Title of Invention	RECLAMATION AND PROCESSING OF SILICON WAFERS FROM ELECTRICALLY REJECTED SOLAR CELLS.
Abstract	Reclamation of silicon wafers from rejected solar cells minimize the recurring losses. The invention provides a suitable chemical composition which is applied at room temperature to the rejected monocrystaline silicon solar cells to remove sintered silver, silver- aluminium and aluminium metal pastes from the cells completely. A re-processing sequence is adapted for converting the reclaimed wafers Into good solar cells at a reasonably good manufacturing yield. These cells were used for manufacture of photovoltaic modules. The re-processed modules were then subjected to type tests (Thermal cycling, Sun-soaking etc) to confirm their long term durability. It has been found that this development has a significant impact on the overall cost of manufacturing operation considering that the photovoltaic industry is facing a severe shortage of silicon feedstock raw material.

Title of Invention

RECLAMATION AND PROCESSING OF SILICON WAFERS FROM ELECTRICALLY REJECTED SOLAR CELLS.

Abstract

Reclamation of silicon wafers from rejected solar cells minimize the recurring losses. The invention provides a suitable chemical composition which is applied at room temperature to the rejected monocrystaline silicon solar cells to remove sintered silver, silver- aluminium and aluminium metal pastes from the cells completely. A re-processing sequence is adapted for converting the reclaimed wafers Into good solar cells at a reasonably good manufacturing yield. These cells were used for manufacture of photovoltaic modules. The re-processed modules were then subjected to type tests (Thermal cycling, Sun-soaking etc) to confirm their long term durability. It has been found that this development has a significant impact on the overall cost of manufacturing operation considering that the photovoltaic industry is facing a severe shortage of silicon feedstock raw material.

