ELECTRODES FOR PHOTOVOLTAIC CELLS AND METHODS FOR MANUFACTURE THEREOF
BACKGROUND
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 m a photovoltaic cell
Photovoltaic systems convert light into electncity for a vanety of applications Photovoltaic systems are commonly known as "solar cells", so named for their ability to produce electncity from sunlight Power production by photovoltaic systems may offer a number ot advantages over other systems of generating electncity These advantages are low operating costs, high reliability, modulanty, low construction costs, as well as environmental benefits
Solar cells convert light mto electncity by exploiting the photovoltaic effect that exists at semiconductor junctions Accordmgly, solar cells generally compnse semiconductor layers to produce an electron current The semiconductor layers absorb incoming light to produce excited electrons In addition to the semiconductor lavers, i.olai cells generally include a transparent cover or other encapsulant, an anti-icilective layer, a tront 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
111 recent years progress has been made on the development of orgamc and morganic-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 mcludes 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 tniodide/iodide
Dye-sensitized films, however, suffer from several technical drawbacks One technical drawback is that a large transport distance results m 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 m the photovoltaic cell The reduced species are generally present in an electrolyte that m 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
Dunng 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 b> 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 teclinical drawback of the current dye-sensitized solar cell is that the poor electron conduction of the T1O2 film consisting of randomly interconnected nano-
particles T1O2 films are generally used as electron transporters m solar cells Further, m solar cells (photovoltaic cells) it is difficult to maximize the interfacial area of the T1O2 electron transporter for optimal loading of the dye Yet another drawback stems from the randomness of T1O2 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 fi-om entenng 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 poie 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 diffiise to the substrate while at the same tune reducmg 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
SUMMARY
Disclosed herein is an article compnsmg a substrate, and a columnar structure, wherein the columnar structure compnses 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 compnses a substrate upon which is disposed an electron transport coating, wherein the electron transport coating compnses a columnar structure
Disclosed herein is a method compnsmg contacting a substrate with a plasma in an expanding thermal plasma generator, wherein the chamber pressure is about 30 milhtorr to about 300 milhtorr, and wherein the plasma comprises a reactive species
and oxygen, and disposing a semiconducting coatmg up on the substrate, wherem the semiconducting coatmg compnses a columnar microstructure
DETAILED DESCRIPTION OF FIGURES
Figure 1 illustrates one exemplary embodiment of a cross-section of an electrode 10 incorporating an electron transport coatmg on a substrate 12,
Figure 2 is a scamung electron micrograph showmg a titamum dioxide coating disposed upon a substrate 12,
Figure 3 is a schematic depiction a columnar structure disposed upon a substrate 12,
Figure 4 illustrates an example of a system 100 for forming the electron transport coating 14, and
Figure 5 illustrates one exemplary embodiment of a cross-section of a photovoltaic cell compnsing an electrode 10 that compnses an electron transport coatmg disposed upon a substrate 12
DETAILED DESCRIPTION OF EMBODIMENTS
Disclosed herein is an article compnsing a coatmg that has a colonmar structure The coating can advantageously be converted mto an electron transport coatmg for use m a photovoltaic cell The coating can also be used as an electrode m a photovoltaic cell Such an electrode generally compnses 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 b& detailed later, the longitudmal axis is parallel to the z-direction which is perpendicular to the surface of the substrate This configuration ad\ antageously 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 miUitorr (mT)
