Emerging Design Principles for Enhanced Solar Energy Utilization

Subscriber access provided by Iowa State University | Library

Feature Article

Emerging Design Principles for Enhanced Solar Energy Utilization with Singlet Fission Melissa K. Gish, Natalie A. Pace, Garry Rumbles, and Justin C. Johnson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10876 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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

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

The Journal of Physical Chemistry

Emerging Design Principles for Enhanced Solar Energy Utilization with Singlet Fission Melissa K. Gish,† Natalie A. Pace,†,‡ Garry Rumbles†,‡,§ and Justin C. Johnson†* †

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado, 80401, United States ‡

Department of Chemistry, University of Colorado Boulder, Boulder, Colorado

§

Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, Colorado

Corresponding Authors *E-mail: [email protected]

Abstract. Singlet fission (SF), the generation of two triplet excitons per the absorption of one photon, is a promising strategy for increasing the efficiencies of solar cells beyond the theoretical Shockley-Queisser limit of 34%. Upon photon absorption by an SF molecule, the initially created singlet excited state (S 1 ) interacts with a neighboring chromophore and is first transformed into a triplet pair (TT), which can be subsequently separated into independent triplet excitons (2T 1 ). These independent triplet excitons can be harvested through triplet charge extraction or triplet energy transfer to an acceptor. Research on SF systems has revealed rates and efficiencies of triplet formation and triplet pair decorrelation that are strongly dependent on interchromophore coupling, which is dictated by molecular structure and the resulting geometrical arrangement of chromophores adopted in covalent (e.g., dimers) and noncovalent (e.g., films and crystals) systems. Incorporation of SF materials into realistic device architectures introduces a host of new challenges to consider regarding the efficient extraction of triplets generated through SF. In this Feature, we review our work that has led to some degree of understanding and control of inter- and intra-molecular SF rates placed in the context of solar energy harvesting architectures, including dye-sensitized solar cells, conjugated polymer films, and ligand-exchanged quantum dots. We emphasize the importance of understanding and manipulating interactions between SF molecules with each other and with the charge or energy collectors across an interface in order to strike a kinetic balance that leads to efficient utilization of triplet excitons.

1 ACS Paragon Plus Environment

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

1. Introduction As the impacts of climate change are becoming increasingly obvious, it is necessary to shift to renewable energy to continue to meet the world’s energy demands and minimize environmental effects.1 Solar energy is, by far, the most abundant and available renewable energy resource providing 10,000 times more energy than is needed to satisfy global consumption needs.2 However, sunlight is diffuse, and efficiently utilizing this abundant resource remains challenging. Current single-junction photovoltaic devices, which convert sunlight directly to electricity, are constrained to power conversion efficiencies (PCE) within the Shockley-Queisser (S-Q) theoretical limit of ~34%. One proposed method for pushing solar cell efficiencies beyond these limitations is through addition of singlet fission materials into devices.3 Singlet fission (SF) utilizes a single photon in organic materials to transform a singlet exciton into two triplet electron-hole pairs, or excitons.4-5 Generating two triplet excitons with just one photon could push the boundaries of the S-Q limit from ~34% to ~45%, if those triplets are effectively harvested through charge injection or energy transfer at a donor/acceptor interface.6 The potential efficiency increase in power conversion efficiency is applicable to many current photovoltaic technologies, including but not limited to crystalline silicon (c-Si). Other PV platforms, such as dye-sensitized solar cells, currently have lower overall efficiencies than silicon, but may represent an opportunity to understand the fundamental principles of how SF molecules should be incorporated into a conventional device.7 SF may also play a role in other optoelectronic research areas such as spectral shaping via downconversion,8 or nascent fields such as quantum computing. In all cases, the efficient production of two triplets is just the first step toward an application, as harvesting or addressing the triplet excitons via electronic or photonic means is necessary to make practical schemes efficacious. 2 ACS Paragon Plus Environment

Page 2 of 40

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


Singlet fission is an exciton multiplication process analogous to multiple exciton generation (MEG) in semiconductor nanocrystals, or quantum dots (QDs).9-10 MEG involves the absorption of a photon with an energy two or more times that of the band gap (E g ) of the QD to create two or more excitons dependent on the initial photon energy.11-12 Similarly, SF starts with the absorption of a photon to create a singlet exciton with an energy E(S 1 ) (Figure 1, Step 1) roughly twice that of the triplet E(T 1 ). In addition to the isoenergetic case (E(S 1 ) = 2E(T 1 )), SF may proceed as an exothermic (E(S 1 ) > 2E(T 1 )) or endothermic (E(S 1 ) < 2E(T 1 )) process, the latter of which minimizes ultimate energetic loss. The singlet exciton interacts with its neighbor, either through space (i.e. monomers in an aggregate or solid)13-14 or through covalent bonds (i.e. dimers, oligomers)15-28 to create a correlated triplet pair 1(TT) (Figure 1, Step 2), which conserves a net singlet spin.29-31 This triplet pair has been shown in some cases to evolve to a state with quintet character that may resist dissociation.32-33 The details of the evolution are currently unclear, but ultimately we desire the formation of free triplets (Figure 1, Step 3), which can be harvested independently through charge or energy transfer to an acceptor (Figure 1, Step 4a, 4b). Smith and Michl have written two comprehensive reviews on the nuances of SF theory, to which the interested reader can refer for details.4-5



Figure 1. Illustration of SF and triplet charge extraction (4a) or triplet energy transfer (4b) at a donor/acceptor interface (top), and the associated spin diagram (bottom). See text for details. Recent efforts in our group have focused on developing a fundamental understanding of triplet transport and extraction to determine appropriate design principles for SF solar cells. Successful integration of SF materials into solar cell devices entails: (1) Strong photon absorption within the visible region of the solar spectrum (2) Fast and efficient SF that outcompetes parallel and detrimental decay pathways (3) Effective triplet pair decorrelation to avoid annihilation (4a) Fast triplet exciton dissociation to charges at an interface with an acceptor or


Page 4 of 40

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


(4b) Efficient triplet energy transfer to a suitable semiconductor partner for exciton harvesting Several studies have demonstrated operational solar cells using SF materials;34-35 however, parameters affecting the rate and efficiency of SF, as well as the presence of competitive deactivation pathways preclude reaching PCE above the S-Q limit. In addition to variables like driving force for triplet exciton dissociation, harvesting of SF excitons may be affected by crystal morphology, molecular packing, and electronic coupling between neighbors. Whereas manipulating these factors in homogeneous thin film configurations is relatively straightforward, introduction of an interface to extract these triplets complicates our control over nearest neighbor interactions. With less control over the crystal phase and molecular packing compared with neat films, the key to success in this arena is to characterize and tune electronic state energies and intermolecular coupling at nanoscale interfaces and then properly manage the kinetic balance between desirable and deleterious pathways. For example, relatively slow SF in some systems does not impede efficient triplet production; however, introduction of an acceptor may distort the kinetic balance in the neat film by enabling fast singlet exciton dissociation or energy transfer, which act as parasitic pathways when the ultimate goal is exciton multiplication. In other situations, the acceptor may play a supportive role. For example, fast geminate recombination of a triplet pair may reduce long-lived triplet yields, in particular for intramolecular SF systems, but a pathway toward even faster triplet energy transfer or dissociation may mitigate these losses. This Feature article describes our recent research in the context of effective SF solar cell design, which not only leverages our basic understanding of SF but also aims for control of all contributing aspects. First, factors affecting intramolecular and intermolecular SF in solution and in thin films will be presented. Later, we will discuss the incorporation of SF molecules into donor/acceptor assemblies. One line of research explores the competition between SF and 5 ACS Paragon Plus Environment


electron injection at metal oxide interfaces for use in dye-sensitized solar cells (DSSCs). Another approach employs SF molecules attached to QDs for potential application in the enhancement of silicon solar cells. We highlight both the fundamental research necessary to elucidate the photophysical pathways in these systems and the challenges associated with routes toward practical implementation. 2. Singlet Fission 2.1 Intermolecular Singlet Fission in Solid State Films Intermolecular SF has been widely investigated in solutions and thin films of tetracene,13-14, 3644

pentacene,14, 26, 45-52 perylene diimides,53-55 among an expanding list of compounds.20, 22, 31, 56-68

Systematic alterations of the crystal phase, crystal size, and molecular packing in thin films of tetracene and 1,3-diphenylisobenzofurans have allowed us to quantify changes in SF rates and efficiencies.41, 68-73 Intermolecular coupling not only controls the intrinsic SF rate constant in crystals, it can also play a significant role in determining the fate of the nascent triplets. For example, strong coupling appropriate for triplet hopping can improve the chance of triplet pair separation.30 Furthermore, the triplet exciton diffusivity, which is important for transporting excitons toward interfaces, depends on both intermolecular arrangements and the relative S 1 -TT energies, which defines the reversibility of SF to partially repopulate the fast-moving singlet.36 Below, we illustrate two recent examples from our lab regarding the control of SF rate constants via the engineering of intermolecular interactions in polycrystalline films.


