Patent Application
20060110618
This disclosure relates to electrodes for photovoltaic cells and methods for manufacture thereof. In particular this disclosure relates to electrodes having a columnar structure that can be used in a photovoltaic cell.
Photovoltaic systems convert light into electricity for a variety of applications. Photovoltaic systems are commonly known as “solar cells”, so named for their ability to produce electricity from sunlight. Power production by photovoltaic systems may offer a number of advantages over other systems of generating electricity. These advantages are low operating costs, high reliability, modularity, low construction costs, as well as environmental benefits.
Solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. Accordingly, solar cells generally comprise semiconductor layers to produce an electron current. The semiconductor layers absorb incoming light to produce excited electrons. In addition to the semiconductor layers, solar cells generally include a transparent cover or other encapsulant, an anti-reflective layer, a front contact substrate to allow the electrons to enter a circuit, and a back contact substrate to allow the electrons to complete the circuit when excited electrons are injected into the semiconductor layer due to light exposure.
In recent years progress has been made on the development of organic and inorganic-organic hybrid solar cells. These types of solar cells can be advantageously manufactured at a relatively low cost. One low cost solar cell is a dye-sensitized solar cell. A dye-sensitized solar cell generally uses an organic dye to absorb incoming light to produce excited electrons. The dye-sensitized solar cell generally includes two planar conducting substrates arranged in a sandwich configuration. A dye-coated semiconductor film separates the two substrates. The semiconductor film is porous and has a high surface area thereby allowing sufficient dye to be attached as a molecular monolayer on its surface to facilitate efficient light absorption. The remaining intervening space between the substrates and the pores in the semiconductor film (which acts as a sponge) is filled with an organic electrolyte solution containing an oxidation/reduction couple such as triiodide/iodide.
Dye-sensitized films, however, suffer from several technical drawbacks. One technical drawback is that a large transport distance results in substantial recombination or back reactions. Recombination occurs when an electron that has been ejected from a dye recombines with the oxidized absorber. Back reactions occur when a hole ejected into the hole transporter contacts an electron that has been ejected into the electron transporter without traveling through the external circuit.
Furthermore, oxidized dyes formed by the ejection of the electrons are generally reduced by a transfer of electrons from a reduced species in the photovoltaic cell. The reduced species are generally present in an electrolyte that in turn, becomes an oxidized species in the electrolyte (after giving up the electron). This oxidized species has to migrate toward the opposite substrate through the same long and torturous diffusion path. The oxidized species get reduced by receiving the electron from the substrate to complete the circuit.
During the random walk of the electron to the substrate, the electron may travel a significant distance, and the electron may be lost by combining with a component of the electrolyte solution. This is also known as “recombination.” Under irradiation by sunlight, the density of electrons in the semiconductor may be very high such that such losses significantly reduce the maximum voltage and therefore the efficiency achievable by the solar cells. One technique for reducing the travel distance of the electron is to reduce the thickness of the semiconductor film and thus, the distance the electron has to travel to reach a substrate. Disadvantageously, reduction in the thickness of the semiconductor film may reduce the light absorption due to lower dye loading, thereby reducing the efficiency of the solar cell.
Another technical drawback of the current dye-sensitized solar cell is that the poor electron conduction of the TiO2 film consisting of randomly interconnected nano-particles. TiO2 films are generally used as electron transporters in solar cells. Further, in solar cells (photovoltaic cells) it is difficult to maximize the interfacial area of the TiO2 electron transporter for optimal loading of the dye. Yet another drawback stems from the randomness of TiO2 particles that form the film. This randomness or disorder creates a wide distribution in pore sizes. While extremely small pores prevent either dye molecules and/or the hole transporter molecules from entering into the pore, extremely large pores do not provide high surface/volume ratios. The inability of the dye molecules and/or the hole transporter molecules to enter into the pore results in increased recombination and/or back reactions. Similarly, the lack of a high surface to volume ratio promotes increased recombination and/or back reactions. This increased recombination and/or back reactions results in a loss of photon generated current.
It is therefore advantageous to minimize recombination and back-reactions by reducing the travel path of the electron and thereby reduce the length of time it takes for the electron to diffuse to the substrate while at the same time reducing the hole transport distance to another substrate. It is therefore desirable to develop solar cells or photovoltaic cells that have reduced charge transport distances and minimize or prevent recombinations and backreactions, and that can be easily mass-produced.
