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Ver. 1.0
Nathan Heston
AM1.5 Solar Simulator
User Guide
Newport Oriel: (800)714-5393
Overview
The power conversion efficiencies (PCE) of most solar cells are heavily
dependent the light source. Sunlight varies in intensity and spectral distribution
depending on the many factors such as location on the earth and time of day and year. To
account for these factors standard irradiance spectra have been adopted by the research
community. Measurements under these standard conditions should allow for accurate
PCEs comparisons throughout the field. However, accurate simulation of the solar
spectrum is difficult and often improper simulation can introduce large errors in reporting
solar cell performances. For this reason, great care should be taken when making these
measurements.
The acronym AM1.5 stands for air mass 1.5. It represents the typical spectrum
that would be expected after sunlight travels through one and a half “typical” Earth
atmospheres. A power conversion efficiency measurement under AM1.5 conditions
involves measuring the I-V response of the solar cell under illumination from AM1.5
radiation. Analysis of a cell’s I-V curve can be done to determine the cell’s maximum
power output and thus the PCE can be calculated.
Sunlight
Sunlight is essentially the radiation spectrum of a 5800K blackbody, with
differences due to spectral lines and both reflection and absorption by the atmosphere.
Solar radiation varies depending on a number of factors including location and time. To
provide a standard set of conditions under which to measure solar cells, researchers have
adopted two common irradiance spectra, known as Air Mass 0 and Air Mass 1.5. These
spectra reflect the typical conditions under which a solar cell might be used. The first of
these two spectra is referred to as AM0, and it represents the radiation seen outside the
Earth’s atmosphere. Solar cells that are developed for space applications use this
reference. The second, and most common, reference spectrum is AM1.5. AM0 and
AM1.5 indicate the amount of “typical” atmosphere through which the radiation passes
(AM0 passes through zero atmospheres and AM1.5 represents sun light that travels
through approximately one and a half Earth atmospheres). Sometimes a third reference
spectrum (AM1) is used which represents the solar radiation that would be received if
sunlight were directly overhead, but this reference is becoming less common. Figure 1
illustrates the difference between the three.
Caution: This instrument is generates UV light and if proper precautions are not taken
you can damage your eyes. Do not look directly into the beam and use eye protection.
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Figure 1. AM0, AM1, and AM1.5 are differentiated by the amount of atmosphere that
they pass through.
The AM0 and AM1.5 reference irradiance data are plotted in Figure 2. The
curves represent the total power per unit area that is received within an incremental
wavelength range. If the total amount of energy is summed over the frequency range the
intensity of light outside the Earth’s atmosphere is about 1350 W/m2. A similar sum for
AM1.5 radiation received at Earth’s surface results in about 890 W/m2. This is the
typical amount of radiation received on a clear sunny day in the United States, and it
includes contributions from both direct sunlight and scattered light. On a clear sunny day
in Florida the total power of the sun is often ~1000 W/m2 (100mW/cm2). This has
become the standard intensity at which the efficiencies of solar cells are reported and is
often referred to as 1 Sun conditions.
Figure 2. AM0 and AM1.5 are irradiance spectra. AM0 is the spectrum of solar light
that would be seen just outside the Earth’s atmosphere. AM1.5 is the spectrum
seen after sunlight passes at a slant through 1.5 atmospheres.
The energy spectrum shown in Figure 2 shows the peak energy density occurs at
~500 nm. However, in all but the most advanced designs, each photon that is absorbed
by a solar cell can only generate one electron which provides the same energy (Ee=
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eVoperating) regardless of the frequency of the photon. This means that when considering
the absorption spectrum of a material for use in solar cells the important quantity to
consider is the number of photons (per unit area, per unit time, per wavelength increment)
and not the energy. For this reason, it is more common to plot the photon flux density
which tells the number of photons per unit area, per unit time, that are received within an
incremental wavelength range. The AM1.5 solar flux density is plotted in Figure 3.
Integration of the AM1.5 photon flux shows that 50% of the photons occur below 900 nm
and 75% occur below 1350 nm.
Figure 3. Normalized AM1.5 photon flux spectrum illustrating the variation in solar
photon flux with wavelength, where the black trace indicates the photon flux, and
the red trace represents the totals percentage of solar photons below the given
wavelength as obtained by integrating the photon flux.
Solar Simulation
Simulating the solar spectrum is difficult because generating a 5800K blackbody
source is not possible with solid state materials. To simulate the solar spectrum most
solar simulators utilize a Xe arc lamp and modify the output with filters. In a Xe arc
lamp the light is generated as high energy electrons pass through Xe gas forming a small
cloud of plasma. The light emitted from this plasma has a very similar irradiance
spectrum to sunlight, but because the plasma is still in gaseous form even high pressure
Xe lamps have strong emission lines, especially in the region from 850-900 nm.
The AM1.5 solar simulators of the MCCL laboratories are Newport Oriel 66907
power supplies with Newport Oriel 66902 simulator lamp housings (see Figure 4). The
lamp housings use150 Xe arc lamps with collimating lenses. The collimating lens of the
lamp housing modifies the output of the lamp to produce a beam of nearly uniform
energy distribution, however perfect collimation is not possible and care should still be
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taken to properly align solar cells so that they are positioned in the same location as the
power meter. Similar to measuring with the MCCL IPCE it is a good idea to design a
mask of the same size and shape of the active area “pixel” for the most accurate
measurement of the lamp power. The spectral mismatch of the lamp Xe arc lamp is
corrected by use of filters.
Figure 4. MCCL AM1.5 solar simulator setup. 1. Newport 66907 Power Supply. 2.
