Perovskite Photovoltaics: Hybrid Perovskite Thin‐Film Photovoltaics: In Situ Diagnostics and Importance of the Precursor Solvate Phases (Adv. Mater. 2/2017)

Abstract

The solidification of hybrid perovskite (MAPbX3, X = I, Br, Cl) inks is studied in situ by A. Amassian and co‐workers in article 1604113, revealing a remarkably complex process mediated by strong solvent‐solute interactions, the outcome of which is halide‐dependent. The ink is shown to solidify within 15–20s of spinning, forming a highly solvated (60–70 vol%) precursor phase. Crystalline order of the precursor, or lack thereof, strongly impacts the morphological outcome of thermal conversion and the reproducibility of solar cells.

Sequential Formation of Tunable‐Bandgap Mixed‐Halide Lead‐Based Perovskites: In Situ Investigation and Photovoltaic Devices

“…The low q scattering of the resulting film is again ascribed to a colloidal metallate sol. [36] The high Br content systems do not achieve crystalline order, but their scattering features continue to change throughout the drying process, indicating mesoscale reorganization of the as-cast film. PbX2 (X = Br, I) have different crystal structures, depending on the ionic radii and electronegativities of the constituent ions.…”

“…The strength of solvent-PbX2 interactions and the ability of the solvated PbX2 to cocrystallize with the solvent was shown to be mediated by the solvent and the halide. [36,74] We, therefore, turn our attention to investigating the solidification and crystallization of Pb(IxBr1-x)2 from a DMSO solution. The time-evolution of the scattering features ( Figure 1d) when substituting I with Br gradually in Pb(IxBr1-x)2 with x = 0.6, 0.4 and 0 (PbBr2),…”

“…[9,35] To date, the one-step spin-coating approach is the most widely used and facile method to fabricate high-quality hybrid perovskite thin films from solution. [32,[36][37][38][39][40][41][42] However, it suffers from several drawbacks related to poor control of anti-solvent dripping during fast spinning, making less reproducibility of film formation and devices in contrast to two-step sequential deposition. [20] Multi-cation and multi-halide perovskite formulations have been proposed to help overcome film formation challenges too, but these compounds tend to suffer from halide segregation.…”

“…[32,43,44] Consequently, highly non-equilibrium quenching steps, such as anti-solvent dripping has been used to initiate phase transformation and increase the nucleation density and growth rate of these materials prior to crystallization. [32,36,38,43] Such approaches can be labor intensive, and may not be scalable to large area continuous fabrication. By contrast, the two-step sequential coating method exhibits fewer challenges associated to the one-step procedure, allowing better control of the phase transformation and morphology of the perovskite active layer toward achieving high quality and pinhole-free perovskite films.…”

“…[47] The formation of perovskites via one-step spin-coating process has been monitored through several in situ techniques. [32,36,42,43,67,[69][70][71] In the context of the two-step process, our previous investigation into the solidification of PbI2 films from a DMF solution using in situ grazing incidence wide-angle x-ray scattering (GIWAXS) measurements revealed an elaborate sol-gel process involving three unstable PbI2DMF solvated complexes, including two ordered ones, which can spontaneously transform to the perovskite phase at room temperature. [33,45] However, there has not been a direct observation of the formation of PbI2, let alone PbBr2 or Pb(I1-xBrx)2 mixed halide films, from DMSO solution or their conversion to pure or mixed halide perovskite phases.…”