Disclosed herein is an article comprising a substrate; and a columnar structure, wherein the columnar structure comprises a semi-conductor and is disposed upon the substrate in a manner wherein the longitudinal axis of the columnar structures is substantially perpendicular to the substrate.
Disclosed herein is an electrode that comprises a substrate upon which is disposed an electron transport coating, wherein the electron transport coating comprises a columnar structure.
Disclosed herein is a method comprising contacting a substrate with a plasma in an expanding thermal plasma generator, wherein the chamber pressure is about 30 millitorr to about 300 millitorr; and wherein the plasma comprises a reactive species and oxygen; and disposing a semiconducting coating up on the substrate, wherein the semiconducting coating comprises a columnar microstructure.
Disclosed herein is an article comprising a coating that has a columnar structure. The coating can advantageously be converted into an electron transport coating for use in a photovoltaic cell. The coating can also be used as an electrode in a photovoltaic cell. Such an electrode generally comprises a substrate upon which is disposed the electron transport coating having a columnar structure. The columnar structures are disposed upon the substrate with their longitudinal axis substantially perpendicular to the surface of the substrate. As will be detailed later, the longitudinal axis is parallel to the z-direction, which is perpendicular to the surface of the substrate. This configuration advantageously permits rapid electron transport thereby minimizing electron recombination within the photovoltaic cell. In one embodiment, the columnar structures are generated on the substrate by an expanded thermal plasma (ETP) process, which employs a pressure of about 30 to about 300 millitorr (mT).
The electron transport coating can comprise a metal oxide, a semiconductor, carbon nanotubes, or the like. Exemplary electron transport coatings comprise semi-conductors. Suitable examples of semi-conductors are metal oxides. Examples of suitable metal oxides are silicon oxides (SiOx), silicon dioxides (SiO2), titanium oxide (TiOx), titanium dioxide (TiO2), zirconia (Zr2O3), alumina (Al2O3), hafnium oxide (HfO2), or the like, or a combination comprising at least one of the foregoing metal oxides. As noted above, the metal oxide compositions can be stoichiometric. The metal oxide compositions can be non-stoichiometric as well. In another embodiment, some of the oxygen in the metal oxide can be replaced by another anion such as nitrogen. As a result, the columnar structures can also comprise oxynitrides. An example of an oxynitride is a metal oxynitride. Examples of oxynitrides are silicon oxynitride, titanium oxynitride, or the like, or a combination comprising at least one of the foregoing oxynitrides.
In one embodiment, when titanium oxide coatings are employed, a TiOx coating may comprise a stoichiometric TiO2 coating, a non-stoichiometric TiOx coating where x is not equal to 2, or a combination thereof. In another embodiment, a suitable electron transport coating can comprise a combination of titanium dioxide and zirconia.
Examples of suitable n-type semiconductors that may be used to coat the substrate are doped silicon or germanium. Dopants for silicon are arsenic, phosphorus or antimony. Examples of carbon nanotubes are single wall carbon nanotubes, multiwall carbon nanotubes and vapor grown carbon fibers.
An exemplary electron transport coating that comprises columnar structures is one that comprises titanium dioxide (TiO2).
In the
Individual columnar structures can contact each other at any point along their lengths or can be isolated from other columnar structures. In one embodiment, when the columnar structures are isolated, the space between two adjacent columnar structures is greater than or equal to about 5 nm. In another embodiment, the space between two adjacent columnar structures is greater than or equal to about 50 nm. In yet another embodiment, the space between two adjacent columnar structures is greater than or equal to about 100 nm. In yet another embodiment, the space between two adjacent columnar structures is greater than or equal to about 500 nm.
The columnar structures have an aspect ratio greater than or equal to about 2. The aspect ratio as defined herein is the ratio of the length of a columnar structure to the smallest surface dimension of the microstructure. In one embodiment, the columnar structures have an aspect ratio of greater than or equal to about 5. In another embodiment, the columnar structures have an aspect ratio of greater than or equal to about 10. In yet another embodiment, the columnar structures have an aspect ratio of greater than or equal to about 100.
