Substituted Alkylammonium Layered Lead Halide Perovskites: An Experimental and Computational Study

Abstract

Organic-inorganic hybrid metal halide perovskites have emerged as a new class of semiconducting materials with potential applications in solar devices, catalyzed by the remarkable power conversion efficiency of over 21% exhibited by the three-dimensional methylammonium lead iodide perovskites (CH3NH3PbI3). Despite the high-power conversion efficiencies, however, the 3D perovskites have not achieved commercial success due to their poor stability under operating conditions and the toxicity associated with lead. The two-dimensional (2D) halide perovskites have improved stability due to larger hydrophobic organic cations and, in combination with 3D perovskites, have shown promise in solar cells, light-emitting diodes, and photodetectors.

The focus of this thesis is on how the optical properties of 2D layered linear alkylammonium lead bromide perovskites, (CnH2n+1NH3)2PbBr4, changes on substituting, partly or wholly, i) the alkyl chains by alkyl chains of differing lengths ii) Pb2+ ions by Sn2+ ions and iii) Br− ions by either I− or Cl− ions, both experimentally, as well as by computational methods. In addition, an attempt is made to identify the chemical species present and to unravel the complex equilibria in the precursor solution of CH3NH3PbBr3 perovskite and the formation of the CH3NH3PbBr3 from the precursor solution in DMF were investigated; both in the presence and absence of capping agents.

In brief, the formation of mixed alkyl perovskites, (CnH2n+1NH3)(CmH2m+1NH3)PbBr4 (n ≠ m), was confirmed by PXRD and it was found that the c parameter was the mean of the values of the pure perovskites, (CnH2n+1NH3)(CmH2m+1NH3)PbBr4 (n = m). The alkyl chain conformation in these perovskites was established using IR and Raman spectroscopy from the progression bands that arise from the coupling of trans-(CH2) units of the chain. The variation of the optical band gap with alkyl chain length was interpreted using Density Functional Theory (DFT) and shown to be linked to the octahedral tilt angle of the respective perovskite. The metal substituted (C4H9NH3)2SnxPb1-xBr4 perovskites showed a nonlinear variation of the optical band gap. The computed optical band gaps for the structures selected using the Site-Occupation Disorder-Cosine Distance Criterion (SOD-CDC) procedure and the structure order parameter (S_order) are able to capture the experimentally observed variation with Sn content, and it is shown from the projected density of states that the origin of nonlinearity arises due to the Sn and Pb orbital offsets as well as the contribution of spin-orbit coupling that is more significant for the heavier lead. The optical properties of layered lead halide perovskites (CnH2n+1NH3)2PbBr4(1-y)I4y and (CnH2n+1NH3)2PbCl4(1-y)Br4y (n = 4, 8), layered perovskites were investigated at different halide content (y = 0, 0.25, 0.5, 0.75, 1) and shown to exhibit a linear variation. DFT computations, too, show a similar variation of the band gap, and the computed projected density of states shows that the linear variation of band gaps is a consequence of the orbital offsets of the halide p orbitals, whose contributions to the valence and conduction bands is proportional to their stoichiometry in the perovskite.

In CH3NH3PbBr3 perovskite, It is shown that on increasing the PbBr2 concentration, there is the formation of a polymeric lead-bromide-DMF complex that breaks upon the addition of CH3NH3Br to the solution. Further addition of toluene leads to the precipitation of the 3D CH3NH3PbBr3 bulk powders in the absence of capping ligands. CH3NH3PbBr3 nanocrystals and 2D layered perovskites were formed in the colloidal solution with capping ligands. The phase-pure 3D CH3NH3PbBr3 nanocrystals can be obtained by redispersing and re-centrifuging the colloidal solution residue.