School of Chemistry - Research Publications

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    Photophysics and spectroscopy of 1,2-Benzazulene
    Awuku, S ; Bradley, SJ ; Ghiggino, KP ; Steer, RP ; Stevens, AL ; White, JM ; Yeow, C (Elsevier, 2021-12)
    The electronic spectroscopy and photophysics of 1,2-benzazulene (BzAz) have been examined in solution and in thin solid films, with the objective of comparing its intramolecular and intermolecular excited state decay processes with those of azulene. Unlike azulene, the S2 – S0 absorption and fluorescence spectra exhibit a clear mirror image relationship dominated by a single strong Franck-Condon active progression. Picosecond transient absorption spectra and non-linear S2 fluorescence upconversion experiments reveal lifetimes that follow a well-established energy gap law correlation, indicative of a dominant S2 – S1 decay route. Mechanistic interpretations, including the possibility of S2 singlet fission in aggregates, are discussed.
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    Multi-resonance TADF in optical cavities: suppressing excimer emission through efficient energy transfer to the lower polariton states
    Cho, I ; Kendrick, WJ ; Stuart, AN ; Ramkissoon, P ; Ghiggino, KP ; Wong, WWH ; Lakhwani, G (Royal Society of Chemistry, 2023-11-07)
    Thermally activated delayed fluorescence (TADF) emitters suffer from molecular aggregation that limits their applicability in light emitting devices. Aggregation-induced excimer formation often leads to a larger Stokes shift, broader emission spectrum, and reduced emission quantum yields, limiting emitter dye loading to a few weight percent in organic light emitting devices. Here, we demonstrate suppression of excimer emission by dispersing a synthesised multi-resonance TADF emitter dye (OQAO(mes)2) in a PMMA host matrix and embedding the host–guest photoactive layer into an optical cavity. Rabi splitting up to 0.24 eV is obtained at 35 wt% dye loading. Under the strong coupling regime, prompt and delayed emission through excimer states is suppressed due to efficient energy transfer to the lower polariton (LP) states, demonstrated by the blue shift of the emission spectrum and narrowing of the emission linewidth. We also observe an increase in reverse intersystem crossing (RISC) rate constants up to 33% that we attribute to a decrease in activation energy by ≈2kT. This work highlights that strong light-matter interactions can be exploited to overcome aggregation-induced excimer losses providing a pathway towards efficient organic light-emitting diodes with high colour purity and organic semiconductor polariton lasing.
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    Quantifying the Relaxation Dynamics of Higher Electronic Excited States in Perylene.
    Hudson, RJ ; Manian, A ; Hall, CR ; Schmidt, TW ; Russo, SP ; Ghiggino, KP ; Smith, TA (American Chemical Society, 2023-08-31)
    Gating logical operations through high-lying electronic excited states presents opportunities for developing ultrafast, subnanometer computational devices. A lack of molecular systems with sufficiently long-lived higher excited states has hindered practical realization of such devices, but recent studies have reported intriguing photophysics from high-lying excited states of perylene. In this work, we use femtosecond spectroscopy supported by quantum chemical calculations to identify and quantify the relaxation dynamics of monomeric perylene's higher electronic excited states. The 21B2u state is accessed through single-photon absorption at 250 nm, while the optically dark 21Ag state is excited via the 11B3u state. Population of either state results in subpicosecond relaxation to the 11B3u state, and we quantify 21Ag and 21B2u state lifetimes of 340 and 530 fs, respectively. These lifetimes are significantly longer than the singlet fission time constant from the perylene 21B2u state, suggesting that the higher electronic states of perylene may be useful for gating logical operations.
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    A sandwich-like structural model revealed for quasi-2D perovskite films
    Zheng, F ; Hall, CR ; Angmo, D ; Zuo, C ; Rubanov, S ; Wen, Z ; Bradley, SJ ; Hao, X-T ; Gao, M ; Smith, TA ; Ghiggino, KP (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.