Page 6 of 40

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


Figure 2. A. AFM images of two polymorphs of tetracene, Tc I (top), and Tc II (bottom) with small (left) and large (right) crystallites. Scale bars = 2 µm. B. Orientation of dimers in the abplane with an [a b] translation vector of [½ ½] in Tc I (top) and Tc (II) bottom. C. Singlet exciton decay time constants derived from global fit of SVD data for both polymorphs with large and small crystallites from low to high fluence. Colors in panel C correspond to box outlines in panel A. Data were collected at room temperature. Reprinted with permission from Ref. 41. Published by The Royal Society of Chemistry. The most stable polymorph of tetracene (Tc I, Figure 2B, top) has nearest neighbor interactions in the unit cell that are slightly displaced along the c-axis in the crystal structure, creating a slipped herringbone arrangement. Depositing thin films on substrates cooled to liquid nitrogen temperature produces a second polymorph that exhibits tighter packing along the c-axis (Tc II, Figure 2B, bottom).41 SF in the tightly packed Tc II films occurs with a 22 ps time constant in large crystallites, which is much faster than the more stable Tc I at 125 ps. Creating small crystallites significantly decreases the time scale for SF in Tc I to 33 ps but does not strongly affect the SF time for Tc II (τ ~ 31 ps). Closer examination of the ab-plane reveals larger intermolecular separations in Tc I films, which may lead to the slower observed SF compared with the denser unit cell of Tc II. Calculations based on a dimer model of SF interactions contradict these observations and appear to overemphasize the role of symmetry in determining the SF coupling matrix elements, while the influence of delocalization in polyacene crystals may be a more crucial factor.74 The crystallite size dependence found for Tc I may relate 7 ACS Paragon Plus Environment


to the motion of singlet excitons toward crystallite surfaces where broken symmetry and SFenhanced geometries could lead to faster triplet formation.40

Figure 3. A. Structure of 1,3-diphenylisobenzofuran (1) and alkylated derivatives. B. Decay associated spectra at long times for thin films normalized to triplet peak at 21500 cm-1. C. Kinetics associated with the triplet in thin films with λ probe = 21900 cm-1, corresponding to the T 1 -T 5 absorption. Adapted with permission from Ref. 73. Copyright 2017 American Chemical Society. 1,3-Diphenylisobenzofuran (DPIBF) is a model chromophore for SF with optimal energy alignment that has exhibited a 200% SF quantum yield in polycrystalline thin films at 77 K.68 Two polymorphs of DPIBF, α-DPIBF and the thermodynamically stable β-DPIBF, display remarkably different behaviors, despite both adopting a slip-stacked configuration of nearest neighbors in the unit cell. SF is dominant in α-DPIBF configurations, while excimer formation and fluorescence in the β-DPIBF configuration prevent the efficient formation of triplets via 8 ACS Paragon Plus Environment

Page 8 of 40

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


SF.68, 70 Although the photophysical picture is complex, it is increasingly clear that longer-range communication between the slip-stacks, and not within them, may play a significant role in determining the SF rate and enabling independent triplet formation. In designing SF solar cells, ensuring a well-defined crystallinity of the SF material may be desirable but is not assured, given the inherent difficulty in controlling the packing in a heterogeneous assembly or at an interface. To understand the effects of disruption of the typical slip-stacked packing on the typically efficient SF in DPIBF, a series of alkyl-substituted DPIBF molecules (Figure 3A) was investigated.73 Methyl (DPIBF-Me) or t-butyl (DPIBF-tBu) substitution at the para positions, or t-butyl substitution at both meta positions (DPIBF-(tBu) 2 ) of one phenyl ring increases intermolecular distance and prevents long-range crystallization when deposited into thin films, resulting in quasi-amorphous packing. DPIBF-Me is partially crystalline but does not form pristine crystalline layers as is the case with DPIBF. Photoexcitation of the substituted thin films results in a two-phase progression of the excited state in all derivatives. In the simplest case, DPIBF-Me, the S 1 state decays in 17 ps to reveal a broad absorption in the red (Figure 3B), attributed to excimers. On longer time scales, singlet states in SF-facilitating environments undergo SF in 294 ps with a triplet yield of 75% (± 20%). Excimer formation is more prominent with the tBu substitution, though they form more slowly than in DPIBF-Me, with time constants of 35 ps in DPIBF-tBu and 76 ps in DPIBF-(tBu) 2 . Likewise, bulkier substitution leads to slower SF and lower triplet yields, with time scales of 483 ps and 832 ps, and yields of 55% (±15%) and 34% (±15%) for DPIBF-tBu and DPIBF-(tBu) 2 , respectively. The heterogeneity in sites where excitons are initially photogenerated results in thin films that may have several paths for excited state evolution, which includes the parasitic lowenergy excimer pathway.75 SF-active sites are the most prevalent in DPIBF-Me films, but the 9 ACS Paragon Plus Environment


existence of excimer geometries75 in amorphous regions leads to deleterious excited-state deactivation pathways and decreased SF as crystallinity is reduced. Other systems less likely to form excimers may not suffer from this detrimental pathway in amorphous phases.76 2.2 Intramolecular Singlet Fission and Triplet Pair Separation Because strong interactions between molecules are essential for rapid SF and consequently high triplet yields, considering alternative ways of increasing coupling that do not rely on van der Waals interactions is important. Synthesis of covalently bound chromophores constructed into dimers, oligomers, and polymers provides control over SF rates through defined interchromophore geometries.22, 27, 77-78 The strength of coupling thus becomes a function of through-bond and through-space interactions, which can be manipulated by the position of chromophore attachment. Triplet separation is critically important in these systems because the limited geometrical space of dimer or oligomer chains can predispose triplet pairs to fast annihilation. Energetic considerations remain important in dimers, as through-bond delocalization can result in significant S 1 energy depression. Our group recently observed this phenomenon in DPIBF dimers. There, monomers of DPIBF, as in Figure 3, were covalently attached directly through the para, para’ position, creating a dimer with potential for full conjugation through the bridge. The dimer was found to have two low-energy isomers in both the lowest excited singlet and triplet states, one with a near-planar and the other with a twisted geometry about the central bond connecting the halves.25 Selective excitation of these isomers in solution leads to either excited state delocalization over the entire chromophore, or localization on one half of the dimer. The delocalized state in the near-planar isomer stabilizes the S 1 energy more than that of T 1 , increasing the endothermicity of SF, whereas the localized excited state in the twisted case allows SF to proceed with an efficiency of ~30%. The triplet of the planarized 10 ACS Paragon Plus Environment

Page 10 of 40

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


isomer is observed (Φ T < 5%), but it is similarly delocalized over both halves of the dimer, and thus photoexcitation of the near-planar isomer represents a detrimental pathway that inhibits triplet state separation and localization. Other DPIBF dimers with longer bridges or substitution to enforce chromophore twisting exhibit localized S 1 states but poorer coupling and smaller triplet yields.20 Adjusting the properties of the bridge to both modulate the SF coupling and to isolate triplets has become a common theme in recent studies of dimers.16, 26-28, 77 In particular, strategies that incorporate pathways for the eventual formation of well-isolated triplet states after SF have improved chances of efficient triplet harvesting.

Figure 4. A. Molecular structures for repeat units of PBTDO1 -A (left) and -B (right). B. Illustration demonstrating two types of triplet formation in polymer thin films. Purple signifies delocalized singlet excitations, and red denotes a coupled triplet pair in (i) and free triplets in (ii) C. Kinetic scheme demonstrating the two possible types of triplet formation in polymer thin films. Case (i) represents excitation at a site where free triplet formation might be unfavorable, 11 ACS Paragon Plus Environment


while case (ii) represents excitation at a site where free triplet formation might be favorable. GS = ground state. Adapted with permission from Ref. 78. Copyright 2017 American Chemical Society. Pathways to triplet pair separation not obviously available in dimers and small oligomers may be offered by conjugated polymers, which have shown promise as efficient SF materials.79-81 For example, poly(benzothiophene dioxide) (PBTDO1) undergoes intramolecular SF in solution in less than 10 ps with a triplet yield of 170%.79 However, the triplet lifetime is exceedingly short with very few triplet excitations remaining beyond 1 ns. Our group recently elaborated on this original report by studying how the introduction of interchain coupling affects SF in the solid state of PBTDO1.78 Although PBTDO1 forms amorphous films, varying the side chains from linear (PBTDO1-A) to branched (PBTDO1-B) is expected to influence the local microstructure in the solid state by sterically inhibiting close packing of chains. Initial triplet yields in films at 10 ps match those of solvated chains at 130 ± 20% and 150 ± 30% for PBTDO1-A and PBTDO1-B, respectively, which proves that the intrinsic SF process (i.e., production of (TT) species) is intramolecular in nature and does not benefit from interchain interactions. However, at times greater than 1 ns in films, triplet excitations persist for both PBTDO1-A and PBTDO1B, unlike in solution-phase dynamics, where long-lived triplet yields are diminutive. Moreover, there is a two-fold increase in the long-lived triplet yield for PBTDO1-A (24 ± 4%) vs. PBTDO1-B (12 ± 1%). The existence of long-lived triplets in films implicates interchain coupling (i.e. offering a 2D or 3D transport network as opposed to 1D) as the precursor to triplet pair separation. In addition, the enhancement of triplet yield in the less sterically encumbered A polymer suggests that modulating the coupling through side-chain engineering directly correlates with efficiency of triplet separation, without influencing the initial steps in the SF process. Although determination of the local interactions that specifically lead to enhanced interchain triplet hopping is extremely difficult, we can hypothesize that close approach “contact” points 12 ACS Paragon Plus Environment

