Publications by authors named "David Ompong"

Using the first-order perturbation theory and exciton-spin-orbit-photon-molecular-vibration-interaction (ESOPMVI) operator, the rates of reverse intersystem crossing (RISC) and thermally activated delayed fluorescence (TADF) are derived. It is shown that the pre-exponential factor of the rate of TADF is not an absolute constant as is commonly assumed. Instead, it depends on the square of the atomic number and the exchange energy, but it also depends on the triplet excitonic Bohr radius as , which enhances the rate of RISC by 4-6 orders of magnitude higher than the rate of the intersystem crossing, which undermines the dependence of the rate on the atomic number, and hence TADF can occur efficiently in metal-free organic solids.

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Thermal stability, closely associated with the operating temperature, is one of the desired properties for practical applications of organic solar cells (OSCs). In this paper, an OSC of the structure of ITO/PEDOT:PSS/PHT:PCBM/ZnO/Ag was fabricated, and its current-voltage (-) characteristics and operating temperature were measured. The operating temperature of the same OSC was simulated using an analytical model, taking into consideration the heat transfer, charge carrier drift-diffusion and different thermal generation processes.

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In this paper, characterisation of exciton generation is carried out in three bulk-heterojunction organic solar cells (BHJ OSCs)-OSC1: an inverted non-fullerene (NF) BHJ OSC; OSC2: a conventional NF BHJ OSC; and OSC3: a conventional fullerene BHJ OSC. It is found that the overlap of the regions of strong constructive interference of incident and reflected electric fields of electromagnetic waves and those of high photon absorption within the active layer depends on the active layer thickness. An optimal thickness of the active layer can thus be obtained at which this overlap is maximum.

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We derived new expressions for the diffusion length of singlet and triplet excitons by using the Föster and Dexter transfer mechanisms, respectively, and have found that the diffusion lengths of singlet and triplet excitons are comparable. By using the Langevin recombination theory, we derived the rate of recombination of dissociated free charges into their excitonic states. We found that in some organic polymers the probabilities of recombination of free charge carriers back into the singlet and triplet states are approximately 65.

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