Photoexcited Phenyl Ring Twisting in Quinodimethane Dyes Enhances Photovoltaic Performance in Dye-Sensitized Solar Cells

Four metal-free organic quinodimethane-based dyes, which represent molecular building blocks for a new class of DSSC dyes, were studied with the objective to improve their photovoltaic performance via molecular engineering strategies. Such strategies can only be systematic and successful if they are derived from knowledge-based paradigms that relate individual aspects of the molecular structure for a given class of dyes to their photovoltaic properties. The optical and electrochemical properties of these dyes were investigated experimentally by UV-vis absorption and emission spectroscopy, as well as cyclic voltammetry and electrochemical impedance spectroscopy. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on these dyes complement these experiments. Among other results, this concerted experimental and computational study revealed that the dialkylaminophenyl moiety in these dyes exhibits a large twist as a result of its ground-to-excited-state optical transition. In particular, a near-perpendicular (88.98 degrees) twist of the dimethylaminophenyl moiety relative to the pi-bridging unit that connects the three aryl rings in 2 was observed upon formation of its photo-excited-state structure. Moreover, it was discovered that 2 affords the highest V-OC and power-conversion efficiency (PCE) values among these dyes. The particularly high PCE for 2 was found to be due to this twisting of the phenyl ring, which blocks the pathway of electrons from TiO2 to the donor, and hence suppresses undesirable electron recombination at the dye center dot center dot center dot TiO2 interface. This diode-like effect minimizes undesirable electron recombination effects and represents an unprecedented structure property relationship that should be useful for the molecular engineering of larger chromophores in this class of dyes for DSSC applications.