Page 12 of 40

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


between thiophene dioxide subunits facilitates interchain triplet hopping, Figure 4B. Indeed, for many systems with heterogeneous interchromophore coupling, a similar situation may arise in which both triplet pair “traps” as well as avenues for separation are ingrained in the local molecular packing structure.82-84 Controlling the density of strong coupling points between chains may allow for larger free triplet yields. 3. Harvesting Triplets through Interfacial Charge or Energy Transfer Examples we have illustrated thus far have been concerned with generating free triplet pairs quickly via SF and subsequently prolonging their lifetime such that harvesting their energy is plausible. The next step towards realistic SF solar cell devices is incorporating SF molecules at interfaces with charge or energy acceptors. A balance between interchromophore coupling and coupling at the donor/acceptor interface is the primary challenge associated with this endeavor. Introduction of acceptors leads to deleterious deactivation pathways, such as electron transfer from the singlet, which may outcompete SF on the surface. Processes like triplet-charge annihilation may also reduce the triplet lifetime. Our recent studies seek to understand these competing pathways in different types of chromophore assemblies at organic-inorganic interfaces. Fortunately, a reasonably large library of versatile SF molecules is becoming available and can be integrated into many types of assemblies, often with directed interactions through strategic substitution. Here, we will discuss two acceptors for harvesting triplets from SF: metal oxides,85-86 and quantum dots (QD).87 3.1. Metal Oxide Interfaces DSSCs combine semiconductor metal oxide nanoparticles with light-harvesting chromophores to generate electricity.88 Visible light excitation of a dye adsorbed to the metal



oxide surface is followed by excited-state electron injection into the semiconductor conduction band and regeneration of the oxidized chromophore by a redox electrolyte (e.g. I 3 -/I-).1, 7, 89 Typically, DSSCs consist of inorganic dyes (e.g. functionalized ruthenium (II) tris-bipyridine) and a high band-gap n-type semiconductor (e.g. titanium dioxide, TiO 2 and tin oxide, SnO 2 ).90-92 The mechanism of electron injection and recombination on inorganic-based DSSCs is wellstudied.1 An important finding by Zigler and coworkers showed the rate and efficiency of electron injection into TiO 2 from a series of ruthenium dyes depends on the overlap between the excited state manifold of the chromophore with the density of states in the conduction band of the semiconductor.89 In other words, sufficient driving force for electron injection is vital to build a functioning DSSC. The efficiency of DSSCs using conventional chromophores has risen to roughly 14%,93 which falls well below the thermodynamic limit and that of other highperforming PV materials classes. However, many important principles associated with molecular assembly and photoinduced electron transfer have been learned from the DSSC platform. We believe that leveraging this knowledge will facilitate progress toward incorporation of SF-relevant molecules and identification of key principles that transcend a particular device architecture. For example, the broken symmetry and dielectric structure of an interface may influence how SF proceeds.94 In addition, sampling of heterogeneous environments by migration of excitons lateral to the interface may expedite discovery of optimized SF sites, triplet dissociation sites, or trap sites (e.g., excimers). Using SF molecules as sensitizers in DSSCs (SF-DSSCs) would potentially allow for two electron injection events per absorbed photon, increasing photocurrents in the high energy portion of the solar spectrum, and improving device efficiencies by utilizing a simple parallel electrical scheme.95-96 Loading of the dye on the semiconductor surface typically occurs through 14 ACS Paragon Plus Environment

Page 14 of 40

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


binding groups (e.g. carboxylic acids, phosphonic acids) covalently attached to the dye.1 Soaking metal oxide films in a dye solution leads to high surface coverages and as such, DSSCs are relatively easy to assemble. Because all molecules naturally reside at the interface, the potential need for triplet exciton diffusion toward the interface prior to exciton dissociation is not a significant issue. However, this configuration introduces a number of challenges to consider. In polycrystalline thin film configurations, molecules are interacting in ordered three-dimensional lattices, whereas SF dye loading on the metal oxide surface occurs primarily as a disordered monolayer, limiting the dimensionality and homogeneity of interactions. Additionally, singlet injection is often fast and efficient for most dyes, setting up a competition with producing triplets via SF. Finally, the driving force for triplet injection is undoubtedly smaller than that of singlets, and may often be insufficient to dissociate triplet excitons, although studies of DSSCs operating via triplets have been reported.89, 97 Work on SF-DSSCs is in its infancy with only a few examples in the literature.85-86, 98-99 Recent efforts in our group have focused on uncovering the mechanism of SF in DSSC configurations using several common SF molecules.

Figure 5. A. Schematic of SF and interfacial charge transfer in 1,3-diphenylisobenzofuran adsorbed to a TiO 2 /ZrO 2 core/shell film. Approximate conduction band and HOMO/LUMO energies are shown. B. Current density (Jsc) vs. cycles of insulating layer of ZrO 2 for a 40 mM concentration of ZrO 2 precursor solution. Inset shows a 150 mM concentration of precursor. C. Injection yield predicted from kinetic simulation with S 1 injection (blue), T 1 injection (red), and total (gray). The dashed gray line represents a triplet injection efficiency (Φ T ) of 50% and a SF 15 ACS Paragon Plus Environment


time constant (τ SF ) of 5 ps. Adapted with permission from Ref. 86. Copyright 2015 American Chemical Society. The SF model compound, DPIBF, was studied as a chromophore for SF-DSSCs. Both direct surface adhesion to mesoporous TiO 2 without a binding group and covalent deposition with a carboxylic acid-functionalized DPIBF dye were attempted and found to be successful for generating a fairly large photocurrent. Initial experiments revealed fast injection from the singlet excited state within 1 ps, deactivating the SF pathway, which has a time constant of ~30 ps. To slow the singlet injection and favor SF on the surface, a thin layer of an insulator, zirconia (ZrO 2 ) was introduced via chemical bath deposition (Figure 5A). The photocurrent of the DPIBF DSSC was measured as a function of the thickness of the zirconia layer. With an insulating layer of 10-15 Å, the time constant of singlet injection is slowed sufficiently, such that SF is competitive and subsequently the two produced triplet states inject into TiO 2 at a reduced rate. Indeed, the observation of a discontinuity in the photocurrent decay as a function of barrier thickness supports the proposed scheme, Figure 5B. Despite observable SF, the overall triplet yield remained low due to the mixture of α-DPIBF and β-DPIBF morphologies on the surface, assuming only α-DPIBF undergoes SF from the study discussed in Section 2.1. Thus, the overall contribution of SF-born triplets to the photocurrent was relatively small, Figure 5C, and pathways to control local molecular alignment were not pursued, partially due to the relative instability of DPIBF chromophores to photooxidation. This study uncovered the potential utility of SF molecules in DSSC configurations, but also pointed to the challenge of controlling the surface arrangement to maximize SF.


Page 16 of 40

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


Figure 6. A. Molecular structure of TIPS-pentacene functionalized with a carboxylic acid group for surface attachment. B. UV-Vis absorption spectra of TIPS-pentacene in solution and polycrystalline film, and functionalized TIPS-pentacene in solution and adsorbed on TiO 2 . C. Molecular structure of TIPS-tetracene functionalized with a carboxylic acid group for surface attachment. D. UV-Vis absorption spectra of TIPS-tetracene in solution and polycrystalline film, and functionalized TIPS-tetracene in solution and adsorbed on TiO 2 . Reprinted with permission from Ref. 85. Published by The Royal Society of Chemistry. Further studies with functionalized pentacene (Figure 6A) and tetracene (Figure 6C) derivatives attached to TiO 2 were conducted using transient absorption spectroscopy to understand the connection between driving force for electron injection and SF. 6,13Bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) and TIPS-tetracene were adsorbed to the TiO 2 surface through carboxylic acid groups, and were found to exhibit minimal signs of intermolecular coupling as judged by only slight perturbations of their absorption spectra



compared with solution (Figure 6B, 6D). For TIPS-pentacene, this weaker intermolecular coupling increases the SF time constant from 150 fs in films to 6.5 ps on the TiO 2 surface. This reduction in SF rate may be due to a variety of factors, including larger intermolecular separation distance, reduced long range order, and reduced dimensionality of the monolayer compared with that of polycrystalline films. It is possible that SF active configurations are rare in the selfassembled layer, and singlet excitons need to hop to a favorable site with appropriate orientations to undergo SF. The combination of factors influenced by intermolecular coupling and order (e.g., SF driving force, SF coupling, rate of exciton migration) may lead to a complex situation that requires further investigation. Once created, triplets from TIPS-pentacene do not inject into TiO 2 , which is likely due to a lack of sufficient driving force. Although driving force can be difficult to determine using oxidation and reduction potentials derived from measurements made under different conditions, our estimates suggest that triplet exciton dissociation for the TIPS-pentacene/TiO 2 interface is thermodynamically unfavorable by at least 200 meV. TIPS-tetracene/TiO 2 interfaces may have a slightly more favorable driving force for triplet injection. However, the slower intrinsic SF rate (i.e., ∼20 ps in films) favors singlet injection over SF, and an injection time of 2 ps is measured using time-resolved infrared spectroscopy. Atomic layer deposition was used to create an insulating alumina (Al 2 O 3 ) layer on the TiO 2 surface in order to impede singlet injection and favor SF. With this insulating layer, SF in adsorbed TIPS-tetracene was observed to occur with a time constant of ∼40 ps. Again, these triplets do not dissociate and apparently lack the appropriate driving force for electron injection. Approaches toward adjusting the driving force in acene/oxide photoelectrodes have been suggested. The Guldi group has explored alternative metal oxides and electrolyte composition to 18 ACS Paragon Plus Environment

