Orthogonally Functionalized Donor/Acceptor Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: An Approach to Introduce Panchromaticity and Control the Charge Recombination

Organic dyes possessing conjugated π-framework forms closely packed monolayers on photoanode in dye-sensitized solar cell (DSSC), because of the limitation to control the orientation and the extend of intermolecular π–π interaction, self-aggregation of dyes leads to reduced cell performance. In this report, a series of homodimeric (<b>D</b><sub><b>1</b></sub><b>-D</b><sub><b>1</b></sub> and <b>D</b><sub><b>2</b></sub><b>-D</b><sub><b>2</b></sub>) and heterodimeric (<b>D</b><sub><b>1</b></sub><b>-D</b><sub><b>2</b></sub> and <b>D</b><sub><b>2</b></sub><b>-D</b><sub><b>4</b></sub>) donor/acceptor (D/A) dyes containing spiroBiProDOT π-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and correlates the device performance with monomeric dyes (<b>D</b><sub><b>1</b></sub> and <b>D</b><sub><b>2</b></sub>). Both the thiophenes (π-spacer) of spiroBiProDOT were functionalized with same or different donor groups which led to homodimeric and heterodimeric chromophores in a single sensitizer. The homodimeric spiro-dye <b>D</b><sub><b>1</b></sub><b>-D</b><sub><b>1</b></sub> showed higher power conversion efficiency (PCE), of 7.6% with a <i>V</i><sub>oc</sub> and <i>J</i><sub>sc</sub> of 0.672 V and 16.16 mA/cm<sup>2</sup>, respectively. On the other hand, the monomeric <b>D</b><sub><b>1</b></sub> exhibited a PCE of 3.2% (<i>V</i><sub>oc</sub> of 0.64 V and <i>J</i><sub>sc</sub> of 7.2 mA/cm<sup>2</sup>), which is lower by 2.4 fold compared to dimeric analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO<sub>2</sub> due to four sp<sup>3</sup>-centers, which arrest the molecular motions after chemisorption. This study shows a new molecular approach to incorporate two chromophores in the dimeric dye possessing complementary absorption characteristics toward panchromatic absorption. The attenuated charge recombination at TiO<sub>2</sub>/Dye/redox couple interface in case of <b>D</b><sub><b>1</b></sub><b>-D</b><sub><b>1</b></sub>, owing to better passivation of TiO<sub>2</sub> surface, was elucidated through impedance analysis. The FT-IR spectrum of <b>D</b><sub><b>1</b></sub><b>-D</b><sub><b>1</b></sub> adsorbed on TiO<sub>2</sub> film indicated both the carboxylic units were involved in chemisorption which makes strong coupling between dye and TiO<sub>2</sub>.