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    A luminescent solar concentrator ray tracing simulator with a graphical user interface: features and applications
    Zhang, B ; Yang, H ; Warner, T ; Mulvaney, P ; Rosengarten, G ; Wong, WWH ; Ghiggino, KP (IOP PUBLISHING LTD, 2020-07)
    A Monte-Carlo ray tracing simulator with a graphical user interface (MCRTS-GUI) has been developed to provide a quantitative description, performance evaluation and photon loss analysis of luminescent solar concentrators (LSCs). The algorithm is applied to several practical LSC device structures including multiple dyes in the same waveguiding layer, and structures where a dye layer is sandwiched between clear substrates. The effect of the host matrix absorption and the influence of the neighboring layers are investigated. Validations demonstrate that the MCRTS-GUI developed provides a reliable and accurate description of LSC performance. Code for the mixed-dye single layer configuration is converted into a ray-tracing package with a user-friendly interface and is made available as open source software.
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    Brownian Tree‐Shaped Dendrites in Quasi‐2D Perovskite Films and Their Impact on Photovoltaic Performance
    Zheng, F ; Angmo, D ; Hall, CR ; Rubanov, S ; Yuan, F ; Laird, JS ; Gao, M ; Smith, TA ; Ghiggino, KP (Wiley, 2022-05)
    Quasi-2D Ruddlesden–Popper perovskites (RPPs) are candidates for constructing perovskite solar cells (PSCs) with superior stability due to their tolerance to the external environment. Fully understanding the film growth mechanism and structure is crucial to further improve the performance of 2D-PSCs while maintaining device stability. In this work, the origin of Brownian tree-shaped dendrites formed in hot-cast methylammonium chloride (MACl)-doped BA2MAn−1PbnI3n+1 ( = 5) quasi-2D perovskite films are reported. Investigations based on optical, electronic, atomic force, and fluorescence microscopies reveal that the dendrites are assembled from large-n RPPs-dominated grains, while the nondendritic film area is composed of small-n RPPs grains and associated with film surface pits caused by the evaporation of MACl. It is proposed that these dendrites are grown by the diffusion-limited aggregation of the MA-rich intermediate phase domains that initially crystallize from the precursor. The formation of these dendrites in quasi-2D perovskite films upon MACl doping is accompanied by improved organization and crystallinity of the 2D RPPs, which benefits the photovoltaic performance. This work provides new insights into the formation mechanism of quasi-2D perovskite films that should assist device engineering strategies to further improve the performance of 2D PSCs.
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    Spontaneous Formation of a Ligand-Based 2D Capping Layer on the Surface of Quasi-2D Perovskite Films
    Zheng, F ; Raeber, T ; Rubanov, S ; Lee, C ; Seeber, A ; Hall, C ; Smith, TA ; Gao, M ; Angmo, D ; Ghiggino, KP (AMER CHEMICAL SOC, 2022-11-23)
    Two-dimensional (2D) Ruddlesden-Popper phase perovskites (RPPs) are attracting growing attention for photovoltaic applications due to their enhanced stability compared to three-dimensional (3D) perovskites. The superior tolerance of 2D RPPs films to moisture and oxygen is mainly attributed to the hydrophobic nature of the introduced long-chain spacer cations (ligands). In this work, it is revealed that a thin capping layer, consisting of self-assembled butylammonium ligands, is spontaneously formed on the top surface of a quasi-2D perovskite film prepared by conventional one-step hot casting. Based on morphological and crystallographic analyses of both the top/bottom surfaces and the interior of quasi-2D perovskite films, the formation process of the 2D capping layer and the assembly of RPPs, comprising both large and small slab thickness (large-n, small-n), is elucidated. The vertical orientation of RPPs that is required for sufficient charge transport for 2D perovskite solar cells (PSCs) is further verified. We propose that the surface capping layer is directly responsible for the long-term stability of 2D PSCs. This work provides detailed insight into the microstructure of quasi-2D RPPs films that should assist the development of strategies for unlocking the full potential of 2D perovskites for high-performance PSCs and other solid-state electronic devices.
