Squaraine dyes are promising chromophores to harvest visible and near-infrared (NIR) photons. A series of indoline-based unsymmetrical squaraine (SQ) dyes that contain alkyl chains at sp C- and N- atoms of indoline moieties with a carboxylic acid anchoring group were synthesized. The optical and electrochemical properties of the SQ dyes in solution were nearly identical as there was no change in the D-A-D SQ framework; however, remarkable changes with respect to the power conversion efficiencies (PCE) were observed depending upon the position of alkyl groups in the dye. Introduction of alkyl groups to the indoline unit that was away from anchoring unit were helped in more dye loading with controlled organization of dyes on surface, increased charge transfer resistance, long electron lifetime, and hence higher PCE than that of the corresponding isomer in which the alkyl groups funtionalized indoline unit contains the carboxylic acid anchoring group. Careful analysis of incident photon-to-current conversion efficiency (IPCE) profiles indicated the presence of aggregated structure on the TiO surface that contributes to the charge injection in the presence of a coadsorbent. A dye-sensitized solar cell (DSSC) device made out of SQ5 was achieved an efficiency of 9.0%, with an open-circuit potential (V) of 660 mV and short-circuit current density (J) of 19.82 mA/cm, under simulated AM 1.5G illumination (100 mW/cm). The IPCE profile of SQ5 shows an onset near to 750 nm with a good quantum efficiency (>80%) in the range of 550-700 nm, indicating the importance of self-organization of dyes on the TiO surface for an efficient charge injection. This present investigation revealed the importance of position of alkyl groups in the squaraine-based dyes for the better PCE.

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In perovskite solar cells, interfaces play a significant role in determining the device stability and device performance. Here, we introduce a versatile donor–π–acceptor (D–π–A) based organic small molecule (AA1) containing phenothiazine (PTZ) with a long alkyl chain as the donor unit, the vinyl-substituted thiophene moiety as a π bridge, and a rhodanine-(CN)2 moiety as an acceptor unit for the first time, and it was successfully deployed to passivate the defects at the surface and grain boundaries of a dual-cation perovskite absorber. The synthesized organic small molecule was characterized thoroughly using 1H NMR, 13C NMR, FT-IR, UV–vis, CV, TGA, and HRMS studies. The FT-IR spectral analysis and X-ray photoelectron spectroscopy (XPS) analysis confirm the interaction between the organic small molecule and the perovskite absorber. Simulated electrostatic potential surface (EPS) images obtained through the density functional theory (DFT) study reveal higher electron density over the acceptor unit of AA1, which ensures effective perovskite defect passivation. The formation of high-quality perovskite film with enhanced crystallinity, improved grain size, and band energy level alignment leads to effective charge carrier transport. The dual nature (defect passivation and optimized band energy level alignment) of AA1 passivation increases the surface photovoltage (from ∼100 to ∼155 mV) and reduces the defect density and ideality factor (∼1.94 × 1014 cm–3 from ∼7.1 × 1014 cm–3 and ∼1.64 from ∼1.88, respectively). The nonradiative recombination is suppressed along with reduced hysteresis, which leads to higher open-circuit voltage (1.09 V from 1.05 V) and power conversion efficiency. This work highlights the use of a push–pull small organic molecule which ensures effective passivation of undercoordinated Pb2+ defects and improved power conversion efficiency. The superior hydrophobic nature of the molecule results in device robustness. Finally, this all-in-one molecule wrestles the three major challenges commonly seen in perovskite solar cells, i.e., interface improvement, unhindered charge carrier transport, and device stability.

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