and quantum dots and have been discussed extensively elsewhere.194,210,211 Abilities to accurately calculate molecular excited-state properties for champion dye molecules for different solar applications including both solar electricity and solar fuels have been an important development in recent years. Benchmark excited-state calculations can be carried out using high-level ab initio methods such as multireference and other ab initio schemes for small to moderately sized prototype chromophores of a few tens of atoms at least.212,213 Further progress also includes a large number of first-principles calculations of the optical absorption spectra of different molecular light-harvesting dyes typically performed at present using widely available capabilities for time-dependent DFT calculations.214−217 Highly cited work in this direction includes calculations on combined experimental and theoretical investigations of Ru-polypyridyl dyes for DSC applications.218 Such studies can now routinely provide information about the nature of low-energy excited electronic states involved in the initial light-harvesting in lightharvesting molecules with hundreds of atoms or more.220 At the same time, there are emerging capabilities to investigate also dyes explicitly interacting with nano-TiO2 using firstprinciples calculations.198,201,204,219,221 A growing field of research for excited-state dynamics concerns accurate first-principles calculations that provide information about the excited-state evolution beyond the initially excited Franck−Condon region for different types of light-harvesters. On the molecular side, this includes recent studies of transition metal complexes and organic dyes for DSC and solar fuel applications, as well as polymers for OPVs, where electronic and structural dynamics are typically closely coupled. One step in this direction is the computational exploration of excited-state potential energy surfaces that remain difficult to characterize experimentally. This includes both manifolds of excited states and complex multidimensional reaction coor10950

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Figure 11. Molecular electronic structure calculations (top) provide information about light-harvesting properties in many molecular systems, for example, the classical charge transfer nature of metal-to-ligand charge transfer excitations from HOMO (top left) to LUMO (top right) in prototype transition metal complexes.198 Calculated multidimensional potential energy surfaces provide further first-principles information about excited-state properties beyond the initially excited Franck−Condon region that helps to elucidate energetically feasible deactivation pathways also in lightharvesting complexes such as transition metal complexes with nontrivial reaction coordinates. Shown (bottom) is a schematic illustration to extend excited state lifetimes of microsecond regime of Ru-complexes with expanded cages. Reproduced with permission from ref 223. Copyright 2012 Elsevier.

simulations of various flavors. Many approaches that include explicit dynamics have also been proposed and applied at different levels of theoretical description, for instance, so-called first-principles nonadiabatic molecular dynamics202 and various parametrized electron dynamics approaches (vide infra). Theories for electron and energy transfer processes provide a central pillar of electron dynamics in solar energy conversion systems, with basic aspects illustrated in Figure 12. Electron transfer is particularly relevant for charge separation where homogeneous electron transfer for donor−acceptor systems is widely described by the Marcus, or Marcus−Hush, theory for electron transfer195 (Figure 12, left panel), with various extensions to complex systems including explicit quantum

dinates in many common molecular light harvesters (e.g., transition metal complexes) (Figure 11).222,223 This provides significant opportunities to theoretically characterize reaction paths for excited-state evolution and short-lived excited-state minima in terms of both electronic state and structure that are difficult to capture using traditional time-resolved optical spectroscopy. Such theoretical information is also very valuable to combine with emerging opportunities to study structural dynamics with femtosecond time resolution using time-resolved X-ray spectroscopic techniques facilitated by free electron lasers.96,224 Further theoretical information that provides direct insight into dynamical phenomena can be obtained using dynamical 10951

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Figure 12. Schematic representation of some fundamental aspects of electron and energy transfer processes relevant for solar energy conversion dynamics.

effects and interfacial processes involving bands of states.196 Briefly, the electron transfer in a donor−acceptor (D−A) system undergoing electron transfer (D−A → D+−A−) that follows Marcus theory is described by a rate constant, kET, for an activated (Arrhenius-type) reaction where the rate constant expression depends on the electronic coupling between the initial and final states, HAB, the reorganization energy, λ, the temperature T, as well as fundamental constants including the (reduced) Planck’s constant, ℏ, and the Boltzmann constant kb.225 κET =

2π |HAB|2 ℏ

⎛ (λ + ΔG°)2 ⎞ 1 ⎟ exp⎜ − 4λk bT ⎠ 4πλk bT ⎝

The success of such exponential models typically depends on a number of experimental factors, not least including nontrivial influence of the real atomistic structure of linker groups and other parts of the physical system, and deviations from ideal exponential behavior consequently also provide potential insight about local electronic system properties and their relation to the atomistic system structure.227,229 Excitation energy transfer (EET) (Figure 12, right panel) constitutes an alternative fundamental process for excited-state evolution that does not in itself lead to charge separation, but is often an important part of a complete photocycle to, for example, move the excitation through a material such as a molecular crystal or polymer phase to an interface where charge separation can occur, such as a bulk heterojunction in an OPV blend (vide infra). Förster resonance energy transfer (FRET) and Dexter electron exchange (DEE) represent two alternative main mechanisms often invoked for such EET that can be distinguished by their fundamentally different distance dependence. Förster energy transfer230 involves a strongly distantdependent dipole−dipole coupling that makes the experimentally important quantum efficiency, EFRET, describes the fraction of energy transfer events in a donor−acceptor system per excitation event, which also depends on the donor−acceptor distance, r, according to eq 4:

