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The performance of lead halide perovskite optoelectronic devices continues to improve, yet the efficiencies are still well below the radiative limit. To approach the radiative limit, detailed understanding of impurities/defects and precise control over their concentrations are required. In Part I, we demonstrated that the soft Lewis acidity of Pb2+ induces a chemical reaction between PbI2 and aliphatic amines producing Pb-alkylamide bonds which can be subsequently incorporated into thin films. Here, we investigate the consequences of these impurities in methylammonium lead triiodide (MAPbI3) thin films. In particular, we link Pb–alkylamide impurities to an extrinsic degradation pathway resulting in Pb0 formation. The proposed mechanisms proceeds viaβ-C–H proton transfer reactions of the amido Pb species. Metallic PbṖ+/Pb0 defects acting as non-radiative recombination centers may limit the performance of many perovskite layers. However, optimal concentrations of Pb–methylamide impurities in sub-stoichiometric (slight excess of PbI2) MAPbI3 films passivate Pb0 defect formation, shown here to simultaneously correlate to improvements in photoluminescence lifetime. These results elucidate the beneficial properties of Pb–amide impurities in low concentrations and the sensitivity of halide perovskite materials to extrinsic defect chemistry. [ABSTRACT FROM AUTHOR]

Metal halide perovskite materials (ABX3) employed in optoelectronic devices have rapidly achieved high efficiencies. Their performance can be partially attributed to intrinsic defects being electronically benign. Despite the inertness of defects within the bulk, such as ion vacancies, solution processing additives and surface passivation have been shown to enhance material properties. Thus, detailed knowledge of additive chemistry is desirous to elucidate passivation mechanisms and facilitate further improvements. In particular, the soft Lewis acid nature of Pb2+ has been overlooked heretofore as a key feature of amine-lead chemistry in perovskite preparations. In Part I of this study, we demonstrate that acid–base reactions between PbI2 and aliphatic amines yield alkylammonium and Pb–alkylamide species. The alkylammonium and alkylamide products are capable of occupying A-site cation and anion vacancies, respectively. Subsequent incorporation of these impurities at perovskite crystal surfaces or formation of non-perovskite phase inclusions likely influence the perovskite layer morphology and properties which are investigated in Part II. These results reveal a new mechanism by which optoelectronic properties of perovskites can be chemically modified. Additionally, the general concept of acidification of organic molecules solvated to Pb2+ promoting unexpected reactions is applied to explain amine reactions with other additives, such as formic acid. Understanding acid–base reactions induced by PbI2 complexing to protic molecules is useful to comprehend and predict defect/impurity chemistry in a wide variety of perovskite precursor formulations. [ABSTRACT FROM AUTHOR]

An important challenge in the development of optoelectronic devices for energy conversion applications is the search for suitable p-type contact materials. For example, p-type MnTe would be a promising alternative back contact for CdTe solar cells due to their chemical compatibility, but under normal conditions it has a too narrow band gap due to its octahedrally coordinated nickeline (NC) structure. The tetrahedrally coordinated wurtzite (WZ) polymorph of MnTe has not been reported, but it is especially interesting due to its predicted wider band gap and better structural compatibility with CdTe and related II–VI semiconductor materials. Here, we report on the stabilization of WZ-MnTe thin films on amorphous indium zinc oxide (a-IZO) substrates relevant to photovoltaic applications. Optical spectroscopy of the WZ-MnTe films shows a wide direct band gap of Eg = 2.7 eV, while photoelectron spectroscopy (PES) measurements reveal weak p-type doping with a Fermi level of 0.6 eV above the valence band maximum. The results of electron microscopy and PES measurements indicate that WZ-MnTe is stabilized due to interdiffusion at the interface with IZO. The results of this work introduce a substrate stabilized WZ-MnTe polymorph as a potential p-type contact material candidate for future applications in CdTe devices for solar energy conversion and other optoelectronic technologies. [ABSTRACT FROM AUTHOR]