Solution-based smoothing of transparent conductive oxide layers

Abstract

Examples have been disclosed for applying a film of a transparent conductor onto fluorine-doped tin oxide (FTO) to form a multilayer structure that exhibits higher transparency while having lower sheet resistance and a smoother surface than a single, thicker FTO of comparable thickness. Transparent conductors of various compositions may be deposited using such solutions. Examples include Sn:In2O3, Ti:In2O3, Cd2SnO4, and combinations of two or more such materials. One example provides an optical device comprising a substrate, an FTO film on the substrate, and a film of a transparent conductor on the FTO film.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 377,494, entitled "APPLYING A TRANSPARENT CONDUCTIVE FILM TO FLUORINE-DOPED TIN OXIDE," filed September 28, 2022. The entire disclosure of this provisional patent application is incorporated herein by reference in its entirety. [Background technology]

[0002] Optically transparent conductive materials have many applications. For example, solar cells contain an optically transparent conductive layer through which solar radiation passes before being absorbed at a semiconductor junction and creating electron-hole pairs. The optically transparent conductive layer acts as a current collector to collect the charge carriers generated by this process. Optically transparent conductive materials are also used as pixel electrodes in display devices such as liquid crystal display (LCD) panels and organic light-emitting diode (OLED) display panels. The electric field generated across each pixel by controlling the pixel electrode is used to control the emission of light from the display panel through the transparent pixel electrode. [Brief explanation of the drawings]

[0003] [Figure 1] 1 is a flow chart illustrating an example of a method for coating a fluorine-doped tin oxide (FTO) film on a substrate.

[0004] [Figure 2] Transmittance (upper trace) and reflectance (lower trace) for uncoated 8 ohm / square FTO.

[0005] [Figure 3] Transmittance (upper trace) and reflectance (lower trace) of an 8 ohm / square FTO coated with an 89+ / -2 nm thick indium tin oxide (ITO) film.

[0006] [Figure 4] Transmittance (upper trace) and reflectance (lower trace) for an 8 ohm / square FTO coated with a 68+ / -2 nm thick ITO film.

[0007] [Figure 5] Transmittance (upper trace) and reflectance (lower trace) of an 8 ohm / square FTO coated with a 48+ / -4 nm thick ITO film.

[0008] [Figure 6] FIG. 1 shows images of atomic force microscopy (AFM) data for an uncoated 8 ohm / square FTO film.

[0009] [Figure 7] FIG. 10 shows images of AMF data for an 8 ohm / square FTO coated with an ITO film having a thickness of 93+ / -17 nm.

[0010] [Figure 8] Schematic diagram showing an example of an optical device comprising an FTO film coated with an ITO film. DETAILED DESCRIPTION OF THE INVENTION

[0011] As mentioned above, optically transparent conductive materials (hereafter referred to as transparent conductors) have many applications. One commonly used transparent conductor is fluorine-doped tin oxide (FTO). FTO is typically provided in the form of a thin film on a glass structure. However, current FTO films can have properties that pose challenges for some applications. For example, FTO films can have higher resistivity than metallic conductors. This can increase power utilization compared to metallic conductors. Furthermore, FTO films can have relatively high roughness. As an example, an FTO film with a sheet resistance of approximately 8 ohms / square can have a roughness on the order of 33.5 + / - 0.2 nm. This roughness can cause some light scattering and therefore reduce the efficiency of devices utilizing FTO films. In some instances, depending on the device structure and deposition method, FTO films with relatively high roughness may not be compatible with the fabrication of devices with active layers. In the case of solar cells, this roughness can reduce the amount of light available to generate charge carriers. In the case of a display device, this roughness can reduce the apparent image intensity.

[0012] The sheet resistance of an FTO film can be reduced by using a thicker FTO film. However, a thicker FTO layer can result in lower optical transparency. This can reduce the amount of light reaching the semiconductor junction within the solar cell, reducing the efficiency of the solar cell. Similarly, reduced optical transparency can also reduce the efficiency of a display device, as more power may be utilized to generate light to compensate for the reduced transparency. Additionally, using thicker FTO films does not address the issue of surface roughness, as this roughness results from the chemical vapor deposition (CVD) process used to form the film. Furthermore, CVD films typically exhibit relatively high roughness for relatively thick films.

