FIELD OF THE INVENTION
The present invention relates to a method for reducing intensity of incident light
uniformly across entire wavelength range for measuring weak light performance
characteristics of solar cells. The invention further relates to incorporation of a
device into I-V measurement system for reducing intensity of incident light
uniformly across entire wavelength range for measuring solar cell weak light
performance characteristics.
BACKGROUND OF THE INVENTION
It is known that energy output of PV (photovoltaic) systems highly depends on
the irradiation conditions at site over the year, and the weak light performance
of solar cells used in the modules. Therefore, solar cell performance under weak
light conditions which is influenced by the shunt resistance of solar cells becomes
a crucial quality issue. With increased market volumes in photovoltaics,
incorporation of cell selection based on weak light performance (due to poor
shunt resistance) is becoming more and more necessary in standardized testing
procedures for higher energy yields.
In this context, it is crucial to measure solar cell efficiency under weak light
conditions (e.g., at 200 to 400 mW/cm2) and select the cells based on the
measured solar cell shunt values for assembling them into modules.

The following prior patents identified by the inventors from the technical field of
the present invention are hereby incorporated by way of reference.

The patent in Reference no. 1 presents techniques for solar cell electrical
characterization using continuous neutral density filter in front of the solar
simulator having regions of varying light attenuation levels ranging from
transparent to opaque. A Continuous Neutral Density Filter (CNDF), used in this
reference has a range of light attenuation levels (or optical density) from totally
transparent to opaque. A CNDF, used in this reference, is fabricated by
depositing a thin metal layer with varying thickness or paint-brush spray on glass
to achieve different light attenuation or printing a smooth gradation of black to
white image on a transparency sheet using an ink jet printer.
The patent in reference no. 2 relates to reduction in light intensity decay of
optical pumping devices.
The patent in reference no. 3 relates to a device for selectively reducing the
intensity of light in view of an eye or an optical instrument using an eye gear
frame, a power source, a light sensor, two transmissive lenses having an array of
shutter elements, user controls and processing circuitry.

In the present invention, a novel method has been disclosed for reducing the
intensity of incident light uniformly across entire wavelength range for measuring
I-V characteristics of a solar cell at specified low light intensity which is not
discussed in any of the patents to the best of our search.
SUMMARY OF THE INVENTION
The invention disclosed herein describes a method for reducing intensity of
incident light uniformly across the entire wavelength range for measuring weak
light performance characteristics of solar cells. The method utilizes a device
called filters consisting of a metallic plate with a number of holes and their
density, designed to transmit a fraction of the incident light over the test solar
cell uniformly across the entire wavelength range. The device when incorporated
in the regular I-V test set up extends its utility. For example, the test system can
be used to sort solar cells for assembling the modules, based on their weak light
performance characteristics and thereby enhance energy yield of the PV systems
in the field.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for reducing
intensity of incident light uniformly across entire wavelength range for measuring
weak light performance characteristics of solar cells.

Another object of the present invention is to propose an I-v measurement
system for reducing intensity of incident light uniformly across entire wavelength
range for measuring weak light performance characteristics of solar cells.
A still another object of the present invention is to propose a method for
reducing intensity of incident light uniformly across entire wavelength range for
measuring weak light performance characteristics of solar cells which provides
low intensity levels over a large area, given a uniform light source.
Yet another object of the present invention is to propose an I-v measurement
system for reducing intensity of incident light uniformly across entire wavelength
range for measuring weak light performance characteristics, which is
inexpensive.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1 shows sun spectrum under different atmospheric conditions AMI.5
Global, AM1.5 Direct and AMO.
Figure 2 shows a comparison of sun's spectrum and a tungsten halogen lamp's
spectrum.
Figure 3 shows the lamp spectra when different filters are placed establishing
that the intensity reduces when different filters are placed, however, the shape
of the spectra remains same.

Figure 4(a),(b) and (c) shows ratio of transmission of light at different
wavelength, with and without filter, which remains nearly same at all
wavelengths from 400-800 nm, suggesting that the same percentage of light is
cut at all the wavelengths (400-800 nm) when filters are placed in the path of
the light source.
Figure 5 shows a schematic of the I-V measurement system for solar cells
incorporating the device to reduce intensity of incident light.
Figure 6 shows the schematic of the device used in the I-V measurement
system.
DETAIL DESCRIPTION OF THE INVENTION
Standard method of reducing the incident light involve use of material media for
example, glass, transparent/translucent films. This invariably modifies the
spectral distribution of the transmitted light. In the present invention the light
intensity of the transmitted light is reduced by uniformly blocking the incident
light beam by an opaque Stainless Steel material of the filter plate. This method
reduces the intensity of the transmitted light without modifying its spectral
distribution.
STC testing of solar cells is performed at an intensity of 100 mW/cm2, AM1.5G
spectrum and 25°C cell temperature. Under the actual field conditions, the solar
cell operates under the sun and its spectrum is shown in figure 1.
The spectrum of a halogen lamp used for testing solar cell is shown in the curve
depicted in figure 2.

