CONDUCTOR LAYER

FIELD OF THE INVENTION

The present invention relates to an improved thin-film solar cell and a method for fabricating a thin-film solar cell having nanostructures incorporated onto an absorber layer and a conductor layer.

BACKGROUND OF THE INVENTION

Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell- quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.

A critical element of most thin film solar cells is the transparent electrode (TEL) layer. This layer must transmit light while simultaneously providing a low resistance path for electrical current to flow out of the solar cell. To achieve this effect, the current material used is indium-tin-oxide (ITO). Indium-tin-oxide (ITO) has many shortcomings, as it is scarce, relatively brittle, having delamination problem and moreover it is costly to be used in practice. It has been reported [1-7] that a random carbon nanotube network film can be explored for application where a low sheet resistance and high optical transparency in the visible and infrared spectral range are essential. These films are made by solution processing and highly flexible. Recent works demonstrated that both Multiwall wall nanotube (MWNT) films and Single wall nano tube (SWNT) films can be used as transparent anodes for solar cells. Rowell et al. [5] have developed a transfer-printing method for producing SWNT films on flexible substrates and demonstrated that these can be used as transparent electrodes for polymer-fullerene bulk hetero junction solar cells.

Emitter contacts for solar cells are usually realized by using conductive transparent metal oxides, such as: Sn02, ITO, Zn ₂ 0 ₄ , CdSn0 ₄ , ln ₂ 0 ₃ , ZnO:AI, as well as CdO, ZnO and RuSi0 ₄ . In order to integrate solar cells into PV modules or for more convenient measurements execution, additional metal contacts attached to transparent conducting oxide (TCO) are applied. The most popular among listed TCO compounds is indium tin oxide (ITO). ITO is a mixture of tin (IV) oxide: Sn0 ₂ and indium (III) oxide (ln ₂ 0 ₃ ) so called ITO.

Besides, another important factor which contributes for the low efficiency on the solar cells is a recombination of the electron-hole in the absorber layer. Recombination of the electron and hole on the absorber layer of solar cell limits the efficiency of the entire cell. Placing nanostructured materials on the absorber layer may reduce the recombination process. Different lengths of the nanostructured material may collect the charge from a wide area of the absorber layer. Another critical element of most thin film solar cells is the transparent electrode layer. This layer must transmit light while simultaneously providing a low resistance path for electrical current to flow out of the solar cell. This transparent conducting layer has limitation of flexibility, availability and high cost manufacturability.

Therefore, there is a need for an improved thin-film solar cell and a method for fabricating a thin-film solar cell having nanostructures incorporated onto an absorber layer and a conductor layer. The present invention further introduces carbon nanotubes (CNT) based solar cell where CNT used as charge transporters for efficiently removing charge carriers from the absorber layer to reduce the rate of electron-hole recombination in the absorber layer. The present invention further provides a considerable reduction of materials with even greater efficiency and economically during operation.

SUMMARY OF THE INVENTION

The present invention provides a method for fabricating a thin-film solar cell having nanostructures incorporated onto an absorber layer and a conductor layer comprising providing a seed layer on top of a back contact layer on a substrate, annealing the seed layer into a plurality of seeds, growing a plurality of nanostructures that will follow the seeds, depositing a photo reactive material to form an absorber layer, providing a window layer on top of the absorber layer, providing a laser printing to form a plurality of connectors on top of the window layer, growing a plurality of nanostructures in between the connectors, encapsulating connectors and nanostructures to form a conductor layer; and providing an encapsulation of the thin-film solar cell.

In one of the embodiment of the present invention, nanostructures grown will follow the seeds are carbon nanotubes via plasma enhanced chemical vapor deposition process. Nanostructures grown in between the connectors are connected are carbon nanowires.

In yet another embodiment of the present invention, the connectors are co-fired from a laser to form grid line and get distinct seeds on the edge of the grid line.

In another of the embodiment of the present invention, nanostructures grown in between the connectors are connected to collect the electron from the surface of the window layer and to allow the electron to bring out from the solar cell. The conductor layer is a transparent conductive layer.

A thin-film solar cell comprising a substrate, a back contact layer, an absorber layer formed on the back contact layer having a plurality of carbon nanotubes grown which will follow a seed layer, a photo reactive material deposited on the nanostructures, a window layer formed on the top of the photo reactive material of the absorber layer and a conductor layer encapsulated formed on the window layer having a plurality of connectors, a plurality of carbon nanowires grown in between the connectors wherein the absorber layer is in electrical communication with the back contact layer and the conductor layer. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

Figure 1 illustrates a schematic diagram of formation of an absorber layer of the thin-film layer solar cell in accordance of the present invention.

Figure 2 illustrates a schematic diagram of formation of a conductor layer of the thin-film layer solar cell in accordance of the present invention

Figure 3 illustrates an encapsulated thin-film layer solar cell in accordance of the present invention.

DETAILED DESCRIPTIONS OF THE INVENTION

The present invention will now be described in detail in connection with specific embodiments with reference to the accompanying drawings.

Generally, solar cell and method disclosed herein provide improved solar cell for converting light, including in particular sunlight, to electrical energy. This solar cell has a wide range of applications, including in both terrestrial and extra-terrestrial settings, and can be incorporated into panels, arrays, flexible films, sheets or other products.

