FIELD OF THE INVENTION:
The present invention relates to a portable non-destructive device for quick measurement of average diffused reflectance of textured surface area of silicon wafers used in solar cells manufacturing industry. The invention further relates to a method of measuring average diffused reflectance of textured surface area of silicon wafers.
BACKGROUND OF THE INVENTION:
Majority of solar cell manufactures use either mono crystalline or multi crystalline Si wafers as the basic input material for production of solar cells. Typically, the as-cut mono or multi crystalline silicon wafers generally received from wafer manufactures reflect about 30-40% of incident light. The amount of light lost due to this reflection from the silicon surface is quite substantial and affects the solar cell efficiency. Hence, in the solar cell manufacturing process, various surface preparation procedures are used to reduce the reflection to a minimum. In this process step, the surface of silicon wafer is treated with chemicals or etched with reactive ions to make the surface rough/textured so that the diffused reflection is reduced to a minimum leading to maximum light absorption. Typically, diffused reflectance from the textured Si wafer is in the range: 10-15%. To maintain a very high yield of solar cells, it is required to maintain a consistent texturisation process. For this purpose, the quality of texturisation needs to be monitored on a regular basis during manufacturing. In order to determine the quality of texturisation, a simple non-destructive tool is required for an operator on the manufacturing bay to measure the diffused reflectance of textured wafers.

US Patent 4, 563, 642 disclosed an apparatus for non-destructively measuring characteristics of a semi conductor wafer with a junction comprising means for radiating a frequency chargeable pulsated photo beam to the wafer, means for taking out the resulting photovoltage by capacitance complying, and means for calculating the carrier lifetime and cutoff frequency from the chopping frequency dependence characteristics of its photovoltage.
EP 200030808673 20031009 (Publication No. EP 1556990) disclosed a non¬destructive analysis method for determining the quality of a solar cell based on Raman spectroscopy by which significant structural parameters (FWHM) relating to the structural properties of a processed absorber layer is determined in a solar cell by establishing a correlation between structural parameters with electric parameters (Voc) which are relevant in terms of the expected electrical properties of the finished processed solar cell.
None of said prior documents discloses a technique for measuring diffused reflectance of silicon wafer, and at the same time, the cited process equipments are not cost effective.
Standard laboratory equipment, like spectrophotometer fitted with an integrating sphere, is known which measures the diffused reflectance. This method is very accurate but has the following disadvantages:
• Destructive in nature (125 mm x 125 mm wafer has to be cut to small size).
• Time consuming, as it has to measure reflectance at all wavelengths in the visible region. Approximately time taken to run one sample is 10-15 minutes.

• It is not portable.
• Needs to be operated in air-conditioned and clean atmosphere.
• Equipment is very expensive (More than Rs. 30-35 lakhs).
A photovoltaic solar cell generates electricity by absorbing solar radiation/light and also can act as a photo detector. The short circuit current generated by the cell is directly proportional to the light intensity falling on it. The response of the solar cell is good even in very low light intensity and it generators photocurrent.
To circumvent some of the above disadvantages, it is necessary to provide a non¬destructive and non- contact technique for diffused reflectance measurement of crystalline silicon wafers/solid surface area, in particular large surface area.
OBJECTS OF THE INVENTION:
It is therefore on object of the invention is to propose a portable, non-destructive, non-contact device and a corresponding method for measuring average diffused reflectance of a textured crystalline silicon wafer/solid surface.
Another object of the invention is to propose a portable, non-destructive, non-contact device, which is simple, cost effective and takes a less time for measuring diffused reflectance of textured silicon wafer.

SUMMARY OF INVENTION:
According to the invention, the solar cells are used as the detector to detect light reflected from the surface of the textured silicon wafer. The electrical signal is detected in the form of a short circuit current and displayed on an ampere meter. As the current generated by the detector cells is directly proportional to the reflectivity of the silicon wafer, the quality of texturisation can be compared by comparing the value of current on the ampere meter reading. Less the value of the current, less is the reflected light from the wafer and better is the quality of texturisation of the wafer.
In the present invention solar cells are used as photo detectors. A halogen lamp has been selected as the light source as the spectrum of the halogen lamp matches well with the solar spectrum. The photocurrent generated by the detector cells is very sensitive to the intensity of light from the source. Therefore a regulated supply has been used to power the lamp to keep the intensity of light constant to enhance the measurement accuracy. All the control electronics and the optics have been housed in a suitable box and care has been taken to make the box light proof to reduce stray light interference with the actual signal.
In the proposed device, a collimated light beam from the halogen lamp is allowed to fall on the textured silicon wafer mounted on a sample holder and the intensity of the reflected light from silicon wafer is measured in terms of short circuit current generated by the detector cells. As the light is reflected uniformly, over a sufficiently large area, from the textured wafer, the current measured by the • detector cells is observed as directly proportional to the reflectance of the textured surface. Hence the current generated by the detector can be used as the qualification test for the quality of texturisation.

