^ A THIN-FILM PHOTOVOLTAIC CELL

Technical Field

[0001] The present disclosure relates to a thin-film photovoltaic cell with an enhanced p-n type semiconductor junction area with back contacts for both ends of the p-n junction. The PV cell of the present disclosure includes an array of nano pillars of a type of conductors (p-type) built on a substrate and covered with an n-type semiconductor material.

Background of the invention

[0002] High solar-energy conversion efficiency, i.e. converting solar energy to electrical power, using photovoltaic cells typically requires absorption of most of the incident light across the solar spectrum. Silicon based photovoltaic cells (PV) have been in existence for a long time and are being used extensively for solar-energy conversion. In addition, thin film PV cells with Cadmium Telluride and Copper-Indium-Gallium Selenide (CIGS) have also been in use to tap solar energy and converting the same into electrical power.

[0003] Typically, thin film PV cells contain five layers. The layers being a substrate, back contact, p-type conductor, n-type conductor and the top contact. Typically, transparent conducting oxides (TCO), like Indium Tin Oxide or Aluminum doped Zinc Oxide are used as top contact material. On top of the TCO there are metallic strips to collect the current carriers. At every interface between two layers, there is an inherent resistance to the flow of current carriers. Also, in such thin film solar cells, since the n-type conductor, TCO are located on top of the p-type absorber, some amount of solar radiation is cut off and the absorber layer gets only fewer photons for energy conversion.

[0004] The known PV cells have two-dimensional p-n type conductor junctions, thus limiting the efficiency of charge separation at the junctions. In addition, the low angle solar radiation is not absorbed by PV cells limiting the number of hours in a day the cell is useful for generating power.

[0005] In addition, in an arrangement of PV cell, where n-type conductor is positioned above p-type conductor, the charge carriers need to travel first along the thickness of the n-type conductor and then along the TCO before getting picked "-' up by the silver strips. The probability of all the charge carriers that are formed at the junction reaching the current collector becomes low.

[0006] Another factor that contribute to loss of energy in such known PV cells, is the longer distance that the charge carriers need traverse before they get picked up by the TCO and then by the metal conductor.

Objects of the present invention

[0007] The primary object of the present invention is to provide a thin-film photovoltaic cell with enhanced p-n type semiconductor junction areas.

[0008] An object of the present invention is to provide a PV cell that uses back contacts for both n- and p-type semiconductors to increase the number of photons that reach the p-type absorber.

[0009] Another object of the present invention is to provide a PV cell with a suitable configuration in which the volume of photon-absorbing particles that are exposed to light is increased by increasing the density of the nano particles.

[0010] Yet another object of the present invention is to provide a PV cell in which no TCO layer is used to filter the passage of photons.

[0011] Still another object of the present invention is to provide a PV cell to harvest the solar energy even at lower angles.

Brief Description of the drawings

[0012] FIG.l is a perspective view of the photovoltaic cell of the present invention.

[0013] FIG.2 is a schematic expression of the various steps involved in the method of preparation of PV Cell of the present invention.

Summary of the present invention

[0014] The present invention provides a thin-film photovoltaic cell with an enhanced p-n type semiconductor junction area with back contacts for both ends of the p-n junction. The PV cell includes an array of nano pillars of a type of conductors (p-type) built on a substrate and covered with an n-type semiconductor material rendering enhanced p-n type semiconductor junction areas. The present w invention also provides a method for the preparation of the high-volume photon-absorbing photovoltaic (PV) cell.

Detailed description of the invention

[0015] Accordingly the present disclosure provides a thin-film PV cell 10, with an enhanced p-n type semiconductor junction area, comprising, a substrate 11, with a surface that is metallized with an appropriate element. The substrate 11 is made of a rigid or flexible insulating material, selected from a glass, a polymer or a combination thereof. The substrate 11 can be made in any suitable shape or configuration and in an exemplary embodiment as shown in FIG. 1; the shape of the substrate 11 is shown as having a rectangular shape. The metallized surface of the substrate 11 is defined by gold, copper, carbon or any other suitable material. The metallized surface is provided with a terminal node, which acts as a first set of back contact electrode 12.

[0016] The metallized surface of the PV cell 10 is coated with a layer 14 an insulator material such as SiCh, and AI2O3, except the terminal node 12. Metallic lines 13 are drawn on the layer 14 along x and y axes of the substrate 11. A metallic line 13a is drawn along the axis x, of the substrate 11 and one end of the metallic line 13a is terminated as a second contact electrode 15. A series of metallic lines 13b are branched from the metallic line 13a and drawn along the axis-y of the substrate 11. The metallic lines 13b are perpendicular to the axis x of the substrate 11.

[0017] The first and second electrode contacts 12 and 15 are arranged on the substrate 11 and form the two contacts for the PV cell 10. In an exemplary embodiment the metallic lines 13a and 13b are drawn with gold and any other suitable conducting materials can be used.

[0018] Trenches 15 are created through the layer 14 along both x and y axes of the substrate 11. The Trenches 15 are made to extend through the layer 14 and expose the metallic portion of the substrate that is connected to the first contact electrode 12. The diameter and height of the trenches 15 can be suitably modified considering the size and the capacity of the PV cell 10. w

[0019] An array of nanostructures such as nanopillars, nanorods, or nanowalls of a type of conductors 16 (p-type) are arranged in the trenches 15. In this disclosure, nanostructures 16 are made of Cadmium telluride (CdTe), Copper-Indium-Gallium Selenide (CIGS) or any other photovoltaic material. It is also within the scope of the invention to use other suitable compounds to build the nanostructures. The nanostructures 16 can be grown either by using sputtering or chemical bath process. The nanostructures 16 can be formed that are square in cross section, cylindrical, or other shapes may be provided. The nanostructures are symmetrical, asymmetrical or combination thereof.

