School of Chemistry - Research Publications
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ItemLight losses from scattering in luminescent solar concentrator waveguides(OPTICAL SOC AMER, 2017-04-01)The reductions in the transmission of emission originating from a fluorophore dissolved in a polymer matrix due to light scattering were compared in two forms of planar waveguides used as luminescent solar concentrators: a thin film of poly(methylmethacrylate) (PMMA) spin-coated on a glass plate and a solid PMMA plate of the same dimensions. The losses attributable to light scattering encountered in the waveguide consisting of the thin film of polymer coated on a glass plate were not detectable within experimental uncertainty, whereas the losses in the solid polymer plate were significant. The losses in the solid plate are interpreted as arising from light-scattering centers comprising minute bubbles of vapor/gas, incomplete polymerization or water clusters that are introduced during or after the thermally induced polymerization process.
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ItemRevealing the Role of Methylammonium Chloride for Improving the Performance of 2D Perovskite Solar Cells(American Chemical Society, 2020-06-10)Layered perovskite films, composed of two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs), show improved stability compared to their conventional three-dimensional (3D) counterparts in perovskite solar cells (PSCs). However, 2D PSCs exhibit a lower power conversion efficiency (PCE), which has been attributed to compositional inhomogeneity and nonuniform alignment of the 2D perovskite phases. Methylammonium chloride (MACl) has been adopted as an additive to improve the PCE and the operational stability of 2D PSCs, although the role of MACl in performance enhancement is unclear. In this work, time- and spatially resolved fluorescence and absorption techniques have been applied to study the composition and charge carrier dynamics in MACl-doped BA2MA4Pb5I16 (⟨n⟩ = 5) layered perovskite films. The inhomogeneous phase orientation distribution in the direction orthogonal to the substrate for undoped layered perovskite films undergoes reorganization upon MACl doping. Based on structural and crystallographic analyses, it is revealed that MACl can facilitate the crystallization of small-n 2D perovskite phases at the cost of consuming an increased amount of BA cations. Consequently, an increase in the thickness of large-n 2D perovskite phases accompanies their enhanced perpendicular alignment ([101] crystalline orientation) to the substrate, which facilitates charge carrier transport and collection by electrodes. The defect passivation of the MACl-doped layered perovskite film provided by the small-n phase is also beneficial to the photovoltaic performance of the PSC device. A maximum PCE (∼14.3%) was achieved at 6 mol % MACl doping, with this optimum level influenced by the increased interfacial roughness of the layered perovskite film caused by the edges of small-n perovskite flakes emerging on the front surface.
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ItemMorphological Requirements for Nanoscale Electric Field Buildup in a Bulk Heterojunction Solar Cell(AMER CHEMICAL SOC, 2021-01-14)The morphology of organic semiconductors is critical to their function in optoelectronic devices and is particularly crucial in the donor-acceptor mixture that comprises the bulk heterojunction of organic solar cells. Here, energy landscapes can play integral roles in charge photogeneration, and recently have been shown to drive the accumulation of charge carriers away from the interface, resulting in the buildup of large nanoscale electric fields, much like a capacitor. In this work we combine morphological and spectroscopic data to outline the requirements for this interdomain charge accumulation, finding that this effect is driven by a three-phase morphology that creates an energetic cascade for charge carriers. By adjusting annealing conditions, we show that domain purity, but not size, is critical for an electro-absorption feature to grow-in. This demonstrates that the energy landscape around the interface shapes the movement of charges and that pure domains are required for charge carrier buildup that results in reduced recombination and large interdomain nanoscale electric fields.
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ItemResolving the Mechanisms of Photocurrent Improvement in Ternary Organic Solar Cells(AMER CHEMICAL SOC, 2019-08-01)The ultralow band gap small-molecule IEICO-4F has been employed as a secondary acceptor in both fullerene-based (PTB7-Th:PC71BM) and nonfullerene-based (PBDB-T:ITIC) ternary organic solar cells (OSCs). Structural characterization methods combined with ultrafast spectroscopy have been applied to resolve the mechanisms, leading to the observed improvement in device efficiency upon addition of IEICO-4F. It is shown that IEICO-4F forms ternary mixed domains in the host systems and improves the device efficiency by broadening the absorption spectral range and enhancing both charge separation and charge transport. The enhanced crystallinity of the semiconductor polymer electron donors in the presence of the EIECO-4 provides additional channels for ultrafast charge transfer and transport compared to binary systems. The optimum ternary blend formulations required to improve device efficiencies are reported. This work provides new insights into the fabrication of high-performance ternary OSCs.
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ItemOptimizing the Crystallinity and Phase Separation of PTB7:PC71BM Films by Modified Graphene Oxide(American Chemical Society, 2018-02-08)A facile method is proposed to obtain modified shorn graphene oxide (DDAB-sGO) with improved dispersion in organic solvents. Didodecyl dimethylammonium bromide (DDAB)-sGO, which exhibits good dispersibility in the nonpolar solvent o-dichlorobenzene, was obtained via the sono-Fenton reaction and DDAB ionic functionalization. DDAB-sGO was used in the preparation of conjugated polymer:fullerene blend composites. UV–visible absorption spectra, steady-state photoluminescence spectra, fluorescence decay, and grazing incidence X-ray scattering measurements were applied to characterize morphologies, structural features, and charge-transport characteristics of the composites. Doped into poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) conjugated polymer blends, DDAB-sGO is shown to facilitate increased crystallinity and phase separation of PTB7 and PC71BM to achieve a more optimal morphology for bulk heterojunction solar cells, resulting in a ∼12% enhancement in power conversion efficiency over the undoped PTB7:PC71BM blend.