Full Text	FIELD OF INVENTION: The present Invention relates to an improved process for reclamation and reprocessing of silicon wafers from electrically rejected mono crystalline silicon solar cells. BACKGROUND OF INVENTION Electrical rejection of processed crystalline silicon solar cells is unavoidable as Solar cell manufacturing is highly process-dependent and vulnerable to many factors. As a result, not all the processed wafers are converted into good solar cells during normal manufacturing process where, often, a distribution of different classes (of output power) of products are produced. The reasons of electrical failures and low solar cell output power may be: 1) Inadequate drying of organic solvents during metal grid sintering, 2) Rupture of p-n junction, 3) Cross contamination of back (Aluminium) and front (Silver) paste, 4) Edge shunting effects and 5) Variation in p-n junction profile. In the conventional manufacture of Silicon solar cells, virgin mono or multi-crystalline silicon wafers are generally used. Some solar cell manufacturers try to use semiconductor reject (IC-grade, polished or unpolished) wafers to supplement the input wafers as the photovoltaic industry is facing a severe shortage of raw silicon material for solar cell manufacture worldwide. Mono crystalline solar cells are conventionally manufactured using CZ silicon wafers of size 125125 mm2. Raw silicon wafers are texturised in alkali solution followed by p-n junction formation in a diffusion furnace, anti-reflection coating in Plasma Enhanced Chemical Vapour Deposition (PECVD) tube furnace, metalisation in an automatic 3- stage screen printer and metal sintering in a belt furnace. Metalisation helps in electrical interconnection of solar cells through tinned copper interconnects. Silver paste is used for metalisation of grid pattern in the front cathode surface and Aluminium and Silver- aluminium pastes for metalisation on the back anode surface. The pastes contain organic solvents and glass frit so that they are screen printable on silicon and help in forming a solid bond with the contact surface. Not all processed silicon wafers are converted into good solar cells. The normal manufacturing processes result in a distribution of different classes (power output) of solar cells. In a normal manufacturing line around 1.5% of the processed solar cells are rejected due to low power output (below 1.6-watt) which are classified as "electrical rejects". Several causes for this rejection are attributable. a) Organic solvents are not dried adequately before entry into the firing zone in the belt furnace. b) Lower or higher firing temperature resulting in insufficient metalisation bond or rupture of p-n junction. c) Cross contamination of back aluminium or silver-aluminium pastes with front surface. d) Edge shunting effects caused in plasma etching process. e) Variations in junction depth profile in the diffusion process. WO 2005/029569, WO 2003/027649, WO 2004/07269 disclose system and method for reclamation, inspection of silicon wafers including detection of metals in semiconductor wafers. US patent numbers US 7008874, US 5855735, US 6451696, US 5622875, and US 64006923 disclose various methods for removal of thin films from semiconductor-grade wafers by chemical etch and polishing both sides of the wafer with abrasive slurry to reclaim the substrate wafers. All IC-grade/semiconductor devices use thicker (500-800 microns) silicon substrates and after reclaim these are reprocessed to fabricate semiconductor devices. However, all the existing processes for salvage and reuse are still costly and complicated. Thus, a need exists to provide a process adapting cheaper and rejected inputs to produce good quality of solar cells. OBJECT OF INVENTION It is, therefore, an object of the Invention to propose an improved process to reclaim silicon wafers from totally electrically rejected, sintered solar cells by adopting a chemical cleaning process for removing the sintered metal pastes from both the surfaces of the solar cells, and reprocess the reclaimed silicon wafers into good quality solar cells using a modified solar cell manufacturing process. Another object of the invention is to propose an improved process to reclaim silicon wafers from totally electrically rejected, sintered solar cells which is capable of removing the sintered metal pastes from both the surfaces of the rejected solar cells. Yet another object of the invention is to propose an improved process to reclaim silicon wafers from totally electrically rejected, sintered solar cells which adapts a chemical cleaning process. A further object of the invention is to propose an improved process to reclaim silicon wafers from totally electrically rejected, sintered solar cells which ensures producing a good quality solar cell by reprocessing the reclaimed silicon wafers. A still further object of the invention is to propose an improved process to reclaim silicon wafers from totally electrically rejected, sintered solar cells which uses a modified solar cell manufacturing method adaptable to continuous