Page 18 of 40

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


promote triplet injection from pentacene derivatives. In particular, they found that tailoring the quasi Fermi level of the semiconductor through indium doped zinc oxide (IZO), as well as addition of Li+ cations, facilitated electron injection from triplet states generated by intersystem crossing (ISC)98 and SF.99 However, despite carrier multiplication efficiencies as high as 130%, device efficiencies remained low at 0.03% and 0.06% for SF devices and 1.5% for charge injection from ISC. Substitution of SF dyes to adjust redox potentials and the utilization of other oxide acceptors remain unexplored but represent promising avenues toward more efficient triplet injection. If electron injection from the triplet state after SF does occur, the cation left behind may interact with remaining triplet states and reduce the efficiency of the second triplet dissociation. However, triplet exciton hopping, cross-surface charge transfer,100 and redox mediation should help to alleviate negative effects. Lessons learned from DSSC approaches in the related field of triplet-triplet annihilation upconversion (TTA-UC) may also find utility in SF devices. In particular, the formation of dye multilayers101 may provide intermolecular coupling pathways beneficial for both faster SF and triplet pair separation prior to charge injection. 3.2. Quantum Dot/Ligand Assemblies The archetypal SF utilization scheme involves a high-performing solar cell, such as c-Si, functionalized with a blue/green absorbing layer that produces and transfers two triplet excitons per absorbed photon to the solar cell across the organic/semiconductor interface. The challenges associated with the direct transfer at this interface are significant, and no clear demonstrations of this operating principle have been shown.35 Quantum dots (QDs) that have been ligandexchanged with SF molecules, particularly acenes, are of significant interest because they enable an alternative pathway by which incident photons can be utilized to minimize losses due to thermalization.87, 102-104 SF followed by triplet energy transfer to the QD could result in emission 19 ACS Paragon Plus Environment


of two low energy photons, effectively doubling the photoluminescence quantum efficiency over direct excitation of the QD. If properly matched to the band gap, the photons emitted by the QD could be reabsorbed by silicon, for example,8, 105 leading to the desirable outcome described above. We note that the reverse process of TTA-UC may also benefit from similar ligandexchanged QD samples, as near-IR absorption by the core of the QD has been shown to lead to triplet energy transfer to a surface-bound acene ligand.106-107 TTA-UC between two ligands creates a high-energy singlet on an emitter molecule that radiates a visible photon. In this fashion, photons below the band gap of a particular semiconductor can still be harvested.

Figure 7. Schematic of energy flow in three different sizes of PbS QDs sensitized with TIPStetracene ligands. A. Photophysical processes occurring after photoexcitation of TIPS-tetracene attached to QD with E g = 1.45 eV. B. Photophysical processes occurring after photoexcitation of TIPS-tetracene attached to QD with E g = 1.18 eV. C. Photophysical processes occurring after photoexcitation of TIPS-tetracene attached to QD with E g = 0.95 eV. Reprinted with permission from Ref 87. Copyright 2018 American Chemical Society. A plethora of studies now exist in the literature that characterize dynamics between QDs and photoactive ligands.87, 102-104, 108-109 In terms of SF and subsequent triplet energy transfer or dissociation, the ligand-exchanged QD offers the unique opportunity to control energy offsets


Page 20 of 40

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


strictly through variations in core QD size. The dependence of triplet energy transfer back and forth across the interface on relative alignment of molecular and QD energy levels is a key parameter for facilitating high emission efficiencies from QDs coupled to SF-born triplets. In addition, ligand exchange has become fairly sophisticated such that coverage and packing density can be controlled,110-111 which provides the opportunity to tune local intermolecular interactions at the surface. QD surface trap states can also be characterized and to some extent controlled,112-113 such that their role in energy transfer processes might be elucidated. Our group recently correlated PbS QD size with energy transfer rates to TIPS-tetracene ligands.87 Quantum confinement effects lead to an increase in the band gap of the QD with decreasing size. Here, QDs with band gaps E g of 0.95 eV, 1.18 eV, and 1.45 eV were synthesized and capped, both partially and fully, with TIPS-tetracene ligands. Photoexcitation of the lowest energy singlet of the TIPS-tetracene ligands leads to rapid energy transfer to the 1S exciton of the PbS QD within ∼2 ps. This fast singlet energy transfer renders SF noncompetitive, even in the optimistic situation of strong intermolecular coupling at the surface. However, triplets do form on the ligands on the 50-200 ns time scale due to reverse exciton transfer from the QDs. Transfer to the triplet state of the ligand occurs in the 1.18 eV and 1.45 eV band gap QDs (TIPS-tetracene E T ∼ 1.2 eV), but there is insufficient driving force for this transfer in the largest QD, where E g =0.95 eV, Figure 7. In the two cases where triplet energy transfer is possible, there is a recycling process between population in the T 1 state of the ligand and the 1S state of the QD, creating a quasi-equilibrium that extends the lifetime of the exciton from ~1.5-2 µs to up to 30 µs for a partially exchanged QD, where triplet-triplet annihilation is suppressed. We note that in the case of SF-born triplets, unidirectional triplet energy transfer to the QD is



desired, and at a rate that is controlled such that Auger recombination (active when two excitons are present in the QD) is minimized. Work on a similar system by Bender et. al. with PbS QDs and TIPS-pentacene functionalized with a phenyl spacer (to prevent TTA-UC) suggested that triplet energy transfer between QDs and acenes is surface-state mediated.114 These surface-states may facilitate triplet energy transfer or hinder it depending on the location and energy relative to the surface-bound acenes, offering another potential route to control excited state dynamics. Much like acene/metal oxide interfaces, controlling the orientation and coupling between the surface-bound molecules on QDs is crucial for manipulating the excited state pathways toward SF; however, much remains to be understood about methods for manipulating and characterizing local molecular ordering at the QD surface. Progress in this area should unlock some of the substantial versatility of QD/ligand systems and potentially allow triplet excitons generated by SF to be transferred to QDs to more efficiently harness the full solar spectrum. 4. Conclusions and Outlook Singlet fission materials have the potential to push solar cell efficiencies beyond conventional limits in a variety of architectures that have advantages over existing tandem cells. Utilizing SF to overcome these theoretical limits not only requires a yield of triplets that approaches 200%, but also an advanced understanding of the factors that may improve or hinder device performance. Having demonstrated efficient SF and its dependence on molecular interactions, we have explored SF in the context of the other relevant solar harvesting processes of triplet exciton dissociation and triplet energy transfer. We have undertaken this task for both intramolecular and intermolecular SF and found unique challenges in both situations. For intramolecular SF systems, in which efficient SF is ensured by defined interchromophore 22 ACS Paragon Plus Environment

Page 22 of 40

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


interactions, we find the need to enable pathways for triplet pair separation, which is essential for enhancing triplet lifetimes and engendering independent triplet exciton formation. For intermolecular SF, triplet pair separation is relatively facile, but the efficient generation of triplets requires manipulation of subtle interchromophore interactions in a noncovalent and often non-crystalline environment. Preliminary experimental explorations of SF solar cell architectures have proved challenging but informative. Attaching functionalized acenes to metal oxide semiconductors in a dyesensitized solar cell configuration introduces new competitive pathways detrimental to efficient SF, particularly, injection from excited singlet states. Additionally, if SF occurs on the surface, the driving force for triplet charge injection into the semiconductor conduction band is low. Prospects for multiple triplets to dissociate will improve if SF rates are enhanced with optimized intermolecular coupling schemes and if triplet charge transfer is made exoergic through tuning of dye and/or semiconductor redox potentials. Ligand-exchanged quantum dots can accept triplet energy and subsequently emit photons, and they also offer unique opportunities for systematic control of dynamics through size variation. However, energy transfer from the singlet state of the acene ligand is fast if it is directly coupled to the QD surface, which outcompetes SF. Nonetheless, we find that the QD quickly converts the singlet exciton to a triplet that can transfer across the QD/ligand interface efficiently and reversibly. Manipulation of energy transfer rates via surface functionalization of the QD or enhanced SF rates in the ligand shell may ameliorate the loss mechanisms encountered thus far. Efforts by several groups in this area have already begun. The fundamental challenges associated with multiple exciton harvesting are most likely to be overcome through systematic and rational control of the molecular and nanoscale semiconductor 23 ACS Paragon Plus Environment


properties. Fortunately, the toolbox of SF molecules and nanoscale acceptors is well-developed and ever growing as researchers join efforts in this burgeoning field. As is often the case, the rewards for intensive dynamical investigations on novel molecules and materials are likely to extend beyond the target of ultraefficient photovoltaics toward fields that may leverage triplet excitons for other important optoelectronic or photoelectrochemical schemes.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work was authored by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences. We acknowledge Obadiah Reid for a critical reading of the manuscript and all prior contributors to the published work we presented. The views expressed in the article do not necessarily represent 24 ACS Paragon Plus Environment

Page 24 of 40

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


the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

REFERENCES 1.

