Theoretical Study On The Mechanism Of Dye Aggregation In Dye-sensitized Solar Cells

Most organic dyes are prone to form aggregates on the photoanode?typically nanocrystalline TiO₂?.Although some aggregates are reported to be benign because of the formed compact dye layer on the TiO₂ surface,the strong intermolecular interaction within the compact dye layer leads to lateral charge transfer between the dye layers as well as excited-state quenching,which is adverse to the process of electron injection into the TiO₂conduction band and thus reduces the photocurrent,photovoltage and the overall photovoltaic performance.Herein,we use density functional theory?DFT?,density functional tight binding?DFTB?method and ab initio molecular dynamics simulations to study the aggregation effects of a series of organic sensitizers and the relationship between molecular configurations and aggregation effects.?1?We investigate the aggregation effects of two organic dyes,WS-2 and WS-6,which were used as sensitizers in dye sensitized solar cells?DSSCs?.The calculated results demonstrate that WS-6 is less prone to aggregation due to a hexyl substituent on the thiophene ring compared to WS-2.Importantly,our results predict strong aggregation interactions induce larger electronic coupling between the stacking dimers,which may be detrimental for electronic injection from dye to TiO₂ and partly responsible for the loss of photo-voltaic efficiency.The deeper understanding of the dye aggregation effects shed lights on a better knowledge about the complex factors determining the function of DSSC and rational design of high efficiency sensitizers?2?We explored the aggregation effects of LS-1 and IQ₄ by comparing their optical properties and intermolecular electronic couplings.The calculated absorption spectra are in good agreement with the experimental observations and reveal them to be evidently affected by the dimerization.Furthermore,molecular dynamics simulations show that steric hindrance induced by the diphenylquinoxaline unit in IQ₄ can elongate the distances between intermolecular?units or electron donors,which are responsible for the fact that the intermolecular electronic coupling of LS-1 is about 10 times larger than that of IQ₄.More importantly,the aggregated IQ₄ remains almost perpendicular to the TiO₂ surface,whereas LS-1 gradually tilts during the dynamic simulation,impacting electron injection and recombination in several ways,which clarifies why IQ₄ leads to larger photocurrent and higher conversion efficiency.?3?The underlying influence factors of different donor groups on power conversion efficiency?PCE?for DSSCs have been investigated by considering dye regeneration and aggregation of the four phenothiazine-based dyes?TP,TTP,EP and EEP?.By comparing the orbital energy levels between electron-donating units and PTZ moiety,the driving force of TP dye regeneration is superior to EEP dye regeneration.Additionally,the calculated average values of dynamic intermolecular lateral charge transfer rate k for?TP?₂ are nearly one order magnitude smaller than those of?EEP?₂,revealing more robust?-?stacking interaction induced by the donor unit of EEP.The main reasons of higher conversion efficiency in TP-based DSSC are superior driving force of dye regeneration and smaller aggregation effects.The deeper insights into regeneration process and the aggregation mechanism induced by different donor units on conversion efficiency encourage the researchers to balance various properties in designing the novel components for DSSCs.The theoretical results are helpful to understand the dye aggregation effects and design high-efficiency self-anti-aggregation sensitizers.