Figure 1 illustrates one exemplary embodiment of a cross-section of an electrode 10 incorporating an electron transport coating on a substrate 12 The electrode 10 may optionally include an mterlayer 16 disposed between the substrate 12 and the electron transport coating 14 depending upon the application The substrate 12 can be optionally pretreated to create an optional hydrophobic mterlayer 16 on the surface In one embodiment, the mterlayer 16 can be an electncally conductive layer Surface properties of the substrate 12 can be enhanced by use of adhesion promoters such as silicon or titanium alkoxides, which help to improve adhesion as well as cohesion between binding surfaces The mterlayer 16 may thus function as an adhesion layer between the substrate 12 and the electron transport coating 14 to promote adhesion between these layers, or may function to reduce stress between the substrate 12 and overlying layers, including the electron transport coating 14 The mterlayer 16 may optionally comprise sublayers where one sublayer functions to reduce stress between the substrate 12 and the electron transport coating 14, and the other sublayer functions to promote adhesion between the substrate 12 and the electron transport coating 14 Alternatively, the mterlayer 16 may provide both the functions of an adhesion layer and to reduce stress
The electron transport coating can compnse a metal oxide, a semiconductor, carbon nanotubes or the like Exemplary electron transport coatings compnse semiconductors Suitable examples of semi-conductors are metal oxides Examples of suitable metal oxides are silicon oxides (SiOx), silicon dioxides (S1O2), titanium oxide (TiOx), titanium dioxide (T1O2), zirconia (Zr2O3), alumina (AI2O3), 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 stoichiometnc The metal oxide compositions can be non-stoichiometnc as well In another embodiment, some of the oxygen in the metal oxide can be replaced by another amon such as nitrogen As a result, the columnar structures can also compnse oxymtndes An example of an oxynitnde is a metal oxynitnde Examples of oxymtndes are silicon oxynitnde, titanium oxynitnde, or the like, or a combination compnsing at least one of the foregoing oxymtndes
In one embodiment, when titanium oxide coatmgs are employed, a TiOx coating may compnse a stoichiometric T1O2 coatmg, a non-stoichiometnc TiOx coating where x is not equal to 2, or a combination thereof In another embodiment, a suitable electron transport coatmg 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 arsemc, 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 compnses columnar structures is one that compnses titanium dioxide (T1O2) Figure 2 shows an electron micrograph of columnar structures of titanium dioxide as seen m a scanning electron microscope As may be seen in the micrograph the titanium dioxide columnar structures are disposed substantially perpendicular to the surface of the substrate The structures shown m the Figure 2 are ordered
Figure 3 IS a schematic showing the columnar structures disposed upon the substrate The cross-sectional area of the column can have any geometry such as cylindrical, rectangular, square, or polygonal In the Figure 3, the exemplary schematic has a rectangular cross-section
In the Figure 3, the x and y axis he parallel to the surface of the substrate that contacts the columnar structure The columnar structure thus extends upwards from the surface of the substrate along the z axis It is generally desirable for the smallest dimension in the x-y plane to be greater than or equal to about 5 nm When the columnar structure is square or rectangular, it is generally desirable for the width of the columnar structure (as measured in the x-direction) to be greater than or equal to about 25 nm In one embodiment, it is desirable for the width of the columnar structure to be greater than or equal to about 50 nm It is generally desirable for the breadth of the columnar structure (as measured m the y-direction) to be greater than or equal to about 5 nm In one embodiment, it is generally desirable for the breadth of the columnar structure to be greater than or equal to about 25 nm In another
embodiment, it is generally desirable for the breadth of the columnar structure to be greater than or equal to about 50 nm It is generally desirable for the length of the columnar structure (as measured in the z-direction) to be greater than or equal to about 5 nanometers In one embodiment, it is desirable for the length of the columnar structures to be gieater than or equal to about 15 nanometers In another embodiment. It is desirable for the length of the columnar structure to be greater than or equal to about 50 nanometers In yet another embodiment it is desirable for the length of the columnar structure to be greater than or equal to about 100 nanometers
Individual columnar structures can contact each other at any point along their lengths or can be isolated from other columnar structures In one embodiment, \^'hen 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 bet\\ een 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 vanety 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 m the Figure 3, a first section 9 (measured along the z direction) of the columnar structure having a thickness of up to about 0 2 micrometer can be
