Title of Invention

AN IMPROVED DOUBLE JUNCTION AMORPHOUS SILICON CELLS AND A METHOD FOR PREPARING THE SAME

Abstract

There is suggested an improved double junction amorphous silicon solar cell comprising different p and ultra thin n-type silicon alloy films, wherein a ultra thin highly conducting n-type micro-crystalline layer is formed on the intrinsic layer of the top cell and a wide gap ultra thin highly conducting n-type micro-crystalline alloy layer is formed on the intrinsic layer of the bottom cell and a method for the preparation of the same.

Full Text

FIELD OF INVENTION The present invention is related to double junction solar photovoltaic cells involving p and n-type layers of amorphous and microcrystaliine hydrogenated silicon and its alloys This invention relates to the use of p-type amorphous and ultra thin n-type microcrystaliine silicon oxide films. This invention also relates to enhancement of short circuit current density (Jsc) and conversion efficiency (r\) of the double junction amorphous silicon photovoltaic cells incorporating said thin films, wide band gap semiconductor alloy materials. BACKGROUND OF THE INVENTION A conventional double junction photovoltaic cell includes a front contact such as tin oxide disposed on the substrate followed by two p-i-n junction stacked one above the other, one acts as top cell and another as bottom cell, and a back contact made of, for example, a metal such as aluminum which serves as back reflector of light and as well as collection electrode, In an amorphous silicon alloy p-i-n/p-i-n structured photovoltaic cell, the intrinsic layer is the photovoltaically active layer That is the intrinsic layer is one in which light is absorbed to create carriers that are collected to produce the photo-generated current in the cell. Some of the incident light is absorbed by the n and p-type layers but generates carriers of shorter lifetime and recombines before they can be collected. Hence, absorption in the doped layers does not contribute to the photo-generated current Therefore minimization of absorption in the doped layers enhances the short circuit current and also the fill factor of p-i-n photovoltaic cells In a double junction cell the unabsorbed light by the intrinsic layer of the top cell passes into the bottom cell and absorbed by the bottom intrinsic layer The p-layer of the top cell absorbs the shorter wave length part (390-450 nm) of the visible spectrum. The absorption loss in the p-layer can therefore be adjusted by using wide band gap p-layer. The i-layer of the top cell that comprises a-Si-H having a thickness ~700Å, for example, has an optical band gap of 1.7 eV and an absorption coefficient which is such that not all the incident light is absorbed in one pass through the thickness of the i-layer. Thus, the long wavelength light makes passes through the n-layer of the top cell to the bottom cell. Usually ~300Å thick, highly conducting and having low activation energy n-layer have been used. In addition the absorption of this layer should be less for which the optical gap of n-layer should be wide. The i-layer of the bottom cell is adjusted in order to match the currents of the top and bottom cells. Details of Prior Art: The performance including conversion efficiency of double junction a-Si solar cells depends on the quality of the materials, individual thickness of each layer and their interfaces. The conventional double junction a-Si solar cell is shown in figure 1 (a) together with the thickness of each layer. The problems of some of the layers shown in figure 1(a) are given below. a. p-a-SiC:H layer: This layer has wide band gap, low electrical conductivity and relative high activation energy. The a-Si solar cell fabricated with it has good blue response but the fill factor is lowered by lower electrical conductivity. b. n-µc-Si:H layer: This is used at the tunnel junction for improving tunneling and reducing resistance. The problem is that the minimum thickness required is ~300 Å. This reduces the intensity of light getting into the bottom cell. The tunneling is also deteriorates. Objects of the Invention: The present invention is intended to enhance the short circuit current and the conversion efficiency of a double junction amorphous silicon photo-voltaic cell. More particularly, the object of this invention is to provide improved double junction amorphous silicon photo-voltaic cell having wide band gap amorphous silicon alloy p-layers to minimize absorption of incident radiation at the window layers and ultra thin highly conducting silicon micro-crystalline n-layer at the tunnel junction to minimize the absorption of radiation while making passes through the n-layer to the bottom cell. Further, the