Optimization of Sol–Gel-Formed ZnO:Al Processing Parameters by Observation of Dopant Ion Location Using Solid-State ²⁷Al NMR Spectrometry

Abstract

Studies on the properties of zinc oxide (ZnO) have been well-reviewed, 1À3 with many applications proposed such as thin-film solar cells 1 and optoelectronic materials. 3 The performance of doped ZnO as a transparent conducting oxide (TCO) prepared using techniques such as magnetron sputtering, chemical vapor deposition, and pulsed laser deposition has provided a solid body of knowledge. 1 Zn is cheap and abundant, making doped ZnO a favored candidate to replace TCOs based on indium tin oxide (ITO) for thin-film photovoltaic applications. Typical dopants used to prepare ZnO-based TCOs include F, B, Al, Ga, In, and Sn. The solÀgel route has been proposed as a cost-effective alternative to vacuumassisted deposition of films for large-area TCOs, and a number of studies have examined the influence of processing details (heating profile, temperature, solvents, etc.) on film properties and performance. 4À20 However, despite the identification of optimum processing conditions, there is limited understanding as to how and why the processing variables influence electrical properties such as resistivity. Previous studies have shown, for example, that the texturing and resistivity of solutionprocessed films are sensitive to both the concentration 5 and heating profiles used to calcine the films. 6,7 Rapid thermal annealing appears to result in highly (002)-oriented films with low resistivities, 7,8 agreeing with other observations that higher conductivities and transmittances occur in preferred (002)-oriented films. 10 However, other studies appear to suggest that more random orientations are preferred. 11 There is general agreement that ZnO:Al TCOs with low resistivities appear to require around 1 at. % added Al dopant and postsynthesis annealing at ca. 400À500°C. 8À14 Estimates of lattice deformation made from X-ray diffraction (XRD) data using Bragg's equation to attempt to quantify the effective dopant concentration suggest that the effective Al concentration is much lower than the added Al concentration. 12 A dopant ion introduced to modify the electronic properties of a material needs to be incorporated into the crystal structure of the host material (either in lattice sites or interstitially). In the case of a ZnO:Al TCO, the Al 3+ ion is required to occupy a Zn 2+ lattice site in order to provide a free electron (charge carrier) and enhance the conductivity of the ZnO. A simplistic representation is shown, 1, in which an Al 3+ ion occupies the site of a Zn 2+ ion, producing a charged defect. 14 A quantum chemical approach has been applied to calculate the structural, electrical, and electronic properties of ZnO due to the Al doping and explains the increase in the n-type electrical conductivity. 15 To ABSTRACT: We report the discrimination of Al doping sites in solÀgel-formed ZnO powders by solid-state 27 Al nuclear magnetic resonance (NMR) spectrometry. A degree of control of dopant placement is demonstrated by modifying sol precursors and processing parameters. ZnO powders containing 1À8 at. % aluminum ions were ...

Polyol synthesized aluminum doped zinc oxide nanoparticles - influence of the hydration ratio on crystal growth, dopant incorporation and electrical properties

“…However we have to note, that the integration of solid-state 27 Al-NMR spectra signals does not provide reliable quantitative data. For all probes we identified amounts smaller 0.2 at.% for Al4 sub that is in accordance with the findings of Avadhut et al [25] and other authors who identified just small amounts of in ZnO solubilized aluminum by 27 Al-NMR [24,28,32]. With increasing hydration ratio the amount of crystalline located aluminum decreases from 0.15 at.% to 0.05 at.% Al and the amount for amorphous located aluminum increases from 1.85 at.% up to 1.95 at.% Al.…”

“…Independent from the hydration ratio the initially inserted aluminum concentration of 2 at.% could be found within the limits of the standard deviation (Table 1). Al-nuclear magnetic resonance spectroscopy ( 27 Al-NMR) the distribution of the aluminum atoms in the host lattice of ZnO can be depicted and characterized [23,24,25,26,27,28]. Related to the crystallographic composition of the wurtzite-structured zinc oxide, different lattice sites can be occupied by the dopant.…”

Relation between synthesis conditions, dopant position and charge carriers in aluminium-doped ZnO nanoparticles

“…In comparison with other findings, 23,25 Fig . 8 presents the high-resolution O 1s spectrum acquired on 0.5 at% ZnO:Al spherical nanocrystals.…”

“…27 The octahedral coordination, where the Al 3+ ion behaves as an acceptor, would decrease conductivity. 23,25,28 Thus, the Al 3+ dopant should be placed in a substitutional, tetrahedral position to effectively modify the electronic properties of the ZnO host material. Even though the Al 3+ ion is smaller than the Zn 2+ ion, it has a preference for the octahedral site in most Al-containing oxides.…”

Structural investigation of aluminium doped ZnO nanoparticles by solid-state NMR spectroscopy

“…Liquid phase preparation of transparent conductive AZO nanocrystals is dominated by sol-gel techniques. [8][9][10][11][12] A number of studies have examined the influence of solvent, temperature, 13 crystallinity, 14 doping concentration 13,[15][16][17][18] and heating profile 18 on film properties, which slightly differ with respect to the resistivity and optical transparency. Despite the complexity of the material there is a consensus that an optimum doping level exists, which however is difficult to compare because the authors often only refer to the Al to Zn ratio in the start mixture.…”