As can be seen from the spectra of figures 1 and 2, that the spectrum of the
halogen lamp used to test the solar cell is similar to the solar spectrum.
In the present invention, a method is devised to reduce the light intensity
without varying the light spectrum. In this method, a metallic sheet containing a
number of identical holes (filter) are used. The percentage transmission of light
through the metallic sheet has been reduced by changing the hole size and their
density i.e. by varying the net open area of the sheet. The change in light
intensity is measured by measuring the short circuit current of the silicon solar
cell detector. To demonstrate the method, three filters/devices have been made
using different hole size and their density to obtain 80%,60% and 40% net light
transmission values. In the light source of an I-V measurement the metallic sheet
is mounted at an optimum distance from the light source and the solar cell under
test.
The percentage light transmission through the filters is given in the table below.

The spectrum of the light at various intensities is now shown in figure 3. The
spectrum of the light source under various conditions has been taken from an
Optical Emission Spectrometer.

In figures 4(a) to 4(c), spectrum of transmitted light after incorporating the
filters in the light assembly is shown. It is seen from the figures that ratio of light
intensity with and without filter, remains the same at each transmitted
wavelength showing that the filters are not altering the spectral content.
Therefore, the light intensity is reduced but the light spectrum does not alter.
Therefore, this invention provides an inexpensive way of varying the intensity of
light uniformly at all wavelength. This can be used for testing solar cells at low
intensities as well. Some shunt paths, in poor cells, are dominant at low
intensities while they may not so dominant at higher intensities. Therefore, cells
when tested under low intensity levels can be sorted out due to poor weak light
performance. This sort technique can thus be utilized in industries for better
quality control.

WE CLAIM :
1. A method for reducing intensity of incident light uniformly across entire
wavelength range for measuring weak light performance characteristics of
solar cells, comprising the steps of:
performing a STC testing of a plurality of solar cells at an intensity of 100
mw/cm2, AM1.5G spectrum and 25°C cell temperature which being the
actual operating conditions for the solar cells;
performing a second testing of the solar cells by using spectrum of a
known halogen lamp used for solar cell testing;
configurating a plurality of devices to reduce the light intensity of light
spectrum consisting of a metallic sheet having a plurality of identical
holes, the hole sizes including their density being varied for each of said
plurality of device;
providing an I-V measurement system and incorporating said plurality of
devices for each phase of testing, wherein in each case the device is
disposed at an optimum distance both from the light source and the solar
cell under test, and
measuring the percentage of light transmission through each device;
wherein the spectrum of the transmitted light after incorporating said
devices or without any of the said devices exhibiting that ratio of light
intensity remains same at each transmitted wavelength although light
intensity is uniformly reduced.
2. The method as claimed in claim 1, wherein the spectrum of the light
source under different conditions is obtained by an optical emission
spectrometer.

3. The method as claimed in claim 1, wherein the method allows
simultaneous identification of solar cells with poor weak light performance
during the testing process to improve the quality control standards.

ABSTRACT

The invention relates to a method for reducing intensity of incident light
uniformly across entire wavelength range for measuring weak light
performance characteristics of solar cells, comprising the steps of :
performing an STC testing of a plurality of solar cells at an intensity of 100
mW/cm2, AM1.5G spectrum and 25°C cell temperature which being the
actual operating conditions for the solar cells; performing a second
testing of the solar cells by using spectrum of a known halogen lamp used
for solar cell testing; configurating a plurality of devices to reduce the light
intensity of light spectrum consisting of a metallic sheet having a plurality
of identical holes, the hole sizes including their density being varied for
each of said plurality of device; providing an I-V measurement system and
incorporating said plurality of devices for each phase of testing, wherein in
each case the device is disposed at an optimum distance both from the
light source and the solar cell under test, and measuring the percentage
of light transmission through each device; wherein the spectrum of the
transmitted light after incorporating said devices or without any of the
said devices exhibiting that ratio of light intensity remains same at each
transmitted wavelength although light intensity is uniformly reduced.