Session L27: Invited Session: Quantum Design of Low-Dimensional Materials Structures for Enhanced Solar Energy Conversion

Use of First-Principles Theory to Identify materials and nano structures for next-generation solar cells

Three genomic-like material design approaches are explored for finding energy relevant semiconductors:\textit{(i) Search of nanostructure combinations leading to intermediate band solar ce}lls: Within the ideology of intermediate band solar cells (IBSC) based on quantum dots, it is presently unknown which combination of dot material + matrix material + substrate satisfies the energetic criteria enabling IBSC. Using the modern theory of nano structures (based on atomistic pseudo potentials in plane waves), we examine various combinations, finding that some of the ``usual suspect'' long believed to be ideal, are, in fact inappropriate. (\textit{ii) Finding new inorganic absorbers with first-principles:} Standard compilations of inorganic compounds reveal thousands of candidate materials that were unexplored for their potential as PV absorbers, among others, because of the absence of a quantifiable ``Design Principle'' that sorts out various materials. The common Schockley - Queisser criteria gives a universal, gap vs efficiency curve which does not distinguish different types of gaps (direct-allowed vs direct-forbidden vs-indirect) nor does it account for non radiative recombination. A simple treatment, called ``Spectroscopically Limited Maximum efficiency'' (SLME) accounts for such factors and can be calculated for hundreds of compounds (using the GW approach), providing insight into previously unrecognized candidates, as well as to the mechanisms at work causing absorption enhancement.(iii) I\textit{nverse Band structure search for direct gap Si-Ge nano structures: }Using a genomic approach to pseudo potential configurational search we identify a Si-Ge nano structures that have direct band gaps, solving the long-standing dilemma of crystalline Column IV absorbers. In collaboration with L. Yu, V. Popescu, J.W. Luo and M. Davezac.   

Photovoltaic effect for narrow-gap Mott insulators

Solar cells, based on conventional band-semiconductors, have low efficiency for conversion of solar into electrical energy. The main reason is that the excess energy of the photon absorbed by an electron/hole pair beyond the band-gap becomes heat through electron-phonon scattering and phonon emission; through these processes electrons and holes relax to their band edges within a characteristic time scale of the order of $10^{-12}-10^{-13}$ secs. We will discuss that a narrow-gap Mott insulator can produce a significant photovoltaic effect and, more importantly, if appropriately chosen it can lead to solar cells of high efficiency. In this case, a single solar photon can produce multiple electron/hole (doublon/hole) pairs, an effect known as impact ionization, faster than other relaxation processes such as relaxation through phonons. It has been proposed previously that this process could lead to an efficient solar cell using band-gap semiconductors; however, the characteristic time-scale for impact ionization is comparable to that for electron-phonon relaxation in band-gap semiconductors. The reason that a Mott insulator can behave differently is that the large Coulomb repulsion present in a Mott insulator leads to a large enhancement of the impact ionization rate. Provided that this enhancement does occur in an appropriately chosen Mott insulator, it can be demonstrated that the efficiency can improve significantly over conventional band-insulators. At present, we are doing calculations on specific transition-metal-oxide based materials believed to be Mott-insulators using extensions of the density functional theory (hybrid functionals) in combination with many-body perturbation theory. Our goal is to determine a promising candidate with suitable band structure and transition matrix elements leading to fast transition rates for impact ionization to occur in a time-scale faster than other relaxation processes.   

Shaping anomalous molecular electroluminescence by resonant nanocavity plasmons

Exciton formation and decay of molecules near metallic nanostructures is important for the control of energy conversion at the nanoscale and plasmonic devices. While extensive research has been carried out to such end using photon-excited techniques, complementary insights into the optical transitions and plasmon-exciton coupling can be obtained through electron excitations by a scanning tunneling microscope [1]. In this talk, we shall describe such reversed process of light-to-electricity conversion through single-molecule electroluminescence that demonstrates the critical role of nanocavity plasmons in the generation and decay of molecular excitons. By tuning the resonant interplay between molecular excitons and plasmons in nearby metallic nanostructures, we demonstrate that plasmons can do much more beyond intensity enhancement [2]: the emission band of molecular fluorescence can be effectively tuned by resonant nanocavity plasmons. New optoelectronic effects including resonant hot-electroluminescence and upconversion electroluminescence from higher vibronic levels of singlet excited states have been realized for porphyrin molecules near metals, which breaks the Kasha's rule and conventional Franck-Condon distribution. The highly confined nanocavity plasmons can behave like a strong coherent optical source with tunable frequency, and can be used to actively control the radiative channels of molecular emitters near metals over a wide spectral range. We shall also discuss critical factors that are responsible for the realization of single-molecule electroluminescence [3]. \\[4pt] [1] Z.C. Dong, et al., Phys. Rev. Lett. 92, 086801 (2004). \\[0pt] [2] Z.C. Dong, et al., Nat. Photonics 4, 50 (2010). \\[0pt] [3] Y. Zhang, et al., Phys. Rev. Lett. (submitted).   

Quantum Photocell: Using Quantum Coherence to Reduce Radiative Recombination and Increase Efficiency

Laser and photocell quantum heat engines (QHEs) are powered by thermal light and governed by the laws of quantum thermodynamics. We here show how to use quantum coherence (PRL, 104, 207701 (2010)) induced by quantum noise (PNAS, 108, 15097 (2011)) to improve the efficiency of a laser or photocell QHE. Surprisingly, this coherence can be induced by the same noisy (thermal) emission and absorption processes that drive the QHE. Furthermore, this noise-induced coherence can be robust against environmental decoherence. Application of the ideas to photosynthesis (Nature, 446, 782-786 (2007)) will also be discussed.