Ashford, D. L.; Gish, M. K.; Vannucci, A. K.; Brennaman, M. K.; Templeton, J. L.;

Papanikolas, J. M.; Meyer, T. J. Molecular Chromophore-Catalyst Assemblies for Solar Fuel Applications. Chem. Rev. 2015, 115, 13006-13049. 2.

Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.;

House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J. Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells. J. Am. Chem. Soc. 2016, 138, 13085-13102. 3.

Rao, A.; Friend, R. H. Harnessing Singlet Exciton Fission to Break the Shockley-

Queisser Limit. Nat. Rev. Mat. 2017, 2, 17063. 4.

Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, 6891-6936.

5.

Smith, M. B.; Michl, J. Recent Advances in Singlet Fission. Annu. Rev. Phys. Chem.

2013, 64, 361-386. 6.

Tayebjee, M. J. Y.; McCamey, D. R.; Schmidt, T. W. Beyond Shockley-Queisser:

Molecular Approaches to High-Efficiency Photovoltaics. J. Phys. Chem. Lett. 2015, 6, 23672378. 7.

Paci, I.; Johnson, J. C.; Chen, X.; Rana, G.; Popovic, D.; David, D. E.; Nozik, A. J.;

Ratner, M. A.; Michl, J. Singlet Fission for Dye-Sensitized Solar Cells: Can a Suitable Sensitizer Be Found? J. Am. Chem. Soc. 2006, 128, 16546-16553. 8.

Futscher, M. H.; Rao, A.; Ehrler, B. The Potential of Singlet Fission Photon Multiplers as

an Alternative to Silicon-Based Tandem Solar Cells. ACS Energy Lett. 2018, 3, 2587-2592.



9.

Beard, M. C.; Johnson, J. C.; Luther, J. M.; Nozik, A. J. Multiple Exciton Generation in

Quantum Dots Versus Singlet Fission in Molecular Chromophores for Solar Photon Conversion. Phil. Trans. R. Soc. A 2015, 373, 20140412. 10.

Beard, M. C.; Blackburn, J. L.; Johnson, J. C.; Rumbles, G. Status and Prognosis of

Future Generation Photoconversion to Photovoltaics and Solar Fuels. ACS Energy Lett. 2016, 1, 344-347. 11.

Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev,

A.; Efros, A. L. Highly Efficient Multiple Exciton Generation in Colloidal PbS and PbSe Quantum Dots. Nano Lett. 2005, 5, 865-871. 12.

Nozik, A. J. Multiple Exciton Generation in Semiconductor Quantum Dots. Chem. Phys.

Lett. 2008, 457, 3-11. 13.

Burdett, J. J.; Muller, A. M.; Gosztola, D.; Bardeen, C. J. Excited State Dynamics in

Solid and Monomeric Tetracene: The Roles of Superradiance and Exciton Fission. J. Chem. Phys. 2010, 133, 144506. 14.

Zimmerman, P. M.; Bell, F.; Casanova, D.; Head-Gordon, M. Mechanism for Singlet

Fission in Pentacene and Tetracene: From Single Exciton to Two Triplets. J. Am. Chem. Soc. 2011, 133, 19944-52. 15.

Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.; Haiges, R.; Krylov, A. I.; Bradforth,

S. E.; Thompson, M. E. Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer. J. Am. Chem. Soc. 2016, 138, 617-27. 16.

Korovina, N. V.; Joy, J.; Feng, X.; Feltenberger, C.; Krylov, A. I.; Bradforth, S. E.;

Thompson, M. E. Linker-Dependent Singlet Fission in Tetracene Dimers. J. Am. Chem. Soc. 2018, 140, 10179-10190. 17.

Cook, J. D.; Carey, T. J.; Damrauer, N. H. Solution-Phase Singlet Fission in a

Structurally Well-Defined Norbornyl-Bridged Tetracene Dimer. J. Phys. Chem. A 2016, 120, 4473-4481. 18.

Feng, X.; Krylov, A. I. On Couplings and Excimers: Lessons from Studies of Singlet

Fission in Covalently Linked Tetracene Dimers. Phys. Chem. Chem. Phys. 2016, 18, 7751-7761. 19.

Greyson, E. C.; Vura-Weis, J.; Michl, J.; Ratner, M. A. Maximizing Singlet Fission in

Organic Dimers: Theoretical Investigation of Triplet Yield in the Regime of Localized Excitation and Fast Coherent Electron Transfer. J. Phys. Chem. B 2010, 114, 14168-14177. 26 ACS Paragon Plus Environment

Page 26 of 40

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


20.

Johnson, J. C.; Akdag, A.; Zamadar, M.; Chen, X. D.; Schwerin, A. F.; Paci, I.; Smith,

M. B.; Havlas, Z.; Miller, J. R.; Ratner, M. A., et al. Toward Designed Singlet Fission: Solution Photophysics of Two Indirectly Coupled Covalent Dimers of 1,3-Diphenylisobenzofuran. J. Phys. Chem. B 2013, 117, 4680-4695. 21.

Lindquist, R. J.; Lefler, K. M.; Brown, K. E.; Dyar, S. M.; Marguiles, E. A.; Young, R.

M.; Wasielewski, M. R. Energy Flow Dynamics within Cofacial and Slip-Stacked Perylene-3,4Dicarboximide Dimer Models of P-Aggregates. J. Am. Chem. Soc. 2014, 136, 14912-14923. 22.

Margulies, E. A.; Miller, C. E.; Wu, Y.; Ma, L.; Schatz, G. C.; Young, R. M.;

Wasielewski, M. R. Enabling Singlet Fission by Controlling Intramolecular Charge Transfer in Pi-Stacked Covalent Terrylenediimide Dimers. Nat. Chem. 2016, 8, 1120-1125. 23.

Mirjani, F.; Renaud, N.; Gorczak, N.; Grozema, F. C. Theoretical Investigation of Singlet

Fission in Molecular Dimers: The Role of Charge Transfer States and Quantum Interference. J. Phys. Chem. C. 2014, 118, 14192-14199. 24.

Muller, A. M.; Avlasevich, Y. S.; Mullen, K.; Bardeen, C. J. Evidence for Exciton

Fission and Fusion in a Covalently Linked Tetracene Dimer. Chem. Phys. Lett. 2006, 421, 518522. 25.

Schrauben, J. N.; Akdag, A.; Wen, J.; Havlas, Z.; Ryerson, J. L.; Smith, M. B.; Michl, J.;

Johnson, J. C. Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran. J. Phys. Chem. A 2016, 120, 3473-3483. 26.

Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E. T.; Casillas, R.; Basel, B. S.;

Thoss, M.; Tykwinski, R. R.; Guldi, D. M. Singlet Fission in Pentacene Dimers. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5235-5330. 27.

Sanders, S. N.; Kumarasamy, E.; Pun, A.B.; Tuan Trinh, M.; Choi, B.;

Xia, J.; Taffet, E.J.; Low, J.Z.; Miller, J.R.; Roy X., et al. Quantitative Intramolecular Singlet Fission in Bipentacenes. J. Am. Chem. Soc. 2015, 137, 8965-8972. 28.

Kumarasamy, E., Sanders, S. N.; Tayebjee, M.J.Y.; Asadpoordarvish, A.; Hele, T.J.H.;

Fuemmeler, E.G.; Pun, A.B.; Yablon, L.M.; Low, J.Z.; Paley, D.W., et al. Tuning Singlet Fission in Pi-Bridge-Pi Chromophores. J. Am. Chem. Soc. 2017, 139, 12488-12494. 29.

Yong, C. K.; Musser, A.J.; Bayliss, S.L.; Lukman, S.; Tamura, H.; Bubnova, O.; Hallani,

R.K.; Meneau, A.; Resel, R.; Maruyama, M.; et al. The Entangled Triplet Pair State in Acene and Heteroacene Materials. Nat. Comm. 2017, 8, 15953. 27 ACS Paragon Plus Environment


30.

Pensack, R. D.; Ostroumov, E. E.; Tilley, A. J.; Mazza, S.; Grieco, C.; Thorley, K. J.;

Asbury, J. B.; Seferos, D. S.; Anthony, J. E.; Scholes, G. D. Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission. J. Phys. Chem. Lett. 2016, 7, 2370-2375. 31.

Breen, I.; Tempelaar, R.; Bizimana, L. A.; Kloss, B.; Reichman, D. R.; Turner, D. B.

Triplet Separation Drives Singlet Fission after Femtosecond Correlated Triplet Pair Production in Rubrene. J. Am. Chem. Soc. 2017, 139, 11745-11751. 32.