object is to provide photo-voltaic cells where the ultra thin wide band gap, highly conducting silicon micro-crystalline alloy n-layer has also been used to minimize the optical absorption and hence to increase the short circuit current. Brief Statement of the Invention: Thus, according to this invention, there is provided an improved double junction amorphous silicon solar cell comprising different p and ultra thin n-type silicon alloy films, wherein a ultra thin highly conducting n-type micro-crystalline layer is formed on the intrinsic layer of the top cell and a wide gap ultra thin highly conducting n-type micro-crystalline alloy layer is formed on the intrinsic layer of the bottom cell. The thickness of the n-layer was reduced from 300Å (conventional) to 180Å by using seeding technique with acceptable opto-electronic properties and the doped hydrogenated micro-crystalline silicon layer (~15oÅ) is deposited on a layer of undoped hydrogenated micro-crystalline silicon film having thickness ~3oA. Importantly, the doped hydrogenated micro-crystalline silicon oxide layer (~150Å) is deposited on a layer of undoped hydrogenated micro-crystalline silicon film having thickness ~3oÅ. It is to be noted that, the opto-electronic properties of the above mentioned material at the required level of thickness i.e. ~180Å for fabrication of the n-layer at the tunnel junction and as the n-layer of the bottom cell, are as follows: (n-µc-Si:H + seed): Optical gap = 2.00 eV; photo-conductivity = 2.57 Scm-1; activation energy = 0.015 eV; (n-µc-SiO:H + seed): Optical gap = 2.14 eV; photo-conductivity = 1.45 Scm-1; activation energy = 0.017 eV; Further according to this invention, there is provided a method for preparing improved double junction amorphous silicon solar cell comprising the following steps: a. preparing the decomposition of silane (SiH4) gas by using hydrogen (H2) as the diluent and phosphine (PH3) as the dopant. Carbon dioxide (CO2) is added to the source gas mixture for providing oxygen, which act as the band gap-widening element, b. depositing a p-a-SiC:H layer of 120Å on same, c. depositing on the layer of step (b) a a-SiC:H layer of 50Å unit, d. depositing on the layer of step (c) a i-a-Si:H layer of about 700Å, e. depositing on the layer of step (d) a n-µc-Si:H layer with said thickness 180Å, f. depositing on the layer of step (e) a p-a-SiO:H layer of 75A unit, g. depositing on the layer of step (f) a layer of a-SiO:H layer of 5oÅ, h. depositing on the layer of step (g) a layer of i-a-Si:H of about 3600A followed by (i) depositing on the layer of step (h), a layer of seed + n-µc-SiO:H of about 180Å with thickness 180Å and finally depositing the metallic layer. The doped hydrogenated micro-crystalline silicon layer (~15oÅ) is deposited on a (ayer of undoped hydrogenated micro-crystalline silicon film having thickness ~3oÅ and the doped hydrogenated micro-crystalline silicon oxide layer (~150Å) is deposited on a layer of undoped hydrogenated micro-crystalline silicon film having thickness ~30Å. The micro-crystalline layer has been prepared by the decomposition of silane (SiH4) gas by using hydrogen (H2) as the diluent and phosphine (PH3) as the dopant and carbon dioxide (CO2) is added to the source gas mixture for providing oxygen, which act as the band gap-widening element. The film has been deposited at a relatively low substrate temperature (200°C) and at a low RF-power density 14mW/cm2 and the chamber pressure was kept fixed at 1.0 Torr. The details of the Invention relate to improvements made by using ultra thin n-uc-Si:H layer at the tunnel junction, ultra thin n-uc-SiO:H layer as the bottom layer of the bottom cell and p-a-SiO:H as the window layer at the bottom cell. BRIEF SUMMARY OF THE INVENTION In accordance with a Ist embodiment of the invention, there is disclosed herein a novel method called seeding technique of fabricating an ultra thin wide band gap n-!ayer of hydrogenated microcrystalline silicon and its alloy having acceptable conductivity and activation energy by the use of RF-PECVD (13.56 MHz) process. In 2nd embodiment of the invention, there is also disclosed herein a method of fabricating a double junction a-Si solar cell using the above mentioned n-layers at the tunnel junction and as the bottom n-layer along with the improved PV parameters. The improvement of the short circuit current and the conversion efficiency in the cell comprises the addition of ultra thin, highly conducting n-layer. The idea of using different type of p-type hydrogenated amorphous silicon alloys as the window layers of the top and bottom cells is for utilizing the best out of each layer for improving the PV parameters. