Remarkable hole transport properties of Spiro[fluorene-9,9′-xanthene] derivatives containing natural amino acid substituents for perovskite photovoltaics

Hole transporting material (HTM) highly affect efficiency and stability of perovskite solar cells (PSCs). Therefore, suitable HTMs should be designed to effectively extract and transfer holes in PSC devices. Herein, several HTMs were engineered composed of Spiro[fluorene-9,9′-xanthene] (SFX) core functionalized with natural twenty-one amino acids at the ortho, meta, and para positions of two SFX phenyl rings. Density functional theory (DFT) computations and Marcus hopping theory were performed to investigate various properties of these HTM derivatives. Highest occupied molecular orbitals (HOMOs) of all HTM samples, except for SFX-Asparagine, SFX-Glutamic acid, SFX-Glutamine, and SFX-Valine, were located between valence band of formamidinium lead iodide (FAPbI₃) perovskite and energy level of Ag cathode, which suggested these HTMs had efficient hole extraction and transport capabilities. Absorption spectra revealed that only SFX-Selenocysteine absorption occurred in visible region (λ_abs^max = 800 nm), while absorption peaks all other molecules took place in ultraviolet (UV) area, similar to Spiro-OMeTAD. This was advantageous as it ensured that the HTMs had not any competition with perovskite material for sunlight absorption. Hole reorganization energies of all SFX-based materials, except for SFX-Serine and SFX-Histidine, were smaller than electron reorganization energies. Furthermore, hole mobility (μ_h) values of all HTMs were larger than (as big as 1000 times and higher) both of calculated (μ_h = 5.65 × 10⁻³ cm²V⁻¹s⁻¹) and experimental (μ_h = 4.53 × 10⁻⁴ cm²V⁻¹s⁻¹) values for Spiro-OMeTAD. This extraordinary result proved that all of the SFX-based molecules developed here had a high potential as alternative and more affordable HTMs to Spiro-OMeTAD.