FIELD OF INVENTION
The present invention generally relates to a non-destructive method for measurement
of quantum efficiency (QE) of tandem junction thin-film photo Voltaic (PV) modules. In
particular the invention relates to measurement of QE of tandem junction PV module
using short pass and long pass optical filters. The test method disclosed in this
invention would find use in any solar cell research and development facility, pilot
production facility and industry.
BACKGROUND OF THE INVENTION
The quantum efficiency measurement is a practical tool to analyse properties of solar
cell device, like thickness of semiconductor layers and their interfaces, internal electric
field distribution, contaminations, etc. through measurement of device output current as
a function of wavelength of the incident photons at a fixed light intensity.
Multi-junction amorphous silicon (a-Si) solar cell modules have been in commercial
production at several manufacturing companies worldwide for more than a decade now.
The structures of these modules vary from tandem junction to triple junction on a
variety of substrates such as glass, stainless steel and plastic film. A very
important criterion for good performance of these modules is the current matching
between the constituent cells. This can be determined from a measurement of
Quantum Efficiency (QE) of the constituent cells in the module. This is one of the most
important tools for evaluating the parameters of multi-junction a-Si solar cells including
the current densities generated by the individual junctions.
Presently, there is no commercial QE measurement set up available for thin film PV
modules. The majority of QE set ups available have been developed for use in research
laboratories where mostly cells of small size of about 1 to 4 cm2 are fabricated and
subjected to QE tests. The requirements of the industry are, however, different where
large area cells and modules are made and are required to be tested non-destructively
with reproducibility of the measurements in a minimum time.
Patent search did not yield any information on any commercial set up for QE
measurement similar to that of the present invention.
There is therefore, a need for providing a non-destructive method of QE measurement
of a larger area thin film tandem junction PV module.
The present QE measurement set up was developed in-house, prompted by the
requirements of process optimization during the course of development of tandem
junction thin film amorphous silicon solar cell modules.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide a non-destructive method for the
measurement of QE on a complete large area (1' x 3') of a-Si tandem junction thin-film
PV module wherein the constituent cells are segregated optically using combination of
short-pass (blue) and long-pass (red) optical filters. These filters are used sequentially
for optical flooding (equivalent to electrical shorting) of one constituent cell and thereby
measuring the response of the other cell against an incident monochromatic wavelength
of light in the range of 400 to 800 nm.
As an example, the test method has been very useful in optimization of the process
parameters of the a-Si/a-Si tandem junction modules. In the case of a-Si/a-Si tandem
junction modules, current matching of the series connected constituent cells was
attempted through variation of thickness of the absorber layers in both the cells. Hence,
in the present method, no special sample preparation is required and a randomly
selected portion of the active solar cell module is employed for measurement of the QE.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to development of a non-destructive method for
measurement of quantum efficiency (QE) of tandem junction thin-film module using
short pass and long pass optical filters for segregating the response of individual
junction. The filters are designed and fabricated to illuminate the constituent cells
sequentially to electrically short one cell and measure the spectral response of the other
cell. In the present invention, QE measurements of large area modules can be
performed non-destructively. This is done by covering the module completely with an
opaque cover having an aperture in it and positioning the module in such a way that
monochromatic light spot falls at the point of interest on the module surface.
In QE measurements a quartz tungsten halogen (QTH) lamp serves as light source a
monochromator provides monochromatic light in the range of 400 to 800 nm (suitable
for amorphous silicon tandem junction module) at an interval of 5-20 nm; the light
beam is chopped at preset frequency different from integral multiple or division of AC
mains frequency (say at 27 Hz). A set of lenses focuses the light on to test device,
(e.g. a solar cell module) and a calibrated reference detector. The light generated
current by the test device (i.e. tandem junction module) and a calibrated reference
detector is preamplified by a transimpedance OPAMP and recorded by lock-in amplifier
(LIA) at a reference frequency preset by the optical chopper. The output of the LIA is
read out by use of a PC based data acquisition (DAQ) card. Using the wavelength
dependent responses of test device and detector measured by LIA along with detector
responsivity data, the QE of the test device is calculated.