(2)

The fundamental ET expression has long served as an important basic model for electron transfer reactions useful for understanding different aspects of the (free) energy driving force and electronic coupling in a variety of experimentally investigated donor−acceptor systems, and includes investigations into maximum electron transfer rates at the point where the reaction is effectively nonactivated and reduced rates in the so-called inverted Marcus region,226 schematically illustrated in Figure 12. A lot of fundamental theoretical work has also been devoted to electron transfer in donor−acceptor systems based on explicit quantum chemical calculations including considerations of the role of molecular linker groups of varying length and chemical nature, for example, in terms of bridge-mediated hopping and superexchange contributions to electron transfer rates (Figure 12, middle panel).227−229 Long-range electron transfer rate constants, for example, for donor−acceptor systems that incorporate molecular linker groups, are often related to an exponential decrease of a fundamental rate constant, k0, with the distance, RDA, between the donor and acceptor units, and where the β-parameter gives a useful measure of how rapidly the rate decreases with distance for a variety of molecular bridges according to eq 3: k = k 0·exp( −βRDA )

E FRET =

1 1 + (r /R 0)6

(4)

In this expression, the Förster radius, R0, describes the distance corresponding to 50% energy transfer efficiency.231 The Dexter exchange mechanism232 involves an electron exchange mechanism where the rate, kDEE, falls off exponentially with the donor−acceptor distance, r, according to the expression in eq 5: ⎡ −2r ⎤ kDEE ∝ J ·exp⎢ ⎣ L ⎥⎦

(5)

This rate expression involves the spectral overlap integral, J, as well as a distance normalization constant, L. The exponential

(3) 10952

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Figure 13. Schematic of a dye-sensitized solar cell (DSC) (left) showing the metal oxide nanoparticles (green balls) and attached dye molecules (in this case, a ruthenium complex; small yellow balls). Light absorption by the dye results in electron injection (green arrow), and for optimized solar cell performance electron−hole recombination (red arrow) has to be minimized (magnified atomistic perspective on right), while net electron transport through the mesostructured TiO2 film as well as oxidized dye regeneration through hole conduction via the electrolyte should be optimized (orange arrow). The DSC schematic is reproduced with permission from ref 280. Copyright 2014 Lund University.

troscopic information for interfacial electron transfer down to a few-femtosecond time-scale also came from core hole clock and related experiments using X-ray techniques.245,246 A full quantum mechanical model directly relevant for ultrafast interfacial electron transfer was developed early on by May, Willig, and co-workers to rationalize several ultrafast electron transfer measurements.247−250 Progress toward accurate atomistic quantum chemical calculations and simulations was initially prohibitive due to the inherently large system sizes associated with realistic dye−semiconductor interfaces. Explicit quantum chemical calculations of key interfacial processes, such as excitations and interfacial electronic couplings for both a nanoparticle/cluster251 and a periodic surface, constituted early steps toward a fundamental understanding of interfacial electronic properties and processes.252 Emerging capabilities for explicitly dynamical techniques including nonadiabatic firstprinciples dynamics simulations introduced by Prezhdo and coworkers have provided significant and very instructive perspectives of heterogeneous electron transfer for several dye−semiconductor interfaces253,254 as has already been reviewed extensively.221,255,256 There are also several other dynamical approaches focusing on various aspects of electron dynamics in complex systems by Batista,257 Thoss,258−260 Troisi,203 Micha,261 Tavernelli and Rothlisberger,9,262 and many others. Making accurate theoretical predictions of materials properties and ultrafast dynamical properties of solar energy conversion systems has developed significantly in the past decade as an increasingly active area of research for most types of emerging solar energy conversion technologies. Clear challenges still remain to treat the full complexity of many complex materials and interfaces. Recent reviews and showcase papers illustrate that many theoretical investigations and advanced calculations have been carried out recently also for QD-SCs,263 OMHPs,264 and OPVs,205 as well as solar fuels and photocatalysis.256 Comparisons between the different solar energy conversion approaches show that there are in many cases functional similarities, for example, in terms of the need to combine initial light-harvesting components with interfacial or