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is the schematic representation of the structures of the double junction p-i-n/ p-i-n structured amorphous silicon photovoltaic ceils. Figure l(a) represents the schematic of the double junction p-i-n/ p-i-n structured amorphous silicon photovoltaic cells using p-a-SiC:H as the window layer for both top and bottom cell. Figure l(b) represents the schematic of the double junction p-i-n/ p-i-n structured amorphous silicon photovoltaic cells using p-a-SiO.H as the window layer for both top and bottom cell. Figure l(c) represents the schematic of the double junction p-i-n/ p-i-n structured amorphous silicon photovoltaic cells using p-a-SiC:H as the window layer for the top andenergy = 0.017 eV; p-a-SiO:H as the bottom cell. Various layers that comprise the photovoltaic cell have been deposited on a glass substrate that was coated with a transparent conducting oxide (TCO) layer The subsequent layer deposited has been given in the diagram. DESCRIPTION OF THE PREFERRED EMBODIMENT (a) Ultra thin microcrystalline n-layer Figure 1 © shows one embodiment of the present invention wherein a ultra thin highly conducting n-type microcrystalline layer is formed on the intrinsic layer of the top cell and a wide gap ultra thin highly conducting n-type microcrystalline alloy layer :s formed on the intrinsic layer of the bottom celt The sample has been prepared in one chamber of a multichamber RF-PECVD system (13.56 MHz) with base vacuum -l0-9 Torr. The layer has been deposited by che decomposition of silane (SiR4) gas by using hydrogen (H2) as the diluent and phosphine (PH3) as the dopant. Carbon dioxide (C02) is added to the source gas mixture for providing oxygen, which act as the band gap-widening element The, "Mm has been deposited at a relatively low substrate temperature (200°C) and at a low rf-power density 14 mW/cm2. The chamber pressure was kept fixed at 1.0 Torr. The thickness of the n-layer was reduced from 300Å (conventional) to 180Å by using seeding technique with acceptable optoelectronic properties In this technique the doped hydrogenated microcrystalline silicon layer (-150Å) is deposited on a layer of undoped hydrogenated microcrystalline silicon film having thickness ~30Å. In the same way the doped hydrogenated microcrystalline silicon oxidelayer (-150Å) is deposited on a layer of undoped hydrogenated microcrystalline silicon film having thickness ~30Å. The optoelectronic properties of the above mentioned material at the required level of thickness i.e. ~ 180A for fabrication of the n-layer at the tunnel junction and as the n- layer of the bottom cell, are as follows: (n-µc-Si:H + seed): Optical gap= 2.00 eV; photoconductivity = 2.57 Scm-1; activation energy = 0.015 eV; (n-µc-SiO:H + seed): Optical gap= 2.14 eV; photoconductivity = 1.45 Scm-1; activation energy = 0.017 eV; (a) Fabrication of double junction photovoltaic cell Figure 1© represents the schematic diagram of the present invention. TCO coated glass was taken as a substrate for fabricating the p-l-n structured photovoltaic cell. P-type layer acts as the window layer for the solar spectrum incident on the photovoltaic cell. Wide band gap window layer allows more light to enter into the active layer. In the present invention we have used wide band gap (2.00 eV) p-type amorphous silicon carbide alloy material as the window layer of the top cell and oxide alloy material for the bottom cell. We have introduced an undoped amorphous silicon carbide alloy sandwiched between the window and active layer at the top cell and an undoped amorphous silicon oxide alloy sandwiched between the window and active layer at the bottom cell. These layers acts as the buffer layers for compensating the mismatch in band gap between the wide band gap window and relatively lower band gap intrinsic layer (1.70 eV). The intrinsic layer is highly photosensitive (photo gain ~ 106) and acts as the active layer in which light is absorbed to create carriers that are collected to produce the photo-generated current in the cell. The ultra thin n-layers, discussed earlier, has been deposited on the intrinsic layers. From the Quantum Efficiency measurement it is observed that the blue and the red response is better due to the use of wide band gap p-type hydrogenated silicon carbide at the top cell and wide band gap p-type hydrogenated silicon oxide film at the bottom cell, respectively. Also the short circuit current improved due to the use of ultra thin (~18nm) n-type microcrystalline hydrogenated silicon at the tunnel junction and ultra thin (~19nm) n-type microcrystalline hydrogenated silicon oxide film as the last layer bottom cell. Below we compare the PV parameters of the present invention (Figure 1©) with the conventional cell