First principles study of Bi dopen CdTe thin film solar cells: electronic and optical properties

Nowadays, efficiency improvement of solar cells is one of the most important issues in photovoltaic systems and CdTe is one of the most promising thin film photovoltaic materials we can found. CdTe reported efficiencies in solar energy conversion have been as good as that found in polycrystalline Si thin film cell [1], besides CdTe can be easily produced at industrial scale

Enhancement of optical absorption in Ga-chalcopirite-based intermediate-band materials for high efficiency solar cells

We present absorption properties enhancement for two CuGaS2-based intermediate-band materials, as promising compounds for high efficiency, lower-cost photovoltaic devices. Previous band diagrams calculations predicted that these materials present a partially filled localized band within the band gap of the host semiconductor, which would increase the absorption of low-energy photons, creating additional electron–hole pairs respect to a conventional semiconductor. This could ideally result in an increase of the photocurrent of the cell without the fall of the open-circuit voltage. In this paper we show, using density functional methods, the effect of this intermediate band on the optical properties of the derived alloys. We highlight the significant enhancement of the absorption coefficient observed in the most intense range of the solar emission and we study the reflectance and transmittance properties of the materials in order to understand the effect of the thickness of the sample on the optical properties. We compare two different substituents of the Ga atoms in CuGaS2, namely, Ti and Cr atoms, able to form the intermediate-band material, and their interest for photovoltaic applications

Theoretical study of band alignment in nano-porous ZnO interacting with substituted Phthalocyanines

The aim of this work is the theoretical study of the band alignment between the two components of a hybrid organic-inorganic solar-cell. The working organic molecules are metal tetra-sulphonated phthalocyanines (M-Pc) and the inorganic material is nano-porous ZnO growth in the 001 direction. The theoretical calculations are being made using the density functional theory (DFT) using a GGA functional with the SIESTA code, which projects electron wave functions and density onto a real space grid and uses as basis set a linear combination of numerical, finite-range localized atomic orbitals. We also used the DFT+U method included in the code that allows a semi-empirical inclusion of electronic correlations in the description of electronic spectra for systems such as zinc oxide

Electronic and atomic structure of complex defects in Al- and Ga-highly doped ZnO films

Point defects in Ga- and Al-doped ZnO thin films are studied by means of first principles electronic structure calculations. Candidate defects are identified to explain recently observed differences in electrical and spectroscopical behavior of both systems. Substitutional doping in Ga-ZnO explain the metallic behavior of the electrical properties. Complexes of interstitial oxygen with substitutional Ga can behave as acceptor and cause partial compensation, as well as gap states below the conduction band minimum as observed in photoemission experiments. Zn vacancies can also act as compensating acceptors. On the other hand, the semiconducting behavior of Al-ZnO and the small variation in the optical gap compared with pure ZnO, can be explained by almost complete compensation between acceptor Zn vacancies and substitutional Al donors. Interstitial Al can also be donor levels and can be the origin of the small band observed in photoemission experiments below the Fermi level. Combinations of substitutional Al with interstitial oxygen can act simultaneously as compensating acceptor and generator o the mentioned photoemission band. The theoretical calculations have been done using density functional theory (DFT) within the generalized gradient approximation with onsite Coulomb interaction. In selected cases, DFT calculations with semilocal-exact exchange hybrid functionals have been performed. Results explain photoelectron spectra of Ga-ZnO and Al-ZnO at the corresponding doping levels

Electronic structure and optical properties in ZnO:M(Co, Cd). Effect of band-gap variation

ZnO doped with some transitions metals (ZnO:M) has several significant potential application. ZnO:Co is proposed to be used in advanced spintronic devices due to its high Curie temperature and large magnetic moments per transition metal. ZnO:Cd has potential applications in short-wavelength optoelectronic devices. This work focuses on an ab-initio study of the electronic and optical properties of ZnO:M doped with Co, and Cd. Theoretical calculations have been done with different computational codes, using the density functional theory (DFT) at the GGA and GGA+U level. The latter introduces a Hubbard term correction in the “d” levels of the Zn and Co. We used different supercells in order to have different realistic dilution levels which can be achieved in experiments. Doping effects on the features of the optical absorption are also studied and analysed in this work