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    A critical analysis of luminescent solar concentrator terminology and results
    Warner, T ; Ghiggino, KP ; Rosengarten, G (Elsevier, 2022-11-01)
    Recent efforts to synchronize laboratory protocols for measuring luminescent solar concentrator (LSC) efficiencies, and to use consistent terminology, has been driven by a prior lack of consensus on both terminology and the reporting of experimental results. This analysis seeks to understand how terminology in the field of luminescent solar concentrators has been used in the past, and to recommend the most clear and consistent terminology that should be used for future publications. LSCs for electrical power generation, referred to as luminescent solar concentrator photovoltaics (LSC-PV), should be treated as integrated photovoltaic cells. Consequently, the power conversion efficiency (PCE) and external quantum efficiency (EQELSC(λ)) should be reported, as they would be with any other photovoltaic device. The term "optical efficiency" has been defined inconsistently throughout the literature, with several different efficiency expressions, and thus it should be interpreted with great care. The highest values for the three most common LSC efficiency metrics have been examined in detail and their validity and significance have been assessed. Among the examined results, it has been assessed that the highest reliable results are a PCE of 7.1%, an optical power efficiency (ηopt) of 7.8% and an external photon efficiency (ηext) of 6.8%. A significant number of highest efficiencies reported have used questionable calculation methods meaning those efficiency values should not be used as a comparison for current and future publications. Additionally, reporting optical power efficiency is not recommended unless optical power is relevant to the device application.
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    Medium effects on the fluorescence of Imide-substituted naphthalene diimides
    Pervin, R ; Manian, A ; Chen, Z ; Christofferson, AJ ; Owyong, TC ; Bradley, SJ ; White, JM ; Ghiggino, KP ; Russo, SP ; Wong, WWH (Elsevier, 2023-03-01)
    Naphthalene diimides (NDIs) are a common class of chromophores used in photon harvesting applications due to their functional malleability through substitution of the NDI core. However, some derivatives with substitution at the imide position of the NDI core only become emissive in electron-rich aromatic solvents. This study examines this phenomenon from both an experimental and theoretical perspective, in order to understand how NDIs interact with each other and the surrounding medium upon photoexcitation. We report the photophysical properties of cyclohexyl and several aromatic imide-substituted NDI derivatives, and show that fluorescence properties are strongly influenced by solvation in more electron-rich aromatic solvents (e.g. toluene, xylene, mesitylene). Theoretical modeling supports strong interactions, including ground state charge-transfer complexation, with aromatic solvents. In solid poly(methyl methacrylate) (PMMA) and poly(styrene) (PS) film media, both aggregation and complexation are shown to contribute to absorption and emission properties. The results also demonstrate that aromatic imide substituents not only act to provide steric bulk to the NDI chromophore but participate in interactions with the surrounding medium that affect the overall photophysical properties.
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    Alkyl Chain Length-Dependent Amine-Induced Crystallization for Efficient Interface Passivation of Perovskite Solar Cells
    Zhao, P ; Subbiah, J ; Zhang, B ; Hutchison, JA ; Ahluwalia, G ; Mitchell, V ; Ghiggino, KP ; Jones, DJ (WILEY, 2023-07)
    Abstract Efficient surface passivation of perovskite solar cells (PSC) using treatment with ammonium salts is demonstrated as an efficient method to enhance the device performance, owing to the affinity between the amine group and [PbI6]4− octahedron. However, due to their high solubility in polar solvents (DMF/DMSO), ammonium salts are more difficult to use in passivation of the interface between the electron transport layer and perovskite thin film in n‐i‐p structured PSCs. In this report, this work successfully links the amine group with a fullerene through a series of increasing carbon chain length, from two to twelve methylene units (FC‐X, X = 2, 6, 12), and then introduce the synthesized molecules as interface passivation layers into SnO2‐based planar n‐i‐p PSCs. Results show that the interface passivation effect is highly dependent on the side‐chain length, and the longer chain length amine‐functionalized fullerene is more beneficial for the device performance. A power conversion efficiency as high as 21.2% is achieved by using FC‐12. The surface energy, perovskite crystallite size and electron transfer capacity correlate with the linker chain length. This work develops an amine‐induced anchored crystallization of perovskite to unravel the mechanism of this passivation effect. As expected, enhanced device stability is also observed in the FC‐12 passivated PSCs.