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Synthesis of K2se Solar Cell Dopant in Liquid NH3 by Solvated Electron T Transfer to Elemental Selenium ⁎ D

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Synthesis of K2se Solar Cell Dopant in Liquid NH3 by Solvated Electron T Transfer to Elemental Selenium ⁎ D Electrochemistry Communications 93 (2018) 44–48 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom Synthesis of K2Se solar cell dopant in liquid NH3 by solvated electron T transfer to elemental selenium ⁎ D. Colombaraa,b, , A.-M. Gonçalvesb, A. Etcheberryb a Université du Luxembourg, Physics and Materials Science Research Unit – 41, Rue du Brill, L-4422 Belvaux, Luxembourg b Université de Versailles, Institut Lavoisier de Versailles – 45, Avenue des États Unis, 78000 Versailles, France ARTICLE INFO ABSTRACT Keywords: This study explores the rich chemistry of elemental selenium reduction to monoselenide anions. The simplest Solvated electron possible homogeneous electron transfer occurs with free electrons, which is only possible in plasmas; however, Liquid ammonia alkali metals in liquid ammonia can supply unbound electrons at much lower temperatures, allowing in situ Microelectrode analysis. Here, solvated electrons reduce elemental selenium to K2Se, a compound relevant for alkali metal Alkali metal selenide extrinsic doping doping of Cu(In,Ga)Se solar cell material. It is proposed that the reaction follows pseudo first-order kinetics Chalcopyrite 2 with an inner-sphere or outer-sphere oxidation semi reaction mechanism depending on the concentration of sol- Copper indium gallium diselenide vated electrons. 1. Introduction the 1930s as powerful reducing agents in both organic and inorganic synthesis [20–23]. It was first proposed by Kraus [24, 25] and has Alkali metals play a crucial role in the world's most efficient solar subsequently been accepted that alkali metals dissociate at least par- cell devices based on the chalcopyrite Cu(In,Ga)Se2 (CIGS) semi- tially into alkali cations and solvated electrons under the action of li- conductor, where they act as extrinsic dopants [1–3]. This has been quid ammonia (reaction 1), where Ak is a generic alkali element: known since the 1990s, when sodium was shown to dramatically im- NH 3 +− +− prove the solar cell's open circuit voltage. The mechanisms by which Ak(s) ⎯→⎯⎯⎯ Ak(NH33 ) ⇌ {Ak e } (NH ) ⇌ Ak(NH33 ) + e (NH ) (1) ff sodium a ects the composition [4, 5], structure [6, 7], and optoelec- Computational work by Chandra and Marx [26] and Hoffmann et al. tronic properties [8, 9] of CIGS are still under debate. Recently, po- [19] confirms that sufficiently diluted solutions contain unbound tassium [10, 11] and the heavier alkali metals [12] have increased electron species near empty cavities of the solvent. Therefore, such performance up to the staggering 22.9% record value established by systems contain the smallest possible anion [27] and constitute the Solar Frontier [13]. ultimate environment for electron transfer processes in homogeneous It was originally suggested by Braunger et al. that sodium segregates media. These are in contrast to electron transfers to or from the surface fi as selenides at CIGS grain boundaries [14], and this was then con rmed of electrodes immersed in solution, which form a heterogeneous in- by atom probe tomography [15]. Importantly, alkali metals are thought terface [28, 29]. to catalyse the oxygenation of CIGS grain boundaries, passivating According to Henry Taube's theory, the limiting pathway for elec- structural defects normally present at such interfaces [16]. However, it tron transfer may involve the formation or cleavage of chemical bonds. fi ff is not known if this bene cial e ect is due to the alkali metal cations, If bonds are formed or broken, the process is called inner-sphere electron the (poly)selenide anions or both. Knowledge of these compounds be- transfer. Conversely, during outer-sphere transfer, electrons tunnel ffi yond computational studies [17] is simply insu cient to draw con- through space upon contact with the redox-active species via diffusion clusions in this regard. Therefore, the synthesis of alkali metal selenide or supramolecular interaction [30]. compounds and the study of their redox chemistry are necessary pre- Synthetic research on liquid ammonia is vast, but is mostly devoted fi conditions for fundamental developments in the eld, also in view of to organic synthesis [31, 32]. Inorganic systems have received less at- the as-yet surprising phenomenon of gas-phase alkali metal doping tention, with the focus more on applications than the fundamentals of [18]. chemical kinetics [33, 34]. Solutions of alkali metals and ammonia have been known since the The present study investigates the reduction of elemental selenium pioneering work of Sir Humphry Davy [19] and have been used since ⁎ Corresponding author at: Université du Luxembourg, Physics and Materials Science Research Unit – 41, Rue du Brill, L-4422 Belvaux, Luxembourg. E-mail address: [email protected] (D. Colombara). https://doi.org/10.1016/j.elecom.2018.06.002 Received 24 April 2018; Received in revised form 7 June 2018; Accepted 7 June 2018 1388-2481/ © 2018 Elsevier B.V. All rights reserved. D. Colombara et al. Electrochemistry Communications 93 (2018) 44–48 density jl averaged over the microelectrode surface depends only on the − bulk solvated electron concentration [ebulk ] and on its diffusion coefficient D, as shown in Eq. (2) [41]. 