manufacturing of industrial solar cells. A still another object of the invention is to propose an improved process to reclaim silicon wafers from totally electrically rejected, sintered solar ceils which is cheap and simple. SUMMARY OF INVENTION Accordingly, there is provided a chemical process for removing the sintered metal (silver, silver-aluminium, aluminium) pastes from the electrically rejected mono crystalline silicon solar cells to reclaim the silicon wafers. A modified solar cell manufacturing process is employed for reprocessing the reclaimed silicon wafers into good solar cells. Outdoor sun soaking tests and thermal cycling tests are performed on photovoltaic modules fabricated with the reclaimed solar cells to confirm the long term stability. This method is also useable for converting low grade solar cells (1.4- watts-1.6-watts) into higher grade solar cells after removing the metal pastes and re-processing into fresh solar cells. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The illustrations accompanying this invention are as follows: Figure 1- shows a process flow chart for Silicon wafer Reclamation Figure 2- schematically illustrates a modified Solar Cell manufacturing method Figure 3- Yield distribution in reprocessed solar cells Figure 4- pictorial views of rejected solar cell, reclaimed silicon wafer and re-processed solar cell. Figure 5- a graphical representation of sun-soaking data on Photovoltaic modules with reclaimed solar cells Figure 6- pictorial views of 75-w and 150-w PV modules fabricated using reprocessed solar cells. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION Mono crystalline solar cells are conventionally manufactured using CZ silicon wafers of size 125125 mm2. Raw silicon wafers are texturised in alkali solution followed by p-n junction formation in a diffusion furnace, anti-reflection coating in Plasma Enhanced Chemical Vapour Deposition (PECVD) tube furnace, metalisation in an automatic 3- stage screen printer and metal sintering in a belt furnace. Metalisation helps in electrical interconnection of solar cells through tinned copper interconnects. Silver paste Is used for metalisation of grid pattern in the front cathode surface and Aluminium and Silver- aluminium pastes for metalisation on the back anode surface. The pastes contain organic solvents and glass frit so that they are screen printable on silicon and help in forming a solid bond with the contact surface. Not all processed silicon wafers are converted into good solar cells. The normal manufacturing processes result in a distribution of different classes (power output) of solar cells. In a normal manufacturing line around 1.5% of the processed solar cells are rejected due to low power output (below 1.6-watt) which are classified as "electrical rejects". Several causes for this rejection are attributable. a) Organic solvents are not dried adequately before entry into the firing zone in the belt furnace. b) Lower or higher firing temperature resulting in insufficient metalisation bond or rupture of p-n junction. c) Cross contamination of back aluminium or silver-aluminium pastes with front surface. d) Edge shunting effects caused in plasma etching process. e) Variations in junction depth profile in the diffusion process. In a first-aspect, the invention proposes a novel chemical process to remove the sintered metal pastes from the rejected solar cells. Several chemical etchant compositions have been studied with a view to identify suitable chemicals for dissolving sintered thick films of aluminium, silver and silver-aluminium. Hydrochloric acid (HCI), hydro fluoric acid (HF), nitric acid (HNO3) and ortho-phosphoric acid (H3PO4) were selected as potential chemical etchants. A series of experiments were performed with different combinations of above acids and dilutions to dissolve the respective metal pastes. 37% HCI solution was found to remove aluminium paste completely at room temperature. As the mixture of HF and HNO3 solution generated corrosive fumes and tended to dissolve the basic silicon itself, a dilute mixture of HNO3 and H3PO4 was preferred as an efficient etchant for silver at elevated temperature. After these basic chemical cleaning steps, the wafers were rinsed thoroughly in running De-Ionised (DI) water and scrubbed, followed by a running DI water rinse. These steps completely removed all the metal pastes from the silicon solar cell. The wafers were rinsed in acetone and dried. The process flow chart for reclamation of silicon wafers from rejected solar cells is illustrated in Fig. 1. This process is found to be simple, adaptable, and reproducible. Adequate safety measures were taken during the chemical cleaning processes. In order to reprocess the reclaimed silicon wafers to produce a good quality of solar cells, the invention provides an improvement in the normal manufacturing process sequence (shown in Fig. 2). The following improvements were introduced . a) Enhancing alkali etching time to facilitate complete removal of existing n-layer and any traces of metal pastes, if present. b) Introducing Silicon