Photo-excited Intramolecular Charge Transfer In Dye Sensitizers : Predictive In-silico Screening for Dye Sensitized Solar Cell Devices

Efficient photo-induced intramolecular charge transfer (ICT) from donor to acceptor in dye molecules is the functional basis and key property in the working of dye-sensitized solar cell (DSSC). To understand the ICT process in photo-excited dye molecules, we analyse the electronic properties and structural parameters of a chosen set of experimentally synthesized donor-acceptor (D-A) and donor-$\pi$--spacer-acceptor (D-$\pi$-A) type dye molecules in their ground, excited and cationic states. The correlation between structural modification and charge redistribution in different parts of the molecule helps to identify the extent of $\pi$-conjugation and spatial rearrangement of electron density localization along the molecular skeleton. We find that prominent twisting of several groups and resulting molecular bond rearrangements in larger parts of the molecule promotes efficient donor to acceptor ICT, such as in D-A type, ADEKA1 and C275 dyes. Thus based on modest computation of structural and electronic properties of dye molecules in their respective ground, excited and cationic states, we identify the desired structural changes that facilitate tunable intramolecular charge transfer to highlight a simple and direct prescription to screen out probable efficient dye molecules among many samples. Motivated by recent experimental evidence of capturing the structural view of the excited-state charge transfer in molecules, we provide a fresh outlook towards predictive and systematic computational screening and design of dye molecules, complementing parallel experimental approaches in the development of state-of-the-art DSSC.