The device can be used in photovoltaic solar cell manufacturing industry to measure the relative diffused reflectance of the textured silicon wafers for sorting good and bad quality of texturisation. Testing process of wafer texturisation in solar cell manufacturing industry needs to be very quick for example, lees than 15 seconds per wafer. Thus, the device can be used in a solar cell industry to test the quality of texturisation from batch to batch. As the testing method is non¬destructive, wafers of sizes from 5 to 6 sq. inches can be tested with great ease.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS:
The proposed invention will be better understood from the following description with reference to the accompanying drawing in which
Figure 1 represents a schematic of the diffused Reflectance measurement equipment
DETAILED DESCRIPTION OF THE INVENTION:
As shown in Figure 1, a 12 volt regulated power supply (1) is connected to supply line (S). The 12 V power supply (1) is used to power the Halogen Lamp (2). A collimated light beam through a lens (L) from the halogen lamp falls on a mounted textured silicon wafer sample (3), the intensity of the reflected light from the wafer is measured in terms of short circuit in mA by a Digital mA Meter (5). The short circuit current is generated by the solar cells used as detectors (4). From the measured current the reflectance is measured,

as the reflectance being directly proportional to the measured current produced by the detector cells (4). Thus, the quality of texturisation of the silicon wafer is determined by this simple device and method.
The present invention is illustrated with the following test datas and example.

Anti Reflection Coating
Table 1 clearly indicates that the current generated by the detector cells is directly proportional to the reflectance of the silicon wafer. Lower the ID better will be solar cell efficiency.
Example:
10 numbers of samples from a mixed batch of good and bad texturisation were studied using this set-up. The comparison of diffused reflectance in terms of detector current is given in the table below;

ID (ref.) = 0.7 mA (Reference wafer) corresponding to average diffused reflectance = 15-16%.

The texturisation will be accepted if ID (ref.) - ID (sample) > 0, and will be rejected if ID (ref.) - ID (sample) < 0.
Table 2 shows that the samples with different quality of texturisation can be segregated. It is obvious that wafers with bad quality of texturisation give higher value of ID. Thus wafers have been compared with a reference sample (ID (ref.) = 0.7 mA), as determined by the user. The rejected silicon wafers need not go for next process steps and can be retextured to achieve proper quality of texturisation.

By using this non-destructive testing method, it is possible to reduce the wastage of costly silicon wafers and save the add-on cost of further processes resulting in huge savings.
The invention as described and illustrated herein should not be read and construed in a restrictive manner as various adaptations, alterations and modifications are possible within the scope and limit of the defined claims as appended.

We Claim:
1. A portable device for measurement of diffused reflectance of textured
silicon wafers used for solar cell production, the device comprising:
- a 12 volt power supply (1) to supply power to a halogen lamp (2) to produce a collimated light beam through a lens (2) to fall on a textured silicon wafer (3) and a reflected light with light intensity is generated;
- a solar detector cell (4) generates a short circuit photo current which is displayed in a digital mA meter (5), characterized in that the intensity of the reflected light being proportional to said short circuit current, the diffused reflectance of the silicon wafers is directly measured by the device, and in that the diffused reflectance of textured silicon wafer is carried out by comparing a real-time detector current (ID) expressed in unit mA, and a reference detector current (ID- ref.)

2. The device as claimed in claim 1, wherein the photovoltaic solar cell (4) is enabled to generate electricity by absorbing solar radiation, and acts as a photo detector of the reflected light from a textured silicon wafer surface.
3. The device as claimed in claim 1, wherein the 12 volt power supply is a regulated power supply to enable accurate measurement of diffused reflectance of the textured silicon wafer by providing a constant light intensity independent of any mains power supply voltage fluctuations.

4. The device as claimed in claim 1, wherein the device is enabled to
measure diffused reflectance of silicon wafers coated with anti reflection
coating (ARC) of silicon nitride with low reflectance (5-6%).
5. The device as claimed in any of the preceding claims wherein a textured
silicon wafer is accepted when ID(ref)- Io(sample) is > 0 and is rejected
when ID(ref) - ID (sample) is < 0.

ABSTRACT

TITLE: A PORTABLE DEVICE FOR MEASUREMENT OF DIFFUSED REFLECTANCE OF TEXTURED SILICON WAFERS USED FOR SOLAR CELL PRODUCTION
The invention relates to a portable device for measurement of diffused reflectance of textured silicon wafers used for solar cell production, the device comprising a 12 volt power supply (1) to supply power to a halogen lamp (2) to produce a collimated light beam through a lens (2) to fall on a textured silicon wafer (3) and a reflected light with light intensity is generated; a solar detector cell (4) generates a short circuit photo current which is displayed in a digital mA meter (5). The intensity of the reflected light being proportional to said short circuit current, the diffused reflectance of the silicon wafers is directly measured by the device, and in that the diffused reflectance of textured silicon wafer is carried out by comparing a real-time detector current (ID) expressed in unit mA, and a reference detector current (ID- ref.)