Potential Induced Degradation

Potential-induced degradation (PID) has been identified as a serious issue for silicon solar modules reliability and both crystalline silicon and thin-film PV modules are susceptible to PID [1]. In grid-connected PV systems, the panels are typically connected in series to generate a high voltage output while the module frames are grounded for electrical safety reasons. However, this architecture creates a high electric potential difference between the solar cells within the modules and the module frame. The high potential difference can cause leakage currents to flow from the module frame to the solar cells, which is known to cause PID. As the system voltage is trending towards even higher voltages of up to 1500 V, it is apparent that PID should be addressed.

What directly causes PID is still unknown. It is known that PID can be influenced by a number of factors such as the properties of the antireflection coating, encapsulation materials, module construction and the system topologies. The surrounding environment condition such as temperature, light intensity or humidity, can also affect the degradation extent. The leakage current can be induced in a typical PV system between the active circuit and the frame of modules. This is known to cause PID. The leakage current can travel through the modules including the surface and the bulk regions. Most detrimental leakage patch is known to be along the surface of the front glass and through the bulk of front glass and the encapsulant.

A number of testing methods were developed to test for PID susceptibility. However, the testing on the identical modules may result in different power losses from PID and the testing method is still unclear to have the correlation to the outdoor PID testing condition. IEC 62804-1 is the primary test procedure standard. It is tested under 60 °C and 85% relative humidity and with an applied voltage equal to the module’s maximum rated system voltage. Another widely used testing method uses a conductive foil (e.g. Al or Cu) on top of the (mini)-module and applies a potential difference between the foil and the solar cell.

 

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Figure 1: Schematic illustration of the cross-section of a PV module indicating the potential difference between the cell and the grounded frame. This potential difference can be in excess of 1000 V depending on the grounding of the PV system. This potential difference can result in the drift of e.g. Na atoms which can result in degradation in the solar cell performance.

 

[1]         W. Luo, Y. S. Khoo, P. Hacke, V. Naumann, D. Lausch, S. P. Harvey, J. P. Singh, J. Chai, Y. Wang, A. G. Aberle, and S. Ramakrishna, “Potential-induced degradation in photovoltaic modules: a critical review,” Energy & Environmental Science, vol. 10, no. 1, pp. 43-68, Jan 2017. Available: http://pubs.rsc.org/en/content/articlehtml/2017/ee/c6ee02271e