Density Functional Theory Study on the Donating Strength of Donor Systems in Dye-Sensitized Solar Cells

Abstract

The electron-donating strengths of donor (D) moieties in thirteen donor-pi-acceptor systems (D1-pi-A to D13-pi-A wherein -pi- and A represent butadiene and cyanoacrylic acid units, respectively) have been studied using B3LYP/cc-pVDZ level density functional theory (DFT) calculations. The selected D moieties are encountered as a part of an organic sensitizer molecule in dye-sensitized solar cell (DSSC) applications. When the D moiety is joined with pi-A, a certain amount of electron donation from D to A occurs leading to an increase in electron density at the A site of D-pi-A compared to the A site of pi-A. This electron reorganization is quantified in terms of a change in molecular electrostatic potential (MESP) minimum (Delta V-mA) at the acceptor site, the CN group of the cyanoacrylic acid. The Delta V-mA is always negative, in the range of -11.0 to -2.6 kcal mol(-1) which provides a quick assessment of the rank order of the electron-donating nature of the D moieties in the ground state of D-pi-A. The optical and photovoltaic properties of D and D-pi-A systems are also determined at the TD-CAM-B3LYP/cc-pVDZ//B3LYP/cc-pVDZ level. An absorption red shift (Delta lambda(max)) in the range of 81-242 nm is observed when D moieties change to D-pi-A systems. The ground state property Delta V-mA showed a strong linear correlation with the excited state property Delta lambda(max). Furthermore, Delta V-mA is found to be proportional to the open-circuit voltage (V-OC). The resemblance of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the D-pi-A system with the respective energies of donor and pi-A systems shows that the donor tunes the HOMO, while pi-A tunes the LUMO. Among the thirteen D-pi-A systems, N,N-dialkylaniline, and julolidine are rated as the best donors for the photovoltaic applications. This study shows that the MESP based assessment of the donating strength of donor systems offers a powerful rational design strategy for the development of efficient dyes for DSSC applications.