To measure the QE characteristics of a tandem cell module, first, the bottom cell is
effectively shorted by illuminating the test device by red light transmitted through the
long pass filter and the photocurrent response of the top cell due to chopped
monochromatic light of variable wavelength (400 to 800 nm) is measured by LIA and
recorded by PC through a built in DAQ card. Similarly, the response to monochromatic
light of the bottom cell is measured from 400 to 800 nm by effectively shorting the top
cell using blue light through the short pass filter.
In a preferred embodiment the present invention provides a non-destructive method for
measurement of quantum efficiency (QE) of thin-film tandem junction photo Voltaic
(PV) module, comprising the steps of covering the PV module under test with an
opaque cover having an aperture and positioning the module so that QHT /
monochromatic light spot from monochromator falls at the desired point on the
amorphous silicon (a-Si) tandem junction module surface; focussing the light from said
monochromator on the test module with the help of a lens system (3); focussing the
light from said monochromator on the test module with the help of a chopper (4), lens
system (3) and a bias light source (5); saturating the bottom junction and the top
junction of said tandem junction PV module with the help of a filter assembly (10);
amplifying the feeble response of light generated current by the test module and a
calibrated reference detector at a reference frequency preset by the optical chopper
(4), with the help of a preamplifier and recording it in a locking amplifier (LIA); reading
the output of said LIA with the help of a PC based data acquisition card (DAQ); and
determining the QE of the test module using the wavelength dependent responses of
test module and detector measured by LIA along with detector responsivity data.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with the reference to the accompanying drawings
where
Figure 1 shows details of hardware used in the test set up for measurement of QE
of large area a-Si/a-Si tandem junction modules.
Figure 2 illustrates the I-V characteristics of a typical large area double
junction a-Si module.
Figure 3 shows QE-?, plots for top and bottom junctions
of a-Si/a-Si tandem thin film module.
The QE measurement set-up for large area a-Si tandem junction modules is shown in
Fig. 1. It has a quartz-tungsten-halogen (QTH) lamp (1) and a monochromator (2).
The QTH lamp (1) can be Model No. 66187 made by Oriel Corporation, USA, and the
monochromator (2) can be of Oriel Corporation, USA, Model No. 77250. The setup is
provided with a set of lenses (3) and a light beam chopper (4). The chopper can be of
Princeton Applied Research, USA, Model No. 196.
The light beam chopper (4), the lens system (3) and a bias light source (5) are
housed in an enclosure (6) painted black from inside to reduce internal
reflection/ avoid stray light. The bias light source (5) used is a 24 V, 250 W
QTH lamp. A small aperture (7) (about 2 cm2 in size) made at the end of the
enclosure lets the monochromatic beam (spot size 0.6 cm2 approximately) of
light fall on the chosen sample (8), which in this case is a large area, tandem
junction module processed at BHEL- ASSCP. The module is wrapped in thick
black paper and placed just behind the aperture in the enclosure. The black
wrapper has an aperture (7) exactly matching with that in the enclosure so as
to allow monochromatic light beam as well as the bias light fall on the exposed
area. The output from the test device is coupled to a lock-in amplifier (9)
(Stanford Research Systems, USA, Model No. SR 510) through a pre-amplifier
cum current to voltage converter.
The large area (1 ft X 3 ft) double junction a-Si modules have the following device
structure: Glass /TCO / p a-SiC:H / i a-Si:H / n µc-Si / p a-SiC:H / i a-Si:H /n a-Si:H /
Ag. The a-Si and the uc-Si layers are made in stationary mode in the same deposition
chamber using RF PECVD (13.6 MHz) method.
A typical double junction module, which has an initial aperture area efficiency of ~6.5%
and short-circuit current (Isc) of 1.20 A (Fig.2) is selected for studying the current
generation in the top and the bottom junctions using the QE measurement set-up
described above. The module has 14 cells (each of area ~180 cm2) connected internally
in series and the monochromatic light beam is allowed to fall on the cell under test.
A filter assembly (10) comprising two filters used in the study are: a) a long-pass (red)
filter (80% total transmission between 600 and 900 nm) to saturate the bottom
junction and b) a short-pass (blue) filter (60% total transmission between 400 and 600
nm) to saturate the top junction. No special sample preparation is resorted to and a
randomly selected part of the active solar cell module is employed for measurement of
QE.
