Simultaneous Control of Self-Assembly and Photon Harvesting Window in NIR-Sensitive Squaraine Dyes for Next-Generation Bifacial Solar Cells

Sensitizers utilized in dye-sensitized solar cells (DSSCs) play a crucial role in solar energy harvesting, and their capability to harvest photons in the wide-wavelength region encompassing visible to near-infrared regions is highly desirable. In addition to this, electron injection along with self-assembly of the dyes also plays a rather important role, and therefore their precise control is highly desirable and challenging too. This study deals with the molecular engineering approaches directed toward the molecular design and synthesis of two unsymmetrical squaraine dyes (SQ-260 and SQ-261), introducing extended π-conjugated moieties in the dye molecular framework of the reference dye SQ-258. Further, the bifacial DSSCs were fabricated using the dyes, and their photophysical and photovoltaic properties were investigated comprehensively. SQ-258, bearing a typical unsymmetrical squaraine dye structure, exhibited higher dye aggregation, a lower energy barrier for dye regeneration, and a relatively narrow photon harvesting window. SQ-260, possessing a cyanoacrylic acid moiety as its anchoring group, solved the problem of lower electron injection, but it exhibited higher aggregation. Finally, SQ-261 was logically designed by incorporating a 1,3-indandione moiety in the central squaric acid core and a cyanoacrylic acid anchoring group in the terminal indole ring, thereby allowing the simultaneous control of aggregation, greater electron injection, and wide-wavelength photon harvesting. However, SQ-261 displayed a lower power conversion efficiency (PCE), mainly due to a very low driving force for electron injection (0.17 eV). This lower driving force has been attributed to the lower band gap (Eg) of SQ-261, which is a consequence of its highly red-shifted absorption edge.