School of Chemistry - Research Publications

Permanent URI for this collection

Search Results

Now showing 1 - 10 of 1338
  • Item
    Thumbnail Image
    Time-resolved emission microscopy of light-induced aggregation of luminescent polymers
    Xu, Y ; Zhou, J ; Smith, TA (IOP Publishing, 2019-12-23)
    Photon pressure has been used to induce the aggregation from solution of a series of photoluminescent conjugated polyelectrolytes containing tetraphenylethene units. These polymers show steady-state and time-resolved emission properties that are dependent on the local chromophore environment that can be influenced by the degree of intra- and inter-molecular interactions, which enables the photoaggregation process to be monitored by time-resolved fluorescence imaging techniques. Structural differences in the polymer lead to variations in the photo-induced aggregation behaviour.
  • Item
    Thumbnail Image
    Light losses from scattering in luminescent solar concentrator waveguides
    Breukers, RD ; Smith, GJ ; Stirrat, HL ; Swanson, AJ ; Smith, TA ; Ghiggino, KP ; Raymond, SG ; Winch, NM ; Clarke, DJ ; Kay, AJ (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.
  • Item
    Thumbnail Image
    Excitonic Processes in a Conjugated Polyelectrolyte Complex
    Nitneth, DT ; Hutchison, JA ; Ghiggino, KP (CSIRO Publishing, 2020)
    In aqueous solution, a di-sulfonated phenylenevinylene polymer (DPS-PPV) forms a complex with non-ionic poly(vinyl alcohol) (PVA) leading to absorption spectroscopic shifts and a dramatic (6-fold) increase in DPS-PPV fluorescence intensity. Spectroscopic investigations demonstrate that the complexation with PVA and other neutral polymers results in conformational changes in the DPS-PPV chains that lead to the removal of non-fluorescent energy traps and results in the observed increase in fluorescence in the bulk solution. Single molecule fluorescence measurements of DPS-PPV chains dispersed on glass and in PVA films confirm that efficient exciton energy transfer occurs within each photo-excited DPS-PPV chain and that the observed increase in fluorescence intensity in the PVA film environment is also associated with fewer quenching sites. The results highlight the importance of conjugated polyelectrolyte conformation on exciton relaxation pathways.
  • Item
    Thumbnail Image
    Revealing the Role of Methylammonium Chloride for Improving the Performance of 2D Perovskite Solar Cells
    Zheng, F ; Zuo, C ; Niu, M ; Zhou, C ; Bradley, SJ ; Hall, CR ; Xu, W ; Wen, X ; Hao, X ; Gao, M ; Smith, TA ; Ghiggino, KP (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.
  • Item
    Thumbnail Image
    Morphological Requirements for Nanoscale Electric Field Buildup in a Bulk Heterojunction Solar Cell
    Schwarz, KN ; Mitchell, VD ; Khan, S-U-Z ; Lee, C ; Reinhold, A ; Smith, TA ; Ghiggino, KP ; Jones, DJ ; Rand, BP ; Scholes, GD (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.
  • Item
    Thumbnail Image
    Resolving the Mechanisms of Photocurrent Improvement in Ternary Organic Solar Cells
    Bi, PQ ; Hall, CR ; Yin, H ; So, SK ; Smith, TA ; Ghiggino, KP ; Hao, XT (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.
  • Item
    Thumbnail Image
    Optimizing the Crystallinity and Phase Separation of PTB7:PC71BM Films by Modified Graphene Oxide
    Lv, C-K ; Zheng, F ; Yang, X-Y ; Bi, P-Q ; Niu, M-S ; Wang, Y-Z ; Smith, TA ; Ghiggino, KP ; Hao, X-T (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.
  • Item
    Thumbnail Image
    Spatially Resolved Photophysical Dynamics in Perovskite Microplates Fabricated Using an Antisolvent Treatment
    Xu, W-L ; Niu, M-S ; Yang, X-Y ; Bi, P-Q ; Zhang, K-N ; Xiong, C ; Yuan, H-C ; Smith, TA ; Ghiggino, KP ; Hao, X-T (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.
  • Item
    Thumbnail Image
    Spectroscopic and Dynamic Properties of Electronically Excited Pendant Porphyrin Polymers with Backbones of Differing Flexibility
    Stevens, AL ; Awuku, S ; Ghiggino, KP ; Hao, Y ; Novakovic, S ; Steer, RP ; White, JM (American Chemical Society, 2020-12-24)
    A zinc porphyrin-pendant norbornene polymer with a rigid backbone characterized by a 2:1 E/Z isomeric structure ratio has been synthesized, and its spectroscopic and photophysical properties are examined. Zinc tetraphenylporphyrin, the porphyrin-substituted norbornene monomer, and a previously reported zinc porphyrin-pendant polymer with a flexible polymethylene backbone have been used as comparators. Unlike its flexible counterpart, the rigid norbornene polymer exhibits clear exciton splitting of its Soret band, much more rapid relaxation rates of its excited singlet states, and a very small yield of an unusually short-lived triplet state. Unlike the flexible pendant polymer, which exhibits excimeric S2 fluorescence as a result of chromophore rotation, anti-Kasha emission from the norbornene polymer originates primarily from the unperturbed porphyrin E region. The low triplet yield in the polymer is attributed to greatly increased rates of competing internal conversion within the singlet manifold. Nevertheless, upconverted delayed fluorescence that is quenched by oxygen is observed upon intense steady-state Q-band excitation of degassed polymer solutions, signaling direct triplet involvement. Consistent with the polymer’s rigid structure, this biexcitonic process is assigned to ultrafast singlet exciton migration and triplet–triplet annihilation following absorption of a second photon by the small steady-state concentration of polymer triplets.
  • Item
    Thumbnail Image
    Poly(3-hexylthiophene) coated graphene oxide for improved performance of bulk heterojunction polymer solar cells
    Zheng, F ; Yang, X-Y ; Bi, P-Q ; Niu, M-S ; Lv, C-K ; Feng, L ; Qin, W ; Wang, Y-Z ; Hao, X-T ; Ghiggino, KP (Elsevier, 2017-05-01)
    An effective method for preparing poly(3-hexylthiophene) (P3HT) coated graphene oxide (GO), (P-GO), based on an ethanol mediated mixing and solvent evaporation method is described. P-GO exhibits good dispersibility in the non-polar solvent o-dichlorobenzene (DCB) allowing the preparation of polymer blend composites. P-GO was doped into P3HT: PCBM blends by solution mixing and shown to facilitate phase separation of P3HT and PCBM in P3HT: PCBM blend films to achieve a more optimum morphology for polymer photovoltaic cells. Bulk heterojunction P3HT: PCBM solar cells exhibit ∼18% power conversion efficiency enhancement in the presence of P-GO.