LED Solar Simulators - A Spectral Adjustment Procedure for Tandem Solar Cells

While recent development successes in the field of Perovskite/Silicon tandem solar cells are increasing the relevance of this technology for terrestrial applications, the demand for precise characterization methods for reliable efficiency measurements of such solar cells is accordingly growing. In contrast to the measurement of single junction solar cells, the measurement of tandem solar cells requires a spectral adjustment of the solar simulator spectrum, such that each sub cell of the tandem device generates the same current as it would under reference conditions. Due to their high number of spectrally different light sources current LED based solar simulators can be used to meet this requirement, but they are often not designed for tandem solar cell measurements. Therefore, in this paper we present a new spectral adjustment procedure which enables the use of LED based solar simulators for the measurement of tandem solar cells. The method is based on an initial measurement of the simulator spectrum, where the measurement procedure we introduce considers the thermal sensitivity of the LEDs when calibrating their spectra. Using this approach, the spectra of the single LEDs are not characterized by measuring them separately, but within a full LED spectrum by varying the intensity of the LED under investigation systematically. Since the high number of spectrally different light sources in a LED simulator results in an infinite number of possible spectra for the tandem cell measurement, an additional constraint for the selection of a suitable simulator spectrum is introduced in addition to the sub cell current condition - the spectral similarity of the simulator spectrum to the reference spectrum. An iterative calculation method based on nonlinear optimizations is utilized for the calculation of a suitable simulator spectrum. The relevance of our procedure is confirmed by a measurement comparison with a 3-source-solar simulator at Fraunhofer ISE CalLab, where excellent agreement is achieved in the measurement of a stable, III-V tandem which serves as reasonable proxy for a state-of-the-art Perovskite/Silicon tandem solar cell.