We demonstrate a facile two-step low-pressure vapor deposition of methylammonium lead iodide (MAPbI 3 ) perovskite films in a single reactor. Continuous, polycrystalline lead iodide (PbI 2 ) films were deposited in the first step and successfully converted to high quality perovskite films in the second step during exposure of PbI 2 films to methylammonium iodide (MAI) vapor. A complete conversion was realized after 90 min of exposure with an average grain size of 3.70 ± 1.80 μm. The perovskite conversion starts at the PbI 2 surface through the intercalation reaction of PbI 2 and MAI vapor molecules and progresses toward the PbI 2 /substrate interface. The coverage and quality of the perovskite thin film are controlled by that of the predeposited PbI 2 film. The absorbance measurements confirmed air stability of the fully converted perovskite for 21 days, ascribed to its superior morphology and grain size. Finally, a planar single-junction perovskite solar cell with no additives or additional interfacial engineering was fabricated and tested under open-air conditions, yielding a best power conversion efficiency of 11.7%. The solar cell maintains 85% of its performance up to 13 days in the open air with a relative humidity up to 80%.

The effect of polarization modulation of the gate dielectric on the performance of metal-oxidesemiconductor field-effect transistors has been investigated for more than a decade. However, there are no comparable studies in the area of organic field-effect transistors (FETs) using polymer ferroelectric dielectrics, where the effect of polarization rotation by 90 • is examined on the FET characteristics. We demonstrate the effect of polarization rotation in a relaxor ferroelectric dielectric, poly(vinylidene fluoride trifluorethylene (PVDF-TrFE), on the performance of small molecule based organic FETs. The subthreshold swing and other transistor parameters in organic FETs can be controlled in a reversible fashion by switching the polarization direction in the PVDF-TrFE layer. X-ray diffraction and electron microscopy images from PVDF-TrFE reveal changes in the ferroelectric phase and domain size, respectively, upon rotating the external electric field by 90 •. The structural changes corroborate density-functional theoretical studies of an oligomer of the ferroelectric molecule in the presence of an applied electric field. The strategies enumerated here for polarization orientation of the polymer ferroelectric dielectric are applicable for a wide range of polymeric and organic transistors.

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%).

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