PID in Thin Film Modules
In continuation of our previous article on “Introduction to Potential Induced Degradation”, we have compiled research and data, to help our readers understand this mechanism in thin-film modules. In this article, we will also talk about some common performance issues faced by plants with thin film modules and the corrective measures that can be taken to improve performance from these plants.
Thin-film modules hit the solar PV market in India as the second generation of PV modules in 2010. Between the years 2011-2014, the total installed capacity of thin-film modules in India soared to ~150 MWp. Thin-film modules were contributing to ~47% of total installed capacity in 2011 but by 2014, the contribution of thin-film modules fell below 5%, eventually phasing out. Today, we have thin-film plants across multiple locations, which have been facing various under-performance issues, with PID being very prominent.
Mechanism of PID in thin-film modules
Amorphous Silicon (a-Si), CIGS and CdTe all have been reported to be suffering from PID when the cells are negatively biased due to the migration of Na+ ions. Two different scenarios can be observed based on the moisture ingression level:
- In absence of moisture ingression: Na+ is reduced to Na. This leads to darkening effect in Sn2O ( Tin oxide) and ZnO ( Zinc oxide) TCO films. This affects the electrical performance to great extent but can be reversed with reverse bias.
- In presence of moisture ingression: Typically in humid environments, a combination of Na+ reduction and moisture ingression results in non-reversible electrochemical corrosion of SnO2 based TCO film with bar graph pattern.The chemical reaction occurs via three separate steps and it is as follows:
- Na+ reduction: Na+ + e- ↔ Na
- Formation of elemental hydrogen: H2O + Na ↔ NaOH + H
- Oxidation reduction reaction: 4H + SnO2 ↔ Sn + 2H2O
As result of these reaction adhesion of TCO is disrupted. The accumulation of Na also raises the mechanical stress at the interface, which beyond a certain point, will cause the TCO film to crack and delaminate.
Moisture ingression also depends on the quality of packaging of the modules apart from the environmental conditions (such as temperature and humidity). Compared to the SnO2 based TCO films, ZnO is largely immune to the chemical attack by atomic hydrogen. Therefore de-lamination effects would not be observed in thin film modules with a ZnO based TCO layer.
Detection of PID in thin-film modules
In case of CdTe, CIS and CIGS modules, PID can be detected in the plant through a combination of IV curves, IV data and EL images. In case of a-Si modules, on-site EL images are difficult to capture and therefore we have to rely only on the IV curves for detecting PID. Since the primary cause of PID is the presence of negative bias, higher degradation in the modules towards the negative end of the strings combined with reduced fill factor in those modules is an indicator of PID in the modules.
The EL image below show CIS modules suffering from PID
The graph below shows IV curves for module at the positive end, middle and negative end of a string for A-Si technology. For this string, the average degradation for the positive half of the modules was ~16% and for the negative half of the modules was ~47%, indicating the effect of negative bias.
What to do if you observe PID in the thin-film modules?
In case you observed PID in the modules on the site, you need to check for moisture ingression and measure the power output of all modules in randomly selected strings. Based on what you find, 2 potential measures can be taken:
- In absence of moisture ingression, reverse potential can be applied on the modules to reverse PID. They may be done by using a float controller device
- In presence of moisture ingression, very little can be done to recover power of affected modules. However, we can measure the degradation level and variation in power of in the modules within the same string. If significant mis-match is observed then the modules can be re-distributed to ensure that modules with similar current are connected in the same string. Gain through such re-binning exercise can be simulated and the exercise must be carried out only if the cost-benefit analysis is favourable
As an asset manager or an IPP, 5 things you need to know before taking over a thin-film plant are:
- Since thin-film modules have a frame-less design, they are in general more susceptible to corrosion, which can have serious impact on performance, and hence must be checked
- Being frameless, modules are generally installed using clamps because of which, the possibility of cracks and breakages in the glass is high, which should be checked. Wind load analysis should also be carried out
- Thorough visual inspection is must for breakage, damage analysis and corrosion
- Since the thin film modules degrade much faster, a detailed degradation analysis is must for making more accurate projections about the generation
- Module insulation and leakage analysis is important to identify and prevent insulation issues in the plant
As an IPP or EPC, 5 things you need to know before purchasing thin-film modules are:
- Thin film modules have higher LID so that should be ensured on sample basis and through test reports (possibly through third party agencies) before procuring
- Generally thin film modules have glass to glass design with the back glass being non tempered which is more susceptible to cracks. Mechanical load test and hail impact test should be carried out on sample basis to ascertain mechanical strength of modules
- Since these modules are heavier, they are difficult to carry and need heavier structure, thereby increasing the overall cost of installation
- Thin film modules in general have more bubble formation in the laminates and higher leakage currents and therefore a thorough visual inspection and insulation testing is must
- Since these modules are mostly installed using clamps, wind load analysis should be carried out to validate the strength of fixture