Low-cost novel X-shaped hole transport materials for efficient perovskite solar cells: Molecular modelling of the core and schiff base effects on photovoltaic and photophysical properties

Abstract

Eight molecules with D-π-D molecular motifs coded HTM(1-4)a,b were designed as efficient symmetric hole transporting materials for perovskite solar cells. These HTMs are composed of different core moieties, such as spiro [fluorene-9,9′-xanthene]-diol (SFO), spiro [fluorene-9,9′-xanthene]-dimethoxy (SFM), benzo [c][1,2,5]thiadiazole (BTD), and biphenyl (BP) and are gaining lot of attention because of their low-cost, reproducible electrical and optical properties in device performance. Detailed information about the energetic of molecular levels (GSOP/ESOP), first singlet excitation energy (E0-0), equilibrium molecular geometry, charge separation, charge transfer, reorganization energies, polarizability and hyperpolarizability, density of states (DOS), solubility and stability of these molecules was attained by systematically performing molecular modeling calculations using density functional theory (DFT) and time dependent-DFT calculations utilizing hybrid density functional B3LYP using the basis set 6–31g(d,p) level of theory as a successful method for predicting the photophysical and photovoltaic properties of these conjugated systems. The results showed that not only the core moieties, but also Schiff-base linkage extended conjugation, have an effect on the photophysical and photovoltaic properties of the proposed HTMs. It was demonstrated that HTMs incorporating SFO (HTM1a,b), SFM (HTM2a,b), and BP (HTM4a,b) achieved better charge transport, high stability values (ηa = 2.11–2.40 eV), low electron-hole binding energy (Eb = 0.16–0.21 eV) than HTM3a,b with BTD core, which improves hole mobility and decreases recombination, all of which improved device photocurrent, which can be attributed to the extended π-conjugation in SFO, SFM and BP cores. These findings are strikingly similar to those of Spiro-OMeTAD (ηa = 2.45 eV, Eb = 0.16 eV), indicating that these HTMs are promising candidates for efficient and cost-effective PSCs. Interestingly, when the stability of HTM(1-4)a and HTM(1-4)b was compared, it was clear that HTM(1-4)a has lower stability than HTM(1-4)b, which could be attributed to the presence of Schiff base CN bonds in HTM(1-4)b, which reduces the molecules stability when compared to HTM(1-4)b, owing to a strong C–C linkage. Meanwhile, HTM(1-4)a outperforms HTM(1-4)b in terms of electronic performance and hole mobility.