[0013] Thus, examples have been disclosed that address the above-mentioned concerns by applying a transparent conductor film onto FTO or other materials. Generally, a solution of a chemical agent for forming a film of transparent conductive material is applied to a substrate containing a film of FTO or other transparent conductor. The solution may be deposited using various methods, such as slot-die coating, doctor blade coating, dip coating, or spin coating. After applying the chemical agent solution, the substrate is heated to cure the film, thereby reacting the chemical agent on the substrate and forming a solid-state film of transparent conductor on the FTO or other material. Transparent conductors of various compositions may be deposited using such solutions. Examples include Sn:In2O3, Ti:In2O3, Cd2SnO4, and combinations of two or more such materials. The resulting multilayer structure exhibits high transparency while having a lower sheet resistance and a smoother surface than a single, thicker FTO of comparable thickness. The relatively low sheet resistance can help improve the power efficiency of optical devices. Furthermore, by smoothing the surface of a CVD-deposited transparent conductor, the disclosed examples can provide a transparent conductor film with lower light scattering than a CVD-deposited transparent conductor lacking an overlying transparent conductor film. This can help improve the transmittance of the transparent conductor film, which can increase the efficiency of optical devices. By creating a smooth surface, the disclosed examples can provide a relatively thick transparent conductor that has better conductivity than thinner films, while avoiding the increased roughness typically associated with such thick films.

[0014] The following example is described with respect to the formation of indium tin oxide (Sn:In2O3, also referred to as ITO) on a CVD-deposited FTO film. However, Ti:In2O3, Cd2SnO4, and combinations of any two or more of these materials (including ITO) may also be used. Furthermore, a film of a transparent conductor may be deposited on any suitable CVD-deposited transparent conductive oxide (TCO), such as FTO, ITO, antimony-doped tin oxide (Sb-SnO2, ATO), and aluminum-doped zinc oxide (Al-ZnO, AZO). A multilayer structure including a film of a transparent conductor (e.g., Sn:In2O3, Ti:In2O3, Cd2SnO4) deposited on a CVD-deposited TCO film may also be referred to as a TCO film stack.

[0015] In a further example, a solution-processed FTO film may be deposited on a CVD-deposited FTO film. This can increase transmittance, decrease sheet resistance, and provide a smoother surface than a CVD-deposited FTO film of comparable thickness. Solution-processed FTO films can be formed using any suitable method. As an example, an FTO precursor solution may be prepared by dissolving SnCl in tetrahydrofuran (THF) and adding a fluorinating agent. Any suitable fluorinating agent may be used. Examples include trifluoroacetic acid, triethylamine trihydrofluoride, and hydrofluoric acid (HF). In some examples, the ratio of F ions to metal ions in the FTO precursor solution is between 5% and 20%. In other examples, ratios outside this range may be used. Furthermore, in other examples, salts other than tin chloride and / or solvents other than THF may be used.

[0016] Transparent conductive film precursor solutions, methods for preparing precursor solutions, and methods for applying precursor solutions to form transparent conductive films are described in U.S. Patent Application Publication No. 2022 / 0102639, the disclosure of which is incorporated by reference in its entirety. Examples of conductive film precursor solutions may include any suitable precursor. A precursor solution for forming an Sn:In2O3 film may include an In2O3 precursor and an SnO2 precursor. A precursor solution for forming a Ti:In2O3 film may include an In2O3 precursor and a TiO2 precursor. Furthermore, a precursor solution for forming a Cd2SnO4 film may include a CdO precursor and an SnO2 precursor. Examples of In2O3 precursors include indium nitrate, indium halides, and combinations thereof. Examples of SnO precursors include tin halides (e.g., tin fluoride, tin chloride, tin bromide, or tin iodide), tin chloride hydrate, tin nitrate, tin nitrate hydrate, tin acetate, tin sulfate, and combinations thereof. Examples of TiO precursors include titanium halides (e.g., titanium fluoride, titanium chloride, titanium bromide, titanium iodide, and combinations thereof). Examples of CdO precursors include cadmium halides (e.g., cadmium fluoride, cadmium chloride, cadmium bromide, cadmium iodide, and combinations thereof). In some examples, the precursor compounds are selected to be water-soluble. For example, one or more precursors may be dissolved in water to form a precursor solution. In some examples, heat and / or stirring may be used to aid in dissolving the precursor compounds. The precursor solution may include any suitable concentration of the selected ion. In some examples, the precursor solution may contain 0.1M to 0.9M In. 3+ In some examples, the precursor solution may have a concentration of 0.1M to 0.9M Sn 2+ In other examples, ion concentrations outside these ranges may be utilized.

