VASPKIT: A user-friendly interface facilitating high-throughput computing and analysis using VASP code

Quantum-dot-sensitized solar cells (QDSCs) are a promising low-cost alternative to existing photovoltaic technologies such as crystalline silicon and thin inorganic films The absorption spectrum of quantum dots (QDs) can be tailored by controlling their size, and QDs can be produced by low-cost methods Nanostructures such as mesoporous films, nanorods, nanowires, nanotubes and nanosheets with high microscopic surface area, redox electrolytes and solid-state hole conductors are borrowed from standard dye-sensitized solar cells (DSCs) to fabricate electron conductor/QD monolayer/hole conductor junctions with high optical absorbance Herein we focus on recent developments in the field of mono- and polydisperse QDSCs Stability issues are adressed, coating methods are presented, performance is reviewed and special emphasis is given to the importance of energy-level alignment to increase the light to electric power conversion efficiency

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Quantum‐Dot‐Sensitized Solar Cells

The modern version of the KKR (Korringa–Kohn–Rostoker) method represents the electronic structure of a system directly and efficiently in terms of its single-particle Green's function (GF). This is in contrast to its original version and many other traditional wave-function-based all-electron band structure methods dealing with periodically ordered solids. Direct access to the GF results in several appealing features. In addition, a wide applicability of the method is achieved by employing multiple scattering theory. The basic ideas behind the resulting KKR-GF method are outlined and the different techniques to deal with the underlying multiple scattering problem are reviewed. Furthermore, various applications of the KKR-GF method are reviewed in some detail to demonstrate the remarkable flexibility of the approach. Special attention is devoted to the numerous developments of the KKR-GF method, that have been contributed in recent years by a number of work groups, in particular in the following fields: embedding schemes for atoms, clusters and surfaces, magnetic response functions and anisotropy, electronic and spin-dependent transport, dynamical mean field theory, various kinds of spectroscopies, as well as first-principles determination of model parameters.

https://iopscience.iop.org/article/10.1088/0034-4885/74/9/096501/pdf

Calculating condensed matter properties using the KKR-Green's function method—recent developments and applications

Recent developments in the ABINIT software package

The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. Nanostructured materials and certain alloys have been employed in the practical implementation of these devices. This Progress Article reviews the range of different approaches and discusses how to resolve the remaining technical issues.

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Understanding intermediate-band solar cells

The intermediate-band solar cell (IBSC) concept has been recently proposed to enhance the current gain from the solar spectrum whilst maintaining a large open-circuit voltage. Its main idea is to introduce a partially occupied intermediate band (IB) between the valence band (VB) and conduction band (CB) of the semiconductor absorber, thereby increasing the photocurrent by the additional $\text{VB}\ensuremath{\rightarrow}\text{IB}$ and $\text{IB}\ensuremath{\rightarrow}\text{CB}$ absorptions. The confined electron levels of self-assembled quantum dots (QDs) were proposed as potential candidates for the implementation of such an IB. Here we report experimental and theoretical investigations on ${\text{In}}_{y}{\text{Ga}}_{1\ensuremath{-}y}\text{As}$ dots in a ${\text{GaAs}}_{1\ensuremath{-}x}{\text{P}}_{x}$ matrix, examining its suitability for acting as IBSCs. The system has the advantage of allowing strain symmetrization within the structure, thus enabling the growth of a large number of defect-free QD layers, despite the significant size mismatch between the dot material and the surrounding matrix. We examine the various conditions related to the optimum functionality of the IBSC, in particular those connected to the optical and electronic properties of the system. We find that the intensity of absorption between QD-confined electron states and host CB is weak because of their localized-to-delocalized character. Regarding the position of the IB within the matrix band gap, we find that, whereas strain symmetrization can indeed permit growth of multiple dot layers, the current repertoire of ${\text{GaAs}}_{1\ensuremath{-}x}{\text{P}}_{x}$ barrier materials, as well as ${\text{In}}_{y}{\text{Ga}}_{1\ensuremath{-}y}\text{As}$ dot materials, does not satisfy the ideal energetic locations for the IB. We conclude that other QD systems must be considered for QD-IBSC implementations.

