The amount of energy incident onto Earth from our sun is so large that covering only 0.1% of the earth's surface with solar cells that are 10% efficient could generate enough power to meet the current demands of the world's population. However, harnessing this energy with inexpensive materials and low cost manufacturing technologies remains an important challenge. While steady progress has been made in solar cells based on the p-n junction diode, the cost of producing electricity from sunlight is still 4-5 times more expensive than competitive technologies. The availability of sustainable and inexpensive energy sources strongly influences the quality of human life. For this reason, new developments in solar-toelectric conversion methods would impact everyone's lives.
We propose to establish an interdisciplinary research team that will focus on assembly and characterization of quantum-dot sensitized solar cells (QDSSCs) as well as on synthesis and characterization of its components. The QDSSC photoelectrode will be a macroscopic ensemble of 1-10 nm diameter semiconductor nanoparticles (e.g., CdSe, CdS, PbSe and Si ) attached to the periphery of 10-100 nm diameter wide band gap (ZnO and TiO2) semiconducting nanowires grown on a transparent conducting substrate. A thin layer of liquid electrolyte containing a redox couple will be sandwiched between this photoelectrode and a counter electrode to form the QDSSC. The macroscopic ensemble of nanometer size heterointerfaces between semiconducting nanoparticles and nanowires presents significant advantages both for light absorption and for charge separation, the two critical steps in solar-to-electric energy conversion. We propose to build solar cells that take advantage of (i) fast electron transfer across large interfacial areas, (ii) the ability to tune the absorption spectrum of nanoparticles to increase overlap with the solar spectrum and (iii) the possibility to generate multiple carriers per photon. This geometry is advantageous because photogenerated electrons in the smaller particle may be transferred to the wire and rapidly transported to a collection electrode while the hole is either transferred to an electrolyte or a hole conducting material. We propose to investigate (i) methods for synthesizing the components of the solar cell, (ii) methods for assembling these components into a functional device, and (iii) characterization of the solar cell, its components and the heterointerfaces, with particular emphasis on the interfacial electronic structure and electron transfer.
Intellectual Merit - The scientific underpinnings of photovoltaic technology are multidisciplinary and cut across traditional boundaries between physics, chemistry, engineering and materials science. The proposed research brings together fundamental studies in synthesis and characterization of nanoparticles and nanosystems, and organizes these efforts in a common goal, discovery of novel efficient solar-toelectric energy conversion methods. To accomplish our goals, we bring together an interdisciplinary team of investigators with expertise ranging from synthesis and assembly of nanostructures (Aydil, Kortshagen, Norris) to materials, interface and photophysical characterization (Aydil, Zhu, Norris) and solar cell design and development (Aydil). The research activities in each of these areas have the potential to advance our knowledge and understanding in interfacial chemistry and physics of semiconductors as well as in synthesis of nanostructured materials. Furthermore, the combination of the advances in these individual fields has the potential to lead to novel solar cells that can reduce our dependence on fossil fuels and the negative effects of burning fossil fuels on the environment.
Broader Impact - New developments in solar-to-electric conversion methods are of great interest to a wide scientific and non-scientific audience. Thus, the proposed project will not only have a broad impact but will also serve as an excellent vehicle for educating students and the general public; the project is a very concrete example of how nanotechnology can address one of the most pressing problems of the 21st century, availability of renewable energy. The PIs will supervise or co-supervise undergraduate students; host high school teachers, international visitors and faculty members from undergraduate institutions; visit high schools; and mentor students in university wide programs designed to increase the participation of disadvantaged or underrepresented groups in science and engineering. Specific examples of the PIs outreach activities are emphasized in Section 4, Results from Prior NSF Support. Such activities will continue with the present project.
The proposed research addresses several of the eight high-risk/high-reward research and education themes outlined in the NSF-NSE announcement. In the order of priority these are (1) "Nanoscale Devices and Systems Architecture," (2) "Nanoscale Structures, Novel Phenomena and Quantum Control," and (3) "Manufacturing processes at the Nanoscale."
