The Impact of the Crystallization Processes on the Structural and Optical Properties of Hybrid Perovskite Films for Photovoltaics

Abstract

We investigate the relationship between structural and optical properties of organo-lead mixed halide perovskite films as a function of the crystallization mechanism. For methylammonium lead tri-iodide, the organic cations rearrange within the inorganic cage, moving from crystals grown in a mesoporous scaffold to larger, oriented crystals grown on a flat substrate. This reduces the strain felt by the bonds forming the cage and affects the motion of the organic cation in it, influencing the electronic transition at the onset of the optical absorption spectrum of the semiconductor. Moreover, we demonstrate that in mixed-halide perovskite, though Cl− ions are not present in a detectable concentration in the unit cell, they drive the crystallization dynamics. This induces a preferential order during crystallization, from a molecular, i.e., organic–inorganic moieties arrangement, to a nano-mesoscopic level, i.e., larger crystals with anisotropic shape. Finally, we show that while Cl is mainly expelled from flat films made of large crystals, in the presence of an oxide mesoporous scaffold they are partially retained in the composite.

Influence of the substrate on the bulk properties of hybrid lead halide perovskite films

“…42 We did not observed a large difference in the PL wavelength for the films contrary to the large variations reported before . 20,36,34 The measured PL wavelengths do not correlate to the observed lattice parameters, which are directly related to the Pb-Pb interatomic distances. The Theoretical studies predict a variation of around 0.02 eV/degree for bang gap energy with Pb-I-Pb angle.…”

“…7,22,23,24 It has been shown that the perovskite film is intrinsically heterogeneous in crystal size, and that Cl plays an important role in improving the crystallization process. 20 However, the influence of the underneath layer on the stability, lattice parameters, orientation and the structure-properties relationship needs further understanding.…”

“…30 , 31 , 32 Also large variability of the photoluminescence (PL) wavelength for MAPbI 3 films has been reported remarking that mechanisms leading to these changes are not well understood yet. 33 Shifts of the PL peak up to 20 nm have been observed and related to grain size, either controlled by the substrate morphology 20,34,35 or by the synthesis for aggregated perovskite dots. 36 On the other hand, theoretical studies correlate the optical band gap, and therefore PL wavelength, mainly with Pb-I-Pb bond angle and Pb-I bond length which could be controlled through the steric size of the molecular cation without altering the metal−halide chemistry.…”

Crystal Morphologies of Organolead Trihalide in Mesoscopic/Planar Perovskite Solar Cells

“…Scanning electron microscopy (SEM) is commonly used to study OTP films, but the SEM has significant limitations in terms of resolution and the ability to identify crystal phases. In this context, Petrozza and coworkers 15 have characterized MAPbI 3 and MAPbI 3−x Cl x in the form of planar films and inside mesoporous TiO 2 using Raman spectroscopy and X-ray diffraction (XRD). They highlight the differences in the way the OTP crystallizes in the two forms and how it affects the onset of optical absorption in OTPs and other properties such as texture.…”

Improving the Morphology of the Perovskite Absorber Layer in Hybrid Organic/Inorganic Halide Perovskite MAPbI3 Solar Cells

“…Tidhar et al [8] discussed the crystallization of methyl ammonium lead halide perovskites and its implications in photovoltaic applications. Grancini et al [9] also studied the impacts of the crystallization processes on the structural and optical properties of hybrid perovskite films. Liang et al [10] presented additive enhanced crystallization of solution-processed perovskite for highly efficient planar heterojunction solar cells.…”