Session A10: Invited Session: Computational Assessment of a Sustainable Energy Future: The Earth-abundant Materials Approach

Computational Design of Solar Energy Harvesting Materials Made of Earth-Abundant Elements

Very large-scale deployment of photovoltaic (PV) technology based on both the first and second generation solar cells posts serious questions on the materials supply as they rely on either high-purity and high-quality silicon crystals or rare elements such as indium and tellurium. ``Ancient'' PV materials made of earth-abundant elements, such as oxides and sulfides of copper and iron, have attracted resurgent interests. There is also intensive research devoted to the search for ``modern'' earth-abundant PV materials, with a recent promising example being Cu$_{2}$ZnSnSe$_{4}$. Computational approaches play a key role in this endeavor by guiding the screening and optimization of the materials toward high device performance. In this paper, I will focus on two aspects of computational design of earth-abundant PV materials. First, I will discuss the methods for accurately predicting band gaps of semiconductor materials. The emphasis will be on the performance of hybrid functional method on different classes of materials. Based on these understandings, I will discuss how to tune the band gap of a material to match the solar spectrum. For example, one could reduce of the band gap of anatase to 1.5 eV by the chemical codoping approach. Second, I will discuss the methods for accurate computation of defect properties, which is important as the defectiveness is intrinsic to the low-cost synthesized materials. I will introduce a method for calculation of defect formation energies by minimizing the error due to the ``band-gap problem'' of the density functional theory. I will also discuss approaches to mitigating the effects of defects, e.g., by passivation.   

Computational approaches to finding earth-abundant thermoelectric materials

Good thermoelectric materials should possess a combination of seemingly incompatible properties, such as high electronic mobility and low lattice thermal conductivity. Therefore, search for crystalline materials with glass-like thermal conductivity has been an active field of research. Several cubic I-V-VI$_2$ semiconductors, the paradigm for which is AgSbTe$_2$, have been shown to exhibit minimal values of lattice thermal conductivity at ambient temperatures when the phonon mean free path equals the interatomic distance. These modes are due to the existence of highly polarizable lone $s^2$ electron pairs on the group V cations. Electrostatic repulsion between the lone $s^2$ pairs and the valence charge on group VI anions tends to favor locally distorted bonding configurations and may lead to unstable phonons. We present the results of first-principles density functional theory (DFT) calculations of phonon dispersion and electron-phonon interactions in cubic I-V-VI$_2$ semiconductors, where the group I elements are Cu, Ag, Au or alkali metals, the group V elements are P, As, or Bi, and the group VI elements are S, Se, or Te. Compounds that have only marginally stable phonons have extremely large Gr\"uneisen parameters that result in a thermal conductivity limited by Umklapp processes to values at the amorphous limit above 200 K. Following the {\it ab initio\/} calculation, we synthesized AgSbTe$_2$, AgSbSe$_2$, AgBiTe$_2$, NaSbTe$_2$, NaSbSe$_2$, and NaBiTe$_2$ and report their thermal conductivity and specific heat: in all cases, the experiments confirm the theory.   

Solar energy into fuels - the importance of interface catalysis

Finding sustainable energy solutions for the future will rely heavily on the energy influx from the sun. One convenient way of storing solar energy is by transforming that energy into a chemical form - like a fuel. The efficiency of such a transformation will require catalysts that are optimized for specific reactions, and we will need to find new catalysts for a number of processes, if we are to successfully synthesize fuels from sunlight. A fundamental insight into the way the catalysts work at the molecular level is an essential ingredient if one wants to speed up the discovery process. In this presentation I will discuss some of the challenges in catalyst discovery. In particular, I will focus on the conversion of syngas to methanol, an important sub-reaction in the biomass to fuels process.   

Key electronic processes in organic solar cells: a theoretical perspective

In this contribution, we discuss state-of-the-art quantum-chemical approaches used to derive the microscopic parameters and model the key electronic processes in organic solar cells. We illustrate the application of recently developed computational methods by computing the electronic couplings and the rates of exciton dissociation and charge recombination in several model donor-acceptor complexes. The contributions of both intra-molecular and inter-molecular vibrations to the electron-vibrational interaction will be discussed in detail. The impact on the charge-transport characteristics of the interplay between electron-vibration coupling and electronic coupling is investigated in the framework of band, disorder, and semi-classical models.   

Harvesting the energy from the sun: insights from ab initio materials modeling

Can quantum and atomistic simulations make valuable contributions to the search of materials for sustainable energy sources? We will address this question by discussing some specific examples where quantum simulations have been used to predict optical properties of nanostructured semiconductors and of complex oxides in contact with water. We will also discuss issues related to the comparisons with increasingly complex experiment.