Effect of donors on intramolecular photoinduced charge-transfer in π-conjugated donor-acceptor solutions for solar cells applications

Photoinduced charge transfer kinetics are essential in assessing the appropriateness of organic materials for solar cell applications. This study utilized femtosecond-to-microsecond time-resolved spectroscopy to examine the excited-state kinetics of a series of π-conjugated donor–acceptor polymers featuring the potent electron-withdrawing unit ((E)-1,2-di(thiazol-2-yl) diazene) (ATZ) alongside three distinct donor moieties: thieno [3,2-b] thiophene (TT), 2,2′-bithiophene (BT), and (E)-1,2-di(thiophen-2-yl) ethene (DTV). Steady-state absorption, fluorescence, density functional theory (DFT) calculations, and transient absorption (TA) spectroscopy were conducted to clarify their photophysical behavior in the presence of 1,4-dicyanobenzene (DCB), a recognized electron acceptor. The findings indicate that intramolecular charge transfer from the donor segments (TT, BT, and DTV) to DCB predominates the excited-state deactivation mechanisms. DFT studies reveal that the highest occupied molecular orbitals (HOMOs) are delocalized along the polymer backbone, whereas the lowest unoccupied molecular orbitals (LUMOs) are predominantly localized on the donor units, hence promoting efficient electron transfer to DCB. These findings elucidate the charge-transfer mechanisms of ATz-based donor–acceptor polymers and underscore their potential for solar applications.