It is desirable for the columnar structure to have a variety of porous structures. The columnar structure can have a nanoporous portion wherein the pores have a size of about 1 to about 10 nm. The pore size as referred to herein is the pore diameter. The columnar structure can also have porous portions that have pores larger than 10 nm. A given columnar structure can have alternating porous and nanoporous portions if desired. For example in the
The titanium dioxide comprises a crystalline anatase phase, a brookite phase, a rutile phase, or a combination comprising at least one of the foregoing crystalline phases and has a high surface area of greater than or equal to about 5 square meters per gram (m2/gm). In one embodiment, the surface area of the columnar structure is greater than or equal to about 100 m2/gm. In another embodiment, the surface area of the columnar structure is greater than or equal to about 200 m2/gm. In yet another embodiment, the surface area of the columnar structure is greater than or equal to about 500 m2/gm. In yet another embodiment, the surface area of the columnar structure is greater than or equal to about 1,000 m2/gm.
It is desirable for the electron transport coating to have a thickness of about 5 nanometers to about 1 millimeter. In one embodiment, the electron transport coating has a thickness of about 100 nanometers to about 0.5 millimeter. An exemplary coating has a thickness of about 10 micrometers.
The substrate 12 can comprise any material that is flexible or rigid. An exemplary substrate 12 is a flexible substrate. A flexible substrate generally has a modulus of elasticity of less than or equal to about 105 gigapascals (GPa) at room temperature. The substrate 12 can be optically transparent if desired. In one embodiment, the substrate can also be electrically conductive if desired. In another embodiment, the substrate can be electrically insulating but can be coated with an interlayer that is electrically conducting.
When the substrate is electrically conductive, it is desirable for the substrate to have a bulk volume resistivity of less than or equal to about 1012 ohm-centimeter. In another embodiment, the substrate has a bulk volume resistivity of less than or equal to about 108 ohm-centimeter. In yet another embodiment, the substrate has a bulk volume resistivity of less than or equal to about 105 ohm-centimeter.
In one embodiment, the substrate can have a surface resistivity of less than or equal to about 1,000 ohm/square. In another embodiment, the substrate can have a surface resistivity of less than or equal to about 500 ohm/square. In yet another embodiment, the substrate can have a surface resistivity of less than or equal to about 100 ohm/square.
In another embodiment, the substrate can also be porous. The substrate can have a porosity of about 10 volume percent to about 90 volume percent. It is desirable for the substrate 12 and any interlayers disposed thereon to be capable of withstanding the temperatures involved in the process for depositing the electron transport coating. The substrate 12 can be formed by injection molding, extrusion, cold forming, vacuum forming, blow molding, compression molding, transfer molding, thermal forming, solvent casting, or the like.
Examples of suitable materials for substrates are glass, polymers, foils, or the like, or a combination comprising at least one of the foregoing materials. Examples of suitable glasses are silica, alumina, zirconia, titania, silicon oxynitride, titanium oxynitride, or the like. Examples of suitable polymers are thermoplastic polymers, thermosetting polymers, or a combination comprising at least one of the foregoing polymers. Examples of suitable thermoplastic polymers are polycarbonates, poly(meth)acrylates, polystyrenes, polyolefins, polyimides, polyetherimides, or the like, or a combination comprising at least one of the foregoing thermoplastic polymers. Examples of suitable thermosetting polymers are polyurethanes, polysiloxanes, epoxies, phenolics, or the like or a combination comprising at least one of the foregoing thermosetting polymers.
It is generally desirable to use metal foils as substrates. It is desirable for the foils to have a coefficient of thermal expansion that is favorable to the deposition of the metal oxide. Examples of suitable foils are those manufactured from aluminum, titanium, silver, platinum, zinc, molybdenum, tantalum, or the like, or a combination thereof. Examples of suitable alloys are steel, stainless steel, invar, or the like.
The electron transport coating 14 is advantageously formed by an expanding thermal plasma process (ETP), as discussed in more detail below.
The deposition chamber 111 also optionally comprises a retractable shutter 124. The shutter may be positioned, for example, by a handle 125 or by a computer controlled positioning mechanism. The shutter 124 may also contain a circular aperture to control the diameter of the plasma that emanates from the plasma generation chamber 110 towards the substrate 12. The deposition chamber 111 may also optionally comprise magnets or magnetic field generating coils (not shown) adjacent to chamber walls to direct the flow of the plasma.
