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Welcome to the next teaching unit

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in the chapter "Electrical Properties of a
Solar Cell".

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This unit deals with the test conditions

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under which the parameters of solar cells are
determined.

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You will also learn about solar simulators,

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which are often used for this purpose.

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We have seen in the last learning unit that
the JV characteristic

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and thus the parameters with which we describe
the electrical behavior

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of a solar cell depend on the irradiance and
the temperature.

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Thus, in order to compare different solar cells
or modules,

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their parameters must be measured

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under the same conditions.

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Therefore,

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globally uniform standard

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conditions have been defined under which the
parameters of a solar cell are measured.

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We will now take a closer look at these so-called

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standard test conditions which are briefly
referred to as STC.

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First, the STC contain

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a specification for the spectrum

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and irradiance of the light source.

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Ideally, the solar cell

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would be irradiated with sunlight.

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However,

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since the irradiance and spectrum of sunlight
are subject to strong fluctuations,

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artificial light sources are usually used to
meet the STC.

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The STC are based on the irradiance

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and light spectrum of the sun on a clear summer
day.

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The spectrum specified here is the AM1.5 spectrum:

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To understand what is meant by AM1.5,

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let's take a quick look at the Earth from the
outside.

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The sun emits light with a certain spectral
distribution,

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which is mainly determined by its surface temperature.

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This light hits the earth's atmosphere in space.

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As it passes through the Earth's atmosphere,

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the solar radiation is absorbed to a greater
or lesser extent,

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depending on the wavelength.

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The spectrum of the solar radiation

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is thereby massively changed.

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The longer the path of the light through the
atmosphere,

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the stronger the change.

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To describe the path length through the atmosphere,

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the Airmass AM is defined.

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It is the ratio of the actual path length of
the light

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to the shortest possible path.

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Outside the Earth's atmosphere the spectrum
is still unchanged

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- it is called the AM0 spectrum.

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Sunlight travels the shortest possible path

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when the sun is exactly perpendicular to the
sky - i.e. at the zenith.

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In this case, the air mass is exactly 1.

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In latitudes north or south of the Tropic of
Capricorn

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(i.e. from 23.5° north and south of the equator),

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however, the case of the sun

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being exactly at the zenith does not occur
throughout the year.

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Here,

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the sun is much lower in the sky even in the
summer months,

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which leads to a higher Airmass

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and thus to a stronger attenuation of solar
radiation.

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In addition,

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the air mass varies during the course of the
day,

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so that over the entire year the air mass is
significantly greater than AM1.

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Therefore, an Airmass of 1.5

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has been agreed upon as a standard value

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for the STC.

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At this Airmass

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the sunlight hits the earth's surface

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at an angle of 41.8°.

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Let's take a quick look at the spectral distribution
of sunlight.

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As you probably know, sunlight

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consists of light of different wavelengths.

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The spectrum describes the intensity of the
light

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in dependence of the wavelength.

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Shown here is the spectrum of sunlight in space
- i.e. under AM0 conditions.

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The spectrum has the highest intensity in the
visible range .

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However, the entire spectrum

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of sunlight extends over a much larger range,

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starting in the ultraviolet and extending far
into the infrared.

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Now, as it passes through the atmosphere, the
spectrum changes.

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Some wavelengths are almost completely absorbed,

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others are more or less attenuated.

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We will deal with the processes behind this
in detail later.

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At this point it is only important for us,

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which spectrum results on the earth's surface.

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Shown here is the spectrum that can be measured

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on the Earth's surface on a clear day

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under AM1.5 conditions.

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This spectrum is called AM1.5 spectrum.

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The second specification

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for the STC refers to the irradiance.

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Similar to the spectrum, the irradiance depends

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strongly on the attenuation in the atmosphere.

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For the STC, an irradiance of 1000 W/m² is
defined as the default value.

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This corresponds approximately

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to the irradiance on a clear summer day

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at northern or southern latitude (hence, outside
the tropics).

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The third condition of STC

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refers to the cell temperature.

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Under STC, this must be 25°C.

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You can see here a list

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of all the STC specifications.

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For solar modules, the electrical parameters
under STC

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are usually specified in the data sheet.

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This allows a comparison of different

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solar modules with each other.

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For most solar modules, however,

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the environmental conditions in reality

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are different from the STC.

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In most countries the irradiation is lower

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than 1000 W/m² most of the time.

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In addition, even good solar modules heat up
during normal operation.

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How much a module heats up depends largely

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on the module design and back ventilation.

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However, as we have seen,

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temperature has a major influence on the performance
of a solar module.

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As a reminder: When the temperature rises by
10°,

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the output of a solar module decreases by about
4-5% of the rated output.

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Therefore,

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the temperature that occurs under normal

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operating conditions is of great importance.