decay makes the Dexter mechanism effective only over short distances (typically ∼10 Å), but can be significant in donor− acceptor systems designed on the molecular level to include close D−A interactions. More general and comprehensive descriptions of fundamental process including electron and energy transfer theories and applications can be found in several general texts,196 including extensive discussions of topics of particular relevance such as molecular photochemistry,194 photoinduced dynamics,155 photoinduced electron transfer,197 as well as heterogeneous, supramolecular, and interfacial electron transfer processes.200,233 The emergence of efficient dye-sensitized solar cells characterized by efficient photoinduced interfacial charge separation at molecule−semiconductor interfaces around 1990 provided an important impetus for developing and applying theoretical descriptions of electron and excited-state dynamics in solar energy conversion systems.200 Early experimental evidence for ultrafast photoinduced interfacial electron transfer in DSCs down to the few-femtosecond timescale included much studied and in parts hotly debated results for Ru−dye systems on different electrode materials such as TiO2 and ZnO that challenged the widely adopted Marcus perspective of electron transfer as an activated process.52,57,234−236 Detailed investigations from well-controlled and/or characterized molecule−semiconductor interfaces provided suitable evidence for validating various computational approaches. This included several early time-resolved experiments by Willig and co-workers on perylene and related organic adsorbates anchored on TiO2 via designated anchor groups. The aim of that work was to elucidate ultrafast interfacial injection dynamics, as well as the dependence of the surface electron transfer processes on various anchor and spacer motifs.237−242 Lian and co-workers studied ultrafast injection to TiO2 from a variety of molecular sensitizers including coumarin 343,58 as well as several transition metal complexes of Ru and Re.57 Moser, Grätzel, and Wachtveitl et al. investigated injection from strongly surface coupled alizarin to TiO2 down to a few-femtosecond time-scale.243,244 Complementary spec10953

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supramolecular charge separation in conjunction with energy and electron transfer. In contrast, there can be unique material challenges for both material design and modeling in terms of, for example, morphological control where supramolecular systems for photocatalysis have well-defined molecular structures while OPVs contain a mixture of unbound lightharvesting polymers and fullerene derivative acceptors where the local interfacial structure can be greatly influenced by the preparation conditions. More specific discussions of such solar energy conversion properties are discussed below in connection with the corresponding experimental discussions for the different types of solar energy conversion systems.

3. SOLAR CELL TECHNOLOGIES 3.1. Dye-Sensitized Solar Cells

A dye-sensitized solar cell (DSC or DSSC) consists of a thin film of metal oxide nanoparticles, often TiO2 or ZnO, sensitized to visible light by dye molecules attached to the nanoparticle surface (Figure 13).280 Light absorption by the sensitizer dye results in electron injection into the metal oxide nanoparticle followed by diffusive electron transport through the nanoparticle film. The electrical circuit is closed by a liquid or solid electrolyte, which also has the function of restoring the oxidized sensitizer to its neutral ground state before the next photon is absorbed.265−267 Processes of interest for an efficient solar cell are dye−semiconductor electron injection, electron−hole recombination, regeneration of oxidized dye by the electrolyte redox couple, charge transport through the nanostructured film, and finally extraction of electrons into an external circuit.57,268,269 The time scale of these processes range from femtoseconds to milliseconds and longer; in this Review, we will focus on the initial ultrafast injection and recombination processes. From a solar cell efficiency point of view, these processes are of utmost importance; every photon not resulting in a conduction band electron and every electron−hole pair recombining imply decreased efficiency.270,271 3.1.1. Electron Injection from Sensitizer to Metal Oxide Acceptor. Initial work on excited-state and electron dynamics in dye-sensitized solar cell materials focused on the dye to semiconductor electron injection process. A large body of work identified this process as decisive for efficient lightharvesting and conversion of the light energy to energy-rich electrons.236,267−269,272−279 For efficient utilization of absorbed photons and excited-state energy of the sensitizer, electron injection into the semiconductor has to be significantly faster than the sum of all other excited-state deactivation processes. For many of the sensitizers explored, nanosecond and longer excited-state lifetimes are not unusual, implying that injection times on the few picosecond time scale are sufficient for close to 100% quantum efficiency injection. Nevertheless, from a fundamental and theoretical point of view, the precise mechanism of dye-to-semiconductor electron injection is interesting and has attracted significant attention. We illustrate that with a brief discussion of the theoretical efforts and how results from this work have been corroborated by experiment. A first fundamental consideration concerns the mechanism of dye−semiconductor interfacial electron transfer where quantum chemical calculations can discriminate between the standard excitation-injection two-step mechanism predominant in most applications, and direct interfacial charge transfer excitation in direct injection DSCs that remains an interesting alternative for efficient charge separation (Figure 14).251 Direct

