Pushing the Boundaries of Endothermic Singlet Fission in Organic Semiconductors through Structure and Energetic Control

Singlet exciton fission (SF) is an exciton multiplication process in organic semiconductors in which a photoexcited singlet exciton is converted to two triplet excitons. SF based solar cells help break the Shockley-Quisser limit as high energy photons are used to generate two electron-hole pairs per photon absorbed with power conversion efficiencies (PCE) ~40%. Energetics of the states involved in SF determine the nature of driving force for SF, making it either an entropy-driven endothermic system or enthalpy-driven exothermic system. Many efforts to understand and implement SF in photovoltaics (PV) have been undertaken in the recent years. However, a fundamental understanding of triplet formation and physical parameters that govern the process is incomplete, and of paramount importance. In this dissertation, we study how two key parameters influence endothermic SF: morphology and energetics. SF occurs via two steps in acenes: a photoexcited singlet, S1, decays to form TT, and TT breaks into two free triplets, T1. The role played by morphology or the local geometry of molecules in triplet generation via SF is vital, as the process depends on the placement of adjacent molecular pairs. Morphology, determined by molecular structure as well as sample preparation methods, is thus linked directly to the efficiency of SF. We first demonstrate this in a model system: endothermic SF nanoparticles. Further, we engineer these nanoparticles to suit triplet transfer into inorganics for PV applications. The effect of molecular and crystal structures on SF dynamics is further explored in a new class of photo-stable molecules, thienoanthracenes. We investigate the endothermicity of thienoanthracenes by assessing their singlet and triplet energies. Thereafter, we delve into thermodynamic and kinetic parameters that determine efficient SF in thienoanthracenes. We choose one of the thienoanthracenes as a model system to understand what drives endothermic SF efficiently in it. An entropy based model with a statistical mechanics perspective is built to predict key factors that help assess the thermodynamic feasibility of SF in endothermic systems.Dr. Manmohan Singh Scholarship (2015-2019) Winton Programme for the Physics of Sustainability ERC as part of the European Union’s Horizon 2020 research and innovation programme (grant agreement No 758826

Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission.

Singlet fission is an exciton multiplication process in organic molecules in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley-Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extraction of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorganic PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley-Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a solution-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and time-resolved measurements combined with analytical modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs >95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the solution phase (135 ± 5%) before quantitative harvesting of the triplet excitons (95 ± 5%) via a low concentration of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is critical to engineer the surface of the QD with a triplet transfer ligand and that bimolecular decay of triplets is potentially a major loss pathway which can be controlled via tuning the concentration of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the solution-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between organic semiconductors and QDs, one that can provide clear design rules for new materials.ER

Kinetics of Melanin Polymerization during Enzymatic and Nonenzymatic Oxidation

Melanin is an abundant biopigment in the animal kingdom, but its structure remains poorly understood. This is a substantial impediment to understanding the mechanistic origin of its observed functions. Proposed models of melanin structure include aggregates of both linear and macrocyclic units and noncovalently held monomers. Both models are broadly in agreement with current experimental data. To constrain the structural and kinetic models of melanin, experimental data of high resolution with chemical specificity accompanied by atomistic modeling are required. We have addressed this by obtaining electronic absorption, infrared, and ultraviolet resonance Raman (RR) spectra of melanin at several wavelengths of excitation that are sensitive to small changes in structure. From these experiments, we observed kinetics of the formation of different species en route to melanin polymerization. Exclusive chemical signatures of monomer 3,4-dihydroxyphenylalanine (dopa), intermediate dopachrome (DC), and early-time polymer are established through their vibrational bands at 1292, 1670, and 1616 cm^–1 respectively. Direct evidence of reduced heterogeneity of melanin oligomers in tyrosinase-induced formation is provided from experimental measurements of vibrational bandwidths. Models made with density functional theory show that the linear homopolymeric structures of 5,6-dihydroxyindole can account for experimentally observed wavenumbers and broad bandwidth in Raman spectra of dopa-melanin. We capture resonance Raman (RR) signature of DC, the intermediate stabilized by the enzyme tyrosinase, for the first time in an enzyme-assisted melanization reaction using 488 nm excitation wavelength and propose that this wavelength can be used to probe reaction intermediates of melanin formation in solution

Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission.

