1. Field
The disclosure relates to liquid based films, and more particularly to transparent conductive oxides (TCO) films, for example, conductive AZO TCO films and methods of making the same.
2. Technical Background
A common keystone component for both display and photovoltaic (PV) technologies is the use of low-cost, high quality TCOs. Typically, commercial grade TCOs are deposited on glass substrates either using sputtering, chemical vapor deposition (CVD), or spray pyrolysis among other techniques. All of these previously mentioned manufacturing techniques usually require either a high temperature deposition process, or the use of vacuum systems that may be quite expensive and not compatible with a continuous processing, for example, roll-to-roll manufacturing. In addition, these deposition techniques do not enable printed electronics.
Indium tin oxide (ITO) is currently the industry standard material for TCO films with low resistivity and a high degree of transparency. However, ITO is also well known to be toxic and relatively expensive when compared to the glass substrates due to the high cost of indium. An alternative TCO is aluminum zinc oxide (AZO). AZO, while being non-toxic and lower in cost relative to ITO, does possess a weaker conductivity than the more common ITO material.
Farley (2004) discloses making ZnO films (without any Al doping but with Co, Fe and Mn doping) in order to have a highly ordered film. The applications are not related to transparent conducting films and conductivity is not reported. Huang (2010) discloses ZnO films and methods of making ZnO films and reports the different changes in shape of ZnO nanocrystals using depending on the sol-gel chemistry used.
It would be advantageous to have a method of making an AZO film which is conductive, reduces manufacturing costs and/or can optimize film quality.
Sol-gel based TCOs can be deposited at room temperature via, for example, dip-coating or spin-coating among other techniques under a normal environment, at very low cost, and may be compatible with a large on-draw or roll-to-roll manufacturing process. A method not requiring high temperature deposition or a vacuum environment is advantageous. The second step after deposition is sintering at moderate temperatures, from 300° C. to 600° C., that can be also done in a non-vacuum environment and perhaps can be also implemented on-line or roll-to-roll. The third step is the crystallization step that in this case requires a controlled atmosphere (N2 or Ar) for eg. 5 hours at 300° C. to 600° C. and can be done after the cutting of the glass and in a batch process that can be economical.
All these are potential advantages of a liquid based TCO, however, one of the main difficulties in the implementation of such process is the fact that the solution can be rather unstable due to the hydrolysis leading to precipitation and uneven results over time.
The disclosed methods can be used to make a very stable solution that can produce TCOs of medium electric quality (resistivity of 6.4 10−2 Ohm·cm).
In one embodiment, polar aprotic solvents as a means to prepare stable formulations which enable the formation of conducting transparent AZO films. The polar aprotic solvents have unique ion solvating properties that greatly facilitate the process of making an AZO precursor solution.
In addition a second application is that these solvent systems allow for the introduction of conductive agents like carbon nanotubes, C60, C70, graphene and graphane to be incorporated directly into the TCO film. The data shows that the introduction of one of these agents namely graphene can improve conductivity while maintaining transparency. While not constrained by theory, these conductive bridge agents provide a means for allowing more cross transport of electrons. So, again the present invention is specifically (1) a process and a stable chemical solvent system which enables AZO films to be formed inexpensively and (2) that these formulations allow for the introduction of conductive agents which improve conductivity. The films described herein can be used as “seed layers” for subsequent growth of AZO films over this initial AZO film.
Polar aprotic solvents such as dimethylformamide (DMF) and/or n-methylpyrrolidone (NMP) can be used to produce stable solution based zinc oxide (ZO), aluminum zinc oxide (AZO), and also metal-semiconductor hybrid doped aluminum zinc oxide (hybrid -AZO) solutions. These ion based aprotic polar solvent solutions enable the process of making TCO films.
The films exhibit good optical properties and good conductivity (that still has room for improvement). The use of additional semiconductor and nanometal doping into the liquid form AZO helps to increase the conductivity and may have other effects such as plasmonics.