Nitride anti-reflection coating (ARC) In the manufacturing process which improved the optical and electrical characteristics of the reclaimed wafers. The blue colour of ARC also reduces the visual impact of previous printing marks on the reclaimed wafers. Results of Line Manufacturing process and Power class distribution: A large number of electrically rejected solar cells were chemically cleaned using the above developed process and silicon wafers were reclaimed. These wafers were used for manufacture of solar cells. The distribution of various classes of cells obtained Is shown in Fig. 3. The overall manufacturing yield is above 80% which is significant. The photographs of an electrically rejected solar cell, reclaimed silicon wafer and re-processed solar cell are shown in Fig. 4a, 4b and 4c respectively. Process qualification through type tests The reclaimed solar cells were used in a 36-cell configuration for manufacturing 75-watt, 37-watt (1/2 cut) and 10-watt (1/6th cut) photovoltaic modules using standard PV module encapsulation materials. The following type tests have been carried out on sample modules manufactured out of these reclaimed solar cells. a) Thermal cycling test with temperature alternating between - 25 Deg. C to + 75 Deg. C, 50 cycles. b) Sun-soaking test for 100 days by continuously exposing them to sun light under short-circuit condition. No visual degradations such as de-lamination, air-bubble, cell breakage were observed before and after the tests. The power output variation was found to be within the normal allowed limit of 5% of the initial value which validated the reclamation and solar cell manufacturing process. The power output variation of the module after sun soaking is shown in Fig. 5. The standard 75-watt and 150- watt PV modules were also tested as control samples (Fig. 6) which show comparable results as that from re-processed solar cells (Fig.6). EXAMPLES/ PREFERRED EMBODIMENT: 57,000 nos. of electrically rejected 125-mm pseudo square silicon solar cells were taken up for chemical treatment and removal of metal pastes. 51,506 nos. of silicon wafers have been recovered from them after chemical cleaning. The recovered silicon wafers were then re-processed into solar cells which resulted in the following yield distribution. The total wattage of the recovered solar cells work out to 90 kW approximately. WE CLAIM; 1. An improved process for reclamation and reprocessing of silicon wafers from electrically rejected mono-crystalline silicon solar cells, the process comprising the steps of: chemically cleaning the rejected solar cells to remove the sintered metal pastes to produce reclaimed silicon wafers; and reprocessing the reclaimed silicon wafers adapting a modified solar cell manufacturing method, wherein the step of chemically cleaning comprises: - dipping the rejected solar cell in 30 to 40% HCI (hydrochloric acid) at room temperature; - providing a first rinsing; - dipping the cells in a diluted mixture of HNO3 and H3PO4, at high temperature; - subjecting the solar cells to a second rinsing followed by scrubber wiping, and a third rinsing; and - providing an acetone-dip to the wafers and drying the wafers, to obtain the cleaned wafers, followed by the step of reprocessing the reclaimed wafers by adapting a modified manufacturing method which is characterized by comprising: - enhancing the alkali etching time to ensure complete removal of the n-layers and traces of metal pastes, if existing after chemical cleaning; and - introducing a Silicon-Nitride anti-reflection coating (ARC) to improve the optical and electrical characteristics of the reclaimed wafers. 2. The process as claimed in claim 1, wherein the steps of rinsing are carried out using deionised (DI) water. 3. The process as claimed in claim 1, wherein the wafers are dried using hot air. 4. An improved process for reclamation and reprocessing of silicon wafers from electrically rejected mono-crystalline silicon solar cells, as substantially described and illustrated herein with reference to the accompanying drawings. ABSTRACT TITLE: AN IMPROVED PROCESS FOR RECLAMATION AND REPROCESSING OF SILICON WAFERS FROM ELECTRICALLY REJECTED MONO-CRYSTALINE SILICON SOLAR CELLS Reclamation of silicon wafers from rejected solar cells minimize the recurring losses. The invention provides a suitable chemical composition which is applied at room temperature to the rejected monocrystaline silicon solar cells to remove sintered silver, silver- aluminium and aluminium metal pastes from the cells completely. A re-processing sequence is adapted for converting the reclaimed wafers Into good solar cells at a reasonably good manufacturing yield. These cells were used for manufacture of photovoltaic modules. The re-processed modules were then subjected to type tests (Thermal cycling, Sun-soaking etc) to confirm their long term durability. It has been found that this development has a significant impact on the overall cost of manufacturing operation considering that the photovoltaic industry is facing a severe shortage of silicon feedstock raw material.