A standard silicon detector is used for measurement of intensity of incident
monochromatic light between 400 and 800 nm and the bias light intensity is
kept constant at 25 mW/cm2. Between 400 and 800 nm, at each wavelength at
an interval of 20 nm, the photocurrent generated due to the chopped
monochromatic light is detected for test module as well as standard detector
with the help of lock-in amplifier. These are then normalized with respect to the
output of the standard detector at each monochromatic wavelength and used
in plotting QE-? curves (see Fig 3).
It can be seen from the comparison of QE data (cell min. current density as
6.65 mA/cm2) and the I-V data for the tandem junction a-Si/a-Si module that
module current calculated from the QE data (6.65 mA/cm2 x 180 cm2 =1.19 A)
is in excellent agreement with the short circuit current (=1.19 A) measured
under Spire 240A sun simulator.
Table I: Features of the test set up & test module
WE CLAIM
1. A non-destructive method for measurement of quantum efficiency (QE) of thin-
film tandem junction photo Voltaic (PV) module, comprising the steps of:
- covering the PV module under test with an opaque cover having an aperture and
positioning the module so that QTH / monochromatic light spot from
monochromator (2) falls at the desired point on the amorphous silicon (a-Si)
tandem junction module surface;
- focussing the light from said monochromator on the test module with the help of
a chopper (4), lens system (3) and a bias light source (5);
- saturating the bottom junction and the top junction of said tandem junction PV
module with the help of a filter assembly (10).
- amplifying the feeble response of light generated current by the test module and
a calibrated reference detector at a reference frequency preset by the optical
chopper (4), with the help of a preamplifier and recording it in a locking amplifier
(LIA);
- reading the output of said LIA with the help of a PC based data acquisition card
(DAQ); and
- determining the QE of the test module using the wavelength dependent
responses of test module and detector measured by LIA along with detector
responsivity data.
2. The method as claimed in claim 1, wherein the large area tandem junction PV
module is for e.g. (1 ft X 3 ft) double junction a-Si modules having the following
device structure: Glass /TCO / p a-SiC:H / i a-Si:H / n µc-Si / p a-SiC:H / i a-Si:H
/n a-Si:H / Ag. in a process by BHEL-ASSCP.
3. The method as claimed in the preceding claims, wherein the QTH light source is
of Oriel Corporation, USA, Model No. 66187 and the monochromator 2 is of Oriel
Corporation, USA, Model No. 77250.
4. The method as claimed in the preceding claims, wherein said chopper is made by
Princeton Model No. 196.
5. The method as claimed in the preceding claims, wherein said bias light (5) is a
24 V 250 W QTH lamp.
6. The method as claimed in the preceding claims, wherein said filtered assembly
(10) comprises a red light long pass optical filter transmitting between 600 and
900 nm and a blue light short pass optical filter transmitting between 400 and
600 nm to saturate respectively the bottom and the top junction with photo
carriers for effectively rendering them short electrically.
7. A non-destructive method for measurement of quantum efficiency (QE) of thin-
film tandem junction photo Voltaic (PV) modules, substantially as herein described
and illustrated in the figures of the accompanying drawings.

1. A non-destructive method for measurement of quantum efficiency (QE) of thin-
film tandem junction photo Voltaic (PV) module, comprising the steps of:
- covering the PV module under test with an opaque cover having an aperture and
positioning the module so that QTH / monochromatic light spot from
monochromator (2) falls at the desired point on the amorphous silicon (a-Si)
tandem junction module surface;
- focussing the light from said monochromator on the test module with the help of
a chopper (4), lens system (3) and a bias light source (5);
- saturating the bottom junction and the top junction of said tandem junction PV
module with the help of a filter assembly (10).
- amplifying the feeble response of light generated current by the test module and
a calibrated reference detector at a reference frequency preset by the optical
chopper (4), with the help of a preamplifier and recording it in a locking amplifier
(LIA);
- reading the output of said LIA with the help of a PC based data acquisition card
(DAQ); and
- determining the QE of the test module using the wavelength dependent
responses of test module and detector measured by LIA along with detector
responsivity data.