Figure 14. Basic mechanisms for photoinduced interfacial electron transfer illustrated for a prototype dye-sensitized nano-TiO2 in DSCs. Part (a) shows the normal excitation-injection mechanism with a high energy sensitizer excited state in the wide band limit where it couples resonantly to the band of acceptor states. Part (b) shows activated excitation-injection for a low-energy excited sensitizer state. Part (c) shows a direct charge transfer injection in a low-energy spectral region where neither the sensitizer nor the substrate absorbs light by itself. 10954

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Figure 15. Theoretical investigations of the influence of anchor and spacer groups on interfacial electron transfer. (a) Schematic model of electron transfer through a spacer and anchor part that can either act as an effective tunneling barrier or an active bridge that allows delocalization of the excited (LUMO) sensitizer state toward the band of the acceptor states.290 (b) Example of electronic structure calculation of the interfacial electronic properties from a first-principles calculation involving a sensitizer or a model nano-TiO2 particle with a reasonably well-developed wide band gap semiconductor electronic structure.283 Panels a and b reproduced with permissions from refs 290 and 283. Copyright 2006 and 2009 American Chemical Society and The Royal Society of Chemistry, respectively.

quasi-continuum of acceptor states.198,289 Electron injection from a molecular donor, d, to a band of acceptor levels, k, can often to a good approximation be considered from a Fermi golden rule perspective where the injection time, tinj, can be estimated from a rate expression for kinj:

injection can be completed essentially instantaneously, that is, on the same subfemtosecond time-scale as the sensitizer excitation itself, such as in catechol and related sensitizers, where there is a very strong interfacial electronic coupling and appropriate interfacial electronic level alignment. This can be seen experimentally281 and rationalized theoretically, through the appearance of a low-energy charge transfer band in the optical absorption spectrum.251 In contrast, electron injection from an optically excited sensitizer in the standard excitationinjection scheme is often surmised to take place from molecules that are coupled to the substrate more weakly. These systems have absorption profiles and excited-state properties that are largely similar to those of the dye molecules themselves,265,269 even though there is evidence for some organic adsorbates that there can be significant changes to the excited state and electronic structure properties of molecules bound to metal oxide surfaces even for cases that do not show prominent charge transfer characteristics.282 For the most widely discussed case of photoinduced interfacial electron transfer involving injection from an excited dye state, the interfacial electronic coupling, interfacial electronic alignment (determining the driving energetic force for injection), and the density of acceptor states are key parameters. Much effort has been devoted to investigate how surface electron transfer depends on variations of the interface in terms of different anchor and spacer groups as exemplified in Figure 15.57,235,240,283 Explicit consideration of the nanocrystalline nature of nano-TiO2 and other mesoporous semiconductor materials requires special care in construction of the nanocrystal models that is nontrivial for accurate calculations that are limited in size due to a propensity of surface effects that need to be handled in a realistic way.284−288 Depending on the positioning of electronic levels in the anchor-spacer groups relative to the excited state of the dye and the conduction band, the anchor-spacer can effectively provide either a barrier or a conduit for the photoinduced surface electron transfer. The interfacial electronic coupling is often reflected in calculated interfacial electronic structures in terms of energy broadening and energy shifts of the adsorbate levels as they interact with a

k inj =

2π ℏ

∑ |Vdk|2 ρ(εk) k

(6)

where the electronic coupling element, Vdk, and substrate density of states (DOS), ρ(εk), are central concepts. A range of computational strategies have been developed to predict lightharvesting capabilities and subsequent electron transfer dynamics in such systems based on first-principles calculations, including assessment of the injection rate from an evaluation of the width of the adsorbate DOS of a dye−TiO2 system showing a spread due to the surface interaction (Figure 15),290 or through electron dynamics following a diabatic scheme with a block representation of the levels based on initial localization of separated donor and acceptor levels (Figure 15).209 As mentioned above, more explicit dynamical information has been obtained for several prototype systems such as alizarin− TiO2 interfaces,253 by using computationally expensive but often very illustrative ab initio nonadiabatic molecular dynamics simulations and related dynamical techniques. This work, as well as similar efforts on other types of solar energy conversion systems displaying ultrafast dynamics, has already been extensively reviewed by Prezhdo and co-workers.255,256 Carefully controlled experiments are needed to provide convincing validation of different computational strategies to capture fundamental surface electron transfer characteristics including ultrafast electron injection rates down to shortfemtosecond timescales. This includes issues of temperature dependence as a sign of activated processes, as well as possible coherences and other more explicit dynamical phenomena. Many experimental investigations of ultrafast photoinduced interfacial dynamics are, however, prone to be influenced by factors such as structural and energetic heterogeneity associated with variable surface binding. A series of perylenes attached to TiO2 substrates using different anchor and spacer groups by 10955