[0017] As described above, a transparent conductor film may be deposited on an FTO film or other conductive material (e.g., ATO or AZO) to form a TCO film stack. In other examples, the transparent conductor film may be deposited on any other suitable substrate. Examples include silicon, silica (SiO2), glass, metal, metal alloy, optical crystal, laser crystal, ceramic substrate, and substrates comprising combinations of such materials. In some examples, the substrate is a silicon wafer. In some examples, the substrate is a hydrophobic or hydrophilic glass (such as silicate glass). In some examples, a TCO film stack is deposited on a glass substrate and a photoreactive material is deposited on the TCO-coated glass to form a solar cell. In some examples, a transparent conductor film may be deposited on the photoreactive material to form a solar cell. Examples of photoreactive materials include silicon, perovskite materials, and cadmium telluride (CdTe). Examples of perovskite materials include methylammonium lead halide and inorganic cesium lead halide. In some more specific examples, the perovskite tandem solar panel comprises an FTO film and a film of transparent conductor deposited on the FTO film to smooth the surface and reduce the sheet resistance of the FTO film.

[0018] The transparent conductor film may be deposited on the substrate using any suitable method. Examples include spin coating, roll coating, spray coating, inkjet printing, mist deposition, slot-die coating, dip coating, and doctor blade deposition. As described above, after deposition, the film may be exposed to heat to cure the film. Examples of processing temperatures include temperatures from 50°C to 1000°C or higher. In some examples, heating the film includes exposing the film to a temperature below the annealing temperature of the film. This can help, for example, to evaporate solvents. In some examples, heating the film may additionally or alternatively include annealing the film. For example, the film may be treated at a first temperature for a first period of time, followed by a second temperature for a second period of time.

[0019] FIG. 1 is a flowchart of an example method 100 for coating an FTO film on a substrate. At 102, the method 100 includes applying a precursor solution to the FTO film. In some examples, at 103, the precursor solution includes one or more of indium / tin, indium / titanium, or cadmium / tin. In some examples, at 104, the method includes applying the precursor solution using one or more of slot die coating, dip coating, doctor blade coating, and spin coating. In some examples, at 106, the precursor solution includes one or more of indium nitrate, indium fluoride, indium chloride, indium bromide, or indium iodide. In some examples, at 108, the precursor solution includes one or more of tin fluoride, tin chloride, tin bromide, tin iodide, tin chloride hydrate, tin nitrate, tin nitrate hydrate, tin acetate, or tin sulfate. In some examples, at 110, the precursor solution includes one or more of cadmium fluoride, cadmium chloride, cadmium bromide, and cadmium iodide.

[0020] The method 100 further includes, at 120, heating the substrate with the precursor solution on the FTO film to cure the precursor solution and form a transparent conductive film. In some examples, the method 100 includes, at 122, heating the substrate in air. In other examples, the substrate may be heated in an inert gas (e.g., Ar) or in a vacuum. In some examples, the substrate is heated at 124 to a temperature between 50°C and 600°C.

[0021] The method 100 further includes annealing the substrate comprising the FTO film and the transparent conductive film at 130. In some examples, the method 100 includes annealing the substrate in a reducing environment at 132. For example, forming gas (5% H2-95% Ar or N2) may be utilized. In some examples, the substrate is annealed at 134 at a temperature between 350°C and 1000°C.

[0022] The deposited transparent conductor film may have any suitable thickness. Examples include films having a thickness within a range of 3 to 3,000 nm. In some examples, the transparent conductor film has a thickness of 350 nm or less, 300 nm or less, 250 nm or less, or 100 nm or less. In other examples, thicknesses outside these ranges may be utilized. Because sheet resistance may depend on thickness, a relatively thick film may be useful for reducing sheet resistance. In some examples, an uncoated CVD-deposited FTO film has a sheet resistance of about 8 ohms / square (e.g., 7.5 to 9.0 ohms / square). In some such examples, after being coated with a TCO film, the coated FTO film has a sheet resistance of 6.0 to 8.5 ohms / square. In some examples, an uncoated CVD-deposited FTO film has a sheet resistance of about 15 ohms / square (e.g., 14.0 to 16.0 ohms / square). In some such examples, after being coated with a TCO film, the as-coated FTO film has a sheet resistance of 11.0 to 13.5 ohms / square. In some examples, the sheet resistance of the multilayer structure is in the range of 6.0 to 8.0 ohms / square. In other examples, the sheet resistance may be outside these ranges. In some examples, coating a CVD-deposited FTO film with a transparent conductor film can reduce the sheet resistance of the resulting multilayer structure by 10% or more, 20% or more, or even 30% or more. Specific examples are described in the experimental results below.

