Search for: All records

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.

 

The true molecular conformation and the crystal structure of benzo[ e ]dinaphtho[2,3- a ;1′,2′,3′,4′- ghi ]fluoranthene, 7,14-diphenylnaphtho[1,2,3,4- cde ]bisanthene and 7,16-diphenylnaphtho[1,2,3,4- cde ]helianthrene were determined ab initio by 3D electron diffraction. All three molecules are remarkable polycyclic aromatic hydrocarbons. The molecular conformation of two of these compounds could not be determined via classical spectroscopic methods due to the large size of the molecule and the occurrence of multiple and reciprocally connected aromatic rings. The molecular structure of the third molecule was previously considered provisional. These compounds were isolated as by-products in the synthesis of similar products and were at the same time nanocrystalline and available only in very limited amounts. 3D electron diffraction data, taken from submicrometric single crystals, allowed for direct ab initio structure solution and the unbiased determination of the internal molecular conformation. Detailed synthetic routes and spectroscopic analyses are also discussed. Based on many-body perturbation theory simulations, benzo[ e ]dinaphtho[2,3- a ;1′,2′,3′,4′- ghi ]fluoranthene may be a promising candidate for triplet–triplet annihilation and 7,14-diphenylnaphtho[1,2,3,4- cde ]bisanthene may be a promising candidate for intermolecular singlet fission in the solid state. 

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 a 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. 

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.