Direct tuning of the band gap: Via electronically-active organic cations and large piezoelectric response in one-dimensional hybrid halides from first-principles

Three- (3D) and two-dimensional (2D) organic-inorganic hybrid halides exhibit superior optoelectronic properties, which strongly depend on the [BX₆] inorganic networks. A-Site organic molecules are considered to have a negligible influence on the electronic states around the Fermi level. Here, using the first-principles method, we exploited the ground state properties and band gap engineering through A-site electronically-active organic molecules in 1D GAPbI₃ (GA = C(NH₂)₃). Our results revealed that, from 3D to 1D structures, organic cation GA⁺-based states can directly contribute to the valence band edges. By introducing C₇H₇ ⁺ organic cations into GAPbI₃, the band gap is directly tuned from 2.28 to 0.69 eV, originating from the partially unoccupied C 2p states of C₇H₇ ⁺ forming several conduction bands below the Pb 6p states. The C₇H₇ ⁺-doped material is expected to exhibit significantly absorption in the visible light region. Finally, we predict a large piezoelectric response in GAPbI₃ with d₃₁ = -141.09 and d₃₂ = 146.16 pC N^-1, which is four times higher than that of the most widely used flexible piezoelectric poly(vinylidene fluoride) (PVDF) material. Our findings will provide new insights into low-dimensional hybrid halides and reveal their potential applications in flexible electronics.