Asymmetric Diphenylamine-Functionalized Carbazole-Phosphonic Acid Derivatives as Hole-Transporting Materials Enabling Inverted Perovskite Solar Cells >20% Efficiency

We designed and synthesized two phosphonic acid carbazole derivatives (3-(3,6-bis ((9,9-dimethyl-9H-fluoren-2-yl) (4-methoxyphenyl) amino)-9H-carbazol-9-yl)propyl) phosphonic acid (LS-W) and (3-(3,6-bis((4’-methoxy [1,1’-biphenyl]-4-yl))(4-methoxyphenyl)amino)-9H-carbazol-9-yl) propyl) phosphonic acid (LS-BR). The carbazole unit is used as the core of the molecular structure, and the asymmetric diphenylamine is connected to form a D-π-D-type electron-donating skeleton. Finally, the phosphonic acid group is introduced to modify the skeleton. In this paper, the interaction between LS-W and LS-BR with undoped poly [bis (4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) was studied by using a bilayer hole-transport layer (HTL) device structure. The electron-donating structure of carbazole-asymmetric diphenylamine extends the π-π conjugation unit of the molecule, improves the film-forming properties of the molecule, and can form a stable π-π stacking between the HTL and the perovskite layer, reducing the nonradiative recombination of the interface layer. The LS-BR has a deeper HOMO energy level, which is close to the HOMO energy level of PTAA and matches the valence band maximum (VBM) of perovskite. Theoretical calculations show that LS-BR has a uniform distribution of the electron cloud, and its higher dipole moment promotes intimate contact between the HTL and the ITO layer. The results of open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) show that the introduction of a bilayer hole transport material (HTM) can effectively reduce the charge recombination of the interface layer and delay the aging of the device. Under simulated AM 1.5 G irradiation (100 mW cm^–2), the photoelectric conversion efficiency (PCEs), short-circuit current density (J_SC), open-circuit voltage (V_OC), and fill factor (FF) of the LS-BR/PTAA device were 20.86%, 23.61 mA cm^–2, 1.087 V, and 81.22%, respectively, which were significantly improved compared with those of the pure PTAA device.