The grand challenge in efficiently harvesting and converting solar radiation into electricity lies in engineering materials on multiple length scales with architectures that direct the flow of energy and the transfer and transport of charge, as in naturally occurring light harvesting systems. Organic-inorganic hybrids, prepared from functional, electro-active organic and nanostructured inorganic materials, combine desirable and tunable chemical and physical properties of the constituent organic and inorganic building blocks in a single composite, making them promising systems for solar technologies. Hybrid materials incorporate the low-cost, large-area processing and high absorbance and quantum efficiencies of organic materials with the adjustable optical properties, high carrier conductivities, and good photostability of inorganic nanostructures. Solar photovoltaic and luminescent solar concentrator technologies will be dramatically advanced if the organic and inorganic building blocks of hybrid structures can be positioned and oriented on the nanometer scale to regulate the competitive processes of charge transfer and transport, emission, and energy transfer.
Hybrid organic-inorganic materials promise one of the best architectures for ultra-low-cost photovoltaic devices. Currently, the efficiency of hybrid photovoltaic devices is limited by the availability of red-absorbing, high-mobility organic and inorganic components (to match the solar spectrum and efficiently collect charge) and of composites with structures that achieve high surface area junctions, yet form well-connected organic and inorganic pathways. This project aims to produce significantly improved hybrid structures for photovoltaics. Improved hybrid materials may also enable creation of high-efficiency luminescent solar concentrators, which currently are limited in performance by materials challenges; organic and inorganic materials alone have not been found to satisfy the broad-spectrum collection, near-unity photoluminescence efficiency, low re-absorption, and good photostability required.
This project brings together advances in chemical synthesis, mathematical modeling, and self-organization to control the position and orientation of organic and inorganic building blocks, exploiting advances at the frontier of chemistry, materials science, and mathematics. We will combine precisely controlled 1) molecular and supramolecular dendrimeric systems tailored to assemble with different structural motifs and 2) nanocrystals of tunable size, shape, and composition that self-assemble into single and multi-component superlattices. Structural, optical, and electrical probes will be combined with mathematical modeling of the effects of interface geometry to optimize charge transfer and transport, emission, and energy transfer. The results will enable engineering of organic-inorganic materials that will be integrated in photovoltaic devices and luminescent solar concentrators.
More broadly, the research program will develop new synthetic methods and mathematical formalisms for the self-assembly of hybrid materials with tailored architectures that is important to provide materials with superior structural, electronic, and optical properties. These materials have applications in imaging, therapeutics, and information technology, in addition to energy harvesting. The project's emphasis on mathematical techniques for engineered self-assembling systems offers the potential for impact in robotics and biological systems. The project will also electronically and optically probe and establish mathematical models of the behavior of organic-inorganic heterojunctions key to their application in a range of electronic and optical devices.
In this project we explored new paradigms in materials design for solar energy conversion through the programmed self-assembly of chemically tailored, electro-active organic dendrons and inorganic, semiconductor nanocrystals (NCs) of tunable size, shape and composition. We identified and synthesized several libraries of dendronized perylene bi-imides (PBIs) and naphthalene bisimides (NBIs) and systematically tailored the lengths of the dendron, the linker connecting the dendron to the imide of the PBI or NBI, and the functionalities on the core of molecule, allowing us to tune both their electronic and assembly properties. We discovered unprecedented, complex helical columnar assemblies that depended on the molecular structure and is key to the design of desired molecular stackings for efficient charge transport for photovoltaic devices. We similarly synthesized II-VI and IV-VI inorganic NCs tailored in size; shape, from spheres to cubes to rods; and in composition, preparing a sphere of CdSe inside a rod of CdS. We uncovered the fundamental properties of IV-VI nanorods that are of particular interest for solar photovoltaics and explored dot-in-rod chemistries for luminescent solar concentrators. To achieve efficient charge separation and transport in assemblies containing NCs, we introduced a new, compact ligand chemistry in thiocyanate to replace the large, bulky ligands used in NC synthesis that prevents electronic communication between NCs or between NCs and dendrons in the solid state. The short, environmentally benign thiocyanate ligand allowed us to deposit from solution highly photoconductive NC materials (x103 times larger than those values calculated from recent literature reports) and to realize excellent, high carrier mobilities in II-VI and IV-VI NC thin film transistors. This non-corrosive chemistry allowed us to demonstrate the first flexible NC thin film transistors. Photovoltaic devices are typically modeled by the spatial distribution of their carrier concentrations. Here, we developed a formalism to mathematically model the shape and location of the heterojunction interface between materials, that is of particular interest as we look to organic and inorganic heterojunctions of complex topologies. This project was instrumental in the education of four postdocs, seven graduate students and numerous undergraduate students. Two of the postdocs have continued on to university faculty positions and all of the graduate students have or will soon graduate with their doctorates. We have also shared the enthusiasm and research of organic and inorganic materials, self-assembly, and solar energy devices with K-12 students, their teachers, and the community through programs such as Penn's GEMS (Girls in Mathematics, Engineering and Sciences); Nanoday with the Franklin Institute in Philadelphia for Nano Day; and "Philly Materials Day."
