Electron-Acceptor-Dependent Light Absorption and Charge-Transfer Dynamics in N-Annulated Perylene Dye-Sensitized Solar Cells

Abstract

In this article, we report two metal-free perylene dyes (C269 and C270) featuring the electron acceptors benzothiadiazole–benzoic acid and ethynylbenzothiadiazole–benzoic acid, respectively, in combination with a bis(4-(hexyl)phenyl)amino-capped N-annulated perylene (NP) electron donor. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations revealed that the use of ethynylbenzothiadiazole–benzoic acid can lower the level of the lowest unoccupied molecular orbital (LUMO), reduce the energy gap, and attenuate the Stokes shift of an NP dye. These effects are in good accord with electrochemical and photophysical measurements. When used in sensitized titania solar cells, C270 dye exhibits a reasonably good power conversion efficiency close to 9% at an irradiance of 100 mW cm–2 simulated AM1.5 sunlight. It was also found that, with respect to C269, C270 dye forms a thinner and looser self-assembled dye layer on the surface of titania, accounting for the shorter electron lifetime and lower open-circuit photovoltage for cells made with C270. Our femtosecond transient absorption (fs-TA) measurements confirmed a positive relationship between the driving energy and electron-injection rate despite the close-to-unity electron-injection yields for both dyes. In addition, the target analysis of fs-TA data indicated that, with respect to C269, more electrons are injected from the relaxed excited states for C270 dye with a lower LUMO level.