Full Text

FIELD OF INVENTION:
The present Invention relates to an improved process for reclamation
and reprocessing of silicon wafers from electrically rejected mono
crystalline silicon solar cells.
BACKGROUND OF INVENTION
Electrical rejection of processed crystalline silicon solar cells is
unavoidable as Solar cell manufacturing is highly process-dependent
and vulnerable to many factors. As a result, not all the processed
wafers are converted into good solar cells during normal
manufacturing process where, often, a distribution of different
classes (of output power) of products are produced. The reasons of
electrical failures and low solar cell output power may be: 1)
Inadequate drying of organic solvents during metal grid sintering, 2)
Rupture of p-n junction, 3) Cross contamination of back (Aluminium)
and front (Silver) paste, 4) Edge shunting effects and 5) Variation in
p-n junction profile.

In the conventional manufacture of Silicon solar cells, virgin mono or
multi-crystalline silicon wafers are generally used. Some solar cell
manufacturers try to use semiconductor reject (IC-grade, polished or
unpolished) wafers to supplement the input wafers as the
photovoltaic industry is facing a severe shortage of raw silicon
material for solar cell manufacture worldwide.
Mono crystalline solar cells are conventionally manufactured using CZ
silicon wafers of size 125*125 mm2. Raw silicon wafers are texturised
in alkali solution followed by p-n junction formation in a diffusion
furnace, anti-reflection coating in Plasma Enhanced Chemical Vapour
Deposition (PECVD) tube furnace, metalisation in an automatic 3-
stage screen printer and metal sintering in a belt furnace.
Metalisation helps in electrical interconnection of solar cells through
tinned copper interconnects. Silver paste is used for metalisation of
grid pattern in the front cathode surface and Aluminium and Silver-
aluminium pastes for metalisation on the back anode surface. The
pastes contain organic solvents and glass frit so that they are screen
printable on silicon and help in forming a solid bond with the contact
surface.

Not all processed silicon wafers are converted into good solar cells.
The normal manufacturing processes result in a distribution of
different classes (power output) of solar cells. In a normal
manufacturing line around 1.5% of the processed solar cells are
rejected due to low power output (below 1.6-watt) which are
classified as "electrical rejects". Several causes for this rejection are
attributable.
a) Organic solvents are not dried adequately before entry into the
firing zone in the belt furnace.
b) Lower or higher firing temperature resulting in insufficient
metalisation bond or rupture of p-n junction.
c) Cross contamination of back aluminium or silver-aluminium
pastes with front surface.
d) Edge shunting effects caused in plasma etching process.
e) Variations in junction depth profile in the diffusion process.
WO 2005/029569, WO 2003/027649, WO 2004/07269 disclose
system and method for reclamation, inspection of silicon wafers
including detection of metals in semiconductor wafers. US patent

numbers US 7008874, US 5855735, US 6451696, US 5622875, and
US 64006923 disclose various methods for removal of thin films from
semiconductor-grade wafers by chemical etch and polishing both
sides of the wafer with abrasive slurry to reclaim the substrate
wafers. All IC-grade/semiconductor devices use thicker (500-800
microns) silicon substrates and after reclaim these are reprocessed to
fabricate semiconductor devices.
However, all the existing processes for salvage and reuse are still
costly and complicated. Thus, a need exists to provide a process
adapting cheaper and rejected inputs to produce good quality of
solar cells.
OBJECT OF INVENTION
It is, therefore, an object of the Invention to propose an improved
process to reclaim silicon wafers from totally electrically rejected,
sintered solar cells by adopting a chemical cleaning process for
removing the sintered metal pastes from both the surfaces of the
solar cells, and reprocess the reclaimed silicon wafers into good
quality solar cells using a modified solar cell manufacturing process.

Another object of the invention is to propose an improved process to
reclaim silicon wafers from totally electrically rejected, sintered solar
cells which is capable of removing the sintered metal pastes from
both the surfaces of the rejected solar cells.
Yet another object of the invention is to propose an improved
process to reclaim silicon wafers from totally electrically rejected,
sintered solar cells which adapts a chemical cleaning process.
A further object of the invention is to propose an improved process to
reclaim silicon wafers from totally electrically rejected, sintered solar
cells which ensures producing a good quality solar cell by
reprocessing the reclaimed silicon wafers.
A still further object of the invention is to propose an improved
process to reclaim silicon wafers from totally electrically rejected,
sintered solar cells which uses a modified solar cell manufacturing
method adaptable to continuous manufacturing of industrial solar
cells.
A still another object of the invention is to propose an improved
process to reclaim silicon wafers from totally electrically rejected,
sintered solar ceils which is cheap and simple.