Analysis of SnS2 hyperdoped with V proposed as efficient absorber material

Intermediate-band materials can improve the photovoltaic efficiency of solar cells through the absorption of two subband-gap photons that allow extra electron-hole pair formations. Previous theoretical and experimental findings support the proposal that the layered SnS2 compound, with a band-gap of around 2 eV, is a candidate for an intermediate-band material when it is doped with a specific transition-metal. In this work we characterize vanadium doped SnS2 using density functional theory at the dilution level experimentally found and including a dispersion correction combined with the site-occupancy-disorder method. In order to analyze the electronic characteristics that depend on geometry, two SnS2 polytypes partially substituted with vanadium in symmetry-adapted non-equivalent configurations were studied. In addition the magnetic configurations of vanadium in a SnS2 2H-polytype and its comparison with a 4H-polytype were also characterized. We demonstrate that a narrow intermediate-band is formed, when these dopant atoms are located in different layers. Our theoretical predictions confirm the recent experimental findings in which a paramagnetic intermediate-band material in a SnS2 2H-polytype with 10% vanadium concentration is obtained

Obtaining an intermediate band photovoltaic material through the Bi insertion in CdTe

Defect interaction can take place in CdTe under Te and Bi rich conditions. We demonstrate in this work through first principles calculations, that this phenomenon allows a Jahn Teller distortion to form an isolated half-filled intermediate band in the host semiconductor band-gap. This delocalized energy band supports the experimental deep level reported in the host band-gap of CdTe at a low bismuth concentration. Furthermore, the calculated optical absorption of CdTe:Bi in this work shows a significant subband-gap absorption that also supports the enhancement of the optical absorption found in the previous experimental results

Intermediate band position modulated by Zn addition in Ti doped CuGaS2

Many works have been done recently with the aim of obtaining intermediate band semiconductors, due to the significant importance of improving solar cell efficiency. Intermediate band materials based on CuGaS2 chalcopyrite semiconductor are one of the proposed materials and specifically Ti doped CuGaS2 is a promising structure to form the intermediate band. Here we present an ab-initio study using the density functional theory in this type of intermediate band chalcogens. Several concentrations of Ti and Zn substituting Ga atoms have been studied and their electronic densities of states were obtained. Results demonstrate a chalcopyrite semiconductor band-gap shortening and intermediate band position modulation inside this band-gap by Zn addition

Advanced theoretically engineeing of novel photovoltaic intermediate band materials based on metal doped chalcogenides

In the present work properties of derived chalcogenide compounds are obtained quantum mechanically using beyond Density Functional Theory (DFT) calculations and several candidates are proposed to be novel photovoltaic materials for the design of intermediate band solar cells with enhanced efficiencies. Substituting appropriately some Ga atoms with transition metals can give a compound with a narrow, partially-filled electronic band located inside the host semiconductor forbidden energy band-gap. Optical properties according to the experimental band-gap of the host semiconductor are obtained for both TiCu4Ga3S8 and CrCu4Ga3S8 The band . originated from the 3d states of the transition metal plays a determining role in the absorption of low energy photons. The effect of the strong correlation present in the 3d-Ti electrons of TiCu4Ga3S8 is also studied by first principles. As a result a value of 0.6 eV for U is found. This value turns out to be lower than that of other Ti based compounds. From a subsequent GGA+U calculation the effect of the correlation is reasoned to be not dangerous for the formation of an isolated intermediate band

Theoretical optoelectronic analysis of MgIn2S4 and CdIn2S4 thiospinels: Effect of transition-metal substitution in intermediate-band formation

We analyzed and compared two families of intermediate-band materials derived from indium thiospinels: MgIn2S4, having an inverse spinel structure, and CdIn2S4, a direct spinel. First-principles studies of the electronic structures of these two parent semiconductors were carried out to understand the nature of their band gaps. Optical properties were also analyzed and we found good agreement with experiments. As derivatives of these semiconductors, alloys where transition metals (M=Ti and V) substitute for In atoms at octahedral sites are presented as a class of intermediate-band materials. First, the effect of the substitution on structural parameters is assessed. Then, electronic structures are determined for Mg2MIn3S8 and Cd2MIn3S8 to show that the t2g d states of the transition metal form a partially filled localized band within the band gap of the host semiconductor. The suitability of these compounds as photovoltaic high-efficiency absorbers is discussed. An increase in absorption is assessed by studying the contribution of the transition-metal band toward their optical properties, in the range of higher solar emission, and comparing them with those of the host semiconductors. An analysis of transmittance spectra is carried out to predict the range of optimum thicknesses for samples of this type of thin film absorber. We compare, by means of structural, electronic, and optical behavior, Ti and V as substituents, to evaluate the resulting alloys for potential photovoltaic applications