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According to the English term

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"normal operating cell temperature",

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this temperature is called NOCT.

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The NOCT is measured using a set of test conditions

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that differ from the STC: Under NOCT conditions,

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the irradiance is 800 W/m²

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and the spectrum is AM1.5 as for STC.

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However, the temperature is not specified

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as the cell temperature, but the ambient temperature.

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This must be 20 °C.

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Since the wind speed also has an effect on
the heating of the module,

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this is also specified with 1 m/s,

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which corresponds to a rather light wind.

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In addition, the module must be rear-ventilated
for this test, i.e.

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(this is)

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the wind also reaches

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the back of the module.

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With the NOCT,

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one now knows the temperature of a specific
module

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that occurs under the conditions just mentioned.

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Typical NOCTs

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of solar modules are around 48°C

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with extreme values between 33°C and 58°C.

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In reality, however,

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both the irradiation and the ambient temperature
vary.

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Therefore, it is also of interest what the
temperature

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is on a particularly hot summer day, for example,

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or on a cold winter day.

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To calculate this temperature,

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the formula shown here can be used.

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You can see that in addition

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to the ambient and NOCT temperatures,

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the irradiance is also included

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in the calculation.

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This is because a solar cell

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delivers larger power

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with higher irradiance and heats up

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due to the corresponding thermalization

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losses in the solar cell.

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We have now become acquainted with two sets
of test conditions

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under which the solar cell parameters can be
determined

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But how are the solar cell parameters

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determined in laboratory practice?

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In principle, it is of course possible to determine
the desired parameters

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in outdoor tests and wait for suitable

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environmental conditions.

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However, these measurements

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usually have to be corrected due to heating
of the modules

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and deviations from the test conditions

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and are thus subject to errors.

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Therefore, in practice, solar simulators are
usually used to determine

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solar cell parameters.

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In general, solar simulators

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have to fulfill several requirements.

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Firstly, the radiation of the solar

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simulator must correspond to the AM1.5 reference
spectrum

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as far as possible over several wavelength
ranges.

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The light sources used in solar simulators

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are predominantly xenon arc lamps, (but also
more and more LED)

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whose spectrum

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has a good match with the AM1.5 spectrum.

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Shown here is the AM1.5 spectrum

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and the spectrum of a xenon arc lamp.

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To classify the spectral deviation,

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the deviation of the

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solar simulator spectrum from the AM1.5 spectrum

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in different wavelength ranges is considered.

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In the individual wavelength ranges, deviations

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upwards (here light blue) and downwards (here
dark blue)

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are added up and the deviation is given in
%.

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We also speak of the so-called spectral mismatch.

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In this way six different wavelength ranges
are analyzed.

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Without filtering, there is a deviation

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of over 50% in the infrared range.

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Therefore, the light of xenon arc lamps is
often

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additionally filtered.

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Generally, solar simulators are divided

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into three classes for classification.

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Class A represents the highest quality class.

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In this class, the deviation must not exceed

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25% in any wavelength range.

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Besides the spectral deviation,

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the spatial uniformity of the irradiation

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is important for the classification of a simulator.

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In class A, the irradiance must not vary

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by more than 2% over the area of the solar
module.

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As a third point, the temporal stability of
the irradiance

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plays a decisive role, as the irradiance

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may only fluctuate minimally for the duration
of the measurement.

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A simulator that receives

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an A classification in all three areas

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is a Class AAA simulator.

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The simplest design of a solar simulator

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is a steady-state simulator.

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In this case, the solar cell

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or the solar module is permanently irradiated.

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The permanent irradiation causes

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a comparatively high-power consumption.

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In addition, the module or cell heats up,

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so cooling is necessary for these simulators.

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Steady-state simulators are mainly used

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for the characterization of solar cells,

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which have a longer response time.

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In addition, they are also suitable, for example,
for investigating

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the light aging behavior of solar cells.

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To avoid heating of the solar cell

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or module, flashed simulators are used.

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Instead of continuous illumination,

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these simulators work with short light flashes

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lasting a few milliseconds.

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It is technically challenging to obtain

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reproducible intensities and spectra from one
flash to another.

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On the other hand, the modules do not heat
up, so there is no need for cooling.

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Let us summarize this learning unit.

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To characterize solar cells and modules,

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the standard test conditions STC are defined,

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which give specifications for irradiance,

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light spectrum and cell temperature.

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To describe the behavior of solar

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modules under normal operating conditions,

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a second set of test conditions is used.

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With the NOCT, the normal operating cell temperature,
it is possible

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to calculate the module temperature under any
environmental conditions.

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Today, the parameters of solar cells and modules
under STC

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are usually determined with the help of a flasher.

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Thank you for your attention.