made nanoporous A second section 11 disposed upon this first vertical section can have a porous structure (with pore sizes greater than 10 nm) for a thickness of up to about 0 2 micrometer In this way, the entire length of the columnar structure can have alternating nanoporous and porous structures In one embodiment (not shown), successive alternating columns can be made nanoporous or porous if desired In another embodiment, several columns in a section of the substrate can be made nanoporous while several columns m an adjacent section of the substrate can be made porous with pore sizes greater than 10 nm if desired
The titanium dioxide comprises a crystalline anatase phase, a brookite phase, a rutile phase, or a combination compnsmg 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 (m^/gm) In one embodiment, the surface area of the columnar structure is greater than or equal to about 100 m^/gm In another embodiment, the surface area of the columnar structure is greater than or equal to about 200 m /gm In yet another embodiment, the surface area of the columnar structure is greater than or equal to about 500 m^/gm In yet another embodiment, the surface area of the columnar structure is greater than or equal to about 1,000 m^/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 compnse any matenal that is flexible or ngid 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 electncally conductive if desired In another embodiment, the substrate can be electncally insulating but can be coated with an mterlayer that is electncally conducting
When the substrate is electncally conductive, it is desirable for the substrate to have a bulk volume resistivity of less than or equal to about 10 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 hav e 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 matenals for substrates are glass, polymers, foils, or the like, or a combination composing at least one of the foregoing matenals Examples of suitable glasses are silica, alumina, zircoma, titania, silicon oxynitnde, titanium oxynitnde, or the like Examples of suitable polymers are thermoplastic pohniers, thermosetting polymers, or a combination composing at least one of the foregoing polymers Examples of suitable thermoplastic polymers are polycarbonates, poly(meth)acrylates, polystyrenes, polyolefins, polyimides, polyethenmides or the like, or a combination compnsing at least one of the foregoing thermoplastic polymers Examples of suitable thermosetting polymers are polyurethanes, polysiloxanes, epoxies, phenolics, or the like or a combination compnsing 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. Sliver, platinum, zmc, molybdenum, tantalum, or the like, or a combination thereof Examples of suitable alloys are steel, stainless steel, mvar, or the like
The electron transport coating 14 is advantageously formed by an expanding thermal plasma process (ETP), as discussed m more detail below Figure 4 illustrates an example of a system 100 for formmg the electron transport coatmg 14 The system 100 compnses a plasma generation chamber 110 and a deposition chamber 111 The deposition chamber 111 contains a substrate 12 mounted on a temperature controlled support 122 The substrate 12 may be a glass substrate, a polymenc substrate or a foil substrate, for example, coated with the optional interlayer 16, shown in the Figure 1 The deposition chamber 111 also contains a door (not shown) for loading and unloading the substrate 12 and an outlet 123 for connecting to a pump The support 122 may be positioned at any position in volume 121 of deposition chamber 111 The substrate 12 may be positioned 10 to 50 cm, for example, and generally about 25 5 cm, from the anode 119 of the plasma generator
The deposition chamber 111 also optionally compnses 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 contam 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 compnse magnets or magnetic field generating coils (not shown) adjacent to chamber walls to direct the flow of the plasma
The deposition chamber HI 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 matenal 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-cylmdncal-conical or comcal-cylindncal 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 mjection nng
The deposition chamber 111 also contains at least one reactant supply Ime 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 evacuatmg 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 compnses three cathodes 113 of which any one can be utilized The cathodes 113 may comprise, for example, thonum 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 forni a plasma in the plasma generation chamber 110, a plasma gas is supplied through plasma gas supply line 117 The plasma gas may suitably compnse 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 gds, then other gasses may be supplied through plural supply lines, if desired Preferably, for the TiOx deposition, the plasma gas compnses argon The plasma gas in plasma generation chamber 110 is maintained at a higher pressure than the pressure
m 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 Imes 112 and 114
