Mixed halide perovskites with chlorine (Cl) content have received significant interest due to better charge transport properties and longer diffusion length compared to pure iodine-based perovskites. The superior properties of Cl-doped perovskites improve solar cell device performance, although the quantification of Cl composition in the perovskite films remain difficult to achieve. Hence, it is difficult to correlate the Cl-quantity with the improved device performance. In this work, we deposited Cl-doped perovskite films through a facile three-and two-step sequential chemical vapor deposition (CVD) where lead halide films were deposited in the first steps of the process and subsequently converted to perovskites. No Cl substitution by iodine was observed during a sequential deposition of lead chloride and lead iodide films which reacted to form a lead chloride iodide phase (PbICl). The substitution of Cl by iodine ions only occurred during the conversion to perovskite phase. Large perovskite grains (greater than 2 µm) were realized when converting a PbI 2 film to perovskite compared to chlorine containing lead halide films, contradicting literature. However, Cl doped perovskite solar cells showed improved device efficiencies as high as 10.87% compared to an un-doped perovskite solar cell (8.76%).
In this article, we used a two-step chemical vapor deposition (CVD) method to synthesize methylammonium lead-tin triiodide perovskite films, MAPb1−xSnxI3, with x varying from 0 to 1. We successfully controlled the concentration of Sn in the perovskite films and used Rutherford backscattering spectroscopy (RBS) to quantify the composition of the precursor films for conversion into perovskite films. According to the RBS results, increasing the SnCl2 source amount in the reaction chamber translate into an increase in Sn concentration in the films. The crystal structure and the optical properties of perovskite films were examined by X-ray diffraction (XRD) and UV-Vis spectrometry. All the perovskite films depicted similar XRD patterns corresponding to a tetragonal structure with I4cm space group despite the precursor films having different crystal structures. The increasing concentration of Sn in the perovskite films linearly decreased the unit volume from about 988.4 Å3 for MAPbI3 to about 983.3 Å3 for MAPb0.39Sn0.61I3, which consequently influenced the optical properties of the films manifested by the decrease in energy bandgap (Eg) and an increase in the disorder in the band gap. The SEM micrographs depicted improvements in the grain size (0.3–1 µm) and surface coverage of the perovskite films compared with the precursor films.
Lead halide thin films, such as lead iodide (PbI2) and lead chloride (PbCl2), are used as precursor films for perovskite preparation, which is frequently achieved by vacuum thermal evaporation but rarely by the low-pressure chemical vapor deposition (CVD) method. Here, we report on the deposition of PbI2 and PbCl2 thin films on glass substrates by employing the low-pressure CVD method. The effect of the substrate temperature on the structure and morphology of the lead halide films is investigated. Crystalline films were realized for both lead halides, with PbI2 films showing high texture compared to the reduced texture of the PbCl2 films. Large lateral grain sizes were observed for the PbI2 films with a flat platelet grain morphology and an average grain size up to 734.2 ± 144.8 nm. PbCl2 films have columnar grains with an average grain size up to 386.7 ± 119.5 nm. The PbI2 films showed a band gap of about 2.4 eV, confirming its semiconducting properties, and the PbCl2 had a wide band gap of 4.3 eV, which shows the insulating properties of this material.
Sequential low-pressure chemical vapor deposition (CVD) offers a controllable and scalable route for the conformal growth of Pb-based perovskite materials, with an improved environmental stability and control of the phase stability of the material. Reducing the dimensionality of the material and alloying with Sn have the additional benefit of mitigating both the instability and environmental challenges associated with Pb. Herein, we report on the sequential CVD of a two-dimensional (2D) Sn−Pb mixed-halide perovskite compound by converting a SnCl 2 /PbCl 2 precursor in a phenylethylamine iodide (PEAI) atmosphere from 80 to 120 °C. A conversion temperature of 100 °C is optimal for the growth of a highly crystalline, uniform, and compact 2D Pb−Sn compound perovskite layer with well-defined grains up to 5 μm, consistent with its optical absorbance and emission properties. Simultaneously, a crystalline PbI 2 layer is produced and attributed to the interruption of the intercalation process. Temperature-dependent photoluminescence measurements show a single excitonic peak from 20 to 360 K for both 2D Sn-only and Sn−Pb compound films, highlighting the phase stability of the material. The observed excitonic peak broadening in the compound film is attributed to growth in the defect density accompanied by a weakening of the exciton−phonon interaction caused by Frenkel-like excitons. This work will contribute toward an understanding of the transport properties of CVDgrown 2D perovskites that will develop the capability of producing 2D-layered field-effect transistors and solar cells.
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