An international research team developed a method to fabricate perovskite thin films using a precursor known as phenethyl ammonium chloride (PEACl) in a simultaneous deposition and passivation step.
“Such a simplified process can enable researchers to make cells with more consistent quality,” Tim Kodalle, the scientist leading experiments completed at Lawrence Berkeley National Laboratory (Berkeley Lab), told pv magazine. “Longer term, the process can potentially reduce the costs and energy consumption of perovskite cell manufacturing.”
The proposed method is said to reduce the number of synthesis steps, while also stabilizing the halide perovskite film for over a month. The team said it used a simplified approach, combining a modified 2D/3D fabrication process with the incorporation of large halides into the 2D/3D perovskite films. The integrated deposition and passivation strategy used PEACl dissolved in isopropyl alcohol (IPA).
The PEACl:IPA solution was deposited as the modified antisolvent (AS) during the 3D halide perovskite thin film process, integrating passivation via 2D perovskite into the deposition step of the 3D perovskite films. The PEACl was added to the antisolvent in the spin coating step. The mixed 2D/3D film was then annealed on a hotplate.
The team compared the performance of the PEACI:IPA-based solar cell with that of a reference device without the PEACI treatment.
“The PEACl-PSCs show significantly improved device performance and reproducibility compared to the control cell,” it said, noting that the performance of the champion cell was verified with a range of tests with results that indicated it had a high stabilized efficiency.
This cell achieved a power conversion efficiency of 20.9%, an open-circuit voltage of 1.13 V, a short-circuit current density of 23.0 mA/cm2, and a fill factor of 80.0%. The benchmark cell reached an efficiency of 19.0%, an open-circuit voltage of 1.10 V, a short-circuit current density of 23.2 mA/cm2, and a fill factor of 73.2%.
Image: Thor Swift/Berkeley Lab
The team attributed the performance improvement to the reduction of the non-radiative recombination rate and improvement of the interface of perovskite and hole transport layer.
To investigate further how the PEACl approach influenced performance as it was unclear how the inclusion of the bulky molecules into the pre-annealed wet film influences nucleation and subsequent crystallization of the perovskite film, a team at Berkeley Lab was asked to carry out a host of in situ measurements.
Several in situ measurement techniques were used. At the Advanced Light Source, photoluminescence (PL) and grazing incidence wide-angle X-ray scattering (GIWAXS) data were collected using a custom-made analytical chamber. Additionally, the team used ultraviolet-visible spectroscopy to observe the evolution of the absorbance of the films as they formed to record differences in the optical properties of the samples, according to Tim Kodalle, the scientist leading the experiments completed at Berkeley Lab.
The researchers then correlated the data streams from the three techniques to analyze the temporal evolution of the crystalline structure of the samples. The team found that PEACl slows down the crystal growth process, which leads to a larger average grain size and narrower grain size distribution. This in turn reduces carrier recombination at grain boundaries and improves the device’s performance and stability.
“The data suggests that during annealing of the wet film, the PEACl diffuses to the surface of the film, forming hydrophobic (quasi-)2D structures that protect the bulk of the perovskite film from humidity-induced degradation,” said the researchers, concluding that the two-step PEACl process can insert a thin protective barrier that keeps out moisture.
The details of the research appear in “An Integrated Deposition and Passivation Strategy for Controlled Crystallization of 2D” published by Advanced Materials. Researchers from the University of Stuttgart, Lawrence Berkeley National Laboratory, Forschungszentrum Jülich, Brandenburg University of Technology, McGill University, and Zentrum für Sonnenenergie und Wasserstoff Forschung collaborated on the project.
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