Modelling of a Fast EQE Measurement System
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Abstract
External Quantum Efficiency (EQE) is an important performance measure parameter of a solar cell. EQE characterization gives deeper insights into the opto-electrical properties, current generation and recombination mechanisms in solar cells. Conventional EQE characterization of solar cells are done using a monochromator and lock-in amplifier. These measurements take a few minutes to completely resolve the spectral response of a solar cell in the visible and near infrared (VIS/NIR) region of the spectrum. Having faster EQE measurement setups will aid in faster solar cell characterization both in research and industrial production environments.
In this work a promising fast EQE measurement prototype called Fast Optical Measurement System (FOMS) developed by Delft Spectral Technologies (DST) is investigated. The FOMS prototype currently can operate only with a modulation scheme (a sequence of images) based on Michelson interferometer. The FOMS prototype hence cannot test other more suitable modulation schemes. To overcome this limitation a model to mimic the working of the FOMS prototype was developed by characterizing the existing setup which involved spectral distribution and intensity variations measurements. Various sub-functions were developed based on the characterization results and they were used to develop the model that could predict the spectrum on each pixel of the image loaded on the system. The developed model takes images corresponding to different modulation schemes as input and predicts the current output for each image. The model was finally validated by generating a new modulation scheme based on monochromatic EQE system. The percentage error in the current generated by the model and the prototype was around 3\% which is within permissible limits.
In the future the developed model can serve as a tool to quickly test new modulation schemes that can make the FOMS prototype faster and more accurate. Further research with respect to developing images based on more suitable modulation schemes can help avoid the mathematical complexities in the Michelson interferometer based modulation scheme. Developing appropriate post-processing steps to complement the modulation schemes will help in obtaining the EQE characteristics of the device under test from the current predicted by the model.