Lead and HTM Free Stable Two-Dimensional Tin Perovskites with Suitable Band Gap for Solar Cell Applications

Organic-inorganic hybrid perovskites have attracted great attention over the last few years as potential light-harvesting materials for efficient and cost-effective solar cells. However, the use of lead iodide in state-of-the-art perovskite devices may demonstrate an obstacle for future commercialization due to toxicity of lead. Herein we report on the synthesis and characterization of low dimensional tin-based perovskites. We found that the use of symmetrical imidazolium-based cations such as benzimidazolium (Bn) and benzodiimidazolium (Bdi) allow the formation of 2D perovskites with relatively narrow band gaps compared to traditional -NH3+ amino groups, with optical band gap values of 1.81 eV and 1.79 eV for Bn2SnI4 and BdiSnI(4) respectively. Furthermore, we demonstrate that the optical properties in this class of perovskites can be tuned by formation of a quasi 2D perovskite with the formula Bn(2)FASn(2)I(7). Additionally, we investigate the change in band gap in the mixed Sn/Pb solid solution Bn2SnxPbx-1I4. Devices fabricated with Bn2SnI4 show promising efficiencies of around 2.3 %.

Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

During the past few years, organic lead halogen perovskites have emerged as a class of highly promising solar cell materials, with certified solar cell efficiencies now surpassing 20%. Concerns have, however, been raised about the possible environmental and legalization problems associated with a new solar cell technology based on a water-soluble lead compound. Replacing lead in the perovskite structure: with a less toxic element, without degrading the favorable photo physical properties, would therefore be of interest. In this paper, the possibility of replacing lead with other metal ions is explored by following the replacement rules of Goldschmidt together with additional quantum mechanical considerations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radius of Sr2+ and Pb2+ are almost identical, suggesting an exchange could be made without affecting the crystal structure. Couple cluster calculations on the metal ions and their halogen salts give the bonding patterns to be sufficiently similar and density functional theory (DFT) revealed the strontium perovskite, CH3NH3SrI3, to be a stable phase, despite the difference in electronegativity between lead and strontium. This is further supported by the existence of binary PM, and SrI2 compounds and the beneficial formation energy of the strontium perovskite. The electronic properties of both CH3NH3SrI3 and CH3NH3PbI3 were simulated and compared, revealing a higher degree of ionic interaction in the metal halogen bound in the strontium perovskite. This is a consequence of the lower electronegativity of strontium, which, together with the lack of d-orbitals in the Valence of Sr2+, results, in a higher band gap. The band gap for the strontium perovskite was estimated to 3.6 eV, which unfortunately is too high for an efficient photo absorber. Initial investigations on experimental synthesis of the strontium perovskite, using wet chemical methods, revealed it to be harder to produce than the lead perovskite This is explained as a:consequence of different bonding patterns in the metal iodine salts, which obstruct the methylammonium intercalation pathway utilized for forming the perovskite. Vapor phase methods are instead suggested as more promising synthesis routes.

American Chemical Society2015

A new formation strategy of hybrid perovskites via room temperature reactive polyiodide melts

Michael Graetzel, Andrey Petrov, Shaik Mohammed Zakeeruddin

Here we introduce a new solvent-free preparation method for hybrid metal halide perovskites involving the direct reaction of metallic lead with polyiodide melts. We discovered new reactive polyiodide melts (RPMs) that can be prepared simply by adding elemental iodine to halide salts of the organic A cations of common hybrid perovskites, e.g. methylammonium iodide (MAI) and formamidinium iodide (FAI), and their corresponding bromide salts MABr and FABr. For MAI/I-2 ratios ranging from 1 : 1 to 1 : 3 they form room temperature ionic liquids containing polyiodide anions and organic counterions. We find that metallic lead can be converted within a few seconds into pure or mixed cation/anion large-grain perovskite films of high electronic quality by a reaction with the RPM. The melts can dissolve also lead derivatives, opening up a realm of opportunities for future development of self-flux growth, liquid phase epitaxy and crystallization of perovskites for solar cell applications.

Royal Soc Chemistry2017

Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

American Chemical Society2015