Theoretical study on the correlations between dye–iodine interactions and open-circuit voltages in dyes containing furan and thiophene

In this study, density functional theory is used to evaluate the noncovalent bond interactions between the dye and iodine. We consider the binding of I2 to multiple electron-donating sites in organic sensitizers containing furan or thiophene at the π-conjugated bridge. The results show that the trend in the stabilization energy based on the second-order perturbation theory analysis of the Fock matrix in NBO increases with O⋯I < S⋯I < N⋯I noncovalent interactions. The trend of interaction energies decreases in the order of C209 > C213 > C214. This trend suggests that the thiophene-containing dyes and the longer π-bridge conjugation allows more stable interactions with the iodine species, thus increasing the concentration of the redox couple at the surface of the semiconductor. This is consistent with the observed open-circuit voltages of the dyes. This work highlights the importance of the use of multiple iodine species interacting with the dye when considering recombination reactions with the electrolyte which ultimately affects the device efficiency.