4FnD [e− ] j = bulk l πr (2) − Taking into account the value of [ebulk ] determined from the mass of K, the volume of condensed NH3 and the microelectrode radius r, D is − − determined to be 9.4 · 10 5 cm2 s 1 for the solvated electron at −50 °C; a value consistent with previous accounts [42, 43]. − Upon addition of selenium, [ebulk ] decreases as shown by the drop in the current plateau (dotted CV, Fig. 2a). Thus, the kinetics of sele- nium reduction can be monitored electrochemically by repeatedly Fig. 1. Schematic diagram of the rig employed for the synthesis and electro- probing the decrease in the stationary current and translating it into chemical kinetic study of the reaction between elemental potassium and sele- − [ebulk ]. nium in liquid ammonia. The kinetic data is converted into first- and second-order integrated rate formats in Fig. 2b. It appears that the reduction of elemental se- − to selenide in liquid ammonia by the action of dissolved elemental lenium to Se2 by the action of solvated electrons in liquid ammonia potassium. This is a special case of electron transfer in which the oxi- proceeds for most of the time with pseudo first-order kinetics with re- dizer undertakes bond cleavage, while the reductant is the solvated spect to the solvated electron. electron itself. Here, the decrease in solvated electron concentration Thanks to the bright distinctive colors of the polyselenide anions during the course of the reaction is monitored by direct measurements: [35, 44–47], inspection of the synthesis run provides a rich wealth of fully reversible solvated electron oxidation via cyclic voltammetry (CV) visual information that simplifies the interpretation of the results at the surface of a microelectrode. (Fig. 2c). A heterogeneous reaction starts at the surface of the Se pellets (reaction (3)): 2. Materials and methods +− +2 − Se2K2e2KSez (s) ++⇌+(NH33 ) (NH ) (NH 3 )z (NH 3 ) (3) The study is carried out in pure liquid ammonia at −50 °C, fol- The elemental selenium employed here is the commercial-grade lowing a route modified from Sharp and Koehler's approach [35]. A black vitreous form. This is mainly composed of Se8 rings, along with standard Schlenck line is connected to a bottle reservoir of ammonia for some distorted chains or opened rings [48], though polymeric rings with up to 1000 atoms have been reported [49]. It is likely that poly- condensation [36](Fig. 1). The reaction vessel was especially designed − selenide ions Se )2 with a range of z (z being the original number and custom-made for this experiment. It is composed of a 50 ml three- z(NH3 of Se atoms in the ring) would form through reaction (3). Although entry cell with a central bore. The entries are for Ar gas purge, insertion − Se )2 species with z ≥ 8 have never been reported [50], kinetically of reagents/electrodes and a security exhaust. A cold finger is placed to z(NH3 rest on the central bore of the cell and a dry ice/acetone mixture is put speaking, they are the most obvious adduct of Sez rings and two solvated electrons, requiring the cleavage of just one bond. inside. The whole cell is also partially submerged in a double-walled − Radical Se ) may form by the action of one solvated electron glass Dewar containing the same cooling mixture. The condensation of z(NH3 − 1 on Sez, followed by capture of a second solvated electron to give ammonia proceeds at a rate of ca. 1 ml·min at a safe pressure of less − − Se )2 . The formation of Se )2 with z > 8 accounts for the than half a bar. A Pt microelectrode (20 μm diameter) is used as the z(NH3 z(NH3 working electrode; in order to ensure an unchanged surface area, the blackish coloration near the surface of the Se pellets, as observed in electrode was calibrated with a ferrocene solution before and after the Fig. 2c at 1 h 40 min (Se8 is red). The lifetime of such a species is short experiments. The reference electrode is a silver wire held in protective under the strongly reducing environment ensured by the large supply of tubing with a bottom frit, to avoid any potential drift due to solvated solvated electrons. Therefore, reaction (3) is followed by further re- electron attack [37]. Stoichiometric amounts of Se shot (2 mm dia- ductions in the homogeneous phase (reaction (4)), as shown by the → → meter) or powder (99.999%, Alfa Aesar) and potassium (99.95%, Alfa color gradient stemming from the Se pellets: black dark red light → fi Aesar) were weighed inside a nitrogen-filled glovebox and placed in brown white (magni ed area in Fig.
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