[0023] Furthermore, the transparent conductor film deposited on the CVD-deposited FTO film has a lower surface roughness than the underlying FTO film. For example, an uncoated FTO film may have a surface roughness in the range of 30-50 nm. After depositing the transparent conductor film, the TCO film stack may have a surface roughness in the range of 0.5-30 nm. In some examples, the transparent conductor film has a surface roughness of 20 nm or less. In some examples, the transparent conductor film has a surface roughness of 10 nm or less. In some examples, the transparent conductor film has a surface roughness of 5 nm or less.

[0024] The resulting multilayer structure can be incorporated into any suitable optical device. Examples include solar cells (e.g., perovskite tandem solar panels) and displays (such as light-emitting diodes (LEDs) and organic LEDs (OLEDs)). As an example, a solar cell can include a photoreactive material deposited on a TCO-coated glass substrate. Because the multilayer structure can exhibit greater transparency than a single, thicker FTO film, such an example can help provide more efficient solar cells.

[0025] Experimental results An Sn:In2O3 (indium tin oxide (ITO)) precursor solution was prepared as disclosed in the aforementioned U.S. Patent Application Publication No. 2022 / 0102639. Examples of precursors are listed above. Before applying the precursor solution to the FTO-coated glass substrate, the ITO precursor solution was filtered using a 0.2 μm syringe filter with a PTFE housing and filter membrane. The FTO-coated glass (NSG TEC glass) was cleaned before deposition by rinsing with acetone, ethanol, and 18 MΩ deionized water. After cleaning, the FTO film on the substrate was hydrophilized, for example, by UV ozone (Novascan) treatment or O2 plasma treatment. The precursor solution was deposited onto the FTO using slot die coating. The stage temperature of the slot die coating apparatus was set to 100°C with a 100 μm gap height, a stage speed of 1 mm / s, and a flow rate of 1 mL / s. The deposited precursor film was cured at 550°C in an atmospheric furnace and allowed to cool slowly in the furnace. The films were then annealed at 500°C for 30 minutes in an atmosphere containing forming gas (5% H2-95% Ar or N2).

[0026] Table 1 shows the difference in sheet resistance between uncoated FTO / glass and FTO / glass coated with an ITO film. [Table 1]

[0027] As shown in Figure 1, a lower sheet resistance was obtained by coating FTO with ITO.

[0028] Table 2 provides further data on the difference in sheet resistance between uncoated FTO / glass and FTO / glass coated with an ITO film, showing that the sheet resistance decreases with increasing thickness. [Table 2]

[0029] As shown in Table 2, the difference in sheet resistance between coated and uncoated samples was reproducible and demonstrated across multiple samples. Furthermore, the reduction in sheet resistance was controllable and tunable. Furthermore, increasing thickness did not adversely affect the optical transmittance of the samples. Figure 2 shows the transmittance (upper line) and reflectance (lower line) for uncoated 8 ohm / sq FTO. Figure 3 shows the transmittance (upper line) and reflectance (lower line) for 8 ohm / sq FTO coated with an 89 nm thick ITO film. Figure 4 shows the transmittance (upper line) and reflectance (lower line) for 8 ohm / sq FTO coated with a 68 nm thick ITO film. Figure 5 shows the transmittance (upper line) and reflectance (lower line) for 8 ohm / sq FTO coated with a 48 nm thick ITO film. As shown in the figure, at 550 nm, the transmittance of the bare FTO, FTO with 89 nm ITO, FTO with 68 nm ITO, and FTO with 48 nm ITO samples is 81%, 83%, 82%, and 81%, respectively.

