Films of semiconductor quantum dots (QDs) are an important emerging class of materials for next-generation solar cells, but the efficiency of cells based on QD films is limited in part by the poor spatial order of the QDs. This project focuses on the development of highly-ordered and electrically-conductive QD films as a new class of electronic materials. With support from both the Electronic and Photonic Materials Program and the Solid State and Materials Chemistry Program in the Division of Materials Research, researchers at the University of California, Irvine (UCI) will make these new materials and study the role of order in charge transport, with the goal of demonstrating greatly improved charge transport in QD films. The project will advance the understanding of transport in functional nanoscale systems, including the role of spatial and energetic order in the emergence of collective mesoscale phenomena such as band formation that depend on strong electronic interactions between nanoscale building blocks. The project will yield new insights into QD self-assembly, interfacial physics, and charge diffusion length, and result in a class of modular, organic-inorganic hybrid QD crystals with transport properties suitable for a variety of QD technologies, including solar cells. Such QD solar cells made by self-assembly from solution can lower the cost of solar electricity and promote solar energy deployment worldwide, with particular benefits to developing communities. The project will also enable a K-12 materials science outreach effort as part of the UCI/Chapman University joint Mathematics, Engineering, Science Achievement (MESA) Program (http://mesa.eng.uci.edu/). The researchers will develop a hands-on QD photophysics MESA program in which students help synthesize QDs and explore their characterization by spectroscopy and electron microscopy. This joint program serves approximately 1,600 elementary, middle and high school students in Orange and Los Angeles Counties, and focuses on promoting STEM education and providing academic enrichment to low-income students from backgrounds with historically low levels of participation in higher education. In addition, several undergraduate students will participate in this research each summer as part of the NSF-funded Chemistry REU and the UCI California Alliance for Minority Participation (CAMP) summer research programs.
PbX (X = S, Se, or Te) quantum dot (QD) thin films represent an important emerging class of absorber layers for next-generation photovoltaics (PV). Remarkable progress has been made in QD PV over the past several years by replacing the long, electrically insulating oleate ligands on as-made PbX QDs with short organic molecules or inorganic ions. Unfortunately, these ligand treatments destroy medium and long range order in the QD films. Partly as a consequence, charge transport is limited to sequential phonon-assisted tunneling, resulting in low carrier mobility, short carrier diffusion lengths, and limited photocurrent from QD devices. One exciting prospect is to assemble QDs into electronically coupled, conductive superlattices (crystals of QDs) in which transport can occur via domain-delocalized states or Bloch-type extended states (mini-bands). Mini-band transport in QD superlattices could yield much larger carrier mobility and diffusion length. With support from both the Electronic and Photonic Materials Program and the Solid State and Materials Chemistry Program in the Division of Materials Research, researchers at the University of California, Irvine will replace the oleate ligands of PbX QDs with conjugated organic "molecular wires" to fabricate a new class of conductive PbX QD hybrid superlattice nanocomposites (HSNs) composed of QDs interconnected with molecular wires. The molecular wires will provide excellent superlattice order, tunable HOMO and LUMO levels, strong electronic coupling via barrier lowering, and large inter-QD spacing that is favorable for superlattice crystallization and the adsorption of co-ligands to passivate surface states. The impact of superlattice order on charge transport, carrier mobility, and diffusion length will be studied using an array of electrical and spectroscopic techniques. The main fundamental goal of the project is to elucidate the degree to which and the mechanisms by which superlattice effects enhance carrier mobility and diffusion length in QD films, while the main applied goal is to make HSNs with minority carrier diffusion lengths of at least 1000 nm, which would enable nearly perfect charge collection and very high conversion efficiency from QD PV.
