Characterization of silicon solar cells by means of their luminescence under high frequency sinusoidal excitations

The dynamic response of solar cells is more complex to be analyzed than the quasi-steady-state (QSS) case because signals are faster and weaker and the analysis is more complex, sometimes challenging, and it requires some corrections in order to reconstruct the static characteristics. As a result, QSS measurements are preferred when measuring I-V curves, sunsVoc characteristics or for luminescence imaging, when integrating times in the range of seconds are used. But dynamic measurements, which are more complex, contain more information than QSS about some internal characteristics that are otherwise hidden. Following this idea, this paper analyses the behavior of the excess carrier concentration in a cell and, consequently, its Voc and PL responses, in open circuit conditions. The excitation is composed of a biasing light that drives the cell to a proper injection-level and a superposed small-signal sinusoidal illumination. The theoretical analysis is based in the classic charge-control model of a pn junction in dynamic conditions, and the small signal decomposition. The data are validated with PC1D simulations for the different scenarios and, in some cases, with experimental measurements. This work proves the existence of a cut-off frequency that is related to the transit time of the device (which is a function of the geometry and the minority carrier diffusivity) and to the effective (recombination) lifetime of the device. Applying excitations of different frequencies and intensities the device can be characterized on by measuring the phase-shift and amplitude dependence on the frequency.