SUMMARY OF INVENTION
Accordingly, there is provided a chemical process for removing the
sintered metal (silver, silver-aluminium, aluminium) pastes from the
electrically rejected mono crystalline silicon solar cells to reclaim the
silicon wafers. A modified solar cell manufacturing process is
employed for reprocessing the reclaimed silicon wafers into good
solar cells. Outdoor sun soaking tests and thermal cycling tests are
performed on photovoltaic modules fabricated with the reclaimed
solar cells to confirm the long term stability.
This method is also useable for converting low grade solar cells (1.4-
watts-1.6-watts) into higher grade solar cells after removing the
metal pastes and re-processing into fresh solar cells.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The illustrations accompanying this invention are as follows:
Figure 1- shows a process flow chart for Silicon wafer Reclamation
Figure 2- schematically illustrates a modified Solar Cell manufacturing
method
Figure 3- Yield distribution in reprocessed solar cells

Figure 4- pictorial views of rejected solar cell, reclaimed silicon wafer
and re-processed solar cell.
Figure 5- a graphical representation of sun-soaking data on
Photovoltaic modules with reclaimed solar cells
Figure 6- pictorial views of 75-w and 150-w PV modules fabricated
using reprocessed solar cells.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF
INVENTION
Mono crystalline solar cells are conventionally manufactured using CZ
silicon wafers of size 125*125 mm2. Raw silicon wafers are texturised
in alkali solution followed by p-n junction formation in a diffusion
furnace, anti-reflection coating in Plasma Enhanced Chemical Vapour
Deposition (PECVD) tube furnace, metalisation in an automatic 3-
stage screen printer and metal sintering in a belt furnace.
Metalisation helps in electrical interconnection of solar cells through
tinned copper interconnects. Silver paste Is used for metalisation of
grid pattern in the front cathode surface and Aluminium and Silver-
aluminium pastes for metalisation on the back anode surface. The
pastes contain organic solvents and glass frit so that they are screen
printable on silicon and help in forming a solid bond with the contact
surface.

Not all processed silicon wafers are converted into good solar cells.
The normal manufacturing processes result in a distribution of
different classes (power output) of solar cells. In a normal
manufacturing line around 1.5% of the processed solar cells are
rejected due to low power output (below 1.6-watt) which are
classified as "electrical rejects". Several causes for this rejection are
attributable.
a) Organic solvents are not dried adequately before entry into the
firing zone in the belt furnace.
b) Lower or higher firing temperature resulting in insufficient
metalisation bond or rupture of p-n junction.
c) Cross contamination of back aluminium or silver-aluminium
pastes with front surface.
d) Edge shunting effects caused in plasma etching process.
e) Variations in junction depth profile in the diffusion process.
In a first-aspect, the invention proposes a novel chemical process to
remove the sintered metal pastes from the rejected solar cells.

Several chemical etchant compositions have been studied with a view
to identify suitable chemicals for dissolving sintered thick films of
aluminium, silver and silver-aluminium. Hydrochloric acid (HCI), hydro
fluoric acid (HF), nitric acid (HNO3) and ortho-phosphoric acid
(H3PO4) were selected as potential chemical etchants. A series of
experiments were performed with different combinations of above
acids and dilutions to dissolve the respective metal pastes. 37% HCI
solution was found to remove aluminium paste completely at room
temperature. As the mixture of HF and HNO3 solution generated
corrosive fumes and tended to dissolve the basic silicon itself, a dilute
mixture of HNO3 and H3PO4 was preferred as an efficient etchant for
silver at elevated temperature. After these basic chemical cleaning
steps, the wafers were rinsed thoroughly in running De-Ionised (DI)
water and scrubbed, followed by a running DI water rinse. These
steps completely removed all the metal pastes from the silicon solar
cell. The wafers were rinsed in acetone and dried. The process flow
chart for reclamation of silicon wafers from rejected solar cells is
illustrated in Fig. 1. This process is found to be simple, adaptable,
and reproducible. Adequate safety measures were taken during the
chemical cleaning processes.

