Maurizia Palummo

Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials

Graphene and monolayer transition metal dichalcogenides (TMDs) are promising materials for next-generation ultrathin optoelectronic devices. Although visually transparent, graphene is an excellent sunlight absorber, achieving 2.3% visible light absorbance in just 3.3 Å thickness. TMD monolayers also hold potential as sunlight absorbers, and may enable ultrathin photovoltaic (PV) devices due to their semiconducting character. In this work, we show that the three TMD monolayers MoS2, MoSe2, and WS2 can absorb up to 5-10% incident sunlight in a thickness of less than 1 nm, thus achieving 1 order of magnitude higher sunlight absorption than GaAs and Si. We further study PV devices based on just two stacked monolayers: (1) a Schottky barrier solar cell between MoS2 and graphene and (2) an excitonic solar cell based on a MoS2/WS2 bilayer. We demonstrate that such 1 nm thick active layers can attain power conversion efficiencies of up to ~1%, corresponding to approximately 1-3 orders of magnitude higher power densities than the best existing ultrathin solar cells. Our work shows that two-dimensional monolayer materials hold yet untapped potential for solar energy absorption and conversion at the nanoscale.

Exciton Radiative Lifetimes in Two-Dimensional Transition Metal Dichalcogenides

Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS2 and in two MX2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 heterobilayers exhibit very long-lived (∼20-30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion.

Ab initio electronic and optical spectra of free-base porphyrins: The role of electronic correlation

We present a theoretical investigation of electronic and optical properties of free-base porphyrins based on density functional theory and many-body perturbation theory. The electronic levels of free-base porphine (H(2)P) and its phenyl derivative, free-base tetraphenylporphyrin (H(2)TPP) are calculated using the ab initio GW approximation for the self-energy. The approach is found to yield results that compare favorably with the available photoemission spectra. The excitonic nature of the optical peaks is revealed by solving the Bethe-Salpeter equation, which provides an accurate description of the experimental absorption spectra. The lowest triplet transition energies are in good agreement with the measured values.

Silicon–Germanium Nanowires: Chemistry and Physics in Play, from Basic Principles to Advanced Applications

Basic Principles to Advanced Applications Michele Amato,*,† Maurizia Palummo,*,‡ Riccardo Rurali,* and Stefano Ossicini* †Institut d’Electronique Fondamentale, UMR8622, CNRS, Universite ́ Paris-Sud, 91405 Orsay, France ‡European Theoretical Spectroscopy Facility (ETSF), Dipartimento di Fisica, Universita ̀ di Roma, “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy Institut de Cieǹcia de Materials de Barcelona (ICMAB−CSIC), Campus de Bellaterra, 08193 Bellaterra, Barcelona, Spain “Centro S”, CNR-Istituto di Nanoscienze, Via Campi 213/A, 41125 Modena, Italy Dipartimento di Scienze e Metodi dell’Ingegneria, Centro Interdipartimentale En&Tech, Universita ̀ di Modena e Reggio Emilia, Via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia, Italy

Optoelectronic properties in monolayers of hybridized graphene and hexagonal boron nitride.

We explain the nature of the electronic band gap and optical absorption spectrum of Carbon Boron Nitride (CBN) hybridized monolayers using density functional theory (DFT), GW and Bethe-Salpeter equation calculations. The CBN optoelectronic properties result from the overall monolayer bandstructure, whose quasiparticle states are controlled by the C domain size and lie at separate energy for C and BN without significant mixing at the band edge, as confirmed by the presence of strongly bound bright exciton states localized within the C domains. The resulting absorption spectra show two marked peaks whose energy and relative intensity vary with composition in agreement with the experiment, with large compensating quasiparticle and excitonic corrections compared to DFT calculations. The band gap and the optical absorption are not regulated by the monolayer composition as customary for bulk semiconductor alloys and cannot be understood as a superposition of the properties of bulk-like C and BN domains as recent experiments suggested.

Optical properties of BN in cubic and layered hexagonal phases

INFM Sezione di Roma-2 and Dipartimento di Fisica, Universita`di Tor Vergata, Via della Ricerca Scientifica 1, I-00133, Rome, Italy~Received 24 January 2001; revised manuscript received 6 April 2001; published 26 June 2001!Linear optical functions of cubic and hexagonal BN have been studied within first principles density func-tional theory in the local density approximation. Calculated energy-loss functions show reasonable agreementwith experiments and previous theoretical results both for h-BN and forc-BN. Discrepancies arise betweentheoretical results and experiments in the imaginary part of the dielectric function for c-BN. Possible expla-nations of this mismatch are proposed and evaluated: lattice constant variations, h-BN contamination inc-BNsamples, and self-energy effects.DOI: 10.1103/PhysRevB.64.035104 PACS number~s!: 78.20.Ci, 68.35.Ja, 71.45.GmI. INTRODUCTION

Semiconducting monolayer materials as a tunable platform for excitonic solar cells

The recent advent of two-dimensional monolayer materials with tunable optical properties and high carrier mobility offers renewed opportunities for efficient, ultrathin excitonic solar cells alternative to those based on conjugated polymer and small molecule donors. Using first-principles density functional theory and many-body calculations, we demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with commonly used acceptors such as PCBM fullerene or semiconducting carbon nanotubes can provide excitonic solar cells with tunable absorber gap, donor-acceptor interface band alignment, and power conversion efficiency, as well as novel device architectures. For the case of CBN-PCBM devices, we predict power conversion efficiency limits in the 10-20% range depending on the CBN monolayer structure. Our results demonstrate the possibility of using monolayer materials in tunable, efficient, ultrathin solar cells in which unexplored exciton and carrier transport regimes are at play.

AB INITIO OPTICAL PROPERTIES OF SI(100)

We compute the linear optical properties of different reconstructions of the clean and hydrogenated Si(100) surface within DFT-LDA, using norm-conserving pseudopotentials. The equilibrium atomic geometries of the surfaces, determined from self-consistent total-energy calculations within the Car-Parrinello scheme, strongly influence reflectance anisotropy spectra, showing differences between the

Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results

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ANALYTICAL EXPRESSIONS FOR THE LOCAL-FIELD FACTOR G(Q) AND THE EXCHANGE-CORRELATION KERNEL KXC(R) OF THE HOMOGENEOUS ELECTRON GAS

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