Theoretical investigation of octahedral tilting and bandgap non‐linearity in monolayer Ruddlesden‐Popper A ₂ Pb _1‐x Ge _x I ₄ perovskites

Abstract

Summary 2D Ruddlesden‐Popper (RP) perovskites (R‐NH3)2An−1BnX3n+1 with tailorable photovoltaic features of low toxicity, reduced dimensionality, and defect tolerance hold promise to overcome the main constraints of perovskite solar cells (PSCs). A systematic investigation of the structural and electronic properties of pure and mixed (R‐NH3)2Pb1−xGexI4 systems was performed with and without spin‐orbit coupling (SOC), by using density functional theory (DFT). The organic spacer cations (R‐NH3+)2 of methylammonium (MA‐CH3NH3+) and Phenylethyl ammonium (PEA‐C6H5[CH2]2NH3+) with x = 0.0, 0.25, 0.50, 0.75, 1.0, resulted in the compositionally modified 32‐permutations by systematically substituting the inorganic cations. Short‐range ordering has categorized all studied compositions of (R‐NH3)2Pb0.50Ge0.50I4 into column and battenberg schemes depending on the Ge defect site. The competing effects of octahedral tilting and lattice distortion with bandgap bowing and SOC, across Pb‐Ge compositional systems, provide a systematic strategy to tune bandgaps. By controlling the cations' composition, these mixed 2D‐perovskites outlined the strategy to achieve the desired bandgap and band positions. A rational design approach of low dimensional perovskites was adopted to tune the electronic properties through the spacer and inorganic cation engineering. Our findings expose the scope of monolayer‐based systems as an ideal choice of absorber for the bottom sub‐cell in all‐perovskites 2D‐3D tandem solar cell, harvesting light from visible (2.4 eV) up to infrared (1.2 eV) in the solar spectrum with features of defect tolerance, quantum and dielectric confinement, low bandgap, low effective masses, and high mobility.

An Overview of Current Printing Technologies for Large-Scale Perovskite Solar Cell Development

“…In this technique, perovskite films are prepared through the thermal evaporation of chemicals and are generally carried out in high-vacuum conditions [15]. Figure 7c shows that the thermal evaporation method is classified into single and multiple-source evaporations.…”

“…A large type A cation will be in the cube centre (usually methyl ammonium ion, CH 3 NH 3 ). Furthermore, the corners are occupied by the small cations of type B (usually Pb), and the cube faces are occupied by the halogen anion of type X (usually I, Cl or Br) [15,16]. In contrast, combining different materials into this structure is essential in determining the photoelectric properties, bandgap, diffusion lengths and mobility [17].…”

“…Generally, TiO2 is used as a mesoporous layer, allowing perovskite material to penetrate the pores of the TiO2 solution and, hence, improving the surface area of the perovskite layer. Additionally, the mesoporous layer supports inhibiting electron-hole pairs' recombination process and improving the PSC's PCE [15,19]. As the perovskite material possesses hole-transporting properties, PSC with a hole-transport material-free structure is also possi- ble.…”