[0030] Table 3 shows data on surface roughness as a function of thickness. In this table, the surface roughness is modeled from scanning ellipsometry data. It is understood that surface roughness can also be a function of the method and equipment used to deposit the precursor solution to form the ITO film. [Table 3]

[0031] As shown in Table 3, applying an ITO film over the FTO film substantially reduced the surface roughness, which may help to avoid refraction losses and therefore increase the efficiency of solar cells, display systems, or other optical systems.

[0032] Figure 6 shows an image of atomic force microscopy (AFM) data for an uncoated 8 ohm / sq FTO film 102. Figure 7 shows an image of AFM data for a coated FTO film 104, which includes an 8 ohm / sq FTO coated with an ITO film having a thickness of 93 + / - 17 nm. As shown, the coated FTO film 104 has a measured maximum roughness of 4 nm, compared to 49 nm for the uncoated FTO film 102.

[0033] FIG. 8 shows an example solar cell 800 including a TCO film stack 804 formed on a substrate 802. The substrate 802 may include any suitable material (e.g., glass). In some examples, the substrate 802 is a hydrophobic or hydrophilic glass (e.g., silicate glass). In other examples, the substrate 802 includes a silicon wafer. The TCO film stack 804 includes an FTO film 806 and a transparent conductor film 808 deposited on the FTO film 806. In other examples, an ATO or AZO film may be used in place of the FTO film 806. The transparent conductor film 808 may include any suitable material, such as Sn:In2O3, Ti:In2O3, Cd2SnO4, or a combination of two or more such materials. The solar cell 800 further includes an electron transport layer 810 on the TCO film stack 804 and a photoreactive layer 812 on the electron transport layer 810. In some examples, photoactive layer 812 includes a perovskite material (such as methylammonium lead halide or an inorganic cesium lead halide). In other examples, photoactive layer 812 includes cadmium telluride. Solar cell 800 further includes a hole transport layer 814 on photoactive layer 812 and a metal conductor layer 816 on hole transport layer 814. Metal conductor layer 816 may include any suitable metal conductor (such as silver or gold).

[0034] In some examples, the TCO film stack 804 has a sheet resistance of 6-8 ohms / square and a thickness of 300 nm or less. Further, in some examples, the transparent conductor film 808 alternatively or additionally has a surface roughness of less than 20 nm. Also, in some such examples, the transparent conductor film 808 alternatively or additionally has a surface roughness of less than 10 nm.

[0035] Another example provides an optical device comprising a substrate, a fluorine-doped tin oxide (FTO) film on the substrate, and a transparent conductor film on the FTO film. In some such examples, the optical device comprises a solar cell. In some such examples, the optical device comprises a display. In some such examples, the transparent conductor film additionally or alternatively comprises an indium tin oxide film. In some such examples, the transparent conductor film additionally or alternatively comprises one or more of Sn:In2O3, Ti:In2O3, or Cd2SnO4. In some examples, the transparent conductor film additionally or alternatively has a sheet resistance of 6 to 8 ohms / square and a thickness of 300 nm or less. In some such examples, the transparent conductor film additionally or alternatively has a surface roughness of less than 20 nm. In some such examples, the transparent conductor film additionally or alternatively has a surface roughness of less than 10 nm.

[0036] Another example provides a method for coating an FTO film on a substrate with a transparent conductive film. The method includes applying a precursor solution to the FTO film, the precursor solution including one or more of indium / tin, indium / titanium, or cadmium / tin; heating the substrate with the precursor solution on the FTO film to harden the precursor solution and form a transparent conductive film; and annealing the FTO film and the substrate with the transparent conductive film. In some such examples, applying the precursor solution includes applying the precursor solution by one or more of slot die coating, dip coating, doctor blade coating, and spin coating. In some such examples, annealing the substrate additionally or alternatively includes annealing the substrate in a reducing environment. In some such examples, heating the substrate to harden the precursor solution additionally or alternatively includes heating the substrate under air. In some such examples, the precursor solution additionally or alternatively includes one or more of indium nitrate, indium fluoride, indium chloride, indium bromide, or indium iodide. In some such examples, the precursor solution additionally or alternatively includes one or more of tin fluoride, tin chloride, tin bromide, tin iodide, tin chloride hydrate, tin nitrate, tin nitrate hydrate, tin acetate, or tin sulfate. In some such examples, the precursor solution additionally or alternatively includes one or more of cadmium fluoride, cadmium chloride, cadmium bromide, or cadmium iodide.