The films are used for two different applications: 1) conductive transparent film and/or 2) controlled rough nanometric surface.
Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments.
Reference will now be made in detail to the present preferred embodiment(s), an examples of which is/are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
The present invention may provide one more of the advantages described below.
The use of aprotic solvents to make a stable ionic TCO film forming precursor solutions, such as, aluminum zinc oxide (AZO) solutions may be valuable to the industry. In addition, the disclosed AZO solutions have not shown any sign of precipitation for several weeks under normal ambient air environment storage. This discovery allows manufacturers ease of use.
These AZO solutions in aprotic solvents produce highly transparent zinc oxide and aluminum zinc oxide films. Furthermore, the aprotic solvents should provide the same or better degree of stability as any other prospective ionic TCO formers such as tin oxides (SnO2).
The transparent films formed from the AZO aprotic solvent precursor solutions present good conductivity (resistivity of 2.32 10−2 Ohm·cm at moment for our best results) and it is believed that with some further optimization they can achieve sputtering grade AZO film conductivity (2.0 10−3 Ohm·cm).
The deposition process is made at “room temperature in normal air conditions” (No vacuum systems are required). This opens the door to the manufacture of printable TCOs which is unique and is clearly distinguishable from prior CVD and PCVD approaches. Conceivably, ink jet printing, spray, ultrasonic mist or even PDMS like stamping of the aprotic solvent TCO precursor solutions can be done on any surface and under ambient conditions.
Due to its liquid form precursors other agents can be added which yield new admixture properties. The degree of solvent proportion control is very tunable and allows easy control of the stoichiometric proportion of admixed agents.
The aprotic solvents are useful for suspension and dissolution of semiconducting and conducting metals such as graphene, carbon nanotubes, silver/gold/platinum nanowires/nanodots and other metallic nanoparticles. The data demonstrates that the addition of conductive and semi-conductive agents like graphene into the AZO formulation can improve conductivity. The aprotic solvent system allows the ability to control of the proportion of graphene doped into the AZO film and that this addition may cause an enhanced conductivity of the film.
The process has a sintering thermal process and a separate crystallization step that can be controlled for a variety of different results in particular when one is interested in the roughness of the substrate for additional light scattering, for example, for photovoltaic applications.
The use of unusual semiconductor/metal nano-particles can lead to optical and electrical different properties as well as increased conductivity.
The films can be used as seed layers for subsequent AZO synthesis by CVD, sputtering, spray pyrolysis, and others type processes.
The films can be prepared on a number of surfaces and substrate geometries and surface textures such as flat glass, glass fiber or Vycor®. Regarding this last point, liquid deposition of the precursor solution using the aprotic solvent system can enable TCOs to be located onto roughened surfaces. This capability may allow coating TCOs conformally over light scattering surfaces without disrupting the desired optical properties.
Here for sake of illustration and proof of principle VWR soda lime glass slides and Corning 1737 glass slides were used, however, other glass compositions, for example, being developed for PV or even HPFS could be used for the same purpose.
The advancement of display systems and the current need for efficient thin-film solar cells sparked a renewed interest on TCOs. Among the TCO's of interest one may mention ITO and AZO. The later has the advantage of being non-toxic and with precursors abundant in nature, in contrast to indium used in ITO.
In one embodiment, conductive AZO films and AZO films doped with graphene have improve conductivity. The use of polar aprotic solvents is advantageous because the aprotic polar solvents have unique solvating properties. Polar aprotic solvents may be described as solvents that share ion dissolving power with protic solvents but lack an acidic hydrogen. These solvents generally have intermediate dielectric constants and polarity. Common characteristics of aprotic solvents are: solvents do not display hydrogen bonding, solvents do not have an acidic hydrogen, and solvents are able to stabilize ions.
As such, it has been found that these solvents do enable the formation of stable ion containing solutions that when deposited and heat treated can yield effective TCO films.
In one embodiment, polar aprotic solvents like DMF can be used to make doped ZnO with Aluminum to produce good optical quality conductive films via a stable solution.
