ZnO has received increasing research interest for optoelectronic device applications because of its wide band gap, large exciton binding energy of ∼60 meV, and ecofriendly nature. It has also shown promising potential as an electrode material in the field of supercapacitors because of its high electrochemical stability, high energy density, and high efficiency of charge storage arising from the fast-reversible redox reaction at the electrode−electrolyte interface.2 Hence, it can display better properties as a battery active material. The fascinating physical properties of ZnO are highly governed by the atomic-scale defects it possesses. The nature and the concentration of defects significantly influence the optical properties of the materials. The optical, electronic, and physicochemical properties of ZnO can also be tailored by some suitable substitution and by varying the particle size.
A lot of theoretical and experimental work has been done to improve the electrical and optoelectronic properties of ZnO by substituting appropriate dopants. Such an introduction of a specific dopant enhances the conductive properties of the material and can hence be used as transparent conductive oxides (TCOs) as electron transporting layers. An example of such a usage can be found in a recent work where Cs-doped ZnO nanoparticles were used as electron transporting layers in colloidal quantum dot solar cells. Improved electrical and optical properties have been reported in ZnO doped with group 13 (IIIA) elements, that is, Al, Ga, and In. These dopant elements have a lower ionization energy than group II elements such as Zn (ns2) and hence they can substitute Zn2+ in the ZnO crystal structure and donate excess electrons, resulting in increased conductivity. Previous experimental and theoretical studies demonstrated that Al-doped ZnO has better optical transmittance in the visible light, whereas Ga-doped ZnO has better conductivity compared to aluminum- and indium-doped ZnO. However, there are some issues with only Al- or Ga-doped ZnO, which restrict its utility for large-scale technological applications. For example, the reaction of Al with oxygen reduces conductivity in the Al-doped ZnO system. On the other hand, doping with a large concentration of Ga in ZnO degrades its photoluminescence properties. Also, higher dopant contents inevitably result in a secondary phase, which would deteriorate the carrier transport properties.
Some theoretical and experimental studies reported the improved structural, optical, electrical, and thermoelectric properties of ZnO co-doped with Al/Ga. The results proved the capability of codoped samples in optoelectronics and the possibility of obtaining codoped ZnO samples with a lower resistivity and a higher thermoelectric power factor compared to conventional only Al- or Ga-doped ZnO.8,9 Thus, it appears that simultaneous substitution of Ga and Al could be an effective method to eliminate the drawbacks of singly (Al or Ga or In) doped ZnO and enhance the various properties. The strength of one dopant can compensate for the shortcoming of another dopant. The codoping of Ga/Al in ZnO offers a balance of performance with respect to only Al- or Ga-doped ZnO. The size of Ga3+ (0.61 Å) is more comparable to that of Zn2+ (0.74 Å) as compared to Al3+ (0.53 Å). Thus, the codoping of Ga3+ and Al3+ at Zn2+ sites results in a lesser lattice distortion and crystal defects in the ZnO lattice. The 3+ valence state of both Al3+ and Ga3+ ions can reduce the latent oxygen vacancies and can generate oxygen interstitials.
Codoping Ga and Al simultaneously in ZnO has been studied.10,11 Various methods have been used to prepare Ga3+/Al3+ codoped ZnO nanomaterials. In literature, mostly thin films and the bulk nanoparticles of these materials were examined detailing the synthesis and optical properties of the same. However, a detailed structural, optoelectronic, and electrochemical analysis of such nanoparticles has not yet been studied. The tailoring of different electronic properties with doping concentration is an extremely important aspect of the functionality of these materials.
One example of such studies is an investigation on assessment of the properties of these modified materials for supercapacitor applications. In this work, various compositions of polycrystalline Ga3+/Al3+ codoped ZnO nanomaterials were prepared using a sol−gel route to ensure a better crystallinity and homogeneity of the samples. Different kinds of defects present in the samples and their correlation with the physicochemical properties have been discussed.