Newport 66902 lamp housing. 3. Collimating Lens and Filter Holder. 4.
Thermopile Power meter Head Unit. 5. Thermopile Control Unit
The MCCL laboratories have two different sets of filters that can be used. The
first set of filters is a three-filter stack(AM0/AM-D/AM1.5). This stack is designed to
reduce the Xe output to an AM1.5 distribution. The second filter is a 2” x 2” square filter
that is known as a AM1.5G filter. This filter is designed to reduce the 150W Xe
spectrum to a replicate the AM1.5 spectrum up to ~900nm. It should be noted that no set
of filters can perfectly remove the spectral lines of the lamp. When reporting high
efficiency cells spectral mismatch factors should be used (which can be obtained from
Newport) or cells should be sent to NREL for certified measurements. The three filter
stack is designed to replicate the solar spectrum over the full range. The transmittance
spectrum for the filter sets is plotted in Figure 5.
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Figure 5. Transmittance spectra of the MCCL solar simulator filters. Black trace is for
the AM1.5G square filter and red trace is the three filter stack. Both produce
similar modification to the Xe spectrum up to ~900nm. Beyond 900 nm the
AM1.5G does not mimic the solar spectrum. As can be seen from the differences
between the two filters, both from Oriel, the resulting spectra will not be identical,
even below 900 nm.
The largest source of errors when reporting solar cell efficiencies likely comes
from under, or overestimating the incident intensity of the AM1.5 spectrum. As
mentioned earlier the standard PCE measurement condition is a 100 mW/cm2 AM1.5
spectrum. When using the three filter stack of the MCCL laboratories the total spectral
power can be measured with a broadband power meter such as either of the two Newport
70260 thermopile detectors. These detectors have a nearly flat response from 200 to
3000nm. Due to the imperfect simulated AM1.5 spectrum produced by the filters and the
imperfect technique of measuring using a thermopile detector, the industry standard has
changed in recent years to calibration through the use of a shortpass filter and a calibrated
photodiode. This allows more accurate calibration by eliminating unwanted portions of
the AM1.5 spectrum, such as the Xe spectral line region, and it reduces error.
Unfortunately this system is expensive and to date we have not purchased a calibrated
photodiode for the MCCL laboratories.
Measuring AM1.5 PCEs
Correctly measuring the AM1.5 PCE of a solar cell is dependent on correctly
characterizing the I-V response of the cell under illumination by an AM1.5 spectrum of
known intensity. The solar simulator and thermopile power meter are used to set the light
intensity. The I-V behavior of the solar cell can be obtained through the use of the
Keithley sourcemeter and LabTracer program installed on the computers. To use the
simulator:
1. Turn the Power Supply on
2. Ensure that the filters are in place on the filter holder and press lamp start
then allow the lamp to warm up for approximately 10 minutes
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3. Using the power meter ensure that the power is 100 mW/cm2 (if you wish
to operate the simulator at other powers please contact the instrument
manager.)
4. Precisely align your solar cell and connect the wire leads. The convention
for bias voltage is to use the positive lead on the solar cell Anode (ITO).
5. Open the computer program called LabTracer (see figure 6)
Figure 6. LabTrace Program Home Screen
6. Click on Setup and select the source tab and adjust the voltage sweep to
run from -1 to +1, and select 100 points (other numbers and ranges can be
used, but you will need to adjust the Excel template linked below.)
7. Once you have the program set up simply click run test.
8. Save the data in a text file. (You can display a graph in LabTracer by
clicking on the graph tab and then define the x-y coordinates of the graph.)
9. Open the data using Microsoft Excel and press Ctrl+A to select the entire
range. Then click Ctrl+C to copy all the data.
10. Use the paste special feature of Excel to paste the data into the template
available here:
Blank Template: http://www.mccl.chem.ufl.edu/Secure/AM1.5-Template.xls
11. The code written in the Template should automatically calculate all the
relevant values such as: Open Circuit Voltage, Short Circuit Current, Short
Circuit Current Density, Fill Factor, and PCE. Make sure that the value on
the template for active area is correct for your cells (the default is
0.25cm2).
12. Before you are familiar with the template you should calculate each of the
values directly from the data to ensure that you are using the template
properly. The relations needed are given in the next section.
13. Another template that I made and you might find useful summarizes the
data from different cells by pulling the data directly off the xls templates.
This template is available here and instructions are in the template file:
Summary Template: http://www.mccl.chem.ufl.edu/Secure/AM1.5-Summary.zip
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Solar Cell Calculations
Calculations for the characteristics of a solar cell are straight forward. An example
of a I-V trace is shown in Figure 7A and a summary of the relations are listed in Figure
7b.
A
B
Figure 7. Solar Cell Characterisitics. A. Illuminated I-V trace of a solar cell showing
the short circuit current, the open circuit voltage and the maximum power point
on the curve. B. List of the relations used in describing solar cell I-V traces.
Solar Simulator Maintenance
The solar simulator should require very little maintenance. The most likely tasks
that will need to be performed are changing the Xe bulb, and calibrating the power meter.
Instructions for changing the Xe bulb can be found in the manual sent from Newport. It
is very important not to touch the bulbs with bare hands because the oil can severely limit
their lifetime. It is also important that the Xe bulbs are operated at 150W-170W range.
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Below this range the bulb may not light and the spectrum will be deficient. Above this
range produces a risk of the bulbs exploding. Calibration of the power meter needs to be
done by sending the unit in to Newport. They recommend this is done every 12 months.
It is a costly procedure (~$500).