Thermoelectrics and Photovoltaics

Thermoelectrics

A significant amount of heat is wasted from industrial processes, home heating and vehicle exhausts that could otherwise be converted to electricity through the use of thermoelectric devices. The interconversion between heat and electricity, through the use of thermoelectrics, is environmentally friendly and highly reliable. With improved efficiency, thermoelectrics could have a significant impact on the energy consumption challenge.

The performance of TEs is characterized by the figure of merit, ZT. ZT=S2σΤ/κ, where S is the Seebeck coefficient, σ is the electronic conductivity and κ is the thermal conductivity, which is controlled by the dynamics of phonons and charge carriers. Limitations to the efficiency of bulk thermoelectrics are largely associated with the intrinsic properties of different classes of bulk materials. Insulators have a low figure of merit (small ZT) because they possess small electrical conductivities. While metals have high electrical conductivities, they also exhibit small ZT’s because the Seebeck coefficient of metals is low and the thermal conductivity, κ, is large. Semiconductors are evidently the best thermoelectric materials because the Seebeck coefficient is large and thermal conductivity in these materials is dominated by the transport of phonons. Hence the goal is to increase the ZT of a material by maximizing the Seebeck coefficient and electronic conductivity, while minimizing thermal conductivity.

Research in the center will be devoted to studying fundamental mechanisms that enable controlling and enhancing the efficiency of solid-state interconversion between heat (phonons) and electricity (charge carriers).

Enhancing ZT requires decoupling S2σ and κ through nano-structural design. Nanostructuring and changes in dimensionality will be used to control carrier and phonon transport, as well as carrier/phonon interactions. Reducing the dimensionality leads to singular features and enhanced response in the densities of electronic states. Nanostructuring, as well as reduced dimensionality, leads to enhanced scattering of mid-range frequency phonons, which are responsible for heat transport. Conjugated single molecules, specifically metal-molecule-metal junctions, will be investigated for TE applications. Because charge transport is controlled by discrete energy levels, the Seebeck coefficient in conjugated molecules is expected to be large; the thermal conductance is expected to be low due to the significant mismatch between the vibrational spectrum of the molecule and the metal. Simulation and modeling will provide critical insights into the relationship between nano-structure and transport mechanisms, and thereby provide guidance for the structural design of the next-generation TE materials.

Photovoltaics


Figure 1: An illustration of the multi-step light-to-electricity converstion processes for organic PV, inorganic PV, and thermoelectrics. The loss mechanisms at each step are also described.

In PVs, the transfer of energy from photons to electrons occurs through a series of identifiable stages: charge separation, diffusion, charge transfer, charge relaxation and finally harvesting (Fig. 1). Each stage is characterized by different length scales (0.1 nm-102 nm) and time scales (10-9-10-15 s) and involves intrinsic energy losses.

Research on inorganic PVs will focus primarily on low dimensional materials, including arrays of quantum dots and rods. Low dimensional and nanostructured materials show exceptional promise for high efficiency energy conversion. These materials will be fabricated using various self-assembly and patterning strategies, including focused ion beam nanopatterning and selective-array epitaxy. By varying the sizes and the spatial locations of dots and rods in 2D and 3D, the interactions between them may be controlled. The densities of electronic states increase with reduced dimensionality. Moreover, the carrier/phonon interactions, photon absorption/emission, electron/hole recombination and transport are necessarily controllable under conditions of reduced dimensionality and spatial organization. Intermediate band semiconductors, specifically dilute semiconductor alloys, will be considered in this study. Intermediate band- semiconductors are advantageous for overcoming intrinsic losses, associated with thermalization and absorption, experienced by p-n junction solar cells. Through a combination of density functional theory (DFT) and molecular simulations, a fundamental understanding of the energy conversion processes will be developed, leading to a series of materials design rules.

Organic PV materials present unique challenges and opportunities for improved efficiency and lower cost. Research will focus on thin-film systems comprising: (1) novel small molecules; (2) conjugated linear chain polymers; (3) dendritic and (4), caged molecules in which the chemical functionalities can be controlled. Self-assembly and patterning strategies will be used to control film morphology (e.g. length scales of phase separation, molecular ordering), which in turn can enhance exciton and charge carrier transport and separation. Molecular dynamics simulations and DFT will accompany chemical synthesis and thin-film processing, and help interpret measurements of ultra-fast energy conversion processes at organic-organic and organic-inorganic interfaces.