Effect of PbI2 deposition rate on two-step Physical Vapor Deposition and Chemical Vapor Deposition all-vacuum prepared perovskite

Organometal halide perovskites (OMHPs) materials are promising absorbers for the manufacture of low cost, high efficiency solar cells. Their high carrier mobility and low recombination rate enhance the efficiency of power conversion. Two alterative synthesis methods have been explained either by one step processing of binary precursor mixture or by sequential deposition of the precursor. However, for the sequential deposition, the impact of lead iodide film crystallinity on perovskite formation has not been investigated comprehensively. In addition, organometal halide perovskite film properties, i.e. grain size, morphology as well as chemical homogeneity depend on the preparation method, which significantly influences its electrical properties.

Researchers lead by Professor Konstantinos Fostiropoulos from Helmholtz Center Berlin for Materials and Energy in Germany prepared polycrystalline lead iodide layers by physical vapor deposition and the impact of the applied deposition rate on the crystallites’ orientation and the morphology of the formed films were investigated. They also demonstrated that for higher deposition rate, the planes of lead iodide film show a higher degree of alignment parallel to the surface of the sample. Their work is now published in peer-reviewed journal, Journal of Solid State Chemistry.

The researchers prepared perovskite layers by a two-step vacuum deposition method. In the first step, lead iodide was deposited by physical vapor deposition at different rates. For increased deposition rate, the film exhibited a lower surface roughness with higher degree of lead iodide planes alignment parallel to substrate’s surface. In the second step, perovskite films were formed by applying a chemical deposition process under a static CH₃NH₃I atmosphere. The high rate of deposited lead iodide films was found to promote the intercalation rate of CH₃NH₃I resulting in the formation of perovskite layers with less residual lead iodide.

Deeper intercalation was realized by prolonging the intercalation time. Hard x-ray photoelectron spectroscopy measurements on this sample indicated the formation of a stoichiometric perovskite film, with a valence band onset of 0.95eV. The influence of residual lead iodide on the perovskite solar cell device attributes was studied with the best performing device being realized for perovskite preparation parameter deposition rate 5.2Ǻ/s and intercalation time (T=65min).

Fostiropoulos notes that in commercial solar cells the devices’ absorber layer is sandwiched between vacuum prepared front and back contacts made of a transparent conductive oxide (TCO) and a metal, respectively. Therefore, for up-scaled industrial solar cells production it makes sense to complete all fabrication steps (substrate/TCO/absorber/metal) under the same controlled conditions avoiding effects of solvents and ambient atmosphere.