## 1 Introduction
This implies disconnecting the module or array and the use of an electronic load or an external power supply to bias the module or string. Finally, I-V curve measurement gives more comprehensive information about the state of a PV module or string. The main characteristic parameters of the device are obtained from the I-V curve. Some of them are inferred directly from the measurement and give information about the performance: short circuit current (\(I_{sc}\)), open circuit voltage (\(V_{\alpha}\)), and maximum power point (\(P_{max}\)) with its associate current (\(I_{mpp}\)) and voltage (\(V_{mpp}\)). Others give information about the physical properties of the device under study, and they are obtained from the I-V curve by applying models. The most widely used are the double or single diode models [21; 22; 23], which supply the parameters: photogenerated current \(I_{L}\), series resistance \(R_{s}\), shunt resistance \(R_{sh}\), diode saturation current \(I_{0}\), and ideality factor m. By analyzing the magnitude and evolution of these parameters, the degradation of a PV device can be quantified. There are several techniques and devices used to measure the I-V curve of PV devices in the field. An excellent review can be found elsewhere [24]. In addition, some authors developed a mobile laboratory to perform I-V curve measurements of PV modules in the plant at standard test conditions according to IEC standard [25; 26]. This has the advantage of ensuring uniform environmental conditions during the tests and it allows a precise quality control of selected PV modules throughout their shelf life. The measurement of the I-V curve of PV modules or strings in a PV plant under illumination implies in most of the cases the disconnection of the module/array in order to sweep the operating point from \(I_{sc}\) to \(V_{\alpha}\). This has the drawback, on the one hand, of requiring the disconnection of the plant and thus production, and on the other, the high voltages and/or currents when strings are measured, which forces taking rigorous personal security measures. In recent times, electronic boards to be installed in photovoltaic modules and inverters have been developed, from which it is possible to make I-V traces without disconnecting the photovoltaic modules. For example, in [27], the authors present two I-V tracing strategies at the photovoltaic module level without the need to disconnect them. One strategy uses electronic boards at the photovoltaic module level, while the second strategy combines electronic boards at the photovoltaic module level together with an electronic string card (common for all photovoltaic modules). In both cases, the measures are of high quality, and the strategies have low costs. The measurement at dark conditions has the clear advantage that it is performed when the plant is not in operation. While the normal operating mode of a plant or PV modules is under illumination, the dark curve gives very important information about the characteristics of the device, and it allows analyzing various failure modes. It has been used for many years to determine solar cell parameters [21; 28; 29] and as a diagnosis to detect certain defects such as mismatch, handling, soldering, or lamination problems in module manufacturing [30]. More recent works propose the use of a dark I-V curve alone or in combination with a light I-V curve for the identification of several failure modes. In [31], complementary analysis of dark and light current voltage characteristic is used to characterize failure modes such as degradation of the electrical circuit of the PV module, mechanical damage to the solar cells, and potential-induced degradation (PID). In [32], the analysis of a dark I-V curve to in situ monitor the degradation of PV modules undergoing thermal cycling and mechanical loading stress testing is proposed. The same group proposes an extension of their method to analyze in situ the temperature dependence of power loss estimations in PID experiments [33].