[0020] The substrate 11 and nanostructures 16 are covered with an n-type semiconductor material 17 such as n-type semiconductor is made of cadmium sulfide(CdS) or any other photovoltaic material. The combination of nanostructures and metallic layer of the substrate 11 forms n-p type junction to the conversion of light energy into an electrical energy.

[0021] Accordingly, the present invention provides a thin-film photovoltaic cell with an enhanced p-n type semiconductor junction area, comprising a first set of back contact electrode disposed on a substrate; a second back contact electrode disposed on the first back contact and separated by an insulating member; a photoresist member arranged to substantially cover the second back contact electrode; plurality of trenches arranged to pass through the photoresist layer to expose the first contact electrode; an array of nanostructures disposed in said trenches to form a /?-type semiconductor; a rc-type semiconductor disposed to cover the .p-type semiconductor, functionally connected to the second back contact; and rc-type and j9-type semiconductors forming an enhanced 3-dimensional junction area.

[0022] The present invention also discloses a method as shown in FIG. 2 for preparing high-volume photon-absorbing photovoltaic cell with back contacts for both ends of the p-n junction, the method comprising the steps of, selecting a suitable substrate and depositing a layer of metal, such as gold on the substrate to form a first contact electrode; covering the substrate with an insulator material such as SiCh; depositing metal lines, preferably gold lines, having w width of about ~50 nm, with intervening gaps between them for the second electrode; covering the surface with photoresist layer; drilling trenches through the photoresist layer to expose the metal surface at the base; building p-type semiconductor nanostructures of suitable diameter and height; etching out the photoresist layer to expose the nanostructures and the second set of back contacts; and covering the PV cell with the n-type semiconductor to form a thin-film photovoltaic cell with an enhanced p-n type semiconductor junction area.

[0023] In the method of the present invention, first contact electrode is a metallic surface formed from gold, copper, carbon or combination thereof. The substrate used for the PV cell, is rigid or flexible formed from glass, polymer or a combination thereof. The nanostructures are advantageously nanorods, nanopillars or nanowires made of suitable photovoltaic material.

Advantages:

[0024] In this configuration, the volume of photon-absorbing particles that are exposed to light can be increased by increasing the density of the nano particles and no TCO layer is used which filters the passage of photons.

[0025] The PV cell of the present disclosure uses back contacts for both n- and p-type semiconductors in the solar cell thereby increasing the number of photons that reach the p-type absorber.

[0026] The surface area of the p-n junction is increased in the range of 60 80% thereby increasing harvesting of solar energy.

[0027] The 3-D structure of the PV cell helps in harvesting the solar energy even at lower angles.

[0028] In the PV cell of the present disclosure, the maximum distance the charge carriers need to travel is less than about 150 nm.

We Claim:

1. A thin-film photovoltaic cell with an enhanced p-n type semiconductor junction area, comprising:

(a) a first set of back contact electrode disposed on a substrate;

(b) a second back contact electrode disposed on the first back contact and separated by an insulating member;

(c) a photoresist member arranged to substantially cover the second back contact electrode;

(d) plurality of trenches arranged to pass through the photoresist layer to expose the first contact electrode;

(e) an array of nanostructures disposed in said trenches to form a p-type semiconductor;

(f) a n-type semiconductor disposed to cover the j?-type semiconductor, functionally connected to the second back contact; and

(g) n-type andj?-type semiconductors forming an enhanced 3-dimensional junction area.

2. The photovoltaic cell as claimed in claim 1, wherein the first contact electrode is a metallic surface formed from gold, copper, carbon or combination thereof.

3. The photovoltaic cell as claimed in claim 1, wherein the substrate is rigid or flexible formed from glass, polymer or a combination thereof.

4. The photovoltaic cell as claimed in claim 1, wherein the second back contact is a plurality of metallic bands with intervening gaps.

5. The photovoltaic cell as claimed in claim 1, wherein the nanostructures are nanorods, nanopillars or nano walls.

6. The photovoltaic cell as claimed in claim 1, wherein the nanostructures are symmetrical, asymmetrical or combination thereof.

7. The photovoltaic cell as claimed in claim 1, wherein the p-type semiconductor is made of Cadmium telluride(CdTe), Copper-Indium-Gallium Selenide (CIGS) or any other photovoltaic material.

8. The photovoltaic cell as claimed in claim 1, wherein the /7-type semiconductor is made of cadmium sulfide(CdS) or any other photovoltaic material.

9. The photovoltaic cell as claimed in claim 1, wherein the array of nanostructures is either j^-type semiconductor or n-type semiconductor.
10. A method for the preparation of a thin-film photovoltaic cell with an enhanced p-n type semiconductor junction area, comprising the steps of:

(a) depositing a metallic layer on a substrate to form a first contact electrode and covering the metallic layer with an insulator;

(b) disposing plurality of metallic bands with intervening gaps on the insulator to form a second contact electrode;

(c) disposing a photoresist member on the insulator;

(d) drilling trenches through the photoresist layer and the insulator to expose the first contact electrode;

(e) constructing an array of j?-type semiconductor nanostructures in the trenches;

(f) etching the photoresist member to expose the nanostructures and the second contact electrode; and

(h) covering rc-type semiconductor on the p-type semiconductor to form an enhanced 3-dimensional junction area .

11. The method as claimed in claim 10, wherein the first contact electrode is a metallic surface formed from gold, copper, carbon or combination thereof.

12. The method as claimed in claim 10, wherein the substrate is rigid or flexible formed from glass, polymer or a combination thereof.

13. The method as claimed in claim 10, wherein the nanostructures are nanorods, nanopillars or nanowires.