November 1, 2008
Transport phenomena at the nanoscale are of interest due to the presence of
both quantum and classical behavior. In this work, we demonstrate that
quantum transport efficiency can be enhanced by a dynamical interplay of the
system Hamiltonian with the pure dephasing dynamics induced by a fluctuating
environment. This is in contrast to fully coherent hopping that leads to
localization in disordered systems, and to highly incoherent transfer that
is eventually suppressed by the quantum Zeno effect. We study these
phenomena in the Fenna-Matthews-Olson protein complex as a prototype for
larger photosynthetic energy transfer systems. We also show that disordered
binary tree structures exhibit enhanced transport in the presence of dephasing.
This phenomena could in principle be applied for the development of materials with
improved exciton transport properties. Our group is beginning work in this direction.
If time is available, I will describe our distributed computing effort for finding novel
candidates for organic photovoltaic devices by the harnessing volunteer CPU time.
Efficient solar-to-electric energy conversion with inexpensive solar cells and materials is one of the most important challenges we face in the 21st century. Crystalline silicon solar cells based on the conventional p-n junction dominate the solar cell market and are commercially available in modules with 15-20% efficiencies. However, they are still too expensive to manufacture which limits their potential for replacing energy from burning fossil fuels. This established technology faces the challenge of discovering innovative methods for making crystalline silicon at lower cost. Thin film solar cells based on various semiconductors such as copper indium gallium selenide (CIGS), cadmium telluride and amorphous silicon reduce the solar cell cost by reducing the amount of photovoltaic material and the amount of energy required to produce the solar cell. However, either their efficiencies are low compared to crystalline silicon or they are difficult to manufacture on large scale. In addition, last decade has produced a number of new ideas and solar cell designs based on inorganic quantum dots and on organic thin films. These ideas are now at the beginning stages of their technological evolution curves and face challenges ranging from establishing fundamental understanding of their operation principles to improving their efficiencies to levels competitive with silicon solar cells. Regardless of the solar cell technology, a number of different challenges must be surpassed to make electricity from solar energy conversion competitive with electricity obtained from burning fossil fuels. This talk will attempt to set the stage for the workshop by providing an overview of various approaches to solar-to-electric energy conversion and by summarizing the scientific challenges that must be addressed to advance the state of the art in photovoltaics.
Organic materials are attractive for application in photovoltaic cells due to their compatibility with lightweight, flexible substrates, and high-throughput processing techniques. Optical absorption in these materials leads to the creation of a bound electron-hole pair known as an exciton. The exciton is mobile, and diffuses to a heterojunction where electron-hole dissociation and photocurrent generation may take place. In most organic materials, the exciton diffusion length is much shorter than the optical absorption length. This “exciton bottleneck” limits the active layer thickness and reduces the absorption efficiency of the cell. Routes around the bottleneck have centered on the use of mixed donor-acceptor morphologies to increase the area of the dissociating interface. While promising, these architectures are difficult to optimize, and can introduce resistance for the collection of photogenerated carriers. This talk will examine an alternate approach to overcome the exciton bottleneck, focusing on the use carefully controlled, graded morphologies in organic photovoltaics.
In order to utilize solar power for the production of electricity and fuel on a massive scale, it will be necessary to develop solar photon conversion systems that have an appropriate combination of high efficiency (delivered watts/m2) and low capital cost ($/m2) to produce solar power that is competitive with coal. One potential, long-term approach to high efficiency is to utilize the unique properties of quantum dot nanostructures to control the relaxation dynamics of photogenerated carriers to produce either enhanced photocurrent through efficient photogenerated electron-hole pair multiplication or enhanced photopotential through hot electron transport and transfer processes. To achieve these desirable effects it is necessary to understand and control the dynamics of hot electron and hole relaxation, cooling, charge transport, and interfacial charge transfer of the photogenerated carriers with femtosecond (fs) to ns time resolution. At NREL, we have been studying these fundamental dynamics in various bulk and nanoscale semiconductors (quantum dots (QDs), quantum rods/wires, and quantum wells) for many years using fs transient absorption, photoluminescence, and THz spectroscopy. Recently, we predicted that the generation of more than one electron-hole pair (existing as excitons in QDs) per absorbed photon would be an efficient process in QDs . This prediction has been confirmed over the past several years in several classes of QDs. We have observed very efficient and ultrafast multiple exciton generation (MEG) from absorbed single high energy photons in Group IV-VI and recently in Si QDs. Efficient MEG has the potential to greatly enhance the conversion efficiency of solar cells that incorporate QDs for both solar.
Harvesting and applications of solar energy requires an understanding of the dynamical response of novel materials on the nanometer scale. We have developed state-of-the-art non-adiabatic molecular dynamics techniques and implemented them within time-dependent density functional theory in order to model the ultrafast photoinduced processes in these materials at the atomistic level, and in real time. The talk will focus on the photo-initiated charge transfer at the molecule-semiconductor interfaces and multiple excitons which can be generated in semiconductor quantum dots in competition with various relaxation processes.
This talk will discuss various issues and approaches in the numerical
simulation of carrier transport in solid state materials, relevant to
the modeling of optical generation / recombination. We will discuss
aspects of deterministic and Monte Carlo methods for the solid state
Boltzmann transport equation as well as the inclusion of quantum
effects in particle based transport simulators.
Using density functional theory (DFT) and time-dependent DFT
quantum-chemical methodologies, we investigate interplay of electronic
properties and conformational dynamics in several optically active
materials. In quantum dots we explore the role of surface ligands on the
electronic structure and observe strong surface-ligand interactions leading
to formation of hybridized states and polarization effects. This opens new
relaxation channels for high energy photoexcitations. Computations of
Ru(II)-bipyridine attached to the semiconductor quantum dot systems
demonstrate possibility of charge separation and energy transfer processes
in the complex. In the amorphous clusters of conjugated polymers, we find
that electron trap states are induced primarily by intra-molecular
configuration disorder, while the hole trap states are generated primarily
from inter-molecular electronic interactions. All these phenomena govern
experimentally observed photoinduced dynamics and define technologically
important properties of materials suitable for solar energy conversion.
Excitons are bound electron-hole pairs, i.e., atomic-H like Bosonic quasiparticles, that determine many optical and optoelectronic properties of solid materials. Exciton formation and dissociation play decisive roles in next generation solar cells. In a conventional p-n junction solar cell, the built-in potential separates the photoexcited electron and hole. In contrast, separating the electron and the hole in an excitonic solar cell requires an energetic driving force at a donor/acceptor (D/A) materials interface. Here, photon absorption creates a localized Frenkel exciton or a delocalized Mott-Wannier exciton in the donor material. Such an exciton migrates to the D/A interface and decays into a charge transfer (CT) exciton: the Coulombically-bound electron and hole are located in spatially separate regions across the interface. Subsequent dissociation of the CT exciton leads to charge carriers and photocurrent. In this talk, I will present our understanding on the exciton dissociation problem from recent experiments and discuss challenges in theoretical/computation treatment of this problem. These challenges arise because one must simultaneous take into account translational symmetry of the donor and acceptor (when the donor and/or acceptor are crystalline materials) and the spatial correlation of the e-h pair.