In order to reprocess the reclaimed silicon wafers to produce a good
quality of solar cells, the invention provides an improvement in the
normal manufacturing process sequence (shown in Fig. 2). The
following improvements were introduced .
a) Enhancing alkali etching time to facilitate complete removal of
existing n-layer and any traces of metal pastes, if present.
b) Introducing Silicon Nitride anti-reflection coating (ARC) In the
manufacturing process which improved the optical and
electrical characteristics of the reclaimed wafers. The blue
colour of ARC also reduces the visual impact of previous
printing marks on the reclaimed wafers.
Results of Line Manufacturing process and Power class
distribution:
A large number of electrically rejected solar cells were chemically
cleaned using the above developed process and silicon wafers were
reclaimed. These wafers were used for manufacture of solar cells.
The distribution of various classes of cells obtained Is shown in Fig. 3.
The overall manufacturing yield is above 80% which is significant.
The photographs of an electrically rejected solar cell, reclaimed
silicon wafer and re-processed solar cell are shown in Fig. 4a, 4b and
4c respectively.

Process qualification through type tests
The reclaimed solar cells were used in a 36-cell configuration for
manufacturing 75-watt, 37-watt (1/2 cut) and 10-watt (1/6th cut)
photovoltaic modules using standard PV module encapsulation
materials. The following type tests have been carried out on sample
modules manufactured out of these reclaimed solar cells.
a) Thermal cycling test with temperature alternating between - 25
Deg. C to + 75 Deg. C, 50 cycles.
b) Sun-soaking test for 100 days by continuously exposing them
to sun light under short-circuit condition.
No visual degradations such as de-lamination, air-bubble, cell
breakage were observed before and after the tests. The power
output variation was found to be within the normal allowed limit of
5% of the initial value which validated the reclamation and solar cell
manufacturing process. The power output variation of the module
after sun soaking is shown in Fig. 5. The standard 75-watt and 150-
watt PV modules were also tested as control samples (Fig. 6) which
show comparable results as that from re-processed solar cells (Fig.6).

EXAMPLES/ PREFERRED EMBODIMENT:
57,000 nos. of electrically rejected 125-mm pseudo square silicon
solar cells were taken up for chemical treatment and removal of
metal pastes. 51,506 nos. of silicon wafers have been recovered from
them after chemical cleaning. The recovered silicon wafers were then
re-processed into solar cells which resulted in the following yield
distribution.

The total wattage of the recovered solar cells work out to 90 kW
approximately.

WE CLAIM;
1. An improved process for reclamation and reprocessing of silicon wafers
from electrically rejected mono-crystalline silicon solar cells, the process
comprising the steps of:
chemically cleaning the rejected solar cells to remove the sintered
metal pastes to produce reclaimed silicon wafers;
and reprocessing the reclaimed silicon wafers adapting a modified
solar cell manufacturing method,
wherein the step of chemically cleaning comprises:
- dipping the rejected solar cell in 30 to 40% HCI (hydrochloric acid)
at room temperature;
- providing a first rinsing;
- dipping the cells in a diluted mixture of HNO3 and H3PO4, at high
temperature;
- subjecting the solar cells to a second rinsing followed by scrubber
wiping, and a third rinsing; and
- providing an acetone-dip to the wafers and drying the wafers, to
obtain the cleaned wafers, followed by the step of reprocessing the
reclaimed wafers by adapting a modified manufacturing method
which is characterized by comprising:
- enhancing the alkali etching time to ensure complete removal of
the n-layers and traces of metal pastes, if existing after chemical
cleaning; and
- introducing a Silicon-Nitride anti-reflection coating (ARC) to
improve the optical and electrical characteristics of the reclaimed
wafers.
2. The process as claimed in claim 1, wherein the steps of rinsing are
carried out using deionised (DI) water.

3. The process as claimed in claim 1, wherein the wafers are dried using
hot air.
4. An improved process for reclamation and reprocessing of silicon wafers
from electrically rejected mono-crystalline silicon solar cells, as
substantially described and illustrated herein with reference to the
accompanying drawings.

