Highly Transparent and Conductive ZnO: Al Thin Films from a Low Temperature Aqueous Solution Approach

Abstract

Transparent conductive oxides (TCO) are a unique class of materials exhibiting optical transparency combined with metallike electrical conductivity and thus are of utmost importance for the rapidly expanding fi elds of transparent electronics and sustainable energy generation. [ 1a-e ] For most applications the standard compound is ITO exhibiting best optoelectronic performance amongst all TCO materials but to ensure a sustainable supply of such materials earth abundant and inexpensive alternatives such as aluminum doped zinc oxide (AZO) are crucial. [ 2 ] In fact this is also refl ected in predicted markets of almost $1 Billion in 2016 for alternative TCOs. [ 3 ] Nowadays, plasma based (magnetron-sputtering), [ 4a ] pulsed laser deposition (PLD), [ 4b ] atomic layer deposition(ALD) [ 4c,d ] or chemical vapor deposition (CVD) [ 4e ] methods are employed on industrial scale to obtain high quality AZO thin fi lms with resistivity of 10 −3 -10 −4 Ω cm and visible transparency > 90%, although instrumental complexity poses high investment costs as well as limits scalability. In this respect low cost non-vacuum methods for AZO thin fi lms are of immense interest.A variety of solution based approaches have been described but to obtain good optoelectronic properties comparable to vacuum deposition techniques, high temperature annealing (300-600 ° C) preferably in vacuum (10 −1 -10 −4 mbar), and for long time (60-90 min) are necessary. Some approaches employ fl ammable or toxic organic solvents (e.g. 2-methoxyethanol), [ 5a-c ] and in the case of electrodeposition conductive substrates are inevitable. [ 5d ] The most straightforward yet challenging approach for deposition of ZnO thin fi lms is aqueous solution growth [ 6 ] (e.g., chemical bath deposition or hydrothermal synthesis) on seeded substrates as illustrated in Figure 1 a. The solution chemistry comprises a water soluble zinc salt and a complexant (usually ammonium salts, ethanolamine, or NH 4 OH) which is also used for adjusting the pH to a basic regime. The crystal growth is associated with the decreasing thermodynamic stability of the zinc complex leading to controlled supersaturation and the retrograde solubility of ZnO upon increased temperature. [ 7 ] Thus, phase pure ZnO thin fi lms can be obtained already at temperatures of 60 ° C. The use of water as solvent makes the method environmental friendly and due to the use of inexpensive chemicals -as mostly water soluble metal salts are employed -it can also be designated as low cost. A number of approaches have been reported to obtain intrinsic, undoped ZnO in the form of nanorod, nanoneedle, or nanopillar thin fi lms [ 8a ] using aqueous solution deposition, but only a few studies attempt to obtain conductive AZO thin fi lms. [ 8b,c ] However, achieving a compact, dense, and highly conductive (<100 Ω /sq) AZO thin fi lm at low process temperatures has not been successful yet, although for most of the applications high conductivity is essential.To overcome this bottleneck, we present a new concept to...

Transparent Electrodes for Efficient Optoelectronics

“…[148] Considering the temperature restriction of several devices, low R sh values are therefore mainly achieved by increasing N e , at the cost of an inflated FCA. Although the FOM values for sputtered and chemical-bath-deposited (CBD) AZO are similar, deposition from solution presents some advantages: it is a simple and low-cost process, [149] and it is gentle, preventing physical damage to the substrates or active device layers where the material is deposited. Next to CBD, ALD and CVD are gentle deposition techniques as well and used for fabrication of several of the transparent electrodes discussed here.…”

Copercolating Networks: An Approach for Realizing High-Performance Transparent Conductors using Multicomponent Nanostructured Networks

“…While materials such as AZO and FTO have shown somewhat lower sheet resistance at a given optical transmittance (compared to ITO) [22,23], the transmittance versus thickness and sheet resistance versus thickness relationships are expected to be qualitatively similar to those of ITO. a) Peumans et al [24], denoted as "ITO theory."…”

Mediator-free interaction of glucose oxidase, as model enzyme for immobilization, with Al-doped and undoped ZnO thin films laser-deposited on polycarbonate supports

“…Aluminum (Al) and gallium (Ga) are n-type dopants that increase the concentration of free electrons, thereby improving the conductivity of ZnO films [7,8]. Al-doping of ZnO (AZO) had shown higher reactivity and optical transmittance than ZnO and is preferred dopant for the fabrication of transducer devices [8,9]. The resistivity is dependent not only on the Al concentration but also on the oxygen partial pressure used during the plasma assisted deposition [10].…”