Tayebjee, M. J. Y.; Sanders, S. N.; Kumarasamy, E.; Campos, L. M.; Sfeir, M. Y.;

McCamey, D. R. Quintet Multiexciton Dynamics in Singlet Fission. Nature Physics 2017, 13, 182-188. 33.

Weiss, L. R.; Bayliss, S. L.; Kraffert, F.; Thorley, K. J.; Anthony, J. E.; Bittl, R.; Friend,

R. H.; Rao, A.; Greenham, N. C.; Behrends, J. Strongly Exchange-Coupled Triplet Pairs in an Organic Semiconductor. Nature Physics 2017, 13, 176-181. 34.

Yang, L.; Tabachnyk, M.; Bayliss, S. L.; Bohm, M. L.; Broch, K.; Greenham, N. C.;

Friend, R. H.; Ehrler, B. Solution-Processable Singlet Fission Photovoltaic Devices. Nano Lett. 2015, 15, 354-358. 35.

MacQueen, R. W.; Liebhaber, M.; Niederhausen, J.; Mews, M.;

Gersmann, C.; Jackle, S.; Jager, K.; Tayebjee, M.J.Y.; Schmidt, T.W.; Rech B., et al. Crystalline Silicon Solar Cells with Tetracene Interlayers: The Path to Silicon Singlet Fission Heterojunction Devices. Mater. Horiz. 2018, 5, 1065-1075. 36.

Wan, Y.; Guo, Z.; Zhu, T.; Yan, S.; Johnson, J. C.; Huang, L. Cooperative Singlet and

Triplet Exciton Transport in Tetracene Crystals Visualized by Ultrafast Microscopy. Nat. Chem. 2015, 7, 785-792. 37.

Burdett, J. J.; Gosztola, D.; Bardeen, C. J. The Dependence of Singlet Exciton Relaxation

on Excitation Density and Temperature in Polycrystalline Tetracene Thin Films: Kinetic Evidence for a Dark Intermediate State and Implications for Singlet Fission. J. Chem. Phys. 2011, 135, 214508. 38.

Grumstrup, E. M.; Johnson, J. C.; Damrauer, N. H. Enhanced Triplet Formation in

Polycrystalline Tetracene Films by Femtosecond Optical-Pulse Shaping. Phys. Rev. Lett. 2010, 105, 257403.


Page 28 of 40

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


39.

Muller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Mullen, K.; Bardeen, C. J. Exciton

Fission and Fusion in Bis(Tetracene) Molecules with Different Covalent Linker Structures. J. Am. Chem. Soc. 2007, 129, 14240-14250. 40.

Piland, G. B.; Bardeen, C. J. How Morphology Affects Singlet Fission in Crystalline

Tetracene. J. Phys. Chem. Lett. 2015, 6, 1841-1876. 41.

Arias, D. H.; Ryerson, J. L.; Cook, J. D.; Damrauer, N. H.; Johnson, J. C. Polymorphism

Influences Singlet Fission Rates in Tetracene Thin Films. Chem Sci 2016, 7, 1185-1191. 42.

Burdett, J. J.; Bardeen, C. J. Quantum Beats in Crystalline Tetracene Delayed

Fluorescence Due to Triplet Pair Coherences Produced by Direct Singlet Fission. J. Am. Chem. Soc. 2012, 134, 8597-607. 43.

Birech, Z.; Schwoerer, M.; Schmeiler, T.; Pflaum, J.; Schwoerer, H. Ultrafast Dynamics

of Excitons in Tetracene Single Crystals. J. Chem. Phys. 2014, 140, 114501. 44.

Tayebjee, M. J. Y.; Clady, R. G. C. R.; Schmidt, T. W. The Exciton Dynamics in

Tetracene Thin Films. Phys. Chem. Chem. Phys. 2013, 15, 14797-14805. 45.

Tabachnyk, M.; Karani, A.H.; Broch, K; Pazos-Outón, L.M; Xiao, J.; Jellicoe, T.C.;

Novák, J.; Harkin, D.; Pearson, A.J.; Rao, A., et al. Efficient Singlet Exciton Fission in Pentacene Prepared from a Soluble Precursor. APL Materials 2016, 4. 46.

Hart, S. M.; Silva, W. R.; Frontiera, R. R. Femtosecond Stimulated Raman Evidence for

Charge-Transfer Character in Pentacene Singlet Fission. Chem. Sci. 2018, 9, 1242-1250. 47.

Wilson, M. W.; Rao, A.; Ehrler, B.; Friend, R. H. Singlet Exciton Fission in

Polycrystalline Pentacene. Acc. Chem. Res. 2013, 46, 1330-1338. 48.

Zimmerman, P. M.; Zhang, Z.; Musgrave, C. B. Singlet Fission in Pentacene through

Multi-Exciton Quantum States. Nat. Chem. 2010, 2, 648-52. 49.

Kaur, I.; Jia, W.; Kopreski, R. P.; Selvarasah, S.; Dokmeci, M. R.; Pramanik, C.;

McGruer, N. E.; Miller, G. P. Substituent Effects in Pentacenes: Gaining Control over HomoLumo Gaps and Photooxidative Resistances. J. Am. Chem. Soc. 2008, 130, 16274-16826. 50.

Wilson, M. W.; Rao, A.; Clark, J.; Kumar, R. S.; Brida, D.; Cerullo, G.; Friend, R. H.

Ultrafast Dynamics of Exciton Fission in Polycrystalline Pentacene. J. Am. Chem. Soc. 2011, 133, 11830-3.



51.

Marciniak, H.; Fiebig, M.; Huth, M.; Schiefer, S.; Nickel, B.; Selmaier, F.; Lochbrunner,

S. Ultrafast Exciton Relaxation in Microcrystalline Pentacene Films. Phys. Rev. Lett. 2007, 99, 176402. 52.

Marciniak, H.; Pugliesi, I.; Nickel, B.; Lochbrunner, S. Ultrafast Singlet and Triplet

Dynamics in Microcrystalline Pentacene Films. Phys. Rev. B 2009, 79, 235318. 53.

Renaud, N.; Grozema, F. C. Intermolecular Vibrational Modes Speed up Singlet Fission

in Perylenediimide Crystals. J. Phys. Chem. Lett. 2015, 6, 360-365. 54.

Eaton, S. W.; Shoer, L. E.; Karlen, S. D.; Dyar, S. M.; Margulies, E. A.; Veldkamp, B.

S.; Ramanan, C.; Hartzler, D. A.; Savikhin, S.; Marks, T. J., et al. Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide. J. Am. Chem. Soc. 2013, 135, 14701-12. 55.

Le, A. K.; Bender, J. A.; Arias, D. H.; Cotton, D. E.; Johnson, J. C.; Roberts, S. T. Singlet

Fission Involves an Interplay between Energetic Driving Force and Electronic Coupling in Perylenediimide Films. J. Am. Chem. Soc. 2018, 140, 814-826. 56.

Musser, A. J.; Al-Hashimi, M.; Maiuri, M.; Brida, D.; Heeney, M.; Cerullo, G.; Friend,

R. H.; Clark, J. Activated Singlet Exciton Fission in a Semiconducting Polymer. J. Am. Chem. Soc. 2013, 135, 12747-12754. 57.

Marguiles, E. A.; Logsdon, J. L.; Miller, C. E.; Ma, L.; Simonoff, E.; Young, R. M.;

Schatz, G. C.; Wasielewski, M. R. Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films. J. Am. Chem. Soc. 2017, 139, 663671. 58.

Hartnett, P. E.; Marguiles, E. A.; Mauck, C. M.; Miller, S. A.; Wu, Y.; Wu, Y.-L.; Marks,

T. J.; Wasielewski, M. R. Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrrole Thin Films. J. Phys. Chem. B 2016, 120, 1357-1366. 59.

Pensack, R. D.; Tilley, A. J.; Parkin, S. R.; Lee, T. S.; Payne, M. M.; Gao, D.; Jahnke, A.

A.; Oblinsky, D. G.; Li, P.-F.; Anthony, J. E.; Seferos, D. S., et al. Exciton Delocalization Drives Rapid Singlet Fission in Nanoparticles of Acene Derivatives. J. Am. Chem. Soc. 2015, 137, 6790-6803. 60.

Wang, C.; Tauber, M. J. High-Yield Singlet Fission in a Zeaxanthin Aggregate Observed

by Picosecond Resonance Raman Spectroscopy. J. Am. Chem. Soc. 2010, 132, 13988-13991.


Page 30 of 40

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


61.

Piland, G. B.; Burdett, J. J.; Kurunthu, D.; Bardeen, C. J. Magnetic Field Effects on

Singlet Fission and Fluorescence Decay Dynamics in Amorphous Rubrene. J. Phys. Chem. C 2013, 117, 1224-1236. 62.

Miller, C. E.; Wasielewski, M. R.; Schatz, G. C. Modeling Singlet Fission in Rylene and

Diketopyrrolopyrrole Derivatives: The Role of the Charge Transfer State in Superexchange and Excimer Formation. J. Phys. Chem. C. 2017, 121, 10345-10350. 63.