The typical process flow to produce these stable liquid based TCO's in this case AZO but also could be ITO, gallium doped zinc oxide (GZO), boron doped zinc oxide (BZO), and fluorine doped zinc oxide (FZO TCO formulations).
Here initially Zinc acetate dehydrate (Zn(CH3COO)2. 2H2O, 99.999% pure from Sigma-Aldrich) is used and dissolved in N,N-dimethylformamide (DMF) at a molar concentration ‘X’. For sake of completeness we use here 0.6 M. Then Aluminum nitrate nanohydrate (Al(NG3)3.9H2O, 99.997% pure from Sigma-Aldrich) is also dissolved in N,N-dimethylformamide (DMF) at a molar concentration ‘Y’. For sake of completeness we use here 0.1 M. The table 1 below shows the physical properties for some of the aprotic solvents, including DMF.
Table 1 shows a description and selected properties of exemplary Polar Aprotic Solvents.
The solution ‘X’ and ‘Y’ are then mixed to the desired atomic concentration of Al in Zn forming a doped solution. Alternatively also one can add additional atom concentrations of metallic nanoparticles (such as gold, platinum, silver, aluminum, cooper, etc) or semiconductor nanoparticles (such as carbon nanotubes/nanodots, graphene, graphene oxide, CdS, CdTe) for enhanced properties that may be conductivity or other physical property. Conceivably, even conductive nanometals like silver nanorods could also be doped into our TCO precursor solution to aid in the formation of a transparent conducting oxide film.
The substrate can be used as it is or it can be prepared to enhance its hydrophilic behavior. For example it was noticed that the 1737 and soda-lime glass ‘wet better’ for the initial deposition layer if its hydrophilic behavior is enhanced by an oxygen plasma cleaning prior to the deposition. Notice that this was found true only for the first deposition layer, after the first layer it seems that the surface become more accepting of the DMF solvent. This step may be optional if one wants to improve the contact of solvent with a substrate. Alternatively, piranha type acid cleaning solutions also enhance the wetting properties of substrates.
The solution, for example a 0.8 atom % of Al doped ZnO solution, is then deposited on a substrate (here glass, semiconductor, metal or other) by spin coating, dip-coating, tape-casting or simply washing the substrate on the solution. Spin-coated was used with velocities ranging from 1000 RPM to 4000 RPM for times from 30 seconds to 60 seconds. In one embodiment, a velocity of 4000 RPM for 30 seconds was used in several samples successfully, all this in a normal environment.
The deposited layer on the substrate is then annealed for a certain temperature and for a period of time. Here, the source of heat can be a simple hot-plate, a tube furnace, a normal oven, an open oven with movement, a flash lamp furnace such as a rapid thermal annealer (RTA), a localized heat source such as a flame or laser. In this example, a hot plate was used where the temperature was measured with an external thermocouple for temperature calibration. The hot-plate was used in a normal environment.
The duration of annealing here may be important as well as the rest time after the deposition and after annealing. In our case, we tried rest time after deposition from 1 minute to 1 hour. Duration of the annealing from 1 minute to 1 hour and rest time after the annealing between layers from 1 minute from 1 hour.
If multiple layers are desired, one should repeat the process of deposition and annealing with their respective times multiple times as indicated in the loop in
After deposition of single and multiple layers the sample is then crystallized (although in some cases one may not want to do that for a particular reason). The crystallization is made in a controlled environment where the atmosphere is controlled. Here several options are available. We obtained our current results by using an Argon atmosphere inside a glove box, where studies made in crystallization in normal air did not lead to good results in terms of conductivity.
Some examples of the optical transmission of different films manufactured with the process can be observed in
These same samples that were optically measured can be observed in
Additional observation of the sample made with 0.8 at % Al in ZnO can be seen in
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In
It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention.
This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/647,815, filed on May 16, 2012, the content of which is relied upon and incorporated herein by reference in its entirety.
Number | Date | Country | |
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61647815 | May 2012 | US |