Globularity-Selected Large Molecules for a New Generation of Multication Perovskites

Area of Science:

• Materials Science
• Photovoltaics
• Solid-State Chemistry

Background:

• Perovskite solar cells (PSCs) utilize perovskites with an APbX3 structure, commonly employing cations like methylammonium (MA) and formamidinium (FA).
• Larger molecular cations (e.g., ethylammonium (EA), guanidinium (GA), imidazolium (IA)) have generally been incompatible with the desired photoactive perovskite phases.

Purpose of the Study:

• To introduce novel molecular descriptors, specifically a "globularity factor", to predict the compatibility of larger molecular cations in perovskite structures.
• To explore the potential of ethylammonium (EA) as a suitable cation for multication 3D perovskites, aiming for improved stability and suppressed halide segregation.

Main Methods:

• Development and application of a "globularity factor" to quantify the deviation of molecular cation shapes from a perfect sphere.
• Experimental incorporation and characterization of methylammonium/ethylammonium (MA/EA) perovskite compositions.

Main Results:

• The globularity factor revealed that ethylammonium (EA) is only slightly larger than formamidinium (FA), suggesting its suitability for perovskite structures.
• Experimental results demonstrated significant incorporation of EA into perovskites, contrary to previous findings with other large cations used only as additives.
• MA/EA perovskites exhibited tunable band gaps from 1.59 to 2.78 eV, achieving highly blue-shifted spectra.
• A specific composition, MA0.5EA0.5PbBr3, achieved a record open-circuit voltage of 1.58 V in a conventional PSC architecture.

Conclusions:

• Ethylammonium (EA) is a viable and beneficial cation for developing high-performance perovskite solar cells.
• The globularity factor provides a valuable tool for designing novel perovskite compositions with tailored properties.
• This research opens avenues for highly efficient, stable, and blue-shifted perovskite solar cells.