Magic-size nanoclusters as low-temperature precursors to nanocrystal and bulk semiconductor films

Washington University, Saint Louis MO

Investigators

Abstract

The research pursued in this project advances semiconductor nanoscience for potential use in solar-energy technologies. As atmospheric carbon dioxide (CO2) levels currently threaten the global ecosystem and human well-being, the development of clean-energy technologies is a major societal necessity. Professor Buhro is providing new methods to prepare more-efficient solar cells and the economical, scalable semiconductor processing required for widespread access to solar electricity. One aspect of the project is to develop semiconductor-nanocrystal films having optimized structures for charge transport in the solar-absorber layers of solar cells. This may increase the performance of the cells. The key to this strategy is the shape of the nanocrystals, which are plate-like or ribbon-like, and stack together compactly. These shapes are capable of transporting charge over large distances. A second aspect of the project is the low-temperature deposition of semiconductor films using solution chemistry. A major drawback to the wider implementation of solar electricity is the high cost of making the semiconductor components of solar cells by the current gas-phase and vacuum-based technologies. Professor Buhro is contributing to the development of much more economical solution-based processing methods. Both aspects of the project rely on so-called "magic-size semiconductor nanoclusters" as key chemical intermediates and precursors. The magic-size nanoclusters are small, discrete molecular units that have the same compositions as the target semiconductor phases, and that produce crystalline semiconductors from solution at low temperatures. By increasing the ability to transport charge and the economy of semiconductor-film fabrication, this research may have broad societal impact as it promotes the availability and practicality of solar-energy technology. Additionally, Professor Buhro is a mentor to a large number female Ph.D. scientists. He works to retain women in STEM fields by incorporating active-learning strategies in introductory courses. In this research project, Professor William Buhro of Washington University in St. Louis is supported by the Macromolecular, Supramolecular, and Nanochemistry (MSN) program to employ stoichiometric magic-size semiconductor nanoclusters and flat (2D), colloidal semiconductor nanocrystals in the advancement of low-temperature, solution-based semiconductor processing and nanocrystal solar cells. Solid-state quantum-dot (QD) solar cells have emerged as a third-generation photovoltaic technology that promises economical, scalable production, and the necessary solar-conversion performance for real-world application. Flat (2D) semiconductor nanocrystals (quantum belts and quantum platelets) have optical properties and interfacial characteristics comparable to colloidal quantum dots. They also have at least one extended length dimension. Professor Buhro is now investigating their use in nanocrystal solar-absorber layers. The architectures of 2D-nanocrystal films consist of bundled semi-ordered domains oriented in three dimensions. The larger lateral nanocrystal dimensions relative to QDs are expected to increase the carrier mean free paths and minimize hopping. The overall film architecture may provide conduction pathways percolating in all three dimensions. These materials may minimizing carrier recombination and increasing the photon-conversion efficiencies of solar cells. A second component of this project addresses molecular semiconductor inks. Ideal molecular inks for semiconductor fabrication should possess stoichiometries matched to the target semiconductor, kinetically labile ligands, and the capacity for low-temperature semiconductor crystallization. Professor Buhro is investigating the newly isolated, stoichiometric magic-size II-VI nanoclusters for this purpose. Strategies for employing such nanoclusters as low-temperature precursors to bulk, polycrystalline semiconductor films are under study. Efforts to complete the structural characterization of these stoichiometric, amine-passivated, magic-size II-VI nanoclusters, and to explore their chemical reactivities and physical properties are also underway. The research pursued in this project is expected to fundamentally advance semiconductor nanoscience impacting solar-energy technologies. Additionally, Professor Buhro mentors a large number female Ph.D. scientists, and is working to retain women by incorporating active-learning strategies into introductory STEM courses.