Singlet fission is an exciton multiplication process in organic molecules in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley-Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extraction of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorganic PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley-Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a solution-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and time-resolved measurements combined with analytical modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs >95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the solution phase (135 ± 5%) before quantitative harvesting of the triplet excitons (95 ± 5%) via a low concentration of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is critical to engineer the surface of the QD with a triplet transfer ligand and that bimolecular decay of triplets is potentially a major loss pathway which can be controlled via tuning the concentration of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the solution-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between organic semiconductors and QDs, one that can provide clear design rules for new materials.ER

Observation of Vibronic -Coupling -Mediated Energy Transfer in Light-Harvesting Nanotubes Stabilized in a Solid-State Matrix

Ultrafast vibrational spectroscopy is employed to obtain real-time structural information of energy transport in double-walled light harvesting nanotubes at room temperature, stabilized in a host matrix to mimic the rigid scaffolds of natural light harvesting systems. We observe evidence of a low-frequency vibrational mode at 315 cm-1 which transfers excitons from the outer wall of the nanotubes to a crossing point through which energy transfer to the inner wall can occur. This mode is furthermore absent in solution phase. Importantly, the coherence of this mode is not transferred to the inner wall upon energy transfer and is only present on the outer wall’s excited state energy surface, highlighting that complete energy transfer between the outer and inner walls does not take place. Isolation of the individual walls of the nanotubes provides evidence that this mode corresponds to a supramolecular motion of the nanotubes. Our results emphasize the importance of the solid state environment in modulating vibronic coupling and directing energy transfer in molecular light-harvesting systems.Winton Program for Physics of Sustainabilit

Elucidation of Excitation Energy Dependent Correlated Triplet Pair Formation Pathways in an Endothermic Singlet Fission System

Singlet fission is the spin-allowed conversion of a photogenerated singlet exciton into two triplet excitons in organic semiconductors, which could enable single-junction photovoltaic cells to break the Shockley–Queisser limit. The conversion of singlets to free triplets is mediated by an intermediate correlated triplet pair (TT) state, but an understanding of how the formation and dissociation of these states depend on energetics and morphology is lacking. In this study, we probe the dynamics of TT states in a model endothermic fission system, TIPS-Tc nanoparticles, which show a mixture of crystalline and disordered regions. We observe the formation of different TT states, with varying yield and different rates of singlet decay, depending on the excitation energy. An emissive TT state is observed to grow in over 1 ns when excited at 480 nm, in contrast to excitation at lower energies where this emissive TT state is not observed. This suggests that the pathway for singlet fission in these nanoparticles is strongly influenced by the initial sub-100 fs relaxation of the photoexcited state away from the Franck–Condon point, with multiple possible TT states. On nanosecond time scales, the TT states are converted to free triplets, which suggests that TT states might diffuse into the disordered regions of the nanoparticles where their breakup to free triplets is favored. The free triplets then decay on μs time scales, despite the confined nature of the system. Our results provide important insights into the mechanism of endothermic singlet fission and the design of nanostructures to harness singlet fission

Research data supporting "Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission"

This dataset consists of graphical and tabular data in an Origin file format. The file includes UV-Vis absorption, PLQE, kinetic modelling, transient PL and absorption, steady-state PL and excitation spectra and magnetic field dependent PL measurement data and analysis. Further information about the data collection methods and analysis is available via the journal JACS, at 10.1021/jacs.9b06584. The Origin file "Analysis.opju" contains the data for all plots presented in the paper and SI titled "Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission", along with additional data surrounding the analysis of the presented data. The file is separated into folders sorted by experiment. Figures used in the paper are prefixed with either "Main Fig" or "SI" followed by a brief description of the figure