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Figure 16. Simulated population dynamics for ultrafast surface electron transfer from perylene (Pe) chromophores with different spacer groups including saturated and unsaturated short hydrocarbon chains. The electron dynamics simulations (in this case neglecting electronic-vibrational coupling) display significant differences in injection rates on the sub-100 fs time scale where even short saturated hydrocarbon spacers (blue lines: PeCH2CH2COOH−TiO2) significantly slow the electron injection as compared to either cases without spacer groups (black lines: PeCOOH− TiO2), or cases with conjugated spacer groups (red lines: PeCHCHCOOH−TiO2). Thick lines present results obtained with a (TiO2)60 cluster model of the substrate, and thin lines correspond to an infinite TiO2 surface model. Reproduced with permission from ref 291. Copyright 2012 American Institute of Physics.

traditionally used ones. Finally, some work on ultrafast dynamics in functional DSCs will be summarized. Given the wide range of different materials that have been investigated, it is not possible to discuss all investigations in detail, but references are given to several reviews that have discussed many further materials and dynamical aspects at greater length. As mentioned above, “real world” systems are often characterized by extensive heterogeneity, in terms of, for example, binding geometry and electronic coupling. Direct comparison with theory is therefore more difficult, but measurements of electron transfer dynamics nevertheless serve the function of identifying bottlenecks in the overall light-to-useful charge conversion to provide valuable information for material and device optimization. Early experimental work on RuN3-sensitized nanostructured TiO2 electrodes from our group236,272,273,301 was aimed at characterizing the electron injection step in the active material that for a long time gave the highest efficiency (∼10%) DSC. Figure 17 illustrates the dye-TiO2 attachment in such as DSC electrode. The electronic structure of the Ru-polypyridyl complexes extensively used as sensitizers includes singlet and triplet metal-to-ligand charge transfer states (1MLCT and 3 MLCT, respectively), accounting for the strong visible absorption.302 Light absorption by these molecules therefore generates the excited 1MLCT state, but within a very short time (∼100 fs) the molecule has relaxed to the lowest 3MLCT state.70,303 This implies that for a complete description of the injection process, injection from both the short-lived 1MLCT and 3MLCT states need to be considered. For efficient electron injection and energy conversion in the sensitized semiconductor system, the energy of the lowest 3MLCT state has to be above the conduction band edge of the semiconductor. The resulting scenario is illustrated by Figure 18, showing the valence and conduction bands of the semiconductor, as well as the ground and excited states of the RuN3 sensitizer. Our own work236,273,276,277,304 showed that following light absorption to

Willig and co-workers provided a good test set for computational validation of ultrafast electron injection models in the socalled wide band limit, that is, with the excited perylene donor level situated well above the conduction band edge of the TiO2 substrate (Figure 16).237,240,290 These investigations highlight the significant and nontrivial role of the anchor and bridge groups as mediators of interfacial electron transfer through a chemical influence on the interfacial electronic coupling that overrides simple arguments based on physical distance between chromophore and surface.260,290,291 Heterogeneous electron transfer was found to take place on a sub-100 fs time scale for a set of several anchor-spacer groups and depended significantly on the particular choice of linker group. Injection with a prototype carboxylic acid anchor group (Figure 16) was determined to take place on a ∼10 fs time scale. Remarkably, the injection with an unsaturated (ethene-type) spacer resulted in a very similar fast injection despite the significantly longer through-linker distance, while a saturated (ethane-type) linker of similar length resulted in a significant slow-down of injection to several tens of femtosecond,238 as could be corroborated computationally.290 Other fundamental aspects of interfacial dynamics and subsequent charge separation that have received theoretical attention include recombination,292 dye regeneration,293,294 and charge transport.295 In this Review, covering several different materials, it is not possible to give a comprehensive account of all of this theoretical and experimental work. Summaries of most aspects of DSC function and processes can be found in many reviews and books.152,266,275,296−300 The focus of this Review is on the ultrafast electron dynamics, which we illustrate by giving a picture of the processes occurring in materials widely used in real solar cells−electron injection, electron− cation recombination, and carrier mobility. Next, we will summarize some work intended to develop sensitizers based on benign and earth abundant elements, and compare the excitedstate/electron dynamics of these sensitizers with that of the 10956

