This paper presents a simple and nondestructive method to determine doping densities and built-in potential of subcells by adapting the well-known capacitance-voltage (C-V) technique to two-terminal (2 T) tandem solar cells. Because of the electrical coupling between the two subcells in a monolithic 2 T tandem solar cell, the standard method using a Mott-Schottky plot (1/C 2 vs V) cannot be applied. Using numerical modeling, it is demonstrated that, by under chosen illumination conditions where only one subcell can absorb the light, it is possible to explore the bias dependence of the capacitance and to extract the parameters of the other subcell if the appropriate frequency conditions are present. This method is experimentally applied to an AlGaAs/Si tandem cell, and parameters of both AlGaAs and Si cells are extracted.Finally, the validity of that method is assessed by the very good agreement obtained when comparing the values extracted from our measurements on the tandem cell to those extracted from measurements on isotype cells and to the values targeted during the fabrication process of the AlGaAs/Si tandem solar cell.
A new setup for recording time dependent photoluminescence signal in both transient and modulated regime was previously assembled. The modulated photoluminescence experiment was achieved in a high frequency domain (up to 10 MHz) while keeping high sensitivity. The methods can thus be applied both to silicon and thin film absorbers. A simulation program was developed for the reconstruction of the experimental data in both time resolved and frequency resolved regimes. In this work we focus on the simulation part and discuss the ability of our setup to identify the carrier recombination paths in thin film solar cells. First results confirm that it should be possible to discriminate between different mechanisms such as radiative, non-radiative recombination and trapping.
Modulated photoluminescence (MPL) is an optoelectronic characterization technique of semiconductor materials. Going to high frequencies enables one to characterize fast phenomena, and so materials with a short lifetime such as chalcogenides or III–V absorbers. Some typical signatures have already been experimentally observed. However, physical mechanisms and quantitative analyses are not well understood yet. Here, using both an analytical approach and a full numerical modeling, we study how the energy position of a defect level, its electron and hole capture cross sections, its density, influence the frequency dependence of the MPL phase. We show that quantitative information can be extracted. We also study the effect of additional surface recombination, and of non homogeneities created by carrier generation profiles or asymmetric top surface and bottom surface recombination velocities, where diffusion of the carriers plays a role and can be limiting at high frequency. Finally we apply our model to an experimental result to extract defect parameters of the sample. Our analysis highlights the usefulness of MPL and the importance of having a proper modeling of the experiment.
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