Within research activities aimed at increasing the efficiency of concentrating photovoltaic systems, the development of direct and indirect diagnostic techniques applied to photovoltaic devices plays a fundamental role. This is because these techniques allow us to obtain essential information for evaluating solar cells and the production process, in terms of both actual achievable performance and identification of any anomalies in the manufacturing processes of photovoltaic devices. Among the direct diagnostic techniques, the measuring the spectral response, i.e., the photocurrent produced as the incident wavelength varies, is of particular importance. Unlike that which happens for single-junction photovoltaic cells, the characterisation of multi-junction cells is more complex as it requires precise light polarisation in order to properly characterise each individual sub-cell, each of which is specialised in the conversion of solar irradiance into a very specific spectral band. The polarisation of light generates a significant photocurrent (greater than the photocurrent produced by the MJ photovoltaic device, which however progressively decreases as the number of junctions increases), and this requires to minimise the noise related to this photocurrent.

In this Line of Activity, research has been directed to the improvement of the light polarisation generation apparatus and the development of a new detection technique of the modulated photocurrent produced by the individual sub-cells. In particular, in order to reduce the noise related to the polarisation of light and allow the spectral response of quadruple junction solar cells to be measured, in addition to replacing the fourth polarisation source with a more stable LED source, a circuit was developed to effectively decouple the continuous component from the modulated component of the measured signal, by replacing the conventional preamplifier and current-to-voltage converter, which also has limitations related to the high dynamics of the signals involved, with a suitable transformer combined with a specific resonant circuit. Some types of decoupling and the non-linear effects produced by the magnetisation of the ferromagnetic material of the transformer were studied. Finally, a simple and effective solution for compensating these nonlinear effects was identified and tested. Finally, the layout of the LED control board was improved so as to make it more suitable for the opto-mechanical configuration of the instrument.

As an indirect diagnostic technique, an analytical technique was developed that, using two different types of photovoltaic cell modelling, allows us to identify some electrical parameters of the cell. Compared to previously published experiences, which used the single diode model as an equivalent electrical circuit of a photovoltaic cell, the new methodology allows us not only to obtain the value of the series resistance and that of the reverse saturation current but also the value of the shunt resistance from the best fitting of the experimental current-to-voltage curve of the cell. In addition, the mathematical modelling was extended to also consider the dual diode model, for more properly simulating the equivalent electrical circuit of the photovoltaic cell.

This model considers two components for determining the dark current: the one produced through the recombination by diffusion of charge carriers and the one from the recombination of charge carriers in the depletion region. This extended model allows us to obtain more information about the quality of the P-N or N-P junction that characterises the photovoltaic device, since the component of the dark current due to the recombination of charge carriers in the depletion region is essentially governed by the concentration of the defects present therein. Therefore, determining the two components of the dark current is an important diagnostic feedback for improving the manufacturing process of solar cells.