PID (Potential Induced Degradation) is a phenomenon that is causing degradation in solar cells due to a negative voltage on the solar cells with respect to the frame of the module. In PV systems, about half of the solar modules operate under such condition in a configuration with modern high voltage inverters.
This negative voltage is physically moving (ionic migration) sodium ions (Na+) through the cover class of the solar module and the EVA sheet towards the solar cells. Once the sodium ions get to the solar cells they diffuse into the cells causing an electrical degradation of the generated current. That is the reason for efficiency loss in the module that can be up to 80%.
Fig 1. Cross section of a conventional c-Si PV module constructed with a glass-EVA encapsulant-cell-EVA encapsulant-backsheet package
Former conventional generations of solar modules
In the first generations of solar modules, manufactures commonly used soda-lime glass containing 13% to 14% Na2O. Also the EVA material did contain significant sodium substances (see solar model anatomy fig 2). Hence, the presence of Na in the solar module encapsulation materials made the system quite vulnerable for PID.
Modern PID free solar modules
For the last 5 years, major efforts have been made by PV module manufactures to try and overcome PID in field installations and power plants. They applied PID preventative measures on 2 levels.
- On cell level
Application of SNx (Silicon Nitride) as antireflective coating is a very dense layer and acts as an important barrier against migration of sodium ions towards the cell.
- On module level
The use of aluminosilicate glass containing less than 3% (weight percentage) of Na2O significantly improves the PID performance. In recent years, PID-resistant EVA materials were also developed with a higher bulk resistivityand they have also shown improved PID-suppressing properties.
Fig 2. Solar module anatomy: module sandwich
How PID resistant are PID free modules?
Initially the improved encapsulation materials will prevent a rapid degradation for PID. Howeverjust like any module, the PID free module will get mechanical and thermal stress in the field. Sodium ions are ample available in the external atmosphere as it is commonly present in salt. Sooner or later these modules will get under mechanical and thermal stress just like the conventional modules did. These phenomena will cause micro cracks and pinholes in the encapsulation materials.
These micro cracks will allow the sodium ions from external sources to migrate through these defects and again find their way to the solar cell. It might take longer but also the labeled “PID free modules” will be susceptible to PID.This statement is also confirmed by independent research of Photovoltaik-InstitutBerlin in their publication at PV Magazine Webinar on PID by Dr. Lars Podlowski.
Due to environmental circumstances such as temperature, humidity and applied voltage even PID free PV modules remain vulnerable for PID.