Busby, E.; Berkelbach, T. C.; Kumar, B.; Chernikov, A.; Zhong, Y.; Hlaing, H.; Zhu, X.

Y.; Heinz, T. F.; Hybertsen, M. S.; Sfeir, M. Y., et al. Multiphonon Relaxation Slows Singlet Fission in Crystalline Hexacene. J. Am. Chem. Soc. 2014, 136, 10654-10660. 64.

Musser, A. J.; Maiuri, M.; Brida, D.; Cerullo, G.; Friend, R. H.; Clark, J. The Nature of

Singlet Exciton Fission in Carotenoid Aggregates. J. Am. Chem. Soc. 2015, 137, 5130-5139. 65.

Eaton, S. W.; Miller, S. A.; Margulies, E. A.; Shoer, L. E.; Schaller, R. D.; Wasielewski,

M. R. Singlet Exciton Fission in Thin Films of Tert-Butyl-Substituted Terrylenes. J Phys Chem A 2015, 119, 4151-61. 66.

Tamai, Y.; Ohkita, H.; Benten, H.; Ito, S. Singlet Fission in Poly(9,9-Di-N-

Octylfluorene) Films. J. Phys. Chem. C 2013, 117, 10277-10284. 67.

Wang, C.; Schlamadinger, D. E.; Desai, V.; Tauber, M. J. Triplet Excitons of Carotenoids

Formed by Singlet Fission in a Membrane. ChemPhysChem 2011, 12, 2891-2894. 68.

Ryerson, J. L.; Schrauben, J. N.; Ferguson, A. J.; Sahoo, S. C.; Naumov, P.; Havlas, Z.;

Michl, J.; Nozik, A. J.; Johnson, J. C. Two Thin Film Polymorphs of the Singlet Fission Compound 1,3-Diphenylisobenzofuran. J. Phys. Chem. C 2014, 118, 12121-12132. 69.

Johnson, J. C.; Nozik, A. J.; Michl, J. High Triplet Yield from Singlet Fission in a Thin

Film in 1,3-Diphenylisobenzofuran. J. Am. Chem. Soc. 2010, 132, 16302-16303. 70.

Schrauben, J. N.; Ryerson, J. L.; Michl, J.; Johnson, J. C. Mechanism of Singlet Fission

in Thin Films of 1,3-Diphenylisobenzofuran. J. Am. Chem. Soc. 2014, 136, 7363-73. 71.

Johnson, J. C.; Nozik, A. J.; Michl, J. The Role of Chromophore Coupling in Singlet

Fission. Acc. Chem. Res. 2013, 46, 1290-1299. 72.

Schwerin, A. F.; Johnson, J. C.; Smith, M. B.; Sreearunothai, P.; Popovic, D.; Cerny, J.;

Havlas, Z.; Paci, I.; Akdag, A.; MacLeod, M. K. et al. Toward Designed Singlet Fission: Electronic States and Photophysics of 1,3-Diphenylisobenzofuran. J. Phys. Chem. A 2010, 114, 1457-1473. 31 ACS Paragon Plus Environment


73.

Dron, P. I.; Michl, J.; Johnson, J. C. Singlet Fission and Excimer Formation in

Disordered Solids of Alkyl-Substituted 1,3-Diphenylbenzofurans. J. Phys. Chem. A 2017, 121, 8596-8603. 74.

Refaely-Abramson, S.; da Jornada, F. H.; Louie, S. G.; Neaton, J. B. Origins of Singlet

Fission in Solid Pentacene from an Ab Initio Green's Function Approach. Phys. Rev. Lett. 2017, 119, 267401. 75.

Dover, C. B.; Gallaher, J. K.; Frazer, L.; Tapping, P. C.; Petty II, A. J.; Crossley, M. J.;

Anthony, J. E.; Kee, T. W.; Schmidt, T. W. Endothermic Singlet Fission Is Hindered by Excimer Formation. Nat. Chem. 2018, 10, 305-310. 76.

Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.; Brutchey,

R. L.; Thompson, M. E.; Bradforth, S. E. Efficient Singlet Fission Discovered in a Disordered Acene Film. J. Am. Chem. Soc. 2012, 134, 6388-6400. 77.

Cook, J. D.; Carey, T. J.; Arias, D. H.; Johnson, J. C.; Damrauer, N. H. Solvent-

Controlled Branching of Localized Versus Delocalized Singlet Exciton States and Equilibration with Charge Transfer in a Structurally Well-Defined Tetracene Dimer. J. Phys. Chem. A 2017, 121, 9229-9242. 78.

Pace, N. P.; Zhang, W.; Arias, D. H.; McCulloch, I.; Rumbles, G.; Johnson, J. C.

Controlling Long-Lived Triplet Generation from Intramolecular Singlet Fission in the Solid State. J. Phys. Chem. Lett. 2017, 8, 6086-6091. 79.

Busby, E.; Xia, J.; Wu, Q.; Low, J. Z.; Song, R.; Miller, J. R.; Zhu, X.-Y.; Campos, L.

M.; Sfeir, M. Y. A Design Strategy for Intramolecular Singlet Fission Mediated by ChargeTransfer States in Donor-Acceptor Organic Materials. Nat. Mater. 2015, 14, 426-433. 80.

Masoomi-Godarzi, S.; Liu, M.; Tachibana, Y.; Goerigk, L.; Ghiggino, K. P.; Smith, T.

A.; Jones, D. J. Solution-Processable, Solid State Donor-Acceptor Materials for Singlet Fission. Adv. Energy Mater. 2018, 1801720. 81.

Hu, J.; Xu, K.; Shen, L.; Wu, Q.; He, G.; Wang, J.-Y.; Pei, J.; Xia, J.; Sfeir, M. Y. New

Insights into the Design of Conjugated Polymers for Intramolecular Singlet Fission. Nat. Comm. 2018, 9. 82.

Pensack, R. D.; Grieco, C.; Purdum, G. E.; Mazza, S. M.; Tilley, A. J.; Ostroumov, E.

E.; Seferos, D. S.; Loo, Y.-L.; Asbury, J. B.; Anthony, J. E., et al. Solution-Processable, Crystalline Materials for Quantitative Singlet Fission. Mater. Horiz. 2017, 4, 915-923. 32 ACS Paragon Plus Environment

Page 32 of 40

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


83.

Pensack, R. D.; Tilley, A. J.; Grieco, C.; Purdum, G. E.; Ostroumov, E. E.; Granger, D.

B.; Oblinsky, D. G.; Dean, J. C.; Doucette, G. S.; Asbury, J. B., et al. Striking the Right Balance of Intermolecular Coupling for High-Efficiency Singlet Fission. Chem. Sci. 2018, 9, 6240-6259. 84.

Lubert-Perquel, D.; Salvadori, E.; Dyson, M.; Stavrinou, P. N.; Montis, R.; Nagashima,

H.; Kobori, Y.; Heutz, S.; Kay, C. W. M. Identifying Triplet Pathways in Dilute Pentacene Films. Nat. Comm. 2018, 9, 4222. 85.

Pace, N. P.; Arias, D. H.; Granger, D. B.; Christensen, S.; Anthony, J. E.; Johnson, J. C.

Dynamics of Singlet Fission and Electron Injection in Self-Assembled Acene Monolayers on Titanium Dioxide. Chem. Sci. 2018, 9, 3004-3013. 86.

Schrauben, J. N.; Zhao, Y.; Mercado, C.; Dron, P. I.; Ryerson, J. L.; Michl, J.; Zhu, K.;

Johnson, J. C. Photocurrent Enhanced by Singlet Fission in a Dye-Sensitized Solar Cell. ACS Appl. Mater. Interfaces 2015, 7, 2286-2293. 87.

Kroupa, D. M.; Arias, D. H.; Blackburn, J. L.; Carroll, G. M.; Granger, D. B.; Anthony,

J. E.; Beard, M. C.; Johnson, J. C. Control of Energy Flow Dynamics between Tetracene Ligands and PbS Quantum Dots by Size Tuning and Ligand Coverage. Nano Lett. 2018, 18, 865-873. 88.

Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells.

Chem. Rev. 2010, 110, 6595-6663. 89.

Zigler, D. F.; Morseth, Z. A.; Wang, L.; Ashford, D. L.; Brennaman, M. K.; Grumstrup,

E. M.; Brigham, E. C.; Gish, M. K.; Dillon, R. J.; Alibabaei, L.Zigler, D. F., et al. Disentangling the Physical Processes Responsible for the Kinetic Complexity in Interfacial Electron Transfer of Excited Ru(II) Polypyridyl Dyes on TiO 2 . J. Am. Chem. Soc. 2016, 138, 4426-4438. 90.

Ardo, S.; Meyer, G. J. Photodriven Heterogeneous Charge Transfer with Transition-

Metal Compounds Anchored to TiO 2 Semiconductor Surfaces. Chem. Soc. Rev. 2009, 38, 115164. 91.

Gish, M. K.; Lapides, A. M.; Brennaman, M. K.; Templeton, J. L.; Meyer, T. J.;

Papanikolas, J. M. Ultrafast Recombination Dynamics in Dye-Sensitized SnO 2 /TiO 2 Core/Shell Films. J. Phys. Chem. Lett. 2016, 7, 5297-5301. 92.

