Gigantic Relevance of Twisted Intramolecular Charge Transfer for Organic Dyes Used in Solar Cells

Within this work, we emphasis on the importance of twisted intramolecular charge transfer (TICT) process in organic dyes based on triphenyl amine moiety to achieve high performance in dye-sensitized solar cells. Through the comparison between two recent made dyes, L1 and L1Fc, on different semiconductors (TiO₂, and ZrO₂), we could spectrally and dynamically detect for the first time the formation of TICT state for L1 on ZrO₂ after localized charge transfer (LCT) state population, and an electron injection process from TICT state on TiO₂. However, for the excited L1Fc dye, the ultrafast electron transfer from ferrocene (Fc) moiety to the L1 unit quenched the formation of TICT state in L1Fc on semiconductors, leading instead to an electron injection process from the LCT state. The electron injection from TICT state in L1 associated with structural rearrangements on TiO₂ leads to slow recombination process and an efficiency improvement of about 325%, compared to solar cells based on L1Fc dye, in which TICT state formation is hindered. Similar electron dynamics are obtained for L1 on TiO₂ upon physically hindering the TICT process by adding polymer matrix. The presence of TICT state for L1 dye and similar triphenyl amine dyes aids to reconstruct the kinetic profile for these dyes on semiconductor surfaces, and to redesign organic dyes accordingly for higher efficiency in solar cells.