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Spectral Mismatch Corrections Video Text Version

This is a text version of the video Spectral Mismatch Corrections, a lecture given as part of the Hands-On Photovoltaic Experience Workshop.

Speaker: Measuring solar cell performance consistently everywhere in the world requires an indoor solar simulator. For most measurements, the solar spectrum is standardized as air mass 1.5 global, or AM 1.5G, which represents the average solar spectrum in temperate climates.

Solar simulators use various lamps to mimic this AM 1.5G solar spectrum. However, these lamps generally do not exactly match the AM 1.5G solar spectrum at all wavelengths which is known as spectral mismatch.

Corrections can be made using the spectral mismatch factor M. This captures the difference in intensity between the AM 1.5G solar spectrum and the lamp spectrum used for solar simulation.

In the graph shown, the wavelength versus an intensity of two common solar simulator lamps is compared to the AM 1.5G spectrum. It is clear that they are spectral mismatch shown by varying light intensities at the given wavelengths when compared to AM 1.5G.

Spectral mismatch calculations allow us to standardize solar simulator measurements across labs and on different days. Using a reference cell with the spectral or spot similar to a test cell will make it easier to account for the spectral mismatch.

Now we can calculate spectral mismatch with an example. Let us assume that we have a silicon reference cell for calibration. We are testing a cell with a larger band gap. We can calculate the spectral mismatch with four terms.

First, we measure the external quantum efficiency (EQE) and spectral response. We multiply the latter with the AM 1.5G solar spectrum to calculate short circuit current. This goes into the numerator of M.

Second, we measure the short circuit current of the test cell under the lamp spectrum. This factor is multiplied with the first short circuit current to calculate M.

Third, we measure the EQE of the test cell and multiply the AM 1.5G solar spectrum to calculate its short circuit current. This goes into the denominator of M.

Fourth, we measure the short circuit current of the reference cell under the lamp spectrum. We divide M by this value.

The net result is a spectral mismatch term M that accounts for differences between both the lamp and reference spectra, plus the differences between test and reference cells.

We can thus divide the measured test cell current by M for more precise comparisons with the literature.

You need two cells and four data sets to measure spectral mismatch M. The cells needed are your reference and test cells. The first data set you'll need is the air mass 1.5 global (AM 1.5G) reference spectrum, which is available on NREL's website.

Second, you'll need your light source spectrum which can be measured by you or the manufacturer. Make sure you have the correct units for radians. They much match the reference spectrum.

Finally, you'll need to know the quantum efficiency (QE) of the reference cell as well as the QE of your test cell.

Now you can get started. We begin by measuring the external quantum efficiency (EQE) of the reference cell. Using this equation.

[Equation on video.]

Once you know the spectral response of the cell multiplied by the AM 1.5G spectrum and a factor proportional to the wavelength to attain the JSC under one-sun. You can multiply by the area of the cell to calculate ISC. We will use this value later.

Next step is to measure the external quantum efficiency of your test cell. Then, you can calculate the SR, JSC, and ISC. We'll use this value later.

Next, measure the short circuit current of the reference cell with the light source you have. If it doesn't match with the ISC obtained in Step 1, adjust the intensity of your source. You can do this by adjusting the distance between the source and the sample.

Now we can measure the short circuit current of the test cell using the light source you have. Then, we calculate the spectral mismatch factor M by plugging in the values from the previous step as shown in the equation here.

[Equation on video.]

Ideally, M should be as close to unity as possible. Once you calculate your spectral mismatch factor M, you can set the one-sun intensity with your reference cell. Then obtain your JV curve.

After you're done measuring the JV curve of your test cell, you can apply the spectral mismatch factor by dividing all current values by M.

[End of audio.]