LeTID Testing To Mitigate Investment Risk

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The PERC technology (Passivated Emitter and Rear Cell) typically boosts the module power by about 10%. This boost however is threatened by a PERC-related degradation mechanisms, which might eat up this gain in the field completely:

In addition to the known Light Induced Degradation (LID) effect, the Light and Elevated Temperature Induced Degradation (LeTID) was reported for modules based on p-type crystalline silicon PERC cells, independent of its Boron-Oxygen content, in 2012 by Ramspeck [1].

Difference between LID and LeTID on mono and multi silicon

Unlike LID, LeTID shows:

1. A longer degradation interval in the field, i.e. years instead of days

2. A larger degradation depth, i.e. up to -10% instead of -1% (multi) and -3% (mono)

3. A recovery to the initial power in the field, but unfortunately again over an interval of several years depending on the local temperatures and irradiations. Figure 1 shows results from field testing by Kersten in 2015 and further results on degradation and recovery intervals in the field by Kersten in 2017 [2].

The mechanism behind LeTID is still not fully understood. The existing theoretical models predict higher degradation depths for multicrystalline than for monocrystalline PERC modules. Despite this, PI Berlin has found similar high degradation depth and durations on monocrystalline PERC modules in laboratory tests as well, as shown in Figure 2.

Further, comparison of LeTID Test results on two multicrystalline modules and 8 monocrystalline modules are shown in Figure 3.

LeTID test results on ten different modules, two multicrystalline modules and 8 monocrystalline modules. The degrees of degradation range from zero to minus four percent. Source: PI Berlin

How to supress LeTID

Some manufacturers have enforced counter-measures to suppress LeTID by either changing their cell process or applying a super-fast degradation and recovery procedure at the end of their cell line using high irradiance and temperatures or even both, e.g. Jinko’s Andrea Viaro presentation under https://www.pv-magazine.com/webinars/secure-your-investment-discover-urgently-required-game-changing-solutions-in-managing-letid/

Electro luminescence images (EL) of a multicrystalline PERC module in the
field with cumulated irradiation data in kWh and power loss in % under accelerating
conditions, i.e. open circuit and thermal insulation of the module backside.
Initial 48h 96h 168h 336h 936h

How to protect your project against LeTID

The current certification procedures does not cover LeTID checks for the module qualification after IEC 61215:2016. The new draft version or this standard intends to include LeTID tests but the procedure is still under discussion. Mitsui Chemicals is involved in this process with an expert from Japan’s National Committee of IEC TC82/WG2.

To prevent commercial damage for the investor’s side, it is recommended to test LeTID sensitivity on samples from the actual shipment lots, see also [3]. PI Berlin and Mitsui recommend and offer laboratory tests to check the LeTID sensitivity on a project by project basis to protect investor risk. At this point in time, LeTID is still not yet fully understood nor is it sufficiently controlled by the photovoltaic community.

[1] K. Ramspeck, et al., “Light induced degradation of rear passivated mc-Si solar cells”, 27th EUPVSEC 2012, pp. 861–865

[2] F. Kersten et al., 31st EUPVSEC (2015), S.1822 and 33rd EUPVSEC (2017), S.1418

[3] https://www.pv-magazine.com/2019/06/15/the-weekend-read-is-perc-still-bankable/

Dr. Tsuyoshi Shioda Researcher (Mitsui Chemicals Inc.)
Dr. Paul Grunow Director (Photovoltaik Institut Berlin Ag)
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