Striking the Right Balance of Intermolecular Coupling for High-Efficiency Singlet Fission

Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet–triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields

Computationally Aided Design of a High-Performance Organic Semiconductor: The Development of a Universal Crystal Engineering Core

Herein, we describe the design and synthesis of a suite of molecules based on a benzodithiophene “universal crystal engineering core”. After computationally screening derivatives, a trialkylsilylethyne-based crystal engineering strategy was employed to tailor the crystal packing for use as the active material in an organic field-effect transistor. Electronic structure calculations were undertaken to reveal derivatives that exhibit exceptional potential for high-efficiency hole transport. The promising theoretical properties are reflected in the preliminary device results, with the computationally optimized material showing simple solution processing, enhanced stability, and a maximum hole mobility of 1.6 cm2 V−1 s−1

Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells

Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells

THE IMPACT OF INTERMOLECULAR INTERACTIONS ON THE THIN-FILM MORPHOLOGY OF NAPHTHALENE TETRACARBOXYLIC DIIMIDES

Molecular semiconductor thin films have garnered significant interest for their use as the active components in lightweight, large-area, and flexible electronic devices. The kinetic constraints associated with rapid film deposition often leave films in non-optimal polycrystalline or amorphous states. Because organic solids are held together by weak non-covalent interactions, proper manipulation of molecular interactions can provide control over the film structure through subsequent post-deposition processing. In this thesis, we develop robust processing-structure-function relationships governing the structural development of a series of naphthalene tetracarboxylic diimide (NTCDI) derivatives through exploring the relative contributions of molecule-molecule, molecule-solvent, and molecule-substrate interactions. Like many other molecular systems, the NTCDI derivatives exhibit polymorphism, or the ability to adopt multiple crystal structures. While subtle changes in chemistry between the NTCDI derivatives do not greatly impact their attainable polymorphs, they do induce changes in the molecule-molecule interactions present between molecular layers within the crystal structures of each derivative; we found the presence of short intermolecular contacts to directly correlate with polymorphic stability of these NTCDI derivatives. This short interlayer contact framework for assessing polymorphic stability is broadly applicable to other molecular semiconductors with disparate chemistries and to a variety of organic materials, including biological building blocks and pharmaceutics. While molecule-molecule interactions can be used as a proxy for polymorphic stability, manipulating molecule-solvent and molecule-substrate interactions can further unravel the rich structural phase-space of the NTCDI derivatives. We demonstrate that controlling the molecule-solvent interactions during solvent-vapor annealing alters the relative growth rates along different crystallographic directions. This results in crystals that can adopt a continuum of habits, ranging from plate-like to needle-like domains, depending on the solvent choice and vapor concentration. Introducing a templating layer prior to deposition effectively modulates molecule-substrate interactions; properly lattice-matched templates can enable heteroepitaxial growth of specific polymorphs that are otherwise inaccessible. By understanding the relative contributions of different types of intermolecular interactions in molecular systems, we demonstrate the ability to unlock the rich structural phase space of the NTCDI derivatives. The processing-structure-function relationships developed within this thesis can assist future materials development, not only for molecular semiconductors, but for organic materials in general

Enhancing Carrier Mobilities in Organic Thin-Film Transistors Through Morphological Changes at the Semiconductor/Dielectric Interface Using Supercritical Carbon Dioxide Processing

Charge-carrier mobilities in poly­(3-hexylthiophene) (P3HT) organic thin-film transistors (OTFTs) increase 5-fold when OTFTs composed of P3HT films on trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (FTS) monolayers supported on SiO₂ dielectric substrates (P3HT/FTS/SiO₂/Si) are subjected to supercritical carbon dioxide (scCO₂) processing. In contrast, carrier mobilities in P3HT/octadecyltrichlorosilane (OTS)/SiO₂ OTFTs processed using scCO₂ are comparable to mobilities measured in as-cast P3HT/OTS/SiO₂/Si devices. Topographical images of the free and buried interfaces of P3HT films reveal that scCO₂ selectively alters the P3HT morphology near the buried P3HT/FTS-SiO₂ interface; identical processing has negligible effects at the P3HT/OTS-SiO₂ interface. A combination of spectroscopic ellipsometry and grazing-incidence X-ray diffraction experiments indicate insignificant change in the orientation distribution of the intermolecular π–π stacking direction of P3HT/FTS with scCO₂ processing. The improved mobilities are instead correlated with enhanced in-plane orientation of the conjugated chain backbone of P3HT after scCO₂ annealing. These findings suggest a strong dependence of polymer processing on the nature of polymer/substrate interface and the important role of backbone orientation toward dictating charge transport of OTFTs

Additive Growth and Crystallization of Polymer Films

We demonstrated a polymeric thin film fabrication process in which molecular-scale crystallization proceeds with additive film growth, by employing an innovative vapor-assisted deposition process termed matrix-assisted pulsed laser evaporation (MAPLE). In comparison to solution-casting commonly adopted for the deposition of polymer thin films, this physical vapor deposition (PVD) methodology can prolong the time scale of film formation and allow for the manipulation of temperature during deposition. For the deposition of molecular and atomic systems, such a PVD manner has been demonstrated to facilitate molecular ordering and delicately manipulate crystalline morphology during film growth. Here, using MAPLE, we deposited thin films of a model polymer, poly­(ethylene oxide) (PEO), atop a temperature-controlled substrate with an average growth rate of less than 10 nm/h. The mechanism of deposition is sequential addition of nanoscale liquid droplets. We discovered that the deposition process leads to the formation of two-dimensional (2D) PEO crystals, composed of monolamellar crystals laterally grown from larger nucleus droplets. The 2D crystalline coverage and crystal thickness of the films can be manipulated with two processing parameters, deposition time, and temperature

Contorted Hexabenzocoronenes with Extended Heterocyclic Moieties Improve Visible-Light Absorption and Performance in Organic Solar Cells

The large band gaps of existing contorted hexabenzocoronene derivatives severely limit visible-light absorption, restricting the photocurrents generated by solar cells utilizing contorted hexabenzocoronene (cHBC). To decrease the band gap and improve the light-harvesting properties, we synthesized cHBC derivatives having extended heterocyclic moieties as peripheral substituents. Tetrabenzofuranyldibenzocoronene (cTBFDBC) and tetrabenzothienodibenzocoronene (cTBTDBC) both exhibit broader absorption of the solar spectrum compared to cHBC, with peak absorbances on the order of 10⁵ cm^–1 in the near-ultraviolet and in the visible. Planar-heterojunction organic solar cells comprising cTBFDBC or cTBTDBC as the donor and C₇₀ as the acceptor surpass those having cHBC in photocurrent generation and power-conversion efficiency. Interestingly, devices containing cTBFDBC/C₇₀ exhibit the highest photocurrents despite cTBTDBC having the smallest band gap of the three cHBC derivatives. X-ray reflectivity of the active layers indicates a rougher donor–acceptor interface when cTBFDBC is employed instead of the other two donors. Consistent with this observation, internal quantum efficiency spectra suggest improved charge transfer at the donor–acceptor interface when cTBFDBCas opposed to cTBTDBC or cHBCis used as the donor

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostatically Interlocked 2D Molecular Packing

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostatically Interlocked 2D Molecular Packin

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostatically Interlocked 2D Molecular Packing

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostatically Interlocked 2D Molecular Packin

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostatically Interlocked 2D Molecular Packing

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostatically Interlocked 2D Molecular Packin