The deposition chamber 111 may also contain an optional nozzle 118. The nozzle 118 provides improved control of the injection, ionization and reaction of the reactants to be deposited on the substrate 12. The nozzle 118 provides for the deposition of a material such as the electron transport coating on the substrate 12 and minimizes or even prevents formation of powdery reactant deposits on the substrate 12. Preferably, the nozzle 118, if employed, has a conical shape with a divergent angle of about 40 degrees and a length of about 10 to 80 cm, preferably about 16 cm. However, the nozzle 118 may alternatively have a variable cross section, such as such as conical-cylindrical-conical or conical-cylindrical. Furthermore, the nozzle 118 may have a divergent angle other than 40 degrees and a length other than 16 centimeter. The nozzle may also be omitted entirely. In the event that the nozzle is omitted, the feed tubes 112 and 114 feed into an injection ring.
The deposition chamber 111 also contains at least one reactant supply line. The number of reactant supply lines may be, for example, two or more. For example, the deposition chamber 111 may contain a first reactant supply line 112 and a second reactant supply line 114 to deposit the electron transport coating film on the substrate 12. The supply lines 112 and 114 preferably communicate with the nozzle 118 and supply reactants into the plasma flowing through the nozzle. The deposition chamber 111 also generally contains vacuum pumps (not shown) for evacuating the chamber 111.
The plasma generation chamber 110 contains at least one cathode 113, a plasma gas supply line 117 and an anode 119. The plasma generation chamber 110 generally comprises three cathodes 113 of which any one can be utilized. The cathodes 113 may comprise, for example, thorium or lanthanum doped tungsten tips. An exemplary tip is a lanthanum doped tungsten tip. The use of lanthanum allows the temperature of the tips to be maintained below the melting point of tungsten, thus avoiding contamination of the plasma with tungsten atoms and lessens changes in the geometry of the tip.
The plasma generation chamber 110 generally includes at least one plasma gas supply line 117. To form a plasma in the plasma generation chamber 110, a plasma gas is supplied through plasma gas supply line 117. The plasma gas may suitably comprise a noble gas, such as argon or helium, or a reactive gas, such as nitrogen, ammonia, carbon dioxide or hydrogen or any mixture thereof. If there is more than one plasma gas, then other gasses may be supplied through plural supply lines, if desired. Preferably, for the TiOx deposition, the plasma gas comprises argon. The plasma gas in plasma generation chamber 110 is maintained at a higher pressure than the pressure in the deposition chamber 111, which is continuously evacuated by a pump. An arc voltage is then applied between the cathode(s) 113 and the anode 119 to generate a plasma in the plasma generation chamber 110. The plasma then extends through the aperture of the anode 119 into the deposition chamber 111 due to the pressure difference between chambers 110 and 111. The reactants are supplied into the plasma through supply lines 112 and 114.
In one embodiment, in one method of manufacturing an electrode 10, a substrate 12 may comprise, for example, a low temperature substrate of polycarbonate with an optional interlayer formed on the polycarbonate. The substrate is provided in the deposition chamber 111 of the system 100 of
Suitable precursors for forming a TiOx film on the substrate are titanium tetrachloride (TiCl4), titanium isopropoxide, titanium butoxide, titanium di-isopropoxide bis (2,4 pentanedionate), titanium (IV) ethoxide, titanium (IV) 2-ethylhexoxide, titanium (IV) isobutoxide, titanium (IV) methoxide, or the like, or a combination comprising at least one of the foregoing precursors. Suitable oxidants are oxygen, nitrous oxide, hydrogen peroxide, ozone, or water.
In one exemplary embodiment, in one method of manufacturing an electron transport coating comprising TiOx on a substrate, a first reactant comprising titanium is reacted with a second reactant comprises oxygen. The first reactant comprises titanium tetrachloride (TiCl4) while the second reactant comprises oxygen. The first and second reactants react to form a TiOx film on the substrate. The flow rate for the TiCl4 reactant is about 0.1 to about 1 standard liters per minute (slm) while the flow rate for the oxygen is about 0.4 to about 10 slm. An exemplary power for the deposition of the TiOx coating is about 20 to about 5000 watts DC power. An exemplary current is about 1 to about 100 amperes, while an exemplary voltage is about 20 to about 50 volts. High quality TiOx films can be deposited at a rate greater than or equal to about 1 micrometer per minute.
In one embodiment, during the expanded thermal plasma process, a chamber pressure of about 30 millitorr (mT) to about 300 mT can be used. In another embodiment, the chamber pressure during the process can be about 35 to about 200 mT. In yet another embodiment, the chamber pressure during the process can be about 45 to about 100 mT.