ABSTRACT

TITLE: AN IMPROVED PROCESS FOR RECLAMATION AND
REPROCESSING OF SILICON WAFERS FROM ELECTRICALLY
REJECTED MONO-CRYSTALINE SILICON SOLAR CELLS
Reclamation of silicon wafers from rejected solar cells minimize the
recurring losses. The invention provides a suitable chemical
composition which is applied at room temperature to the rejected
monocrystaline silicon solar cells to remove sintered silver, silver-
aluminium and aluminium metal pastes from the cells completely. A
re-processing sequence is adapted for converting the reclaimed
wafers Into good solar cells at a reasonably good manufacturing
yield. These cells were used for manufacture of photovoltaic
modules. The re-processed modules were then subjected to type
tests (Thermal cycling, Sun-soaking etc) to confirm their long term
durability. It has been found that this development has a significant
impact on the overall cost of manufacturing operation considering
that the photovoltaic industry is facing a severe shortage of silicon
feedstock raw material.

Documents:

00135-kol-2007-correspondence-1.1.pdf

00135-kol-2007-form-18.pdf

0135-kol-2007-abstract.pdf

0135-kol-2007-assignment.pdf

0135-kol-2007-claims.pdf

0135-kol-2007-correspondence others.pdf

0135-kol-2007-description(complete).pdf

0135-kol-2007-drawings.pdf

0135-kol-2007-form-1.pdf

0135-kol-2007-form-2.pdf

0135-kol-2007-form-3.pdf

135-KOL-2007-(09-11-2012)-CORRESPONDENCE.pdf

135-KOL-2007-(20-06-2012)-ABSTRACT.pdf

135-KOL-2007-(20-06-2012)-AMANDED CLAIMS.pdf

135-KOL-2007-(20-06-2012)-DESCRIPTION (COMPLETE).pdf

135-KOL-2007-(20-06-2012)-DRAWINGS.pdf

135-KOL-2007-(20-06-2012)-EXAMINATION REPORT REPLY RECEIVED.pdf

135-KOL-2007-(20-06-2012)-FORM-1.pdf

135-KOL-2007-(20-06-2012)-FORM-2.pdf

135-KOL-2007-(20-06-2012)-OTHERS.pdf

135-KOL-2007-CORRESPONDENCE 1.1.pdf

135-KOL-2007-CORRESPONDENCE 1.2.pdf

135-KOL-2007-EXAMINATION REPORT.pdf

135-KOL-2007-FORM 18.pdf

135-KOL-2007-FORM 3.pdf

135-KOL-2007-GPA.pdf

135-KOL-2007-GRANTED-ABSTRACT.pdf

135-KOL-2007-GRANTED-CLAIMS.pdf

135-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

135-KOL-2007-GRANTED-DRAWINGS.pdf

135-KOL-2007-GRANTED-FORM 1.pdf

135-KOL-2007-GRANTED-FORM 2.pdf

135-KOL-2007-GRANTED-SPECIFICATION.pdf

135-KOL-2007-OTHERS.pdf

135-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

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Patent Number

254667

Indian Patent Application Number

135/KOL/2007

PG Journal Number

49/2012

Publication Date

07-Dec-2012

Grant Date

04-Dec-2012

Date of Filing

31-Jan-2007

Name of Patentee

BHARAT HEAVY ELECTRICALS LIMITED

Applicant Address

REGIONAL OPERATIONS DIVISION(ROD), PLOT NO: 9/1, DJ BLOCK, 3RD FLOOR, KARUNAMOYEE, SALTLAKE CITY, KOLKATA-700091 HAVING ITS REGISTERED OFFICE AT BHEL HOUSE SIRI FORT

Inventors:

#	Inventor's Name	Inventor's Address
1	RAVI, S	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026.
2	VENKATARAMU M.	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026.
3	PREMACHANDRAN S.K.	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026.
4	TAGORE A.R.	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026.
5	SIVARAM S.	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLATICS DEPT, MYSORE ROAD, BANGALORE-560026.
6	FELIX MENEZES	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026.
7	SAJI SALKALACHEN	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026.

PCT International Classification Number

B24B1/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA