The present invention relates to structures having a transparent conducting overlayer and, more particularly, relates to the formation of structures having a transparent conducting overlayer and metallic layer for solar cell applications and display applications.
Solar cells may be manufactured using a p-i-n type of semiconductor junction. The semiconductor material making up the p-i-n semiconductor junction may be amorphous silicon to reduce cost. An example of one such solar cell 10 is shown in
The TCO 24 may be a relatively thick film (50-100 nm) of indium tin oxide (ITO) or aluminum-doped zinc oxide (ZnO), the latter with the advantage of avoiding the rare element indium. However, these oxide film transparent conducting overlayers require expensive deposition techniques and are brittle, so they cannot be used in flexible solar cells such as may be the optimal type for low-cost mass production.
The various advantages and purposes of the exemplary embodiments as described above and hereafter are achieved by providing, according to a first aspect of the exemplary embodiments, a method including forming a bottom metallic electrode; forming a semiconductor junction on the metallic electrode; forming a transparent conducting overlayer in contact with the semiconductor junction; and forming a metallic layer in contact with the transparent conducting overlayer, wherein the metallic layer is formed by a plating process.
According to a second aspect of the exemplary embodiments, there is provided a method including forming a bottom metallic electrode; forming a semiconductor junction on the metallic electrode, the semiconductor junction being in direct contact with the bottom metallic electrode; forming a transparent conducting overlayer over and in direct contact with the semiconductor junction; and forming a metallic layer over and in direct contact with the transparent conducting overlayer, wherein the metallic layer is formed by a plating process.
According to a third aspect of the exemplary embodiments, there is provided a method including forming a bottom metallic electrode; forming a semiconductor junction on the metallic electrode, the semiconductor junction being in direct contact with the bottom metallic electrode; forming a metallic layer over and in direct contact with the semiconductor junction, wherein the metallic layer is formed by a plating process; and forming a transparent conducting overlayer over and in direct contact with the metallic layer.
The features of the exemplary embodiments believed to be novel and the elements characteristic of the exemplary embodiments are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The exemplary embodiments, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:
Carbon nanotubes have been proposed as an alternative material for the transparent conducting overlayers in solar cells. The carbon nanotubes (CNT) may be in the form of a mat of CNT. While very high transparencies are achievable with CNT, the electrical resistivity of 80% transparent CNT mat films is still unacceptably high, in the 60-250 Ohms/square range, for them to be used as one-for-one replacements for Indium Tin Oxide (ITO) or Al-doped zinc oxide (ZnO) TCO films.
A proposed solution is to form a metallic layer over the CNT mat layer. In a preferred embodiment, the metallic layer is a pattern of busbars and fingers in a highly-conducting metal such as copper over the CNT mat layer. The resulting composite two-component overlayer microstructure forms a transparent conducting overlayer with both a high optical transparency and a low electrical resistance which can functionally replace transparent conducting overlayer oxide layers.
Graphene, an allotrope of carbon, may also be used in place of the CNT mat layer. Graphene typically refers to a single planar sheet of covalently bonded carbon atoms. In essence, graphene is an isolated atomic plane of graphite. Graphene is believed to be formed of a plane of carbon atoms that are sp2-bonded carbon to form a regular hexagonal lattice with an aromatic structure. The thickness of graphene is one atomic layer of carbon. That is, graphene does not form a three-dimensional crystal. However, multiple sheets of graphene may be stacked. A typical graphene “layer” may include a single sheet or multiple sheets of graphene.
In a most preferred embodiment, the metallic layer is formed by a plating process such as by electroplating or electroless plating.
Referring now to
The TCO 212 may include a CNT mat or graphene. CNT mats may be formed by the following process steps. In one process, CNT mats may be formed by the vacuum filtration technique. A purified, diluted nanotube solution of CNTs dispersed in water and a sufactant is filtered through cellulose ester membranes (MF-Millipore Membrane, mixed cellulose esters, Hydrophilic, 0.1 μm, 25 mm) to form a uniform nanotube film. The process involves the pretreatment of membranes with a small amount of DI water and subsequent filtration of different volumes of nanotube solution to achieve varying thickness of nanotube film. The filtration speed is kept as low as possible to achieve films with high degree of uniformity. Filtered nanotube film material on membrane is then allowed to set for 15-20 minutes. 50 mL of water is slowly passed through the film to wash off the surfactant. Post wash, the film is left to dry in air for 15 minutes. The target glass slide on which the nanotube film is to be transferred is made wet with single drop of DI water. The nanotube film side of the membrane is made in contact with the wet glass and a small pressure is applied to make them stick together. The membrane is then slowly dissolved in acetone, leaving nanotube film on the glass slide. The nanotube film is kept in acetone for 30 minutes for complete removal of residual filter paper. The film is transferred to the semiconductor by scooping the film suspended in solution using the semiconductor. CNT films may also be produced by spin-coating, dip-coating, doctor blading or spray coating.
Graphene for the TCO may be made by the following process steps. Graphene transparent electrodes may be made from the growth of graphene by chemical vapor deposition (CVD) on copper foils. A carbon source (ethylene, ethanol etc.) is flowed into a furnace with copper foil heated to greater than 800° C. A single layer of graphene is grown on the copper foil. The graphene is then transferred to the desired substrate via delamination or roll-to-roll transfer. Graphene may also be formed by thermal decomposition, exfoliation from graphite or assembly of graphene films from solution. It should be understood that forming the graphene on structure 200 includes forming the graphene directly on structure 200 as well as forming the graphene separately and then transferring the graphene to structure 200.
The metal busbar architecture can be formed on graphene regardless of the graphene's origin, whether it be CVD, thermal decomposition, exfoliation from graphite or assembly of graphene films from solution.
The foregoing description of the CNT mats and graphene are for purposes of illustration and not limitation. The CNT mats and graphene may be formed on the structure 200 by any other method known now or in the future.
A preferred embodiment is for metallic layer 214 to be in the form of busbars and fingers as shown in
w=60 micrometers
x=0.15 centimeters
L=3 centimeters
I=0.12 centimeters.
It should be understood that the above dimensions are for purposes of illustration only and not limitation.
The metallic layer 214 should be formed by a plating process including electroplating or electroless plating. The use of a plating process is advantageous in that it is a low cost, solution based process and can be performed at room temperature. In the case of electroless plating, a plating seed (typically palladium-based seeds) are patterned on the carbon films surface. The patterning can be done via stamping of an organic layer that is charged to bind to the seeds or by directly stamping the seeds themselves. Once the palladium particles are patterned on the carbon film, the film is dipped in an electroless metal (for example, copper) bath where the patterned areas are metalized to form metallic layer 214. Electroplating may be performed by patterning the surface of the carbon film with a resist layer that only leaves areas of the carbon film exposed where metal is desired. The sample can then be placed in an electroplating bath where a bias is applied to the carbon film, resulting in metallization in the exposed areas of the film to form metallic layer 214.
Referring now to
It is preferred that the metallic layer 314 be in the form of busbars and fingers as shown in
The metallic layer 314 should be formed from a plating process including electroplating and electroless plating as described previously.
The structure 300 has several advantages over the structure 200. The metallic layer 314 will be more stable due to the good contact with the p-i-n semiconductor junction 302. The porosity of the TCO 312 would not be an issue since the TCO is on top and the metallic layer 314 will not be penetrating through the TCO 312.
While the exemplary embodiments shown in
Referring to
The exemplary embodiment of
Referring to
The exemplary embodiment of
The processes for forming the embodiments of the invention are described with respect to
Thereafter, a semiconductor junction (for example, 202 in
At this point in the process flow, the process could diverge and take a left branch or right branch. The left branch describes the process flow for the structure 200 shown in
With respect to the left branch 506 of the process flow, the transparent conducting overlayer (for example, 212 in
Thereafter, the metallic layer (for example, 214 in
With respect to the right branch 512 of the process flow, the metallic layer (for example, 314 in
Thereafter, the transparent conducting overlayer (for example, 312 in
While the exemplary embodiments have been primarily directed to solar cell applications, the exemplary embodiments may also have applicability to display applications, organic photovoltaic cells and inorganic thin-film cells such as copper indium gallium selenide (GIGS); copper, zinc tin sulfide (CZTS); cadmium telluride, etc.
It will be apparent to those skilled in the art having regard to this disclosure that other modifications of the exemplary embodiments beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.