Understanding Precursor Materials Critical for Next-Generation Solar Cells

The Science

In recent years, considerable research has focused on improving the power conversion efficiency of solar cells. Photovoltaics based on metal halide perovskites, or hybrid organic–inorganic perovskites (HOIPs), have been a primary target. Unlike the silicon used in traditional solar cells, HOIPs can be processed directly from solution, opening doors for lower-cost and higher-efficiency cell fabrication. However, the precise mechanisms that form HOIPs are not fully understood.

Using complementary experimental measurements and computational predictions of spectroscopic signals, researchers identified species present in precursor solutions, including iodoplumbates (PbI_x). This study provides insight into reactivity and dynamical processes in solution as well as the chemical properties of these precursors.

The Impact

Properly identifying iodoplumbate species that are present and stable in a perovskite precursor solution is vital. Different species affect solution dynamics in a variety of ways, including the nucleation and crystal growth that form final products favoring high-performing thin films in photovoltaic cells.

The results of this study suggest that only three lead (Pb) species are dominant and thermodynamically stable in dilute Pb perovskite precursor solutions, which provides critical new insights into the species that are most likely responsible for crystal nucleation and growth in these materials.

Summary

In this study, researchers employed both experimental measurements and computational predictions of spectra from several methods—extended X-ray absorption fine-structure, X-ray absorption near-edge, and ultraviolet–visible spectroscopies—to investigate the identity of iodoplumbate species that are present and stable in dilute perovskite precursor solutions composed of a constant concentration of PbI_x with various concentrations of methylammonium iodide in a commonly deployed dimethylformamide (DMF) solvent.

Through both experimental measurements and density functional theory calculations, this work determined that X-ray absorption fine-structure and X-ray absorption near-edge spectroscopies tell a remarkably consistent story about the structural presence and structural stability of lower-order iodoplumbates in dilute precursor solutions. These results convincingly show that MAPbI perovskite precursor solutions in DMF should only be composed of low-order PbI_x species, namely Pb²⁺, PbI⁺, and PbI₂. This study suggests that PbI³⁻ is, at best, metastable and also casts doubt on the stability of high-order iodoplumbate ions such as PbI₄²⁻, PbI₅³⁻, and PbI₆⁴⁻ in precursor salt solutions. Furthermore, should PbI₄²⁻ ions exist in these precursor solutions, it would not be in a symmetrical tetrahedral structure, as suggested in the literature. In questioning the importance of higher-order iodoplumbates, especially PbI₅^3– and PbI₆^4–, this study offers a new perspective in this field.

Contact

Nadia N. Intan, Pacific Northwest National Laboratory, nadia.intan@pnnl.gov

Chris Mundy, Pacific Northwest National Laboratory, chris.mundy@pnnl.gov

Greg Schenter, Pacific Northwest National Laboratory, greg.schenter@pnnl.gov

Funding

Funding for this work includes the following resources:

• The Department of Energy (DOE), Office of Science, Graduate Student Research.
• The DOE Office of Science, Basic Energy Sciences program under Award DE-SC0022305.
• The DOE under contract no. DE-AC05-76RL1830.
• The DOE Office of Science, Basic Energy Sciences program, Division of Chemical Sciences, Geosciences, and Biosciences, project numbers 80818, 16248, and 16249.
• The experimental part of this research used resources from the Advanced Photon Source, an Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory, and was supported by the DOE under contract no. DE-AC02-06CH11357.
• Computing resources were provided by the Advanced Research Computing at Hopkins high-performance computing facilities, which are supported by National Science Foundation grant number OAC 1920103, PNNL’s Institutional Computing program, and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility operated under contract no. DE-AC02-05CH11231.