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The Back Cover picture illustrates the fascinating field of perovskite-based solar cells (PSCs) from the point of view of organic charge carriers materials. Recently, the well-known perovskites attracted considerable interest in the photovoltaic community, as they can provide high conversion efficiencies. To further improve PSCs, organic compounds can be used as hole- and electron-transport materials, which contributed towards the development of this emerging field. More details can be found in the Review by J. L. Delgado et al. on page 3012 in Issue 18, 2015. (DOI: 10.1002/cssc.201500742).

A new series of solvent- and halide-free ammine strontium metal borohydrides Sr(NH₃)_n(BH₄)₂ (n=1, 2, and 4) and further investigations of Ca(NH₃)_n(BH₄)₂ (n=1, 2, 4, and 6) are presented. Crystal structures have been determined by powder XRD and optimized by DFT calculations to evaluate the strength of the dihydrogen bonds. Sr(NH₃)(BH₄)₂ (Pbcn) and Sr(NH₃)₂(BH₄)₂ (Pnc2) are layered structures, whereas M(NH₃)₄(BH₄)₂ (M=Ca and Sr; P2₁/c) are molecular structures connected by dihydrogen bonds. Both series of compounds release NH₃ gas upon thermal treatment if the partial pressure of ammonia is low. Therefore, the strength of the dihydrogen bonds, the structure of the compounds, and the NH₃/BH₄⁻ ratio for M(NH₃)_n(BH₄)_m have little influence on the composition of the released gasses. The composition of the released gas depends mainly on the thermal stability of the ammine metal borohydride and the corresponding metal borohydride.

Storage solution: The structural trends of a series of Sr(NH₃)_n(BH₄)₂ (n=1, 2, and 4) are presented and compared to those of Ca(NH₃)_n(BH₄)₂ (n=1, 2, 4, and 6). The thermal properties are investigated in detail and compared to those of other similar compounds. A new general mechanism for decomposition and gas release from ammine metal borohydrides is proposed.

Herein, we present the precise stoichiometric control of methlyammonium lead iodide perovskite thin-films using high vacuum dual-source vapor-phase deposition. We found that UV/Vis absorption and emission spectra were inadequate for assessing precisely the perovskite composition. Alternatively, inductively coupled plasma mass spectrometry (ICP-MS) is used to give precise, reproducible, quantitative measurements of the I/Pb ratio without systematic errors that often result from varying device thicknesses and morphologies. This controlled deposition method enables better understanding of the evaporation and deposition processes; methylammonium iodide evaporation appears omnidirectional, controlled using the chamber pressure and incorporated in the film through interaction with the unidirectionally evaporated PbI₂. Furthermore, these thin-films were incorporated into solar cell device architectures with the best photovoltaic performance and reproducibility obtained for devices made with stoichiometric perovskite active layers. Additionally, and particularly pertinent to the field of perovskite photovoltaics, we found that the I–V hysteresis was unaffected by varying the film stoichiometry.

Stoichiometric vapor: The vapor phase deposition of methlyammoinum iodide and PbI₂ to form methylammonium lead iodide perovskite thin films for solar cell applications is studied and optimized. The I/Pb ratio is controlled using a custom evaporation system and the films are analyzed by SEM and inductively coupled plasma mass spectrometry. The best photovoltaic performance is obtained for devices made with stoichiometric perovskite active layers.