A Note on Stability of a Dopant for Perovskite Solar Cells: Dimerization of 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine Iodine

The note continues with the density-functional theory (DFT) quantum-chemical understanding of perovskite solar cells at molecular level. In particular, 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine iodine (or BFBAI₂, stoichiometry C₁₄H₁₂F₆I₂N₂) is further calculated—the species is known to improve the power conversion efficiency and device stability. The thermodynamic-stability calculations are performed at the M06-2X/3-21G level with anharmonic vibrational analysis (including vibrational-rotational coupling) for construction of the vibrational-rotational partition functions. The dimerization is shown to be an essential feature of BFBAI₂ (it is based on the formation of two hydrogen bonds). The BFBAI₂ dimerization is described in the terms of the standard Gibbs energy and the related dimerization equilibrium constant. Comparisons are made with the water dimer, commonly used as a model system for hydrogen-bond formation. The equilibrium constants for the water dimerization are consistently lower than for the dimerization of BFBAI₂ - as the presence of two hydrogen bonds in the BFBAI₂ dimer contributes to the larger stabilization. The dimerization of BFBAI₂ upon higher surface coverages represents an additional factor for the layer stabilization as there is decrease in the standard Gibbs energy at moderate temperatures. The dimerization also brings additional features for modulation of surface conditions.