Developing the Family of 3D Organochalcogenide-Halide Perovskites

Non-technical summary

Lead-halide perovskites are a class of semiconductors that show great promise as cheap and efficient solar-cell absorbers. However, accessing the bandgaps required for strong sunlight absorption in high-efficiency solar cells and realizing long-term stability with these materials are outstanding challenges to the implementation of this clean-energy technology.

This project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, explores how incorporating chalcogens (Ch = S, Se) into the perovskite Pb-X (X = Cl, Br, I) framework affects material stability and optical/electronic properties. A key challenge to mixing halides and chalcogenides in a perovskite are the very different synthetic conditions required for chalcogenide perovskites (> 900 C in a furnace) and halide perovskites (near-ambient conditions in solution).

This project studies how to use organochalcogenides (RCh; R = organic group), which readily dissolve in solution, to circumvent this problem. Carefully designed RCh molecules, with R groups of different sizes and shapes, are used to target a wide range of organochalcogenide-halide perovskites. These perovskites are studied for their ability to absorb light and generate long-lived electrons and holes that could be extracted to generate current (in a solar cell) or to make chemical bonds (in photocatalysis).

Overall, this project sets the foundation for synthesizing and using a new family of semiconductors that may combine the stability of the lead-chalcogenides with the optical properties of the lead-halide perovskites. To expose students to the joys of materials synthesis early in their undergraduate careers, freely available educational tools are being developed as part of this project.

In particular, the syntheses of nontoxic perovskites that show visually striking properties are described in simple steps, only using equipment available in undergraduate teaching labs. These lab modules are first tested in a Stanford chemistry course and further refined through feedback from teachers from nearby PUIs and high schools, prior to publishing in journals and the group website.

Technical summary

Lead chalcogenides (e.g., PbS) and lead-halide perovskites (e.g., (CH3NH3)PbI3) have been independently developed as solar absorbers, which motivates the discovery of new materials combining the more-covalent Pb-Ch (Ch = chalcogenide) bonds and the more-ionic Pb-X (X = halide) bonds. Because the typical synthetic conditions for chalcogenide perovskites (>900 C; O2 free) and for halide perovskites (near-ambient conditions) are mutually incompatible, this project explores the use of organochalcogenides (RCh; R = organic group) for incorporating chalcogenides into the halide perovskite framework.

Using RCh ligands with various functional groups, steric profiles, and Ch termini, a wide range of organochalcogenide-halide perovskites with rich compositional diversity are targeted. Through well-established collaborations with experts in the field, the local and long-range structures of the perovskites are investigated and advanced spectroscopic methods and computational analysis are used to interrogate the fundamental optoelectronic properties of these novel semiconductors, including bandgaps, band dispersions, carrier dynamics, lattice dynamics, and trap states.

The RCh-perovskite family provides new handles to tune for potentially increasing the stability of halide perovskites--an outstanding challenge in the field--and for accessing desirable bandgaps and band dispersions for charge extraction in photocatalysis or photovoltaics. New educational tools are developed to introduce young students to the joys of materials discovery and synthesis.

For this pedagogical work, open-source and accessible laboratory modules are developed that describe straightforward syntheses of nontoxic perovskites that show visually striking properties. These modules are used in a Stanford chemistry course and further refined through solicited feedback from teachers from PUIs and high schools and disseminated by publishing in education-oriented journals and on the group website.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.