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achieving efficient electron injection from a sensitizer dye to a metal oxide nanostructured film appears to be a relatively manageable task for chromophore−semiconductor interfaces that give a sufficient driving force for electron injection and interfacial electronic coupling of the excited state with the substrate conduction band. The overwhelming part of the work addressing the injection process has been performed on electrode systems in contact with a solvent. Work on full solar cells is considerably less common, but has certainly been performed312 and was recently reviewed by Douhal and co-workers.152 The reported work on full DSC appears to give conflicting pictures of the relative importance of ultrafast (singlet) and slow (triplet) injection, mainly ultrafast313,314 or mainly slow.315 Experiments on the Ru-dye-based champion cells, with the RuN3 and N719 dyes and an electrolyte composition as for a working cell, constitute an interesting example, suggesting a low yield of singlet fsinjection. From a combination of probing in the visible/NIR (dye cation), mid-IR (electrons in the semiconductor conduction band), and fluorescence (dye 3MLCT state), it was concluded that injection on a 1−1000 ps time scale dominates.294,315 These results can be understood as a consequence of a shift to higher energy of the conduction band, which slows the triplet injection. Singlet injection, which in a sensitized electrode in contact with solvent occurs with a yield of 50−60% and on the sub-50 fs time scale (discussion above), appeared to have a lower yield (10−20%) in both a sensitized electrode (in contact with solvent) and a solar cell (containing electrolyte and redox couple) when the injection was probed by conduction band electron absorption in the infrared.294 The reason for this apparent lower yield of electron injection from the 1MLCT state with infrared probing of electrons could be partial formation of an electron−cation complex that dissociates on a slower picosecond time scale.279,297,316 Durrant and co-workers showed a significant effect of lithium cations (Li+) and tert-butylpyridine (tBP) coadsorbents on the injection kinetics in N719-sensitized DSCs that was attributed to a significant (ca. 2-fold) retardation of the injection dynamics for a 100 meV shift of the conduction band edge toward lower driving force for interfacial electron injection, thus impacting the overall IPCE efficiency in the context of minimization of kinetic redundancy.317 3.1.2. Electron−Cation Recombination. Electron injection from dye to semiconductor is just the first step in a series of processes that eventually lead to a photocurrent and voltage in an external circuit. For the electrons to be extracted in high yield, recombination with the holes on the oxidized dye has to be much slower than the rereduction of the oxidized sensitizer by the redox couple of the hole transport material (HTM). Because this process relies on diffusion of redox components in a liquid electrolyte, or charge migration in a solid HTM, the rate of rereduction may vary greatly depending on the nature of the HTM.266 Electron−hole recombination times on the hundreds of nanosecond and slower time scale are generally sufficient for efficient utilization of the light generated charges. The realization that electron−hole recombination is a process directly related to solar cell efficiency, every recombined electron is a lost electron and lost photocurrent, has motivated work to understand the factors controlling the process.152,275 For Ru-polypyridyl dyes (e.g., RuN3 and the black dye) resulting in very efficient solar cells, electron−cation recombination has been shown to be very slow (microsecond time scale)318−320 and slower than regeneration of the oxidized

Figure 17. Illustration of sensitizer-TiO2 binding in a DSC electrode. Reproduced with permission from ref 220. Copyright 2005 American Chemical Society.

Figure 18. Schematic model of two-state electron injection and structure of RuN3. Following MLCT excitation (at 530 nm) of the RuN3-sensitized TiO2 film, an electron is promoted from a mixed ruthenium NCS state to an excited π* state of the dcbpy-ligand and injected into the conduction band (CB) of the semiconductor. GS: ground state of RuN3. Channel A: electron injection from the non thermalized, singlet 1MLCT excited state. Channels B and C: intersystem crossing (ISC) followed by internal vibrational relaxation in the triplet 3MLCT excited state. Channel D: electron injection from the thermalized, triplet 3MLCT excited state. Reproduced with permission from ref 236. Copyright 2002 American Chemical Society.

the 1MLCT state, ∼50% of the molecules inject electrons directly from this state into the semiconductor conduction band with a characteristic time constant of ∼50 fs. Upon excitation to higher-lying vibrational states of the 1MLCT state, even faster injection occurs (∼20 fs), in competition with vibrational energy relaxation and redistribution. The residual ∼40% of the excited sensitizers relax to the triplet state, from which they inject electrons much more slowly on the 1−100 ps time scale.301,305 Many other Ru-sensitizers,269,306 but also porphyrins,18,307,308 phthalocyanines, and organic dyes,309−311 have by now demonstrated fast and efficient injection. Thus, 10957

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Figure 19. Binding model for Zn-porphyrins to TiO2. The edge-to-edge distance (Ree) is decreased upon tilting dye molecule. Reproduced with permission from ref 307. Copyright 2011 American Chemical Society.

