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Article Cite This: ACS Omega 2019, 4, 11320−11331
http://pubs.acs.org/journal/acsodf
Tellurium Doping and the Structural, Electronic, and Optical Properties of NaYS2(1−x)Te2x Alloys Lahcene Azzouz,*,†,‡ Mohamed Halit,† Zoulikha Charifi,§,∥ and Cheŕ if F. Matta*,‡,⊥ †
Laboratoire Physique des Matériaux, Université Amar Telidji, BP 37G, Laghouat 03000, Algeria Department of Chemistry and Physics, Mount Saint Vincent University, Halifax, Nova Scotia B3M 2J6, Canada § Department of Physics, Faculty of Science and ∥Laboratoire Physique et Chimie des Matériaux, University of M’sila, 28000 M’sila, Algeria ⊥ Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J3, Canada Downloaded via 31.40.210.50 on July 17, 2019 at 19:48:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: New ternary and quaternary NaYS2(1−x)Te2x alloys (with x = 0, 0.33, 0.67, and 1) are proposed as promising candidates for photon energy conversion in photovoltaic applications. The effects of Te doping on crystal, spectral, and optical properties are studied within the framework of periodic density functional theory. Increasing Te content decreases the band gap (Eg) considerably (from 3.96 (x = 0) to 1.62 eV (x = 0.67)) and fits a quadratic model (Eg(x) = 3.96−6.78x + 4.70x2, (r2 = 0.96, n = 4)). The band gap of 1.62 eV makes the NaYS0.67Te1.33 alloy ideal for photovoltaic applications for their ability to absorb in the visible segment of the sunlight spectrum. The calculated exciton binding energies are 9.78 meV for NaYS1.33Te0.67 and 6.06 meV for NaYS0.67Te1.33. These values of the order of the thermal energy at room temperature suggest an easily dissociable hole−electron pair. The family of NaYS2(1−x)Te2x alloys are, therefore, promising candidates for visible photocatalytic devices and worthy of further experimental and theoretical investigations.
1. INTRODUCTION In recent years, photovoltaics and photochemical systems have attracted much attention as clean and renewable sources of energy.1−3 The semiconductors upon which these systems are designed are characterized by a number of properties that include, for example, the band gap (generally below 3 eV, which falls within the visible spectrum), a high absorption coefficient (α > 104 cm−1), a low effective mass (m* < 0.5 m0), an exciton binding energy Eb below 25 meV in ambient temperature, and a high relative dielectric constant (εr) value.4,5 The light conversion steps can be broken down conceptually into light absorption, exciton dissociation, and diffusion of charge carriers, steps that are depicted diagrammatically in Figure 1. In this work, a rational design of new photovoltaic materials is undertaken in advance of synthesis on the basis of optimizing these values by varying the percentage of doping in computational “experiments”. The absorption is modeled theoretically by the interband electronic transitions. A material with a band gap between 1.4 and 3 eV (i.e., in the visible spectrum) is required for photovoltaic or photochemical applications.1,5−7 In this region, a solar cell represents a compromise between a high © 2019 American Chemical Society
photocurrent and a high photovoltage. On the other hand, the energy of such excitons is sufficient to drive certain chemical reactions in photochemical devices4 such as the splitting of water (that require a band gap of approximately 2 eV4). The binding energy of the exciton, denoted as Eb, is the electron−hole energy that must be as low as possible to facilitate dissociation. For thermal energy to be able to dissociate, the exciton Eb should be less than kBT (i.e., approximately 25 meV at room temperature). The commonly used materials in photovoltaic devices have Eb values of
