A DFT Study Of Regeneration Process Of Porphyrin Zinc Analogues In Dye-sensitized Solar Cells

In this work, we use density functional theory (DFT) to investigate three possibleregeneration pathways for a series of Zn-tetraphenylporphines with seven differentsubstituent groups (-CN,-F,-Cl,-H,-PhCH3,-OH and-NH2). The influences ofsubstituent groups on the regeneration are also discussed. We hope that this work willbe helpful in advancing the understanding of the regeneration process and the effectsof substituent groups on the regeneration of zinc porphyrin sensitizers.1. The two most stable [D+…I-] intermediates (DyeIZnand DyeIPy) and the [D…I2-]intermediates (DyeIZn-I and DyeIPy-I) were found using CAM-B3LYP and B3LYP.The calculation results show that the regeneration is a charge-transfer interaction; the[D+…I-] and [D… I2-] intermediates exhibit regular change in their electronicstructures, charge density difference and regeneration energy properties when theelectronic effects of the substituent groups are changed from electron-withdrawing toelectron-donating. The calculation results show that the combination of I-anions withZn atoms to form DyeIZnand DyeIZn-I intermediates is dominant for Por-CN, Por-Fand Por-Cl. For Por-H, Por-PhCH3and Por-OH, the combination of I-anions with thepyrrole of porphine to form DyeIPyand DyeIPy-I intermediates is more favorable. Thetransition states of the formation of [D…I2-] intermediates (the second step of theregeneration) for Cl-, H-, PhCH3-, and OH-substituted porphyrins are found. It is alsofound that the regeneration for Por-CN and Por-F has no activation energy and theregeneration is very fast and the regeneration efficiency may be improved if the effectof electron-withdrawing is strong.2. The alternative regeneration pathways including one-step regeneration and thereductive quenching reaction of excited dye are calculated and discussed. The resultsshow that the two-step regeneration mechanism is a dominant one compared withone-step regeneration. With the electron-withdrawing effect of the substituent groupincreasing, the possibility of the reductive quenching reaction increases.3. The free energy of electron injection for zinc porphyrins has been calculatedusing CAM-B3LYP. We found that zinc porphyrins substituted with theelectron-donating group has more exoenergic free energy and can obtain a higherelectron injection driving force. According to the calculation results it is implied that the design of a novel zincporphyrin needs to consider not only light absorption but also the effects of othersteps in photonic chemical circuit such as regeneration and electron injection.