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  1. Thermally activated delayed fluorescence (TADF) is the internal conversion of triplet excitons into singlet excitons via reverse intersystem crossing (RISC). It improves the efficiency of OLEDs by enabling the harvesting of nonradiative triplet excitons. Multiple resonance (MR) induced TADF chromophores exhibit an additional advantage of high color purity due to their rigid conformation. However, owing to the strict design rules there is a limited number of known MR-TADF chromophores. For applications in full-color high-resolution OLED displays, it is desirable to extend the variety of available chromophores and their color range. We computationally explore the effect of chemical modification on the properties of the MR-TADF chromophore quinolino[3,2,1-de]acridine-5,9-dione (QAD). QAD derivatives are evaluated based on several metrics: The formation energy is associated with the ease of synthesis; The spatial distribution of the frontier orbitals indicates whether a compound remains an MR-TADF chromophore or turns into a donor-acceptor TADF chromophore; The change of the singlet excitation energy compared to the parent compound corresponds to the change in color; The energy difference between the lowest singlet and triplet states corresponds to the barrier to RISC; The reorganization energy is associated with the color purity. Based on these metrics, QAD-6CN is predicted to be amore »promising MR-TADF chromophore with a cyan hue. This demonstrates that computer simulations may aid the design of new MR-TADF chromophores by chemical modification.« less
    Free, publicly-accessible full text available January 1, 2024
  2. Free, publicly-accessible full text available August 1, 2023
  3. Optical upconversion (UC) of low energy photons into high energy photons enables solar cells to harvest photons with energies below the band gap of the absorber, reducing the transmission loss. UC based on triplet–triplet annihilation (TTA) in organic chromophores can upconvert photons from sunlight, albeit with low conversion efficiency. We utilize three energy-based criteria to assess the UC potential of TTA emitters in terms of the quantum yield (QY) and the anti-Stokes shift. The energy loss in the singlet pathway of an emitter encounter complex, where a high energy photon is emitted, determines whether a chromophore may undergo TTA. The energy loss in the triplet pathway, which is the main competing process, impacts the TTA QY. The energy difference between the lowest singlet and triplet excitation states in TTA emitters sets an upper bound for the anti-Stokes shift of TTA-UC. Using the energetic criteria evaluated by time-dependent density functional theory (TDDFT) calculations, we find that benzo[ a ]tetracene, benzo[ a ]pyrene, and their derivatives are promising TTA emitters. The energetics assessment and computer simulations could be used to efficiently discover and design more candidate high-performance TTA emitters.
  4. Abstract

    Singlet fission (SF), the conversion of one singlet exciton into two triplet excitons, could significantly enhance solar cell efficiency. Molecular crystals that undergo SF are scarce. Computational exploration may accelerate the discovery of SF materials. However, many-body perturbation theory (MBPT) calculations of the excitonic properties of molecular crystals are impractical for large-scale materials screening. We use the sure-independence-screening-and-sparsifying-operator (SISSO) machine-learning algorithm to generate computationally efficient models that can predict the MBPT thermodynamic driving force for SF for a dataset of 101 polycyclic aromatic hydrocarbons (PAH101). SISSO generates models by iteratively combining physical primary features. The best models are selected by linear regression with cross-validation. The SISSO models successfully predict the SF driving force with errors below 0.2 eV. Based on the cost, accuracy, and classification performance of SISSO models, we propose a hierarchical materials screening workflow. Three potential SF candidates are found in the PAH101 set.

  5. Abstract Bituminous limestone of the Ediacaran Shibantan Member of the Dengying Formation (551–539 Ma) in the Yangtze Gorges area contains a rare carbonate-hosted Ediacara-type macrofossil assemblage. This assemblage is dominated by the tubular fossil Wutubus Chen et al., 2014 and discoidal fossils, e.g., Hiemalora Fedonkin, 1982 and Aspidella Billings, 1872, but frondose organisms such as Charnia Ford, 1958, Rangea Gürich, 1929, and Arborea Glaessner and Wade, 1966 are also present. Herein, we report four species of Arborea from the Shibantan assemblage, including the type species Arborea arborea (Glaessner in Glaessner and Daily, 1959) Glaessner and Wade, 1966, Arborea denticulata new species, and two unnamed species, Arborea sp. A and Arborea sp. B. Arborea arborea is the most abundant frond in the Shibantan assemblage. Arborea denticulata n. sp. resembles Arborea arborea in general morphology but differs in its fewer primary branches and lower length/width ratio of primary branches. Arborea sp. A and Arborea sp. B are fronds with a Hiemalora -type basal attachment. Sealing by microbial mats and authigenic cementation may have played an important role in the preservation of Arborea in the Shibantan assemblage. The Shibantan material of Arborea extends the stratigraphic, ecological, and taphonomic ranges of this genus. UUID:more »« less
  6. The efficiency of solar cells may be increased by utilizing photons with energies below the band gap of the absorber. This may be enabled by upconversion of low energy photons into high energy photons via triplet–triplet annihilation (TTA) in organic chromophores. The quantum yield of TTA is often low due to competing processes. The singlet pathway, where a high energy photon is emitted, is one of three possible outcomes of an encounter between two triplet excitons. The quintet pathway is often too high in energy to be accessible, leaving the triplet pathway as the main competing process. Using many-body perturbation theory in the GW approximation and the Bethe–Salpeter equation, we calculate the energy release in both the singlet and triplet pathways for 59 chromophores of different chemical families. We find that in most cases the triplet pathway is open and has a larger energy release than the singlet pathway. Thus, the energetics perspective explains why there are so few TTA emitters and why the quantum yield of TTA is typically low. That said, our results also indicate that the performance of emitters from known chemical families may be improved by chemical modifications, such as functionalization with side groups, and thatmore »new chemical families could be explored to discover more TTA emitters.« less
  7. Singlet fission (SF) is a photophysical process considered as a possible scheme to bypass the Shockley–Queisser limit by generating two triplet-state excitons from one high-energy photon. Polyacene crystals, such as tetracene and pentacene, have shown outstanding SF performance both theoretically and experimentally. However, their instability prevents them from being utilized in SF-based photovoltaic devices. In search of practical SF chromophores, we use many-body perturbation theory within the GW approximation and Bethe–Salpeter equation to study the excitonic properties of a family of pyrene-stabilized acenes. We propose a criterion to define the convergence of exciton wave-functions with respect to the fine k-point grid used in the BerkeleyGW code. An open-source Python code is presented to perform exciton wave-function convergence checks and streamline the double Bader analysis of exciton character. We find that the singlet excitons in pyrene-stabilized acenes have a higher degree of charge transfer character than in the corresponding acenes. The pyrene-fused tetracene and pentacene derivatives exhibit comparable excitation energies to their corresponding acenes, making them potential SF candidates. The pyrene-stabilized anthracene derivative is considered as a possible candidate for triplet–triplet annihilation because it yields a lower SF driving force than anthracene.