Jayaraj Nithyanandhan

In DSSCs, dye regeneration efficiency and dye aggregation on the TiO ₂ surface can be modulated by using bulky aromatic donors wrapped with alkyl groups. Introduction of rigid aromatic rings around the donor unit of a dye will directly impact the driving force for electron injection and dye regeneration of a dye. In this work, we designed and synthesized KNS dyes with an auxiliary TAT donor integrated with a visible active squaraine dye. Here, octupolar‐structured auxiliary TAT wrapped with alkyl groups is used as a strong donor and shelter to reduce the dye aggregation and charge recombination process. Further, to improve the light‐harvesting efficiency and incident photon‐to‐current conversion efficiency of DSSC devices fabricated with KNS dyes, the central squaric acid unit has been modified by appending the electron‐withdrawing dicyano group at the central squaric unit, and the trans ‐configured KNS1 dye was converted to cis ‐configured KNS2 dye. The power conversion efficiency of devices based on the KNS dyes was studied with and without 3 equivalents of CDCA by using the I ⁻ /I ₃ ⁻ electrolyte. Out of these devices, the KNS1: CDCA (1:3) based cell exhibited the best PCE of 6.25% with V _OC of 793 mV, J _SC of 11.08 mA cm ⁻² , and ff of 71%.

Indoline (In) and aniline (An) donor-based visible light active unsymmetrical squaraine (SQ) dyes were synthesized for dye-sensitized solar cells (DSSCs), where the position of An and In units was changed with respect to the anchoring group (carboxylic acid) to have In-SQ-An-CO₂H and An-SQ-In-CO₂H sensitizers, AS1–AS5. Linear or branched alkyl groups were functionalized with the N atom of either In or An units to control the aggregation of the dyes on TiO₂. AS1–AS5 exhibit an isomeric π-framework where the squaric acid unit is placed in the middle, where AS2 and AS5 dyes possess the anchoring group connected with the An donor, and AS1, AS3, and AS4 dyes having the anchoring group connected with the In donor. Hence, the conjugation between the middle squaric acid acceptor unit and the anchoring −CO₂H group is short for AS2, AS5, and AK2 and longer for AS1, AS3, and AS4 dyes. AS dyes showed absorption between 501 and 535 nm with extinction coefficients of 1.46–1.61 × 10⁵ M^–1 cm^–1. Further, the isomeric π-framework of An-SQ-In-CO₂H and In-SQ-An-CO₂H exhibited by means of changing the position of In and An units a bathochromic shift in the absorption properties of AS2 and AS5 compared to the AS1, AS3, and AS4 dyes. The DSSC device fabricated with the dyes contains short acceptor-anchoring group distance (AS2 and AS5) showed high photovoltaic performances compared to the dyes having longer distance (AS1, AS3, and AS4) with the iodolyte (I^–/I₃^–) electrolyte. DSSC device efficiencies of 5.49, 6.34, 6.16, and 5.57% have been achieved for AS1, AS2, AS3, and AS4 dyes, respectively; without chenodeoxycholic acid (CDCA), small changes have been observed in the device performance of the AS dyes with CDCA. Significant changes have been noted in the DSSC parameters (open-circuit voltage V_OC, short-circuit current J_SC, fill factor ff, and efficiency η) for the AS5 dye while sensitized with CDCA and showed highest DSSC efficiency of 8.01% in the AS dye series. This study revealed the potential of shorter SQ acceptor-anchoring group distance over the longer one and the importance of alkyl groups on the overall DSSC device performance for the unsymmetrical squaraine dyes.

Squaraine dyes possess sharp far‐red active transition with high extinction coefficient and form aggregates on TiO₂ surface. Aggregation of dyes on TiO₂ has been considered as a detrimental factor for DSSC device performance, which can be controlled by appending alkyl groups to the dye structures. Hence by integrating alkylated (alkyl groups with both in‐plane and out‐of‐plane) aryl group with indoline moiety to make it compatible with other electrolytes and for controlling the dye‐aggregation, a series of squaraine acceptor‐based dyes SQA4‐6 have been designed and synthesized. SQA4‐6 dyes showed absorption between 642 and 653 nm (λ_max), photophysical and electrochemical studies indicated that the HOMO energy levels of this sets of dyes are well aligned with the potentials of I⁻/ and [Co(bpy)₃]^2+/3+ redox shuttles for better dye regeneration process. DSSC device efficiency of 3% has been achieved for SQA5 dye with iodolyte (I⁻/) electrolyte in the presence of 0.3 mM of chenodeoxycholic acid (CDCA). The IPCE profile of DSSC device fabricated with SQA4‐6 dyes indicated the contribution of aggregated structures for the photocurrent generation.

Visible-light-active alkyl group-wrapped unsymmetrical squaraine dyes SD1-SD3 were synthesized, featuring an indoline donor and pyridine and carboxylic acid anchoring groups. Their photophysical, electrochemical, and photovoltaic characteristics were examined by fabricating a dye-sensitized solar cell (DSSC) device. Both carboxylic acid and pyridine anchoring groups containing squaraine dyes SD3 and SD2 possess similar photophysical and electrochemical characteristics. However, their photovoltaic performances were completely different. The SD3 dye with the carboxylic acid anchoring group displayed a DSSC device efficiency of 7.20% (V-OC 0.81 V; J(SC) 12.29 mA/cm(2)) using iodolyte (I-/I-3(-)) electrolyte, compared to SD1 (V-OC 0.659 V; J(SC) 4.97 mA/cm(2); and eta - 2.34%) and SD2 (V-OC 0.629 V; J(SC) 1.68 mA/cm(2); and eta - 0.84%), which were featured with pyridyl anchoring groups. These results were attributed to dye loading on the Lewis and Br & oslash;nsted acidic sites of TiO2 and the importance of aggregated structures for photocurrent generation. In the incident photon-to-current efficiency (IPCE) analysis, SD1 dye-sensitized devices exhibited photocurrent generation from both monomeric and aggregated dyes on the TiO2 surface. In contrast, SD2 showed photocurrent generation solely from aggregated states. Despite the introduction of long alkyl chains to reduce dye aggregation and charge recombination, the results indicated preferential charge injection from only the aggregated SD2 dye on TiO2. Fluorescence-quenching experiments indicated an efficient charge transfer from the aggregated SD2 dye to TiO2 compared to that of the monomeric dye. Cosensitization, a method to enhance the light-harvesting efficiency and photocurrent generation in DSSCs, was explored by simultaneously cosensitizing pyridyl-based dyes (SD1 and SD2) with a blue-colored carboxylic acid-based squaraine dye SD4. IPCE analysis demonstrated that both SD1 and SD4 contributed to generating a photocurrent of 9.11 mA/cm(2). The sequential cosensitization of SD1 and SD4 with the coadsorbent CDCA showed the highest performance, with a V-OC of 0.663 V, a J(SC) of 11.43 mA/cm(2), and an efficiency (eta) of 5.20%.