As noted above, the electrode can be advantageously used in a photovoltaic cell. In one exemplary embodiment, depicted in the schematic (not to scale) in
In one embodiment, the two electrodes have at least one electrically conductive surface or interlayer that is in electrical communication with either the electron transport coating 14 or the hole transporter 312. In one embodiment, the first electrode 10 comprises a first substrate 12 and a first conductive surface (also known as the interlayer) 16 that communicates electrically with the electron transport coating 14 while the second electrode 310 comprises a second substrate 322 and a second conductive surface (or interlayer) 324 that communicates with the hole transporter 312. The substrate 12 can advantageously serve as a mechanical support for the conductive surface 16.
With reference again to the photovoltaic cell 300 of
In one embodiment, the photovoltaic cell 300 of
In another embodiment, the photovoltaic cell 300 of
After the deposition of the columnar structure of TiO2 on the substrate 12 or the electrically conducting interlayer 16, it can be subjected to sintering. Sintering is conducted at a temperature of about 300 to about 500° C. for a time period of 10 minutes to 1 hour. Sintering can be conducted in a vacuum or under a gaseous blanket. A suitable gaseous blanket is oxygen. The sintering facilitates the conversion of the columnar structure from an amorphous form to a crystalline form. Exemplary crystalline forms are anatase, rutile, brooklite, or a combination comprising at least one of the foregoing forms. After sintering the columnar structures can still comprises a minor amorphous phase.
In one embodiment, a photovoltaic cell comprising electrodes manufactured by the method disclosed herein can generate an electrical density of greater than or equal to about 5 milliamperes/cm2. The surface area in square centimeters refers to the flat surface of the cell. In another embodiment, a photovoltaic cell comprising electrodes manufactured by the method disclosed herein can generate an electrical density of greater than or equal to about 10 milliamperes/cm2. In yet another embodiment, a photovoltaic cell comprising electrodes manufactured by the method disclosed herein can generate an electrical density of greater than or equal to about 20 milliamperes/cm2. The columnar structures can be tailored to produce current densities of greater than or equal to about 50 milliamperes/cm2.
Photovoltaic cells comprising electrodes manufactured by the method disclosed herein can have an efficiency of greater than or equal to about 5%. In another embodiment, photovoltaic cells comprising electrodes manufactured by the method disclosed herein can have an efficiency of greater than or equal to about 10%. In yet another embodiment, photovoltaic cells comprising electrodes manufactured by the method disclosed herein can have an efficiency of greater than or equal to about 15%.
The following examples, which are meant to be exemplary, not limiting, illustrate compositions and methods of manufacturing of some of the various embodiments of the electrodes for photovoltaic cell described herein.
Examples 1-3 below were undertaken to demonstrate the formation of columnar structures of titanium dioxide on a glass substrate when the pressure in the reaction chamber of an expanded thermal plasma is varied from about 45 to about 100 millitorr (mT). The titanium dioxide precursors that were used were titanium chloride or titanium isopropoxide. Argon was fed into an expanding thermal plasma generator. Oxygen along with the precursors were fed into the reaction chamber at about 3 centimeters from the anode. The temperature of the substrate during deposition can be varied from about 100 to about 150° C. In these examples, the substrate was maintained at a temperature of about 150° C. The pressure in the reaction chamber was maintained either at 45 mT or at 100 mT. The as-deposited materials were amorphous in nature and upon further sintering at a temperature of 450° C., they were converted into the desired materials with the required stoichiometry thereby forming crystalline columnar structures comprising anatase. The time for the sintering was 30 minutes. The columnar structures obtained upon such annealing are generally completely crystalline. In some instances, the columnar structures have a minor portion of an amorphous phase.
The substrates were subjected to varied reaction conditions to form the titanium dioxide layers, which are described in each of the examples below. The substrates together with the columnar structures were tested in photovoltaic cells and the efficiency was noted. The cell was manufactured as follows.
The glass substrates with the columnar structures disposed thereon were sintered. The glass plates were dyed with Ru dye (N3) at 40° C. for 1 hour. The cell was constructed using a 40 micrometer PRIMACOR® gasket. Acetonitrile was used as electrolyte in the cell. Platinum coated glass was used as counter electrode and the cells were constructed by sandwiching the 2 plates under pressure. The pressure was 2.8 kg/cm2 and the temperature was 90° C. The cells were tested in solar stimulator under standard testing conditions as specified by NREL certified Silicon standards.
In this example, titanium dioxide columnar structures were deposited on a glass substrate. The conditions are shown in the Table 1 below. The sample was translating.