[0037] Another example provides a solar cell comprising a glass substrate and a transparent conductive oxide film stack on the glass substrate, where the transparent conductive oxide film comprises a fluorine-doped tin oxide (FTO) film on the perovskite substrate and a transparent conductor film on the FTO film. The solar cell further comprises an electron transport layer on the transparent conductor film of the conductive oxide film stack and a photoreactive layer on the electron transport layer. In some such examples, the transparent conductor film comprises an indium tin oxide film. In some such examples, the transparent conductor film additionally or alternatively comprises one or more of Sn:In2O3, Ti:In2O3, or Cd2SnO4. In some examples, the transparent conductive oxide film stack additionally or alternatively has a sheet resistance of 6 to 8 ohms / square and a thickness of 300 nm or less. In some such examples, the transparent conductor film additionally or alternatively has a surface roughness of less than 20 nm.

Claims

1. 1. An optical device comprising: A substrate; a fluorine-doped tin oxide (FTO) film on the substrate; a transparent conductor film on the FTO film; An optical device comprising:

2. The optical device of claim 1 , wherein the optical device is a solar cell.

3. 10. The optical device of claim 1, wherein the optical device is a display.

4. 10. The optical device of claim 1, wherein the film of the transparent conductor comprises an indium tin oxide film.

5. 10. The optical device of claim 1, wherein the film of the transparent conductor is Sn:In ２ O ３ , Ti:In ２ O ３ , or Cd ２ SnO ４ , an optical device comprising one or more of:

6. 10. The optical device of claim 1, wherein the film of the transparent conductor has a sheet resistance of 6-8 ohms / square and a thickness of 300 nm or less.

7. 10. The optical device of claim 1, wherein the film of the transparent conductor has a surface roughness of less than 20 nm.

8. 8. The optical device of claim 7, wherein the film of the transparent conductor has a surface roughness of less than 10 nm.

9. 1. A method for coating a fluorine-doped tin oxide (FTO) film on a substrate with a transparent conductive film, comprising: applying a precursor solution to the FTO film, the precursor solution comprising one or more of indium / tin, indium / titanium, or cadmium / tin; heating the substrate with the precursor solution on the FTO film to harden the precursor solution and form the transparent conductive film; annealing the substrate comprising the FTO film and the transparent conductive film; A method comprising:

10. 10. The method of claim 9, wherein applying the precursor solution comprises applying the precursor solution by one or more of slot die coating, dip coating, doctor blade coating, and spin coating.

11. 10. The method of claim 9, wherein annealing the substrate comprises annealing the substrate in a reducing environment.

12. 10. The method of claim 9, wherein heating the substrate to cure the precursor solution comprises heating the substrate in air.

13. 10. The method of claim 9, wherein the precursor solution comprises one or more of indium nitrate, indium fluoride, indium chloride, indium bromide, or indium iodide.

14. 10. The method of claim 9, wherein the precursor solution comprises one or more of tin fluoride, tin chloride, tin bromide, tin iodide, tin chloride hydrate, tin nitrate, tin nitrate hydrate, tin acetate, or tin sulfate.

15. 10. The method of claim 9, wherein the precursor solution comprises one or more of cadmium fluoride, cadmium chloride, cadmium bromide, and cadmium iodide.

16. A solar cell, A glass substrate; a transparent conductive oxide film stack on the glass substrate, a fluorine-doped tin oxide (FTO) film on the perovskite substrate; a transparent conductive oxide film stack comprising: a film of a transparent conductor on the FTO film; an electron transport layer on the film of the transparent conductor of the conductive oxide film stack; a photoreactive layer on the electron transport layer; and A solar cell comprising:

17. 17. The solar cell of claim 16, wherein the film of the transparent conductor comprises an indium tin oxide film.

18. 17. The solar cell of claim 16, wherein the film of the transparent conductor is Sn:In ２ O ３ , Ti:In ２ O ３ , or Cd ２ SnO ４ , a solar cell comprising one or more of:

19. 17. The solar cell of claim 16, wherein the transparent conductive oxide film stack has a sheet resistance of 6-8 ohms / square and a thickness of 300 nm or less.

20. 17. The solar cell of claim 16, wherein the film of the transparent conductor has a surface roughness of less than 20 nm.

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