sensitizer by the redox mediators,321 and therefore generally not a limiting factor for the efficiency of a solar cell based on these dyes. The recombination kinetics has, however, been found to depend strongly on conditions including electrolyte composition (up to a million-fold change in rate) and electron occupation in trap and conduction band states.322 Slow recombination for some champion Ru-polypyridyl sensitizers under optimized conditions often seems to have been extrapolated to suggest that this is also the case for other dyes (see, e.g., ref 18), leading to a picture where variations in solar cell efficiency have been directly correlated to the efficiency and rate of electron injection.18 Electron−cation recombination kinetics is generally observed to be strongly material (sensitizer and metal oxide) dependent and highly nonexponential, covering a very broad time scale from picoseconds to milliseconds.297,323,324 The strong nonexponentiality of the process is generally explained as a consequence of trapping−detrapping from trap states with a wide distribution of energies.323,325,326 However, the strong dependence on sensitizer of the recombination rate as well as theoretical considerations discussed above290 suggest that there are also factors related to, for example, sensitizer structure, exact mode of binding to the metal oxide surface, etc., controlling the recombination dynamics. There is a clear asymmetry between the interfacial injection and recombination in efficient DSCs in terms of the free energy driving force for the process, where a good DSC will have minimized the driving force for injection while there should be a large driving force for recombination. Hupp and co-workers provided early evidence that the large driving force for interfacial recombination can lead to the display of inverted Marcus region-type behavior,327 while Durrant and co-workers have provided evidence that the recombination for a set of investigated Ru-polypyridyl and porphyrin sensitizers is near the peak of the Marcus free energy curve (ΔG ≈ λ ≈ 0.8 eV), although with significant dispersion of the results that indicate other factors to also be potentially significant.328 To provide further insight into the chemical nature of the factors that control the interfacial electron transfer processes, a systematic study was performed of how sensitizer binding to the semiconductor surface controls the electron transfer processes in general and electron−cation recombination in particular.307 By using a series of Zn-porphyrins (Zn-P), several molecular properties of importance for dye−semiconductor binding could be varied systematically, which is used here to exemplify some of the key considerations for controlling the

interfacial charge separation.269,307 The electron transfer dynamics were monitored by transient absorption (TA) spectra and kinetics.307,308 The excited state of the sensitizer was formed within the time resolution (50 ns. From Marcus theory of electron transfer,225,329,330 and its modifications for interfacial electron transfer,331,332 it is expected that the electron transfer rate should have a strong (exponential) distance dependence. If electron transfer between the porphyrin core and the semiconductor occurs via the connecting spacer, as often envisaged, making this spacer longer should slow the transfer. With the help of the sensitizers 2,4,6-Me and BP, this expectation was tested; introduction of the extra phenyl moiety in the biphenyl spacer of TiO2/BP relative to TiO2/2,4,6-Me would result in approximately 1.5 times longer through-bond distance between the porphyrin core and the TiO2 surface and therefore result in considerably smaller electronic coupling and much slower electron injection and recombination. Transient absorption kinetics of the two molecules attached to the TiO2 film showed that the charge recombination process does not meet this expectation; the BP sensitizer with the longer connecting spacer had a much faster recombination rate than 2,4,6-Me.307 Also, the electron injection was faster for BP/TiO2. Thus, both electron injection and recombination were overall faster for the sensitizer with the longer connecting spacer. For an analogue to 2,4,6-Me, lacking the methyl groups on the phenyl substituents on the porphyrin core, the effect was even more pronounced with very fast injection and complete recombination within 500 ps.307 Obviously, electron transfer does not occur as could be anticipated via the spacer connecting the porphyrin core to the TiO2 surface. Instead, a picture was suggested where the single carboxyl anchoring group allows a flexible binding geometry; for some of the porphyrins, depending on structural factors such as length of the spacer group and bulkiness of the porphyrin core, a fraction of the molecules are bound at an angle to the semiconductor surface, and electron transfer occurs through space rather than through the linker group connecting the porphyrin core to the anchoring COOH group.307 This binding model is illustrated in Figure 19. When the tilt angle is changed as a result of a change of porphyrin molecule size or 10958

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Figure 20. Sensitizer-linker-anchor combination with a pyrene chromophore and a tripodal anchor group achieving two-dimensional (lateral and vertical) structural control of surface sensitization of semiconductor surfaces (left). Reproduced with permission from ref 336. Copyright 2009 American Chemical Society. HOMO level of a ruthenium star complex that effectively shields the metal core in all directions also in nanostructured films (right). Reproduced with permission from ref 339. Copyright 2012 The Royal Society of Chemistry.