Theoretical and experimental examination of the intermediate-band concept for strain-balanced (In,Ga)As/Ga(As,P) quantum dot solar cells

Random substitutional A(x)B(1-x) alloys lack formal translational symmetry and thus cannot be described by the language of band-structure dispersion E(k(→)) Yet, many alloy experiments are interpreted phenomenologically precisely by constructs derived from wave vector k(→), eg, effective masses or van Hove singularities Here we use large supercells with randomly distributed A and B atoms, whereby many different local environments are allowed to coexist, and transform the eigenstates into an effective band structure (EBS) in the primitive cell using a spectral decomposition The resulting EBS reveals the extent to which band characteristics are preserved or lost at different compositions, band indices, and k(→) points, showing in (In,Ga)N the rapid disintegration of the valence band Bloch character and in Ga(N,P) the appearance of a pinned impurity band

Effective Band Structure of Random Alloys

Quantum dots (QDs) embedded in a matrix exhibit a coexistence of ``zero-dimensional'' (0D) bound electron and hole states on the dot with ``three-dimensional'' (3D) continuum states of the surrounding matrix. In epitaxial QDs one encounters also ``two-dimensional'' (2D) states of a quantum well-like supporting structure (wetting layer). This coexistence of 0D, 2D, and 3D states leads to interesting electronic consequences explored here using multiband atomistic pseudopotential calculations. We distinguish strained dots (InAs in GaAs) and strain-free dots (InAs in GaSb) finding crucial differences: in the former case ``potential wings'' appear in the electron confining potential in the vicinity of the dot. This results in the appearance of localized electronic states that lie above the threshold of the 3D continuum. Such resonances are ``strain-induced localized states'' (SILSs) appearing in strained systems, whereas in strain-free systems the dot resonances in the continuum are the usual ``virtual bound states'' (VBSs). The SILSs were found to occur regardless of the thickness of the wetting layer and even in interdiffused dots, provided that the interdiffusion length is small compared to the QD size. Thus, the SILSs are well isolated from the environment by the protective potential wings, whereas the VBSs are strongly interacting. These features are seen in our calculated intraband as well as interband absorption spectra. Furthermore, we show that the local barrier created around the dot by these potential wings suppresses the 0D-2D (dot-wetting layer) hybridization of the electron states. Consequently, in contrast to findings of simple model calculations of envelope function, 0D-to-2D ``crossed transitions'' (bound hole-to-wetting layer electron) are practically absent because of their spatially indirect character. On the other hand, since no such barrier exists in the hole confining potential, a strong 0D-2D hybridization is present for the hole states. We show this to be the source for the strong 2D-to-0D crossed transitions determined experimentally.

Coexistence and coupling of zero-dimensional, two-dimensional, and continuum resonances in nanostructures

Quantum dot-based infrared detectors often involve transitions from confined states of the dot to states above the minimum of the conduction band continuum of the matrix We discuss the existence of two types of resonant states within this continuum in self-assembled dots: (i) virtual bound states, which characterize square wells even without strain and (ii) strain-induced localized states The latter emerge due to the appearance of “potential wings” near the dot, related to the curvature of the dots While states (i) do couple to the continuum, states (ii) are sheltered by the wings, giving rise to sharp absorption peaks

Strain-Induced Localized States Within the Matrix Continuum of Self-Assembled Quantum Dots

q point of the two-dimensional Brillouin zone and (ii) are allowed by symmetry to couple at this point � q. In this case, the system manifests two potential wells of opposite attractiveness, such as a well for � states and a barrier for X states. For InP/GaP this leads to the formation of an interface-localized state already in a single heterojunction, lying energetically between theedge of InP and the X edge of GaP. When the InP/GaP quantum well is formed, this single state evolves into a pair of interface-localized states, located deep in the band gap. Because of their mixed � -X character, these interface-localized states possess a strong optical signature. This new understanding allows us to provide a different interpretation to the previously observed photoemission data for InP/GaP quantum wells and dots. We find analogous states in GaAs/AlAs and GaAs/GaP but now these levels are resonant within the continuum of states of the matrix conduction band and are therefore less pronounced.

Voicu Popescu

Papers

Theoretical and experimental examination of the intermediate-band concept for strain-balanced (In,Ga)As/Ga(As,P) quantum dot solar cells

Effective Band Structure of Random Alloys

Coexistence and coupling of zero-dimensional, two-dimensional, and continuum resonances in nanostructures

Strain-Induced Localized States Within the Matrix Continuum of Self-Assembled Quantum Dots

Localized interface states in coherent isovalent semiconductor heterojunctions