Engineering All-Inorganic Quantum Dot Heterojunction Photovoltaics Through Surface Chemical Manipulations

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PI: Kagan, CherieProposal Number: 1236406Institution: University of PennsylvaniaTitle: Engineering All-Inorganic Quantum Dot Heterojunction Photovoltaics Through Surface Chemical ManipulationsColloidal semiconductor quantum (QDs) promise low-cost, large-area, high performance solar photovoltaics (PVs) as they may be processed from solution like inks and they have size, shape, and composition-dependent electronic and optical properties uniquely allowing the design and fabrication of all-inorganic materials from nanoscale building blocks. The challenge is to harness the prized electronic properties of these solution-processable QDs in solid thin films where facile charge transport and doping are required to engineer junctions for efficient solar PVs and more broadly for semiconductor electronic and optoelectronic devices. The large area surface (to volume), stoichiometric balance, and coupling between QDs greatly affect charge carrier type and transport in QD solid thin films. This project will carry out chemical transformations to manipulate the surface chemistry and stoichiometry of QDs in solid state thin films while probing in-situ their surface chemistry and electrical properties to design high mobility, n- and p-type QD thin films and high efficiency pn heterojunction PVs. For example, wet-chemical synthetic methods yield macroscopic quantities of colloidal semiconductor QDs that may be tailored in size, shape, and composition and deposited like inks by spin- casting and dip-coating to form large-area, uniform QD thin films. Here, the PI will exchange the QDs in thin films using the recently developed ammonium thiocyanate treatment to form strongly electronically coupled and nearly bare QDs, providing sites for subsequent chemical transformations. Electrochemical, optical and electrical measurements will be used to characterize the energy levels and carrier type, trapping, and mobility as we modify QD films. High mobility, long carrier lifetime QD films with electronic structures tailored to define large junction offsets will be integrated into solar cells and optimized to form broad band, high absorption QD films for efficient solar PVs.The proposed research activities will develop understanding of the role of surface, stoichiometry, and extrinsic atoms in weakly quantum confined, wider band gap and more strongly quantum confined, smaller band gap QDs. The role of the surface is important in designing high carrier mobility, n- and p-type materials that impact not only solar photovoltaics, but the design of a broad range of sustainable electronic and optoelectronic devices; low-power electronics, energy harvesting thermoelectrics, and low-power light-emitting diodes; pursued from solution-processable QD materials. The fundamental understanding and technological developments will expand knowledge and teaching of nanostructured materials and of processes for chemical transformations and design of materials, applicable to the growing toolbox of nanostructured materials. The program will provide an interdisciplinary research experience for undergraduate and graduate students. It will also provide an opportunity for the students and PI to share the science and engineering of sustainable and nanoscale technologies with K-12 students and teachers, the public, and the scientific community through demos, presentations, and formal and extramural coursework.