In one embodiment, in one method of manufactunng 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 Figure 4 A plasma is generated using a plasma gas supplied by plasma gas line 117 The plasma gas may be, for example, a noble gas In one embodiment, the plasma gas is argon
Suitable precursors for forming a TiOx film on the substrate are titanium tetrachlonde (T1CI4), titanium isopropoxide, titanium butoxide, titanium di-isopropoxide bis (2,4 pentanedionate), titanium (IV) ethoxide, titanium (IV) 2-ethylhexoxide, titamum (IV) isobutoxide, titanium (IV) methoxide, or the like, or a combination compnsmg at least one of the foregoing precursors Suitable oxidants are oxygen, mtrous oxide, hydrogen peroxide, ozone, or water
In one exemplary embodiment, m one method of manufactunng an electron transport coating compnsing TiOx on a substrate, a first reactant compnsmg titamum is reacted with a second reactant compnses oxygen The first reactant compnses titamum tetiachlonde (T1CI4) while the second reactant compnses oxygen The first and second reactants react to form a TiOx film on the substrate The flow rate for the T1CI4 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 dunng the process can be about 35 to about 200 mT In yet another embodiment, the chamber pressure dunng the process can be about 45 to about 100 mT
As noted above, the electrode can be advantageously used m a photovoltaic cell In one exemplary embodiment, depicted in the schematic (not to scale) in Figure 5, a photovoltaic cell 300 composes a first'electrode 10 and a second electrode 310 The second electrode is termed the counter electrode and it may optionally be utilized in a photovoltaic cell application Disposed between the opposing faces of the electrodes are an absorber 318, the electron transport coating 14 and a hole transporter 312 The absorber 318 is in electncal communication with the electron transport coating 14 and the hole transporter 312 respectively The absorber is generally a dye that can absorb electromagnetic radiation and can eject an electron into the electron transport coating 14, while at the same time ejecting a hole into the hole transporter 312
In one embodiment, the two electrodes have at least one electncally conductive surface or mterlayer that is in electncal communication with either the electron transport coating 14 or the hole transporter 312 In one embodiment, the first electrode 10 compnses a first substrate 12 and a first conductive surface (also known as the mterlayer) 16 that communicates electncally with the electron transport coating 14 while the second electrode 310 compnses a second substrate 322 and a second conductive surface (or mterlayer) 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 Figure 5, when light impinges upon the absorber 318, it absorbs the incident radiation and ejects an electron As shown in the Figure 5, the electron is ejected into the electron transport coating 14 and travels to the first electrode 10 A hole is simultaneously ejected mto the hole transporter 312 and travels to the second electrode 310 The electron then travels through an external electncal circuit 24 and recombines with the hole to produce
electncity The external electncal circuit 24 as referenced herein pertains to elements that are in electrical communication with the electrodes and not m coramumcation with the internal components of the photovoltaic cells such as the absorbmg molecule, the electron and hole transporter, the insulating molecules, charge separators or ionic dopants
In one embodiment, the photovoltaic cell 300 of Figure 5 can have the first electrode 10 compnsmg a transparent and/or flexible substrate 12 while the electron transport coating 14 compnses a columnar structure composing titanium dioxide and the second electrode 310 composing a flexible and/or transparent substrate 322 which may be the same or different from the substrate used in the first electrode In one embodiment, the second electrode 310 compnses an electncally conductive surface 324 In another embodiment, the second electrode 310 compnses an electron transport coating that compnses columnar structures that are aligned perpendicular to the surface of the substrate of the second electrode The columnar structures used m the second electrode 310 can compnse a metal oxide or conductive carbon nanotubes
In another embodiment, the photovoltaic cell 300 of Figure 5 can have the first electrode 10 compnsmg a substrate 12 The substrate 12 has disposed upon its surface an electncally conducting mterlayer 16 The electncally conducting mterlayer 16 IS disposed between the substrate 12 and the electron transport coating 14 The electncally conducting mterlayer is therefore m contact with the columnar structure The electncally conducting mterlayer compnses indium tin oxide, F-doped tiansparent oxides, conductive polymers, metallic thm films, metal foils, or a combination compnsmg at least one of the foregoing electncally conductive coatings The electncally conductive interlayer can optionally contact the external circuit 24 The substrate 12 can be transparent, and can be either flexible or ngid