The as-deposited coating was amorphous in nature and upon further sintering at a temperature of 450° C. for 30 minutes was converted into crystalline columnar structures comprising anatase. The coating thickness after sintering was about 8.1 to about 8.6 micrometers. The glass plate thus coated was utilized in a photovoltaic cell and the efficiency of the cell was determined to be 6%. The photovoltaic cell was manufactured as described above.
In this example, titanium dioxide was deposited on a glass substrate. The glass substrate was subjected to a stationary deposition for 4 minutes under the conditions shown in the Table 2.
As in example 1 above, the as-deposited coating was amorphous in nature and upon further sintering at a temperature of 450° C., was converted into crystalline columnar structures comprising anatase. The average coating thickness after sintering was 12.0 micrometers. The glass substrate thus coated was used in a photovoltaic cell. The photovoltaic cell was manufactured as described above. The efficiency of the cell was 7.2%.
In this example, titanium dioxide was deposited on a glass substrate. The glass substrate was first preheated by parking the substrate in front of the arc and then subjected to a stationary deposition for 4 minutes under the conditions shown in the Table 3.
As in example 1 and 2 above, the as-deposited materials was amorphous in nature and upon further sintering at a temperature of 450° C. was converted into crystalline columnar structures comprising anatase TiO2. The coating thickness after sintering was about 6.9 to about 7.2 micrometers. The glass substrate thus coated was used in a photovoltaic cell. The photovoltaic cell was manufactured as described above. The efficiency of the cell was less than 2%.
In this example, titanium dioxide was deposited on a glass substrate. This example was similar to Example 3, except that the glass substrate was not preheated. The glass plate was subjected to a stationary deposition for 4 minutes under the conditions shown in the Table 4.
As in example 1 and 2 above, the as-deposited materials was amorphous in nature and upon further sintering at a temperature of 450° C. was converted into crystalline columnar structures comprising anatase TiO2. The coating thickness after sintering was about 6.15 to about 6.4 micrometers. The glass substrate thus coated was used in a photovoltaic cell. The photovoltaic cell was manufactured as described above. The efficiency of the cell was 4.7%.
This example demonstrates the formation of zinc oxide (ZnO) structures on a glass substrate. The reactive precursor was dimethyl zinc. The dimethyl zinc was introduced into the reaction chamber in a 1.0M solution in heptane. ZnO deposited for 10 minutes in reaction chamber. The reaction conditions are shown in the Table 5.
The average coating thickness was around 12.6 micrometers before sintering. This sample was sintered and a photovoltaic cell was constructed as detailed above. The measured efficiency of the photovoltaic was 0.3%.
This example demonstrates the deposition of a titanium dioxide coating on a titanium foil substrate. The titanium foil was subjected to 25 passes through the reaction chamber with a pause of about 1 minute between each pass. The speed of the titanium foil during each pass through the chamber was 2.5 centimeter per second. The reaction conditions are shown below in the Table 6.
The coating thickness was about 8 to about 10 micrometers after sintering.
This example demonstrates the difference in photovoltaic cell characteristics when sintered electrodes and non-sintered electrodes are used in photovoltaic cells. The sintered and non-sintered electrodes both contain an electron transport coating comprising titanium dioxide columnar structures. The titanium dioxide electron transport coating had an average thickness of about 10 micrometers after sintering. The substrate comprised glass.
Two photovoltaic cells, one having a sintered electron transport coating and one containing a coating that was not subjected to sintering were tested in a photovoltaic cell. The sintering was conducted at 450° C. as described above and resulted in a conversion of amorphous titanium dioxide to crystalline (anatase) titanium dioxide. The deposition of the electron transport coating for the sintered cell has already been described in Example 2 above. The cells were manufactured as described above. The results for the cells are shown in the Table 7 below.
Fill Factor = Max output power/(Short Circuit Current × Open circuit voltage)
From the Table 7, it may be seen that the sintered cell, which has the crystalline electron transport coating has a much higher efficiency of 7.2% than the non sintered cell, which has an efficiency of only 0.4%. It can also be observed that the current density is 21 milliamperes per square centimeter (mA/cm2) for the sintered cell.
From the above examples, it can be seen that the expanding thermal plasma process can be advantageously used to produce electrodes have columnar structures that facilitate increased efficiencies in photovoltaic cells. The columnar structures upon crystallization reduce electron recombination in a photovoltaic cell. The electron transport coatings can also be advantageously deposited at low temperatures.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
