Guiding principle for crystalline Si photovoltaic modules with high tolerance to acetic acid

Abstract

A guiding principle for highly reliable crystalline Si photovoltaic modules, especially those with high tolerance to acetic acid generated by hydrolysis reaction between water vapor and an ethylene-vinyl acetate (EVA) encapsulant, is proposed. Degradation behavior evaluated by the damp heat test strongly depends on Ag finger electrodes and also EVA encapsulants. The acetic acid concentration in EVA on the glass side directly determines the degradation behavior. The most important factor for high tolerance is the type of Ag finger electrode materials when using an EVA encapsulant. Photovoltaic modules using newly developed crystalline Si cells with improved Ag finger electrode materials keep their maximum power of 80% of the initial value even after the damp heat test at 85 °C and 85% relative humidity for 10000 h. The pattern of dark regions in electroluminescence images is also discussed on the basis of the dynamics of acetic acid in the modules.

Degradation of copper‐plated silicon solar cells with damp heat stress

“…It has also been noted that this issue seems to be more prevalent in Ag‐screenprinted cells of earlier vintages, and cells manufactured more recently have undergone up to 10 000 h of DH exposure with EVA encapsulant with minimal degradation 18 . Tuning the microstructure of the Ag paste, or replacement of the vulnerable lead oxide with another material (e.g., bismuth oxide), or the introduction of a sacrificial material, may make it less susceptible to corrosion 37,38 .…”

“…It has also been shown that a non‐permeable backsheet, for example, glass, prevents this degradation, as do non‐EVA encapsulants 16,17 . This degradation seems to affect cells of earlier vintages more than recent vintages, suggesting that recent Ag‐screenprint pastes have been made more tolerant to acetic acid 18 . In all of these studies, it is worth mentioning that degradation is limited to metal components—the underlying cell's antireflection layer, p‐n junction, and wafer are unaffected.…”

Detection of acetic acid produced in photovoltaic modules based on tin film corrosion during damp heat test

“…These times correspond to the reacted mass m AA of 52.5 µg for the 70 nm sensors and 120 µg for the 160 nm sensors. Masuda and Hara 16) determined the acetic acid concentration in EVA by ion chromatography during the DH test using the same module without tin sensors [Module C, Fig. 2(b) in Ref.…”

“…6 also support this observation, since EL images of both sensors of different thicknesses appear bright until about 2000 h, and the dark region expands from the edges to the central region between 2000 and 4000 h. 29) The dark region is where the current injection is blocked because of the increased contact resistance due to electrode corrosion. 16) Therefore, it is considered to also indicate that a certain amount of acetic acid reacted with the electrode there. Considering this, the advantage of the tin sensor we developed is that acetic acid can be detected both before EL images become dark and after EL darkening saturates, by adjusting film thickness.…”

“…6,[11][12][13] The production pathway of acetic acid during the DH test is considered to be the hydrolysis of the EVA encapsulant by water from moisture penetrating into the back sheet of a PV module. 13,14) Then, the generated acetic acid attacks not the pn junction 6,15,16) but the interface layer between the Ag finger electrodes and the Si emitter layers through the corrosion of lead compounds contained in a screen-printed Ag paste, resulting in the increase in contact resistance. [16][17][18][19][20] Therefore, precise measurement and detection of the acetic acid generated is essential for predicting the lifetime of a PV module.…”