Ai, X.; Anderson, N. A.; Guo, J. C.; Lian, T. Q. Electron Injection Dynamics of Ru

Polypyridyl Complexes on SnO 2 Nanocrystallite Thin Films. J. Phys. Chem. B 2005, 109, 70887094.



93.

Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya, M. Highly-Efficient

Dye-Sensitized Solar Cells with Collaborative Sensitization by Silyl-Anchor and CarboxyAnchor Dyes. Chem. Commun. 2015, 51, 15894-15897. 94.

Petelenz, P.; Snamina, M. Locally Broken Crystal Symmetry Facilitates Singlet Exciton

Fission. J. Phys. Chem. Lett. 2016, 7, 1913-1916. 95.

Hanna, M. C.; Nozik, A. J. Solar Conversion Efficiency of Photovoltaic and

Photoelectrolysis Cells with Carrier Multiplication Absorbers. J. Appl. Phys. 2006, 100, 074510. 96.

Jeong, N. C.; Son, H.-J.; Prasittichai, C.; Lee, C. Y.; Jensen, R. A.; Farha, O. K.; Hupp, J.

T. Effective Panchromatic Sensitization of Electrochemical Solar Cells: Strategy and Organizational Rules for Spatial Separation of Complementary Light Harvesters on High-Area Photoelectrodes. J. Am. Chem. Soc. 2012, 134, 19820-19827. 97.

Kallioinen, J.; Benko, G.; Sundstrom, V.; Korppi-Tommola, J. E. I.; Yartsev, A. P.

Electron Transfer from the Singlet and Triplet Excited States of Ru(dcbpy) 2 (NCS) 2 into Nanocrystalline TiO 2 Thin Films. J. Phys. Chem. B 2002, 106, 4396-4404. 98.

Kunzmann, A.; Gruber, M.; Casillas, R.; Tykwinski, R. R.; Costa, R. D.; Guldi, D. M.

Tuning Pentacene Based Dye-Sensitzed Solar Cells. Nanoscale 2018, 10, 8515. 99.

Kunzmann, A.; Gruber, M.; Casillas, R.; Zirzlmeier, J.; Stanzel, M.; Peukert, W.;

Tykwinski, R. R.; Guldi, D. M. Singlet Fission for Photovoltaics with 130% Injection Efficiency. Angew. Chem. Int. Ed. 2018, 57, 10742-10747. 100.

Brennaman, M. K.; Gish, M. K.; Alibabaei, L.; Norris, M. R.; Binstead, R. A.; Nayak, A.;

Lapides, A. M.; Song, W.; Brown, R. J.; Concepcion, J. J., et al. Pathways Following Electron Injection: Medium Effects and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore-Catalyst Assembly on TiO 2 . J. Phys. Chem. C 2018, 122, 13017-13026. 101.

Dilbeck, T.; Wang, J. C.; Zhou, Y.; Olsson, A.; Sykora, M.; Hanson, K. Èlucidating the

Energy- and Electron- Transfer Dynamics of Photon Upconversion in Self-Assembled Bilayers. J. Phys. Chem. C. 2017, 121, 19690-19698. 102.

Ehrler, B.; Wilson, M. W.; Rao, A.; Friend, R. H.; Greenham, N. C. Singlet Exciton

Fission-Sensitized Infrared Quantum Dot Solar Cells. Nano Lett. 2012, 12, 1053-7. 103.

Garakyaraghi, S.; Mongin, C.; Granger, D. B.; Anthony, J. E.; Castellano, F. N. Delayed

Molecular Triplet Generation from Energized Lead Sulfide Quantum Dots. J. Phys. Chem. Lett. 2017, 8, 1458-1463. 34 ACS Paragon Plus Environment

Page 34 of 40

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


104.

Davis, N. J. L. K.; Allardice, J. R.; Xiao, J.; Petty, A. J.; Greenham, N. C.; Anthony, J.

E.; Rao, A. Singlet Fission and Triplet Transfer to PbS Quantum Dots in Tips-Tetracene Carboxylic Acid Ligands. J. Phys. Chem. Lett. 2018, 9, 1454-1460. 105.

Pazos-Outon, L. M.; Lee, J. M.; Futscher, M. H.; Kirch, A.; Tabachnyk, M.; Friend, R.

H.; Ehrler, B. A Silicon-Singlet Fission Tandem Solar Cell Exceeding 100% External Quantum Efficiency with High Spectral Stability. ACS Energy Lett. 2017, 2, 476-480. 106.

Huang, Z.; Li, X.; Mahboub, M.; Hanson, K. M.; Nichols, V. M.; Le, H.; Tang, M. L.;

Bardeen, C. J. Hybrid Molecule-Nanocrystal Photon Upconversion across the Visible and nearInfrared. Nano Lett. 2015, 15, 5552-5557. 107.

Mongin, C.; Garakyaraghi, S.; Razgoniaeva, N.; Zamkov, M.; Castellano, F. N. Direct

Observation of Triplet Energy Transfer from Semiconductor Nanocrystals. Science 2016, 351, 369-372. 108.

Tabachnyk, M.; Ehrler, B.; Gelinas, S.; Bohm, M. L.; Walker, B. J.; Musselman, K. P.;

Greenham, N. C.; Friend, R. H.; Rao, A. Resonant Energy Transfer of Triplet Excitons from Pentacene to PbSe Nanocrystals. Nat. Mater. 2014, 13, 1033-1038. 109.

Thompson, N. J.; Wilson, M. W. B.; Congreve, D. N.; Brown, P. R.; Scherer, J. M.;

Bischof, Thomas S.; Wu, M.; Geva, N.; Welborn, M., et al. Energy Harvesting of Non-Emissive Triplet Excitons in Tetracene by Emissive PbS Nanocrystals. Nat. Mater. 2014, 13, 1039-1043. 110.

Wang, C.; Kodaimati, M.; Lian, S.; Weiss, E. Systematic Control of the Rate of Singlet

Fission within 6,13-Diphenylpentacene Aggregates Adsorbed to PbS Quantum Dots. Faraday Discuss. 2018, Just Accepted. 111.

De Nolf, K.; Cosseddu, S. M.; Jasieniak, J. J.; Drijvers, E.; Martins, J. C.; Infante, I.;

Hens, Z. Binding and Packing in Two-Component Collodial Quantum Dot Ligand Shells: Linear Versus Branched Carboxylates. J. Am. Chem. Soc. 2017, 139, 3456-3464. 112.

Gao, J.; Johnson, J. C. Charge Trapping in Bright and Dark States of Coupled PbS

Quantum Dot Films. ACS Nano 2012, 6, 3292-3303. 113.

Caram, J. R.; Bertram, S. N.; Utzat, H.; Hess, W. R.; Carr, J. A.; Bischof, T. S.; Beyler,

A. P.; Wilson, M. W. B.; Bawendi, M. G. PbS Nanocrystal Emission Is Governed by Multiple Emissive States. Nano Lett. 2016, 16, 6070-6077.



114.

Bender, J. A.; Raulerson, E. K.; Li, X.; Goldzak, T.; Xia, P.; Van Voorhis, T.; Tang, M.

L.; Roberts, S. T. Surface States Mediate Triplet Energy Transfer in Nanocrystal-Acene Composite Systems. J. Am. Chem. Soc. 2018, 140, 7543-7553.


Page 36 of 40

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


TOC Graphic



Biographies and Photos

Melissa K. Gish received her B.S. in Chemistry from the University of Southern California in 2011. She was awarded her Ph.D. in 2018 from the University of North Carolina at Chapel Hill, where she worked with Professor John Papanikolas to study fundamental electron transfer processes in dye-sensitized photoelectrosynthesis cells using ultrafast spectroscopies. She is currently a post-doctoral researcher at the National Renewable Energy Laboratory working with Dr. Justin Johnson to develop and study singlet fission solar cells.

Natalie Pace is currently completing her Ph.D. in physical chemistry from the University of Colorado at Boulder. She received her bachelor's degree in chemistry from Grinnell College. She


Page 38 of 40

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


uses ultrafast spectroscopy to study photoinduced charge transfer from singlet and triplet states at the National Renewable Energy Laboratory.

Garry Rumbles received his PhD from the University of London in 1984. He was a member of faculty in the department of chemistry, Imperial College, London from 1989 to 2001. In 2000 he joined NREL where he is now a senior research fellow. Dr Rumbles is an adjoint professor of chemistry at the University of Colorado Boulder. His research interests are in the field of photochemistry with a focus on photoinduced electron transfer processes in organic semiconductors.

Justin Johnson received his bachelor's degree from Macalester College in 1999 and his Ph.D. in chemistry from the University of California, Berkeley, in 2004. He did postdoctoral work with Dr. Arthur Nozik at National Renewable Energy Laboratory (NREL) and Prof. Josef Michl at the 39 ACS Paragon Plus Environment


University of Colorado, Boulder. He joined the research staff at NREL in 2008 and is currently a senior scientist, investigating the dynamics of photophysical phenomena associated with solar light harvesting, energy storage, and quantum information.


Page 40 of 40

Emerging Design Principles for Enhanced Solar Energy Utilization

Recommend Documents