P. Wahnón

Analysis of metallic intermediate-band formation in photovoltaic materials

The substitution of some transition atoms in III–V-type semiconductors may give rise to a type of high-efficiency photovoltaic material with an isolated intermediate narrow band in the middle of the band gap, capable of absorbing photons of low energies. We have carried out a comparative analysis of the nature and possibility of formation of the intermediate band in two compounds Ga4P3Ti and Ga3P4Ti in terms of density of states, electronic density, and atomic and orbital populations. We found that the intermediate band is formed only in one of these compounds.

V-doped SnS2: a new intermediate band material for a better use of the solar spectrum

Intermediate band materials can boost photovoltaic efficiency through an increase in photocurrent without photovoltage degradation thanks to the use of two sub-bandgap photons to achieve a full electronic transition from the valence band to the conduction band of a semiconductor structure. After having reported in previous works several transition metal-substituted semiconductors as able to achieve the electronic structure needed for this scheme, we propose at present carrying out this substitution in sulfides that have bandgaps of around 2.0 eV and containing octahedrally coordinated cations such as In or Sn. Specifically, the electronic structure of layered SnS2 with Sn partially substituted by vanadium is examined here with first principles quantum methods and seen to give favourable characteristics in this respect. The synthesis of this material in nanocrystalline powder form is then undertaken and achieved using solvothermal chemical methods. The insertion of vanadium in SnS2 is found to produce an absorption spectrum in the UV-Vis-NIR range that displays a new sub-bandgap feature in agreement with the quantum calculations. A photocatalytic reaction-based test verifies that this sub-bandgap absorption produces highly mobile electrons and holes in the material that may be used for the solar energy conversion, giving experimental support to the quantum calculations predictions.

Energetics of formation of TiGa3As4 and TiGa3P4 intermediate band materials

Using density functional theory quantum methods, total energy values and vibrational properties have been computed, and thermodynamic properties evaluated, for Ti-substituted GaAs and GaP, proposed as candidates for intermediate band photovoltaic cells. The calculations predict that the formation of these materials from the binary compounds implies an increase in total energy (that is ascribed largely to the change in coordination undergone by Ti, from six-fold to four-fold), and thus phase separation rather than mixed compound formation would be favored. However, the mentioned increase is not larger (for the arsenide case it is actually smaller) than that predicted for Mn-substituted GaAs, a material which has been experimentally made, and therefore the obtention of these Ti-substituted materials is expected to be feasible as well. Vibrational and disorder entropy contributions to the formation free energy of the ternary compounds have been also computed; they compensate partially for the total energy increase, and indicate that the thermodynamic feasibility of the materials synthesis improves for low Ti concentrations and high temperature conditions.

Band gap control via tuning of inversion degree in CdIn2S4 spinel

Based on theoretical arguments, we propose a possible route for controlling the band-gap in the promising photovoltaic material CdIn2S4. Our ab initio calculations show that the experimental degree of inversion in this spinel (fraction of tetrahedral sites occupied by In) corresponds approximately to the equilibrium value given by the minimum of the theoretical inversion free energy at a typical synthesis temperature. Modification of this temperature, or of the cooling rate after synthesis, is then expected to change the inversion degree, which in turn sensitively tunes the electronic band-gap of the solid, as shown here by screened hybrid functional calculations.

First principles characterization of direct transitions for high efficiency new photovoltaic materials

Some alloys containing a transition metal atom in an III–V host semiconductor show an intermediate half filled band in the middle of the usual semiconductor band gap. The presence of this intermediate band allows to use this material in high efficiency solar cells due to its capability of absorbing low energy photons. In the current work a study of the optoelectronic properties is presented. We mainly focus the work in the obtaining the matrix elements that contribute to direct transitions. We also have analyzed some of the factors on which that process depends. We have also found that some low energy transitions can be found for several points inside the Brillouin zone.

Application of the exact exchange potential method for half metallic intermediate band alloy semiconductor

In this paper we present an analysis of the convergence of the band structure properties, particularly the influence on the modification of the bandgap and bandwidth values in half metallic compounds by the use of the exact exchange formalism. This formalism for general solids has been implemented using a localized basis set of numerical functions to represent the exchange density. The implementation has been carried out using a code which uses a linear combination of confined numerical pseudoatomic functions to represent the Kohn-Sham orbitals. The application of this exact exchange scheme to a half-metallic semiconductor compound, in particular to Ga(4)P(3)Ti, a promising material in the field of high efficiency solar cells, confirms the existence of the isolated intermediate band in this compound.