After the deposition of the columnar structure of Ti02 on the substrate 12 or the electncally conducting interlayer 16, it can be subjected to sintenng Smtenng is conducted at a temperature of about 300 to about 500°C for a time penod of 10 minutes to 1 hour Sintenng can be conducted in a vacuum or under a gaseous blanket A suitable gaseous blanket is oxygen The sintenng 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 smtenng the columnar structures can still compnses a minor amorphous phase
In one embodiment, a photovoltaic cell composing electrodes manufactured by the method disclosed herein can generate an electncal 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 compnsmg electrodes manufactured by the method disclosed herein can generate an electncal density of greater than or equal to about 10 milliamperes/cm' In yet another embodiment, a photovoltaic cell compnsmg electrodes manufactured by the method disclosed herein can generate an electncal 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/cm
Photovoltaic cells compnsmg electrodes manufactured by the method disclosed herein can have an efficiency of greater than or equal to about 5% In another embodiment, photovoltaic cells compnsmg 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 compnsmg 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 manufactunng of some of the vanous embodiments of the electrodes for photovoltaic cell descnbed herein
EXAMPLE
Examples 1 - 3 below were undertaken to demonstrate the formation of columnar structures of titanium dioxide on a glass substrate when the pressure m the reaction chamber of an expanded thermal plasma is vaned from about 45 to about 100 millitorr (mT) The titanium dioxide precursors that were used were titanium chlonde 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 dunng deposition can be vaned from about 100 to about 150°C In these examples, the substrate was maintained at a temperature of about 150°C The pressure m the reaction chamber was maintained either at 45 mT or at 100 mT The as-deposited matenals were amorphous in nature and upon further smtenng at a temperature of 450°C, they were converted into the desired matenals with the required stoichiometry thereby forming crystalline columnar structures compnsing anatase The time for the sintenng 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 vaned reaction conditions to form the titanium dioxide layers, which aie descnbed m 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 Acetonitnle was used as electrolyte in the cell Platinum coated glass was used as counter electrode and the cells were constructed by sandw ichmg the 2 plates under pressure The pressure was 2 8 kg/cm^ and the temperature was 90°C The cells were tested m solar stimulator under standard testing conditions as specified by NREL certified Sihcon standards
Example 1
In this example titanium dioxide columnar structures were deposited on a glass substrate The conditions are shown m the Table 1 below The sample was translating

(Table Removed)

The as-deposited coating was amorphous m nature and upon further smtenng at a temperature of 450°C for 30 minutes was converted into crystallme columnar structures compnsmg anatase The coating thickness after smtenng was about 8 1 to about 8 6 micrometers The glass plate thus coated was utilized m a photovoltaic cell and the efficiency of the cell was determined to be 6% The photovoltaic cell was manufactured as descnbed above
Example 2
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 m the (Table Removed)
As m example 1 above, the as-deposited coating was amorphous m nature and upon farther smtenng at a temperature of 450°C, was converted mto crystalline columnar structures compnsmg anatase The average coating thickness after smtenng was 12 0 micrometers The glass substrate thus coated was used in a photovoltaic cell The photovoltaic cell was manufactured as descnbed above The efficiency of the cell was 7 2%
Example 3
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
(Table Removed)
As m example 1 and 2 above, the as-deposited matenals was amorphous in nature and upon further sintenng at a temperature of 450°C was converted mto crystalline columnar structures compnsing anatase T1O2 The coatmg thickness after sintenng 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 descnbed above The efficiency of the cell was less than 2%
Example 4
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 mmutes under the conditions shown in the (Table Removed)
As in example 1 and 2 above, the as-deposited matenals was amorphous in nature and upon further sintering at a temperature of 450°C was converted into crystalline columnar stiaictures comprising anatase T1O2 The coatmg thickness after sintenng 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 descnbed above The efficiency of the cell was 4 7%
Example 5
