Rational design of pyridine bidentate molecules for surface passivating carbon-based perovskite solar cells

Surface passivation has emerged as an effective and feasible approach to reduce surface defects and suppress undesirable non-radiative recombination, enhancing performance and stability of perovskite solar cells (PSCs). Rational designing passivation molecules are crucial for effective and robust passivation in PSCs. With this regards, we purposely choose a series of pyridine derivatives, amino-pyridine (Py-NH 2 ), thiol-pyridine (Py-SH), 2-amino-5-(trifluoromethyl)pyridine (5-TFMAP), and 2-mercapto-5-(trifluoromethyl)pyridine (5-TFMMP) to investigate the passivation mechanism from both experimental and theoretical perspectives. The density functional theory (DFT) calculation shows that the 5-TFMMP has the highest absorption energy (E ad ) among all the passivators, thus forming the most stable bidentate coordination with the under-coordinated Pb 2+ . Meanwhile, adding −CF 3 group into the pyridine ring changes molecular electrostatic potential and increases defect formation energy of iodine vacancy (V I ) and Pb-I antisite of perovskite films. Among these four passivators, 5-TFMMP molecule combining the trifluoromethyl (−CF 3 ) group and the thiol group in the ring of pyridine ring demonstrates the best passivation effect, confirming from the highest power conversion efficiency (PCE) in carbon-based PSCs. Compared with the original PSCs, PCE of the 5-TFMMP passivated devices increased from 11.72% to 15.00%. Moreover, long-term stability of the 5-TFMMP passivated PSCs has been significantly enhanced attributing to the hydrophobic nature of the −CF 3 group at the interface of perovskite. The PCE of the 5-TFMMP-passivated PSCs has retained 88% of the initial PCE after stored at 30%–40% relative humidity for 60 days, compared with that of less than 30% for the original devices. This work provides valuable insights for proper designing effective passivators from the molecular structure point of view in PSC field.