Band-edges of bismuth-based ternary halide perovskites (A3Bi2I9) through scanning tunneling spectroscopy vis-a-vis impact of defects in limiting the performance of solar cells

We study the role of defects on the band-edges of bismuth-based ternary halide perovskites (A(3)Bi(2)I(9)) formed with cesium and methylammonium (MA) ions at the A-site. We consider a range of precursor-stoichiometries leading to different reaction environments. The intrinsic defects formed during the perovskite formation affect the materials' band-energies. We introduce scanning tunneling spectroscopy (STS) to probe the band-edges. Fermi energy of the perovskites could be seen to remain always closer to valence band (VB) as compared to conduction band (CB) inferring their p-type nature. Acceptor-type defects having a least formation energy have been considered to result the p-type behavior. Upon a variation in the precursor-stoichiometry in forming the perovskites, STS studies evidence a decrease in the band gap of the materials formed with BiI3-rich precursors. The change in band-energies occurs due to formation of deep-defects above the VB-edge; a shift in the CB-edge towards the Fermi energy is due to a change in the contribution of Bi(p) orbitals which form the CB of the perovskite. With methylammonium at the singly-protonated site, the material becomes defect-tolerant as evidenced by absence of deep level defects and an improvement in the parameters of solar cells formed with (CH3NH3)(3)Bi2I9.