Martina Stumpp

Stabilization of Organic–Inorganic Perovskite Layers by Partial Substitution of Iodide by Bromide in Methylammonium Lead Iodide

Thin films of the methylammonium lead halides CH3 NH3 Pb(I1-x Brx )3 are prepared on fluorine-doped tin oxide substrates and exposed to humid air in the dark and under illumination. To characterize the stability of the materials, UV/Vis spectra are acquired at fixed intervals, accompanied by XRD, energy-dispersive X-ray spectroscopy, SEM, and confocal laser scanning microscopy. Different degradation mechanisms are observed depending on the environmental conditions. It is found that bromide can successfully suppress the transformation of the perovskite into the monohydrate, presumably owing to stronger hydrogen-bonding interactions with the organic cation. However, under illumination in humid air, rather rapid decomposition of the perovskites was still observed, which is due to phase segregation. The use of increased bromide content in methylammonium lead halide absorbers is discussed in terms of their application in perovskite solar cells.

Diffusion-controlled electrochemical growth of porous zinc oxide on microstructured electrode band arrays

Films of zinc oxide were prepared by electrochemical deposition at constant electrode potential from an oxygen-saturated aqueous zinc-chloride solution on rotating microband electrode arrays of gold or silver on SiO2/Si wafers. Porous ZnO films were obtained by deposition in the presence of the xanthene dye eosinY as a structure-directing agent. The electrode size, the gap width, the number of electrode bands, the deposition time, and the rotation rate were varied. The growth of ZnO was monitored by the observed current, and the films were subsequently characterized by profilometry and by scanning electron and confocal laser microscopy. Strongly enhanced growth was observed at the edges of the electrode bands. This was explained by models of hemi-cylindrical diffusion revealing inhomogeneous mass transport to the electrodes. The models were extended to processes of electrochemical deposition. A model for diffusion-limited growth on microstructured electrode band arrays is proposed and applied to the present results.

Preparation and characterization of mixed formamidinium lead iodide/methyl ammonium lead bromide layers

In the past few years, solar cells based on layers of organo-metal halide perovskites such as formamidinium and methyl ammonium lead halides gave rise to high-efficiency and low-cost photovoltaics. Further enhancements of their performance can be achieved by chemically modifying the perovskite material. Especially, the long-term stability of these devices can be improved by mixing different components of the perovskite such as the organic cation or the halide anion. This work is focused on the mixture of the well-performing formamidinium lead iodide perovskite with the highly stable methyl ammonium lead bromide. The deposited mixed perovskite layers are investigated by X-ray diffraction, UV/Vis transmission spectroscopy, scanning electron microscopy, and confocal laser scanning microscopy. We confirm the very good miscibility of both materials and demonstrate two different methods to deposit high-quality layers.Graphical abstract