structures (Figure l(a)). Figure l(b) has been fabricated having the structure identical to Figure l(a) employing p-a-SiO:H as the window layers instead of p-a-SiC:H films in order to judge the feasibility of the p-a-SiO:H film as the window layers. The PV parameters of the present invention along with the comparative results of that of the conventional one have been given in Table 1. It can be seen from Table 1 that short circuit current density (Jsc) and conversion efficiency (Η|) of the double junction have been improved in the present invention compared to that of the conventional one. The ultra thin, highly conducting n-µc-Si:H film deposited by seeding technique favors the long wavelength light making passes through the n-layer more to the bottom cell in the present invention giving rise to the increase in the short circuit current of the photovoltaic cell. Table 1: PV parameters of the double junction cells with different window layer (Table Removed) WE CLAIM 1. An improved double junction amorphous silicon solar cell comprising different p and ultra thin n-type silicon alloy films, wherein a ultra thin highly conducting n-type micro-crystalline layer being located on the intrinsic layer of the top cell and a wide gap ultra thin highly conducting n-type micro-crystalline alloy layer is formed on the intrinsic layer of the bottom cell. 2. An improved double junction amorphous silicon solar cell as claimed in claim 1. wherein the thickness of the n-layer was reduced from 300A (conventional) to 180Å by seeding technique with acceptable opto-electronic properties. 3. An improved double junction amorphous silicon solar cell as claimed in claim 2, wherein the doped hydrogenated micro-crystalline silicon layer (-150Å) is deposited on a layer of undoped hydrogenated micro-crystalline silicon film having thickness ~30Å 4. An improved double junction amorphous silicon solar cell as claimed in claims 1 and 2. wherein the doped hydrogenated micro-crystalline silicon oxide layer (~1 50 A) is deposited on a layer of undoped hydrogenated micro-crystalline silicon film having thickness ~30Å. 5. An improved double junction amorphous silicon solar cell as claimed in claims lto 4 wherein, the order of the different layer from the top is as shown in Figure 1C. 6. A method for preparing improved double junction amorphous silicon solar cell comprising the following steps : a. preparing the decomposition of silane (SiH4) gas by using hydrogen (H2) as the diluent and phosphine (PH3) as the dopant. Carbon dioxide (CO2) is added to the source gas mixture for providing oxygen, which act as the band gap-widening element. b. depositing a p-a-SiC:H layer of 120Å on same. c. depositing on the layer of step (b) a a-SiC:H layer of 50Å unit. d. depositing on the layer of step (c ) a I-a-Si:H layer of about 700A e. depositing on the layer of step (d) a n-uc-Si:H layer with said thickness 180A f depositing on the layer of step (e) a p-a-SiO:H layer of 75 Åunit g. depositing on the layer of step (f) a layer of a-SiO:H layer of 50A h. depositing on the layer of step (g) a layer of I-a-Si:H of about 3600A followed by (i) depositing on the layer of step (h). a layer of seed + n-uc-SiO:H of about 180Å with thickness 180Å and finally depositing the metallic layer. 7. A method as claimed in claim 6 wherein the micro-crystalline layer has been prepared by the decomposition of silane (SiH4) gas by using hydrogen (H2) as the diluent and phosphine (PH3) as the dopant. Carbon dioxide (CO;) is added to the source gas mixture for providing oxygen, which act as the band gap-widening element. 8. A method as claimed in claim 7 wherein the film has been deposited at a relatively low substrate temperature (200°C) and at a low RF-power density 14mW/cm and the chamber pressure was kept fixed at 1.0 Torr. 9. An improved double junction amorphous silicon solar cell substantially as herein described with reference to the accompanying drawings. 10. A method for preparing improved double junction amorphous silicon solar cell substantially as herein described with reference to the accompanying drawings.

Documents:

1079-del-2002-abstract.pdf

1079-DEL-2002-Claims-(29-04-2009).pdf

1079-del-2002-claims.pdf

1079-DEL-2002-Correspondence-Others-(24-04-2009).pdf

1079-del-2002-correspondence-others.pdf

1079-del-2002-correspondence-po.pdf

1079-DEL-2002-Description (Complete)-(29-06-2009).pdf

1079-del-2002-description (complete).pdf

1079-del-2002-drawings.pdf

1079-del-2002-form-1.pdf

1079-del-2002-form-13-(04-04-2006).pdf

1079-del-2002-form-13.pdf

1079-del-2002-form-18.pdf

1079-DEL-2002-Form-2-(29-04-2009).pdf

1079-DEL-2002-Form-2-(29-06-2009).pdf

1079-del-2002-form-2.pdf

1079-del-2002-form-3.pdf

1079-del-2002-gpa.pdf