The term nanostructure is used herein to refer to a material structure having a size in at least one dimension (e.g. a diameter of a tube, or a length, width or thickness of another structure) that is less than about 1 micron, in other embodiments less than about 500 nm, about 100 nm, about 20 nm or about 1 nm.

The present invention relates to an improved thin-film solar cell and a method for fabricating a thin-film solar cell having nanostructures incorporated onto an absorber layer and a conductor layer. The nanowires are in corporate on the absorber layer and the conductor layer of a thin-film solar cell. To fabricate the thin-film solar cell of the present invention, a substrate either a polymer, silicon wafer with various thickness or different types of glass is used. This substrate undergoes a cleaning process as required in semiconductor manufacturing. Generally, the substrate on which used in the present invention is electrically conductive or semi- conductive.

Then, a formation of a back contact layer also known as back electrode on the substrate (1A) is provided as illustrated in Figure 1. This formation is achieved by a sputtering process. Molybdenum is preferably used for back contact formation in the present invention. A person skilled in the art is able to use other equivalent processes and other materials such as aluminum, silver, gold, platinum, nickel or any equivalent materials.

Subsequently, a seed layer is formed on top of the back electrode as in step 1B. The seed layer of the present invention is preferably a thin layer of gold. The formation provided via sputtering, evaporation, spin coating or any similar techniques. A seed layer is then masking in a predetermined and specified pattern as in step 1C. Such process is to provide different thickness of the seed layer to grow the nanostructures in the following steps.

Upon masking, the seed layer is etched as in step 1D to reduce the thickness to a value lower than the original seed layer. This is to reduce the thickness of seed layer without presence of mask. After the etching process, the mask is removed as in step 1E. This leaves behind the seed layer with at least two different thicknesses of the top layer. In step 1E, the small portions (boxes) on top of the seed layer are also a part of seed layer but with different thickness. The thickness of these portions is proportional with the seed that will form later after annealing the sample as illustrated in Step 1F.

After removing the mask, an annealing process is performed in step 1F. This process is provided to break down the seed layer into a plurality of different seeds. A person skilled in the art needs to note that the thick seed layer forms into seeds with higher radius while the thin seed layer produces seeds with low radius. Next, nanostructures preferably carbon nanotubes or carbon nanowires are grown by following the plurality of seeds as in step 1G. It will up the seed and then formed beneath the seeds. The process of growing the nanostructures is performed using plasma enhanced chemical vapor deposition process. Finally, photo reactive materials are deposited to form an absorber layer of the solar cell as illustrated in 1H. After depositing the absorber layer, a window layer is formed on top of the photo reactive materials as in 2A. The thickness of the absorber layer and window layer can vary widely and be designed to achieve suitable absorption of incident radiation. Then, a thin layer and thin width of connector materials are printed on top of the window layer as illustrated in step 2B. The connector is co-fired from a laser printing by leaving certain patterns on the surface. This is to achieve a closely connected grid line or bus bar and to get distinct seeds on the edge of the grid line. A top view of the printed and laser fired connector and seeds on the top surface are further illustrated in Figure 2.

Nanostructures preferably carbon nanotubes or carbon nanowires are grown and connected in between the connectors. In this step 2C, horizontal and random nanotubes are grown. In one embodiment, the nanotubes can be substantially vertically oriented.

These nanotubes are highly conducted and capable to collect the electron from the surface. The nanostructures are grown in between the connectors are connected to collect the electron from the surface of the window layer and to allow the electron to bring out from the solar cell. Subsequently, connectors and nanostructures are encapsulated to form a conductor layer as in step 2D. The conductor layer is a transparent conductive layer. The conductive layer is a transparent conductive oxide (TCO) such as indium tin oxide (ITO), fluorinated indium tin oxide, zinc oxide (ZnO) or aluminium doped zinc oxide or a related material. Finally, an encapsulation is provided to whole cell to form a thin-film solar cell as depicted in Figure 3.

The desired size of the nanostructures for the method of the present invention ranges between about 1nm and 100μηι in at least one dimension, preferably between 1 nm and about 500 nm in at least one dimension.

One of the advantages of the present invention provides an absorber layer of a thin-film solar cell with nanowires having different length for collecting the split charges from the entire portion of the photoactive absorber layer. The electrode structure on the top surface of a thin-film solar cell collects and removes the electrons from the cell without blocking the incident of lights. Another advantage of the present invention is that a reduction of the recombination on the absorber layer is provided. Having reduction of the recombination on the absorber layer, the numbers of free electrons to flow through are reduced and hence this reduces the short circuit current flow and in which reduce the efficiency of the cell as a whole. In addition, a transparent conductive layer provides an assistance to flow out electrons from the solar cell with a minimal loss. The nanostructures also can provide light trapping structures and thereby can increase the opportunity for light to be absorbed in the absorber layer.

The foregoing embodiment and advantages are merely exemplary and are not to be construed as limiting the present invention. The description of the embodiments of the present invention is intended to be illustrative and not to limit the scope of the claims and many alternatives, modifications and variations will be apparent to those skilled in the art.

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