Spectral Matching / Spectral Mismatch Error and Reference Spectrum

What is spectral matching?

Spectral matching is a process of comparing the spectral characteristics of a light source with a reference spectrum, such as natural sunlight or a standard spectrum. The degree of similarity between the two spectra is measured by a spectral match factor (SMM), which is a numerical value ranging from 0 to 1. A higher SMM indicates a better match between the light source and the reference spectrum. Spectral matching is important for solar simulators because it affects the accuracy and reliability of testing solar devices and materials under artificial light conditions.

Spectral mismatch error

Spectral mismatch error (MM) is the error that occurs when the output characteristics of a solar cell are measured, due to the mismatch between the spectrum of the measurement light source (solar simulator) and the actual sunlight (Reference Spectrum).

There are three patterns of spectral mismatch error:

1. Spectrum mismatch between solar simulator and reference spectrum (AM1.5G, AM0, etc.).
2. Mismatch in response between reference cell and unknown solar cell.
3. In the case of multi-junction solar cells, the spectral response of each element cell does not match.

These spectral mismatches lead to a kind of measurement error called spectral mismatch error that is quantified by the spectral mismatch factor (SMM). Therefore, Spectral mismatch error needs to be corrected in order to accurately evaluate the efficiency and performance of a solar cell.

Spectral Mismatch Factor (SMM)

In IEC 60904-7:2019, the spectral mismatch factor (SMM) should be determined as follows:

Where:

• Eref(λ): The irradiance per unit bandwidth at a particular wavelength λ, of the reference spectral irradiance distribution (reference spectrum), for example as given in IEC 60904-3;
• Emeas(λ): The irradiance per unit bandwidth at a particular wavelength λ, of the spectral irradiance distribution of the incoming light at the time of measurement (test spectrum);
• Sref(λ): The spectral responsivity of the reference PV device at reference conditions;
• SDUT(λ): The spectral responsivity of the device under test at reference conditions.

[If the relative test spectrum would be identical to relative reference spectrum, then the SMM is 1 and spectral mismatch corrections can be neglected, even if the spectral responsivities of the devices differ.]

Source: IEC 60904-7:2019, Photovoltaic devices - Part 7: Computation of the spectral mismatch correction for measurements of photovoltaic devices.