Hexamethylphosphoramide-Assisted Structure and Morphology Regulation of a PbI₂ Film for Air-Processed Efficient Perovskite Solar Cells via a Two-Step Deposition Method

Abstract

The two-step deposited method with the advantages of high reproducibility and controllability is widely used in fabricating highly efficient perovskite solar cells (PSCs). However, it is still a challenge to fabricate air-processed PSCs through this method due to the existing techniques with glove box circumstance that are not well extended to the ambient air-condition fabrication process. Here, hexamethylphosphoramide (HMPA) used as a solvent additive is incorporated into the first-step PbI 2 precursor solution, which can effectively modify the morphology and crystallinity of the PbI 2 layer, thereby creating more opportunities for the infiltration of the secondstep organic salt. This two-step coating leads to the full conversion from PbI 2 to the perovskite crystal and large grains, reduced defects, and enhanced uniformity for the final perovskite film. Consequently, a champion efficiency of 20.84% of the HMPA-based device fabricated using this procedure is achieved. The HMPA-based device shows long-term stability of the remaining 85.7% of the initial efficiency after 1000 h of storage in an ambient air environment. This method demonstrates the great potential toward air-processed efficient PSCs.

Bandgap Engineering of Two‐Step Processed Perovskite Top Cells for Perovskite‐Based Tandem Photovoltaics

“…The reduced FWHM value indicates an enhanced crystallinity and larger grain sizes of the perovskite film according to the Scherrer equation $$$$\begin{equation} D=\frac{K\lambda }{\beta \cos (\Theta )} \end{equation}$$$$where D is the crystallite size, K a constant, λ the X‐ray wavelength, β the FWHM and Θ the diffraction angle. [ 38,65 ]…”

“…where D is the crystallite size, K a constant, 𝜆 the X-ray wavelength, 𝛽 the FWHM and Θ the diffraction angle. [38,65] According to the SEM images (Figure S11, Supporting Information), all perovskite films (Ref, PbBr 2 , FABr, and PbBr 2 +FABr) exhibit a comparable morphology, yet subtle distinctions are discernible. The reference films and the films with PbBr 2 display more pinholes and present a disorderly appearance in contrast to the films with FABr and the films with PbBr 2 and FABr.…”

“…[37] Additionally, the presence of undercoordinated Pb ions, serving as non-radiative recombination sites, can result in charge carrier recombination and ionic motion, which in turn limits the overall performance of the device. [38][39][40][41][42][43] This effect holds particular significance for solar cells in a p-i-n configuration, where the remaining PbI 2 layer can hinder the effective extraction of charges to the underlying transport layer, possibly resulting in the formation of a counter diode. [44] Incorporation of bromide during the second deposition step poses a challenge with regard to bromide diffusion into the lead halide layer, which arises from the elevated activation energy needed for the migration of bromide anions in comparison to iodide anions.…”

Synergistic Effects of Energy Level Alignment and Trap Passivation via 3,4-Dihydroxyphenethylamine Hydrochloride for Efficient and Air-Stable Perovskite Solar Cells

“…The outstanding performance and low processability cost of organometallic perovskite solar cells (PSCs) have attracted widespread interest from scientists throughout the world. − The performance of PSCs has improved dramatically in the past decade, reaching up to 25% , since the first report in 2009 . This significant development can be associated not only with the beneficial inherent characteristics of perovskite materials, such as adjustable band gap, high carrier mobility, and ultralong carrier diffusion length, − but also with the substantial studies that have been conducted on device engineering, such as composition modification, controlling crystallization, and engineering the interface. − These factors have all played a role in the development of this technology. − …”