Effective Electrochemical n-Type Doping of ZnO Thin films for Optoelectronic Window Applications

Abstract

An effective n-type doping of ZnO thin films electrochemically synthetized was achieved by varying the chloride ion concentration in the starting electrolyte. The ratio between chloride and zinc cations was varied between 0 and 2 while the zinc concentration in the solution was kept constant. When the concentration of chloride in the bath increases an effective n-type doping of ZnO films takes place. n-type doping is evidenced by the rise of donors concentration, obtained from Mott-Schottky measurements, as well as from the blueshift observed in the optical gap owing to the Burstein-Moss effect.ZnO with low resistivity, high transmittance down to the UV spectral range, and good chemical stability under strong reducing environments is a promising transparent conductive oxide (TCO) to be used as transparent electrode for photovoltaic solar cells and electrodes on flat panel displays. 1 This role is presently reserved to conventional TCOs like FTO (Fluorine-doped tin oxide) and ITO (Indium tin oxide). Nevertheless, one of the advantages of ZnO is that can be electrochemically deposited on practically any conductive substrate. Unintentionally doped ZnO thin films are always n-type due to the presence of native donors. 2 However, for some applications a greater conductivity is required and then n-type doping has to be boosted by increasing the concentration of shallow donors. In ZnO, additional ntype doping can be attained by using group III metal elements such as Al, Ga or In, which substitute Zn. The conductivity increase depends upon the nature and amount of the ions incorporated. Up to now, metal elements like Al, 3,4 Ga, 5,6 Ba, 7 Sc, 7 Sn 8 and In 8 have been widely used for this purpose. On the other hand, elements of 1B group (Cu, Ag, Au) act as acceptors when substituting Zn. Recently, p-type behavior has been reported in Cu-doped ZnO films. 9 Further, the substitution of ZnO lattice anions can also result in an increase of the conductivity even though this option has been less explored. Substituting O by VII group elements, such as F or Cl, also results in an effective n-type doping. There are a few studies where fluor 10,11 or chlorine 12-15 replace oxygen. It is always found the doping with extrinsic ions increases conductivity and decreases transparency of films. However, the use of non-metal dopants in substitution to O was suggested as a better way to achieve high carrier concentration and mobility while keeping good transparency, due to the weaker perturbation of the ZnO conduction band expected in this configuration. 12,16 Until now, most of the research on the deposition methods has mainly been focused on vapor-phase techniques. 17 A common characteristic of these kinds of techniques is the high temperature. Temperatures around 900-1100 • C are the most frequently used in vapor−liquid−solid processes, 18,19 while lower temperatures of 400−500 • C are used in free-catalyst and more sophisticated techniques such as metal−organic chemical vapor deposition. 20 Electrochemical deposition of ZnO nanowire a...

Effects of Deposition Temperature on the Electrochemical Deposition of Zinc Oxide Thin Films from a Chloride Solution

“…where h is Planck's constant, m* (0.24 m e ) is the effective mass of the electron, 38) and m e is the free-electron mass. The calculated ¦E g of 0.35 and 1.57 eV for the ZnO deposited at 70 and 80°C did not agree with the measured ¦E g of 0.16 and 0.14 eV, respectively.…”

Development of Inverted Organic Photovoltaics with Anion doped ZnO as an Electron Transporting Layer

“…For improving the conductivity and mobility of ZnO thin film, n-type doping process with group III metallic cation elements such as Al [15,17,20,24], Ga [16,20], and In [17,20] is widely used and that substitute the zinc atoms in ZnO lattice [20,24]. Furthermore, it is also well known that group VII non-metallic anion elements doping such as F [18,29], Cl [19][20][21][22][23][24][25][26][27][28][29], Br [29], and I [29] can also improve the conductivity and mobility. These anion elements can substitute the oxygen atoms in ZnO lattice [20,24,28].…”

“…Generally, the oxygen atoms as vacancies can act as an electron-hole recombination sites in I-OPVs [10,30,31], and results in decrease the performance of I-OPV. Among the various methods to reducing the recombination sites of ZnO ETL, anion doping can be one of the ideal concepts, because the atonic radius of group VII anion is almost similar to the oxygen atom as well as their excess electrons can help to achieving the high mobility [23,26] and conductivity [21,24,26] while maintaining high optical transparency [20,21,23,27], compared to group III cation metallic elements. Above all, anion element doping into ZnO ETL has been conducted by various deposition techniques such as electrochemical deposition [19,21,22,24,25], atomic layer deposition [26], radio frequencymagnetron sputtering [29], metalorganic chemical vapor deposition (MOCVD) [20,28], and pulsed laser deposition [23,27].…”