The addition of fluorine atoms and alkyl chains to aromatic ligand dipole for enhancing stability and photoelectronic properties of formamidinium perovskite surfaces

The long-term stability on perovskite surfaces limits the practical application of perovskite solar cells (PSCs). Adding multifunction ligand can deal with this issue, however, the structure diversity of ligand and perovskite surface leads to the complicated adsorptive interaction, affecting the improvement effect of the stability and photoelectric properties. In this work, strongly electronegative fluorine atoms and alkyl chain with different length are introduced into aromatic ligand dipole to modify the polarity, regulating the phase stability, energy state and photoelectric properties of formamidinium perovskite (FAPbI3) surfaces. The results show the adsorption and interaction energy of positive ligand dipoles on the I-terminated surface decrease with the increase of polarity, which displays stronger ordered π-π stack and adsorption anchoring of ligand dipoles and thus decreasing the formation energy to improve the phase stability. The positive 4-Trifluoromethyl-phenethylammonium (CF3-PEA) with the highest polarity obtains the highest stability and shifts work function (WF) upward from 6.21 eV to the highest 6.46 eV. The negative CF3-PEA adsorbs on the PbI2-terminated surface induced by CF3•••Pb and π•••I interaction, achieving the stable phase with lower formation energy and lowest WF 5.61 eV. This work provides tutorial for optimizing performance of PSCs by the addition of ligand dipoles.