Quantifying Stabilized Phase Purity in Formamidinium-Based Multiple-Cation Hybrid Perovskites

A promising approach for the production of highly efficient and stable hybrid perovskite solar cells is employing mixed-ion materials. Remarkable performances have been reached by materials comprising a stabilized mixture of methylammonium (MA(+)) and formamidinium (FA(+)) as a monovalent cation. We compare and quantify the methods of stabilizing FA(-) based perovskites involving the additional blending of the smaller inorganic cations cesium (Cs+) and rubidium (Rb+), which can lead to an improvement in phase purity of black cubic perovskite modification. Even under excess lead iodide conditions, the presence of a separate PbI2 phase as well as hexagonal phases, which are very common for formamidinium-containing perovskites, can be drastically reduced or even completely prevented. In this aspect, adding both Cs+ and Rb+ showed greater effectivity than only adding Cs+, enabling an increase in the percentage of the cubic phase within the material from 45% in the double-cation FA:MA mixture to 97.8% in the quadruple composition. The impact of admixing inorganic cations on the perovskite crystal structure resulted in enlarged homogeneous crystallite sizes and a less pronounced orientational order and indicated also minor modifications of unit cell sizes. Finally, we discuss the impact of the phase purity on charge-carrier recombination dynamics and solar cell performance.

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

Effect of metal cation replacement on the electronic structure of metalorganic halide perovskites: Replacement of lead with alkaline-earth metals

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

Organic and inorganic lead halogen perovskites, and in particular, CH3NH3PbI3, have during the last years emerged as a class of highly efficient solar cell materials. Herein we introduce metalorganic halogen perovskite materials for energy-relevant applications based on alkaline-earth metals. Based on the classical notion of Goldschmidt's rules and quantum mechanical considerations, the three alkaline-earth metals, Ca, Sr, and Ba, are shown to be able to exchange lead in the perovskite structure. The three alkaline-earth perovskites, CH3NH3CaI3, CH3NH3SrI3, and CH3NH3BaI3, as well as the reference compound, CH3NH3PbI3, are in this paper investigated with density functional theory (DFT) calculations, which predict these compounds to exist as stable perovskite materials, and their electronic properties are explored. A detailed analysis of the projected molecular orbital density of states and electronic band structure from DFT calculations were used for interpretation of the band-gap variations in these materials and for estimation of the effective masses of the electrons and holes. Neglecting spin-orbit effects, the band gap of MACaI(3), MASrI(3), and MABaI(3) were estimated to be 2.95, 3.6, and 3.3 eV, respectively, showing the relative change expected for metal cation exchange. The shifts in the conduction band (CB) edges for the alkaline-earth perovskites were quantified using scalar relativistic DFT calculations and tight-binding analysis, and were compared to the situation in the more extensively studied lead halide perovskite, CH3NH3PbI3, where the change in the work function of the metal is the single most important factor in tuning the CB edge and band gap. The results show that alkaline-earth-based organometallic perovskites will not work as an efficient light absorber in photovoltaic applications but instead could be applicable as charge-selective contact materials. The rather high CB edge and the wide band gap together with the large difference of the electron and hole effective masses make them good candidates for n-type selective layers in hot carrier injection solar cell devices together with some light absorber candidates. The fact that they have similar lattice constants as the lead perovskite and suitable positions of the valence band edges open up the possibility to use them also as thin epitaxial p-type hole selective contacts in combination with the lead halogen perovskite materials. This can lead to both charge selectivity as well as to superior crystal growth of lead perovskite with less contact stress, which is interesting for further investigations.

Amer Physical Soc2016

Effect of metal cation replacement on the electronic structure of metalorganic halide perovskites: Replacement of lead with alkaline-earth metals

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

Amer Physical Soc2016

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