Recent Progress in Solution Processed Aluminum and co-Doped ZnO for Transparent Conductive Oxide Applications

Abstract

With the continuous growth in the optoelectronic industry, the demand for novel and highly efficient materials is also growing. Specifically, the demand for the key component of several optoelectronic devices, i.e., transparent conducting oxides (TCOs), is receiving significant attention. The major reason behind this is the dependence of the current technology on only one material-indium tin oxide (ITO). Even though ITO still remains a highly efficient material, its high cost and the worldwide scarcity of indium creates an urgency for finding an alternative. In this regard, doped zinc oxide (ZnO), in particular, solution-processed aluminum doped ZnO (AZO), is emerging as a leading candidate to replace ITO due to its high abundant and exceptional physical/chemical properties. In this mini review, recent progress in the development of solution-processed AZO is presented. Beside the systematic review of the literature, the solution processable approaches used to synthesize AZO and the effect of aluminum doping content on the functional properties of AZO are also discussed. Moreover, the co-doping strategy (doping of aluminum with other elements) used to further improve the properties of AZO is also discussed and reviewed in this article.

Keywords: aluminum doped zinc oxide; co-doped zinc oxide; doping; sol-gel; solution processable; spin coating; spray pyrolysis; transparent conducting oxides.

Figure 1

Band structures of Al_xZn_1−xO with various amounts of doped Al: (a) x = 0, (b) x = 0.0625, (c) x = 0.125, (d) x = 0.1875. Reproduced with permission from ref. [54].

Figure 2

SEM images for AZO films obtained from spray pyrolysis with varying Al-doping contents: (a) 0 at.% (undoped), (b) 1 at.%, (c) 2 at.%, (d) 3 at.%, (e) 4 at.%, and (f) 5 at.%. Reproduced with permission from ref. [51].

Figure 3

Hall effect results showing the change in carrier concentration, carrier mobility, and resistivity upon doping of ZnO with Al. Reproduced with permission from ref. [52].

Figure 4

XRD spectra of undoped ZnO and AZO films deposited at 400 °C on quartz substrates for various Al doping concentrations. Reproduced with permission from ref. [50].

Figure 5

(a) Simplified schematic of the spray pyrolysis system and the main stages (b) of the ZnO-based film growth process, starting from small sprayed clusters, which undergo nucleation and coalescence, forming a continuous polycrystalline layer that then acquires a light-purplish color during substrate heating (via the hot plate), and finally changes to a darker purple after the rapid thermal annealing (RTA) post-process. Reproduced with permission from ref. [63].

Figure 6

Transmission–reflectance spectra for 10 mol% AZO deposited via AACVD using different solvents. “T or R” refer to transmission or reflectance. Reproduced with permission from ref. [64].

Figure 7

Surface images of undoped and doped ZnO films, (a) undoped, (b) 5% Al doping, (c) 10% Al doping, (d) 25% Al doping, (e) 50% Al doping, and (f) zoomed-in image of the zone indicated by the black rectangular box depicted in (e). Reproduced with permission from ref. [65].

Figure 8

(Left) SEM images of the (a) AGZO, (b) IGZO, and (c) AIZO thin films. (Right) Transmission–reflectance spectra of the AGZO, IGZO, and AIZO films. All the films were deposited at 450 °C. Reproduced with permission from ref. [25].

Figure 9

Optical transmittance of Sm:AZO thin films with different co-doping levels (inset of figure), showing the relationship of (αhυ) 2 and hυ of Sm:AZO thin films with different co-doping levels. Reproduced with permission from ref. [12].

Figure 10

The 2D AFM images of Pr:AZO thin films with different doping levels: (a) 0%, (b) 0.5%, (c) 1%, and (d) 1.5%. Reproduced with permission from ref. [11].

Figure 11

The cross-sectional SEM image of the TAZO thin film, with varying dopant concentrations, annealed at 500 °C for 1 h in air (a–c) and vacuum (d–f). The doping levels are (a,d) Sn = 1 at.% and Al = 0.5 at.%; (b,e) Sn = 1 at.% and Al = 1 at.%; and (c,f) Sn = 3 at.% and Al = 1 at.%. Reproduced with permission from ref. [74].

Figure 12

Variation in resistivity, charge carrier density, and mobility of the ZnO:Al:Ag nanostructures with Ag doping. Reproduced with permission from ref. [75].