Development and implementation of the exact exchange method for semiconductors using a localized basis set

One of the major deficiencies of density functional theory is presented in the approximation of the exchange energy term. An important advance in solving this problem has been the development of orbital-dependent exchange functionals. The exact exchange method is one of the best defined releases of such functionals. Up to now it has been applied in solid systems only using a plane wave representation basis set. In this paper we present a development and implementation of the exact exchange formalism for solid semiconductors using a basis set of localized numerical functions. The implementation of the exact exchange scheme has been carried out in the SIESTA code, as a new path to get the exchange part in the Kohn–Sham energy and potential. This program is an ab initio periodic fully self-consistent density functional code which uses norm-conserving non-local pseudopotentials. Linear combination of confined numerical pseudoatomic orbitals have been used to represent the Kohn–Sham orbitals. The calculation results of the electronic properties of several semiconductor systems using different qualities of the basis set are compared with experimental results and presented in this paper.

First principles calculations of electronic structures and metal mobility of NaxSi46 and NaxSi34 clathrates

Energetics, geometry, electronic band structures, and charge transfer for NaxSi46 and NaxSi34 clathrates with different degrees of cavity filling by sodium, and the mobility of the Na atom inside the different cavities are studied using first principles density functional calculations within the generalized gradient approximation. The stabilization of the clathrate lattice and the cell volume variation upon the inclusion of Na (which appears to move easily in the larger cavities of NaxSi34, thus justifying the experimental observations) are discussed in connection with the onset of the repulsion between Na and Si for distances shorter than ∼3.4 A. For all degrees of filling of the different cavities examined we find that the electron population of the s orbitals in the partially ionized Na atoms increases with a decrease in the size of the cavity, and that the Na states contribute significantly to the density of states at the Fermi level and thus influence the properties of these compounds.

Solution-based synthesis and processing of Sn- and Bi-doped Cu3SbSe4 nanocrystals, nanomaterials and ring-shaped thermoelectric generators

Copper-based chalcogenides that comprise abundant, low-cost, and environmental friendly elements are excellent materials for a number of energy conversion applications, including photovoltaics, photocatalysis, and thermoelectrics (TE). In such applications, the use of solution-processed nanocrystals (NCs) to produce thin films or bulk nanomaterials has associated several potential advantages, such as high material yield and throughput, and composition control with unmatched spatial resolution and cost. Here we report on the production of Cu3SbSe4 (CASe) NCs with tuned amounts of Sn and Bi dopants. After proper ligand removal, as monitored by nuclear magnetic resonance and infrared spectroscopy, these NCs were used to produce dense CASe bulk nanomaterials for solid state TE energy conversion. By adjusting the amount of extrinsic dopants, dimensionless TE figures of merit (ZT) up to 1.26 at 673 K were reached. Such high ZT values are related to an optimized carrier concentration by Sn doping, a minimized lattice thermal conductivity due to efficient phonon scattering at point defects and grain boundaries, and to an increase of the Seebeck coefficient obtained by a modification of the electronic band structure with Bi doping. Nanomaterials were further employed to fabricate ring-shaped TE generators to be coupled to hot pipes, which provided 20 mV and 1 mW per TE element when exposed to a 160 °C temperature gradient. The simple design and good thermal contact associated with the ring geometry and the potential low cost of the material solution processing may allow the fabrication of TE generators with short payback times.

Ground and excited state adiabatic 2A″ and 2A′ potential energy surfaces of the (Kr–O2)+ cluster ion

The seven lowest adiabatic potential energy surfaces (PES) of the (Kr–O2)+ cluster ion in each of the 2A″ and 2A′ symmetries are calculated. The computational method involves configuration interaction calculations in a basis of a thousand projected valence‐bond state functions. It resorts to diagonal corrections of the Hamiltonian matrix prior to configuration interaction and makes use of an l‐dependent pseudopotential for Kr. The results are characterized by the shallowness of the 1 2A″ potential well and the absence of wells in the other PES investigated. The 1 2A″ equilibrium characteristics differ significantly from those proposed in other work. Notable effects on all the PES are observed when the O–O bond is stretched beyond 2.5a0. No 1 2A′−2 2A′ (nor 1 2A″−2 2A″) pseudocrossings are found that could explain, on the basis of mere electronic structure arguments, the available thermal energy charge transfer data. A 2 2A″−1 2A′ crossing actually exists but the related Coriolis coupling mechanism cannot ...