Researchers develop new vacuum deposition process that could reduce costs and allow excellent film quality

Researchers from Italy's CNR-IMM, Università del Salento, Università degli Studi di Catania and University of Bari 'Aldo Moro' have developed an innovative vacuum deposition method to prepare thin CH₃NH₃PbI₃ (MAPbI₃) layers for semitransparent perovskite solar cells.

Schematic of three physical deposition methods. Image from article

This new (patent pending) method to deposit thin perovskite layers for PSC under low vacuum conditions is called LV-PSE (low vacuum proximity space effusion) and can reportedly reduce costs and waste.

The method is based on two-step Low-Vacuum Proximity-Space-Effusion (LV-PSE) that can deliver high-quality at low production costs. The team reports that process parameter optimization was validated by theoretical calculation.

The researchers show that, during the process of CH₃NH₃I (MAI) deposition (second step) on PbI₂ (first step) at a given substrate temperature, the conversion of the PbI₂ film to MAPbI₃ occurs from the top surface inward via an adsorption'incorporation'migration mechanism guided by the gradient of energetic MAI concentration.

The quality of the final layer arises from this progressive conversion that also exploits the lattice order (texture) of the mother PbI₂ layer.

Then, p'i'n solar cells were prepared using ITO/PTAA/MAPbI₃/PCBM-BCP/Al architectures with a photo-active layer thickness of 150 nm. This layer, characterized by an Average Visible Transmittance (AVT) as high as 20%, produced an average efficiency of 14.4% - an impressive result considering the transparency vs. efficiency countertrend that indeed demands to boost the quality of the material.

The demonstrated that a further down scalability of the MAPbI₃ layer is feasible as proved by reducing the thickness down to 80 nm. In this specific case, the devices showed an average efficiency of 12.9% withstanding an AVT of 32.8%. This notable efficiency recorded on those extremely thin layers thus benefits from the exclusive quality of the MAPbI₃ grown with the developed method.

The team stated that the use of LV-PSE and the scaled MAPbI₃ thicknesses, associated with excellent efficiency values, could pave the way for a cost reduction for device fabrication besides allowing a reduction by ∼1/3 of the Pb content inside the devices against environmental concerns.