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ItemSpatially Resolved Photophysical Dynamics in Perovskite Microplates Fabricated Using an Antisolvent Treatment(American Chemical Society, 2017-11-30)Perovskite microplates have important implications in the fields of functional electronics and optoelectronics. We report a facile strategy, antisolvent treatment for the growth of perovskite microplates. The morphology and crystalline quality of the microplates could be controlled by the amount of the chlorobenzene antisolvent used. An appropriate amount of antisolvent facilitates the formation of high-quality perovskite microplates with no residual precursor remaining. Spatially and temporally resolved fluorescence measurements demonstrate the heterogeneity of defect-state density and recombination processes in various perovskite microplate regions. The body center shows higher defect state density when compared with that at the edge or the corner of the microplate. Excessive antisolvent degrades the microplates into smaller particles. The results of this study reveal the factors that influence the crystallization process and photophysical dynamics of perovskite microplates.
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ItemHighly Efficient Luminescent Solar Concentrators by Selective Alignment of Donor-Emitter Fluorophores(AMER CHEMICAL SOC, 2019-04-23)Vertically aligning fluorophores to the surface of a waveguide is known to be an effective approach to improve the optical quantum efficiency (OQE) of luminescent solar concentrators (LSCs). While the chromophore alignment assists waveguiding of the emitted photons to the LSC edges, it also significantly reduces the light-harvesting properties of the LSC. We report here a fluorophore pair consisting of a sphere-shaped energy donor and a rod-shaped emitter that was incorporated in LSCs to provide selective fluorophore alignment to address the reduced incident-light absorption issue. A liquid-crystal polymer matrix was used to perpendicularly align the rod-shaped acceptors to a favorable orientation for light guiding, while the sphere-shaped donor was randomly oriented to maintain its light-absorbing properties. The OQE of LSC devices with this selectively aligned donor-acceptor fluorophore system is 78% without significant loss of light-harvesting capability.
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ItemCompetitive Triplet Formation and Recombination in Crystalline Films of Perylenediimide Derivatives: Implications for Singlet Fission(AMER CHEMICAL SOC, 2020-05-28)Developing photostable compounds that undergo quantitative singlet fission (SF) is a key challenge. As SF necessitates electron transfer between neighboring molecules, the SF rate is highly sensitive to intermolecular coupling in the solid state. We investigate SF in thin films for a series of perylenediimide (PDI) molecules. By adding different substituents at the imide positions, the packing of the molecules in the solid state can be changed. The relationship between SF parameters and the stacked geometry in PDI films is investigated, with two-electron direct coupling found to be the main SF mechanism. Time-resolved emission and transient absorption data show that all of the PDI films undergo SF although with different rates and yields varying from 35 to 200%. The results show that PDI1 and 2, which are stacked PDI pairs twisted out of alignment along the highest occupied molecular orbital to lowest unoccupied molecular orbital transition, exhibit faster and more efficient SF up to 200% yield. We demonstrate that both triplet formation and decay rates are highly sensitive to the ordering of the molecules within a film. The results of this study will assist in the design of optimized structures with a fast SF rate and low recombination rate that are required for useful light harvesting applications.
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ItemFRET-enhanced photoluminescence of perylene diimides by combining molecular aggregation and insulation(Royal Society of Chemistry, 2020-07-14)The photoluminescence quantum yield (ϕPL) of perylene diimide derivatives (PDIs) is often limited by aggregation caused quenching (ACQ) at high concentration or in the neat solid-state. Energy transfer in high dye concentration systems is also a key factor in determining ϕPL as a result of energy funneling to trap sites in the sample. By tuning the substituents, we present two classes of PDIs with aggregation and insulation of the PDI core. By combining these fluorophores in a polymer film, we demonstrate highly emissive samples (85% ϕPL) at high concentration (140 mM or 20% w/w). Experimental and theoretical studies provide insight into why such a combination is necessary to achieve high ϕPL. While insulated fluorophores maintain respectable ϕPL at high concentration, an improved ϕPL can be achieved in the presence of appropriately oriented fluorophore aggregates as emissive traps. The theoretical calculations show that the relative orientation of aggregated monomers can result in energetic separation of localized states from the charge-transfer and bi-excitonic states thereby enabling high ϕPL.
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ItemA sandwich-like structural model revealed for quasi-2D perovskite films(Royal Society of Chemistry, 2021-04-28)The excellent performance and stability of perovskite solar cells (PSCs) based on quasi-2D Ruddlesden–Popper perovskites (RPPs) holds promise for their commercialization. Further improvement in the performance of 2D PSCs requires a detailed understanding of the microstructure of the quasi-2D perovskite films. Based on scanning transmission electron microscopy (STEM), time-resolved photoluminescence, and transient absorption measurements, a new sandwich-like structural model is proposed to describe the phase distribution of RPPs. In contrast to the conventional gradient distribution, it is found that small-n RPPs are sandwiched between large-n RPP phase layers at the front and back sides owing to crystallization initiated from both interfaces during film formation. This sandwich-like distribution profile facilitates excitons funneling from the film interior to both surfaces for dissociation while free carriers transport via large-n channels that permeate the film to ensure efficient charge collection by the corresponding electrodes, which is favorable for high-performance photovoltaics. This discovery provides a new fundamental understanding of the operating principles of 2D PSCs and has valuable implications for the design and optimization strategies of optoelectronic devices based on quasi-2D RPPs films.