binding on both kinetic and stability properties.334,335 Tripodal anchors designed with computational support to match TiO2 substrates were synthesized by Galoppini and spectroscopically characterized by Meyer, demonstrating two-dimensional interfacial control on semiconductor surfaces but also highlighting remaining problems of full functional control in mesoscopic films due to an abundance of necking regions between neighboring nanoparticles.336 One approach to achieve fool-proof sensitizer-substrate functional control also in nanostructured films is to use a star strategy with sensitizers featuring rigid anchor-linker groups in all directions, for example, allowing efficient electronic coupling control and shielding of a metal core. The use of star complexes in particular achieved very effective electronic shielding of the chromophore core with significant blocking of ultrafast surface electron transfer as compared to other linker approaches.337−339 Interfacial electron dynamics involving sensitizers anchored on alternative semiconductor substrates of interest for DSC applicat ions have also been investig ated extensively.266,275,297,340 ZnO has in particular provided a good material for comparison with TiO2 due to its similar band gap (both in magnitude as well as in valence and conduction band edge positions), but different natures of the valence and conduction bands. In particular, the bottom of the conduction band of TiO2 is formed mainly from Ti 3d levels, while the bottom of the ZnO conduction band is formed from Zn 4s and 4p contributions. Calculated interfacial electronic interactions using organic model adsorbates were found to be significant for ZnO as for TiO2, suggesting that ultrafast electron injection could occur for both substrates.252 Lian and co-workers341 and Furube and co-workers342 presented experimental evidence for multiexponential electron injection of Ru polypyridyl complexes on ZnO nanocrystalline thin films with an ultrafast (50 ns) conduction band electrons that can contribute to photocurrent; smaller tilt angle leads to higher amplitude of long-lived electrons. By comparing tilt angles obtained from the SFG measurements with solar cell power conversion efficiency, this correlation was taken one step further; a smaller tilt angle leads to higher cell efficiency, meaning that solar cell power conversion efficiency is directly related to the extent of slow electron−cation recombination, that is, the concentration of long-lived electrons in the conduction band. The results suggest a method to characterize the structure of the sensitizer/semiconductor interface and thus pave the way toward providing DSC materials with predictable electron transfer properties. Several groups have in this context explored advanced anchor strategies to achieve better functional control of the sensitizer− semiconductor interactions with the ultimate goal to achieve more effective interfacial charge transfer properties that may be relevant not only for traditional dye-sensitized solar cells but also for related applications with, for example, photocatalysis on sensitized semiconductor films (Figure 20).333 Gray and coworkers made some early characterizations of the kinetic influence of alternative anchors and linkers for RuN3-type dyes on TiO2 to suggest a significant influence of the interfacial 10959

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Zn2+, due to low surface pH caused by, for example, acidic protons of the sensitizer, was, however, early identified as one potential complication for the determination341 of reliable ultrafast kinetics on ZnO under complex operating conditions.343 Surface electron transfer kinetics has subsequently been investigated for numerous other substrates such as SnO2, with opportunities to vary the interfacial electronic interaction, driving force for injection, and surface passivation.344 Electron injection was found to be nonexponential with subpicosecond and picosecond components, similar to injection to TiO2, although with somewhat higher amplitude of picosecond time scale injection. Recombination was slow on the many nanosecond and slower time scale. Recent work to achieve improved control over the interfacial electron injection and recombination processes has included the use of directed dipoles.345,346 A recent breakthrough was also achieved through the demonstration of the ability to control kinetic pathways for interfacial electron transfer from a semiconductor to a molecule using steric modifications of linker groups investigated by Meyer, Berlinguette, and coworkers.347 3.1.3. Formation of Mobile Charges. Optical spectroscopy can provide detailed information about excited states and intermediates participating in the light-to-charge conversion processes of DSC materials. As can be easily appreciated, electron transport within and between the semiconductor nanostructures and mobility of electrons are important aspects of the function of a solar cell. Optical techniques unfortunately do not provide much information about this. More information is obtained from transient far-infrared conductivity spectra and kinetics measured by time-resolved terahertz (THz) spectroscopy (TRTS).61,316 This technique allows noncontact characterization of photoconductivity with subps time resolution,348,349 where the amplitude of the photoconductivity is a direct measure of the population of injected charge carriers and carrier mobility. In addition, from the shape of the transient conductivity spectrum, it is possible to infer the mechanisms of the charge transport or to distinguish between the response of free charge carriers and localized excitations.350 The strong interaction of THz radiation with free charge carriers in semiconductors makes the TRTS an ideal tool for the investigation of charge carrier dynamics in semiconductors and DSC. The conventional scenario for DSC is that mobile electrons appear in concert with injection. Optical transient absorption studies of Furube and co-workers297,342 suggested that there was a delay in the formation of free charges for a dye−ZnO system. Time-resolved THz spectroscopy, directly probing mobile charges, could provide further insight into the process of free charge formation. A combined optical TA/THz study was used to simultaneously monitor the electron injection through formation of oxidized sensitizer (by TA) and the appearance of mobile charges for two different dyes, a Zn-porphyrin and RuN3, attached to ZnO or TiO2.61,316 For both dyes on TiO2, mobile electrons appear within the time resolution of the experiment (