For single-junction devices, a basic class A spectral match defined in the international standards will suffice. However, for modern multi-junction solar cells, specifically those for space applications, the number of junctions has increased over the decades to the point where there are three and four junction devices, with six junction devices being designed at the time of writing.
Below is an example of a triple-junction solar cell for terrestrial applications (AM1.5 spectrum) which shows how the subcells respond to different wavebands of the spectrum.
Below is an example of a triple-junction solar cell for terrestrial applications (AM1.5 spectrum) which shows how the subcells respond to different wavebands of the spectrum.
(a) The structure of an MJ solar cell. There are six important types of layers: pn junctions, back surface field (BSF) layers, window layers, tunnel junctions, anti-reflective coating and metallic contacts. (b) Graph of spectral irradiance E vs. wavelength λ over the AM1.5 solar spectrum, together with the maximum electricity conversion efficiency for every junction as a function of the wavelength. [1]
The common method of setting a solar simulator is to use a reference cell that gives a known response when exposed to a reference spectrum, which are referred to as "reference standards". The calibration procedure, very basically, consists of measuring the reference cell when exposed to the simulator beam and adjusting the output of the simulator such that the performance under simulator and reference conditions are as closely matched as possible. Using advanced close-match solar simulators this can be as close as within 1% of the target value.
To calibrate a solar simulator for measuring a multi-junction solar cell, the same procedure is carried out with a reference standard for each junction of the device.
The implications for a single-source solar simulator with a basic class A spectral match now become clear. With the performance of multi-junction cells being limited by one junction, a +/-25% deviation in average irradiance across 100nm wavebands can result in a spectrum that is totally unacceptable for accurately measuring such devices. It also becomes impossible to adjust specific parts of the spectrum in order to achieve calibration, as is discussed in the following chapters.
To calibrate a solar simulator for measuring a multi-junction solar cell, the same procedure is carried out with a reference standard for each junction of the device.
The implications for a single-source solar simulator with a basic class A spectral match now become clear. With the performance of multi-junction cells being limited by one junction, a +/-25% deviation in average irradiance across 100nm wavebands can result in a spectrum that is totally unacceptable for accurately measuring such devices. It also becomes impossible to adjust specific parts of the spectrum in order to achieve calibration, as is discussed in the following chapters.