This example demonstrates the formation of zmc oxide (ZnO) structures on a glass substrate The reactive precursor was dimethyl zinc The dimethyl zmc was introduced into the reaction chamber in a 1 OM solution m heptane ZnO deposited for 10 minutes m reaction chamber The reaction conditions are shown in the Table 5
(Table Removed)
The average coating thickness was around 12 6 micrometers before sintenng This sample was sintered and a photovoltaic cell was constructed as detailed above The measured efficiency of the photovoltaic was 0 3%
Example 6
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 titamum foil dunng each pass through the chamber was 2 5 centimeter per second The reaction conditions are shown below in the Table 6
(Table Removed)
The coating thickness was about 8 to about 10 micrometers after sintenng
Example 7
This example demonstrates the difference in photovoltaic cell charactenstics when sintered electrodes and non-smtered electrodes are used in photovoltaic cells The sintered and non-smtered electrodes both contain an electron transport coating compnsing titanium dioxide columnar structures The titanium dioxide electron transport coating had an average thickness of about 10 micrometers after smtenng The substrate compnsed glass
Two photovoltaic cells, one having a sintered electron transport coating and one containing a coating that was not subjected to sintenng were tested in a photovoltaic cell The sintenng was conducted at 450°C as descnbed 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 descnbed m Example 2 above The cells were manufactured as descnbed above The results for the cells are shown in the Table 7 below
(Table Removed)
Fill Factor = Max output power/(Short Circuit Current x 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/cm ) 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 descnbed with reference to exemplary embodiments, it will be understood by those skilled m the art that vanous changes may be made and equivalents may be substituted for elements thereof without departing fi:om the scope of the invention In addition, many modifications may be made to adapt a particular situation or matenal to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the mvention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the mvention will include all embodiments fallmg within the scope of the appended claims.

CLAIMS:
1. An article comprising:
a substrate (12); and
a columnar structure, wherein the columnar structure comprises a semi-conductor and is disposed upon the substrate (12) in a manner wherein the longitudinal axis of the columnar structures is substantially perpendicular to the substrate (12).
2. The article of Claim 1, wherein the substrate (12) is optically transparent, porous and/or electrically conductive.
3. The article of Claim 1, wherein the substrate (12) is optically transparent, non-porous and/or electrically conductive.
4. The article of Claim 1, wherein the substrate (12) comprises glass, polymers, metal foils or a combination thereof
5. The article of Claim 1, wherein the semi-conductor is a metal oxide, and wherein the metal oxide is non-stoichiometric or stoichiometric.
6. The article of Claim 1, wherein the semi-conductor is titanium oxide, and wherein the titanium oxide is non-stoichiometric or stoichiometric.
7. An electrode (10) comprising:
an electrically conductive substrate (12) upon which is disposed an electron transport coating (14), wherein the electron transport coating comprises a columnar structure.
8. The electrode (10) of Claim 7, wherein the electron transport coating
(14) has a columnar structure that comprises a semi-conductor, a conductor, or a
combination thereof
9. The electrode (10) of Claim 8, wherein the semiconductor comprises a metal oxide and wherein the conductor comprises carbon nanotubes.
10. The electrode (10) of Claim 9, wherein the metal oxide is titanium oxide, and wherein the titanium oxide is non-stoichiometric or stoichiometric.
11. The electrode (10) of Claim 7, wherein the columnar structures are formed in an expanding thermal plasmar.
12. The electrode (10) of Claim 7, wherein the electrically conductive substrate (12) comprises a supporting portion and an electrically conductive portion, wherein the electrically conductive portion is in contact with the columnar structure.
13. The electrode (10) of Claim 7, wherein the electrically conductive portion is a coating that comprises indium tin oxide, F-doped transparent oxides, conductive polymers, metallic thin films, metal foils, or a combination comprising at least one of the foregoing.
14. A photovoltaic cell (300) comprising the electrode (10) of any one of Claims 7 through 13.
15. A fuel cell comprising the electrode (10) of any one of Claims 7 through 13.
16. A method comprising:
contacting a substrate (12) with a plasma in an expanding thermal plasma generator, wherein the generator 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 (12), wherein the semiconducting coating comprises a columnar microstructure.
17. The method of Claim 60, further comprising sintering the substrate
(12) at a temperature of about 300 to about 500°C.