Interface Modification with Holistically Designed Push–Pull D–π–A Organic Small Molecule Facilitates Band Alignment Engineering, Efficient Defect Passivation, and Enhanced Hydrophobicity in Mixed Cation Planar Perovskite Solar Cells

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.