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Relationship between Carbon Nitride Structure and Exciton Binding Energies

 

The sun is a virtually inexhaustible natural source of energy, the total solar power over the earth’s surface amounting to 65 Petawatts at any instant, with roughly half of this energy falling within the visible light range. Artificial photosynthesis reproduces all the steps that allow plant to store this energy with a manmade device, inducing chemical reactions. Graphitic carbon nitride, with general formula g-CxNyHz, has been extensively studied as a light-absorbing element in photocatalytic water splitting cells.

Recently, the crystal structures of completely polymerized graphitic g-C3N4 and incompletely polymerized g-C6N9H3 crystals based on the triazine and heptazine monomers have been characterized. The atomic and electronic nature of these structures were evaluated using DFT. The study revealed strongly conjugated structures for the fully polymerized g-C3N4 and planar structures for the incompletely polymerized g-C6N9H3.

The exciton binding energies of the heptazine-based structures are lower than that of their triazine-based analogues and lower for the completely polymerized structures than their incompletely polymerized analogues. The rather low dielectric constant and charge mobilities result in high exciton binding energies and hence low dissociation probabilities for these excitons. This confirms the necessity of a morphology inspired by bulk heterojunction architectures to ensure efficient charge carrier generation. The studied compounds can be considered intermediates between typical inorganic and organic semiconductors in terms of their photoabsorption properties.