274
M. Pravettoni
It can give rise to very slow degradation of maximum power of up to 5% or more
(degradation up to 7% has been reported in hot Mediterranean climate, see [10]). The
origin of LETID is suspected to be a high concentration of weakly bonded hydrogen
near the rear side passivation layers (some authors therefore refer to it as HydrogenInduced Degradation, or HID). With high temperature and illumination, hydrogen
is released and can bond to silicon independently of the dopant species, thus causing
degradation that can occur in both p-type and n-type silicon
13 (see [11]). In reality
a fully satisfying and scientifically robust understanding of LETID is still largely
missing at the time of writing.
Detection/prevention: no method fully agreed to date.
10.4.5.3 Potential-Induced Degradation (PID)
Probably the most sadly famous example of midlife failure is the third one: Potential-Induced Degradation (PID). Several physical phenomena can cause PID, and
unfortunately some are not yet fully understood (see [7]). What is commonly agreed
is that PID is caused by leakage currents that flow due to the large potential difference
in field installations (typically a few kilovolts), between the module frame
14 and the
solar cells within the module (or vice versa). PID is more severe at high temperature
and high humidity. It has been shown that PID can cause up to 30% loss of the initial
power at the system level. It may affect both p-type and n-type modules: on the other
hand, in HJT modules
15 the TCO layer is conductive and experimental results have
shown that this prevents the PID effect.
16
PID can be considered a partially recoverable failure mode. In fact, since PID
originates from the potential difference between the cell and the module frame and
mounting structure, a “PID box” (i.e. a voltage generator connected in series) can
actively reverse this potential difference during the night. This will reverse the current
flow that caused PID and restore the original PID-free state: the process can be
repeated periodically every night to neutralize PID failures.
Detection: electroluminescence in the field (but takes time and is expensive).
Prevention: by dedicated module (e.g. glass-glass, TPO or ionomers instead of EVA
as encapsulant) and system design.
13 LETID effect in HJT modules is under investigation at the time of writing.
14 Glass-glass frameless modules are in principle less prone to PID. Nevertheless, since glass-glass
modules are usually mounted in the field with metallic clamps, a large potential difference between
the cells and the clamp can still generate PID even in glass-glass modules.
15 High-quality HJT modules are not laminated with EVA but with TPO, edge sealing is done with
butyl and not with silicone and glass-glass modules are preferably used, instead of glass-backsheet
modules. These very points lead more likely to PID-free behavior.
16 The counter argument is that current migration into TCO layers (especially migration of Na +
ions, the most popular candidate in theoretical explanations of PID), might lead—depending on
the quality of the TCO—to TCO corrosion. This may be considered to be a side-effect of PID on
those HJT modules, which use low-quality TCO. Good quality TCO layers are dense and so thick
(100 nm, 1000 atoms), that Na + ions cannot easily migrate into the HJT cell.
M. Pravettoni
It can give rise to very slow degradation of maximum power of up to 5% or more
(degradation up to 7% has been reported in hot Mediterranean climate, see [10]). The
origin of LETID is suspected to be a high concentration of weakly bonded hydrogen
near the rear side passivation layers (some authors therefore refer to it as HydrogenInduced Degradation, or HID). With high temperature and illumination, hydrogen
is released and can bond to silicon independently of the dopant species, thus causing
degradation that can occur in both p-type and n-type silicon
13 (see [11]). In reality
a fully satisfying and scientifically robust understanding of LETID is still largely
missing at the time of writing.
Detection/prevention: no method fully agreed to date.
10.4.5.3 Potential-Induced Degradation (PID)
Probably the most sadly famous example of midlife failure is the third one: Potential-Induced Degradation (PID). Several physical phenomena can cause PID, and
unfortunately some are not yet fully understood (see [7]). What is commonly agreed
is that PID is caused by leakage currents that flow due to the large potential difference
in field installations (typically a few kilovolts), between the module frame
14 and the
solar cells within the module (or vice versa). PID is more severe at high temperature
and high humidity. It has been shown that PID can cause up to 30% loss of the initial
power at the system level. It may affect both p-type and n-type modules: on the other
hand, in HJT modules
15 the TCO layer is conductive and experimental results have
shown that this prevents the PID effect.
16
PID can be considered a partially recoverable failure mode. In fact, since PID
originates from the potential difference between the cell and the module frame and
mounting structure, a “PID box” (i.e. a voltage generator connected in series) can
actively reverse this potential difference during the night. This will reverse the current
flow that caused PID and restore the original PID-free state: the process can be
repeated periodically every night to neutralize PID failures.
Detection: electroluminescence in the field (but takes time and is expensive).
Prevention: by dedicated module (e.g. glass-glass, TPO or ionomers instead of EVA
as encapsulant) and system design.
13 LETID effect in HJT modules is under investigation at the time of writing.
14 Glass-glass frameless modules are in principle less prone to PID. Nevertheless, since glass-glass
modules are usually mounted in the field with metallic clamps, a large potential difference between
the cells and the clamp can still generate PID even in glass-glass modules.
15 High-quality HJT modules are not laminated with EVA but with TPO, edge sealing is done with
butyl and not with silicone and glass-glass modules are preferably used, instead of glass-backsheet
modules. These very points lead more likely to PID-free behavior.
16 The counter argument is that current migration into TCO layers (especially migration of Na +
ions, the most popular candidate in theoretical explanations of PID), might lead—depending on
the quality of the TCO—to TCO corrosion. This may be considered to be a side-effect of PID on
those HJT modules, which use low-quality TCO. Good quality TCO layers are dense and so thick
(100 nm, 1000 atoms), that Na + ions cannot easily migrate into the HJT cell.