Heat-assisted recombination-driven defect annihilation: on-site prospect for SHJ module stability

Binbin Xu^1,2, Kai Zhang^1,2, Yanxin Liu^1,2, Karsten Bittkau¹, Andreas Lambertz¹, Uwe Rau^1,2, Kaining Ding^1*

¹IMD-3 Photovoltaics, Forschungszentrum Jülich GmbH, Germany

²Jülich-Aachen Research Alliance (JARA-Energy) and Faculty of Electrical Engineering and Information Technology, RWTH Aachen University, Germany

^*E-mail: k.ding@fz-juelich.de

Abstract

Crystalline silicon (c-Si) solar cells based on silicon heterojunction (SHJ) technology are gaining an increasing share in the rapidly growing global photovoltaic (PV) market. At present, SHJ cells and modules are produced on the gigawatt scale and deployed in large solar plants. Furthermore, new architectures such as heterojunction back-contact (HBC) and perovskite–silicon tandem devices have emerged from SHJ technology. Within this scope, the long-term performance and reliability of SHJ modules are of primary concern to the PV community.

Over the past decade, extensive efforts have been devoted to elucidating the degradation mechanisms of SHJ cells and modules. Degradation may already occur during the storage of cell products before module fabrication. At the module level, the main degradation modes can be classified as ultraviolet-induced degradation (UVID), damp-heat (DH) -induced degradation caused by the heat-assisted water/moisture ingress, potential-induced degradation (PID), and degradation of encapsulation (such as discoloration, delamination). Except for the latter, degradation of the hydrogenated amorphous silicon (a-Si:H) layers has been identified or suggested as a key origin of these effects. In all these cases, a reduction in the open-circuit voltage (V_OC) is commonly observed, linked to the deterioration of surface passivation through the rupture of Si–H bonds and the resulting increase in interfacial dangling bonds.

In contrast to these degradation phenomena, light soaking (in some case the heat-assisted intensive light soaking) treatments have demonstrated a capability to improve the passivation quality of SHJ cells. The transferability of such light-soaking benefits to SHJ modules has also been investigated. Reports from several groups indicate that light soaking can suppress certain degradation modes, especially UVID, in SHJ modules. Even under one-sun light soaking, V_OC losses induced by UV exposure can be partially recovered, which can be attributed to a reverse process that re-saturates interfacial defects. This observation raises the question of whether a similar recovery driven by natural sunlight under outdoor conditions could occur during field operation, for which systematic studies are still scarce. Moreover, most investigations to date have relied on indoor accelerated testing, whereas direct field data remain limited.

This work presents the results of a one-year outdoor exposure of SHJ single-cell laminates operated under either open-circuit (OC) or short-circuit (SC) conditions. To evaluate the influence of the actual UV dose reaching the solar cells on passivation quality, three types of encapsulants—UV-transmitting, UV-cutoff, and UV-down-shifting—were employed. The benefits of UV-cutoff and UV-down-shifting encapsulations demonstrate the occurrence of UVID during in-field operation. Furthermore, all module types exhibited less V_OC loss under OC conditions than under SC, suggesting the existence of a competing mechanism that heals and partially compensates the passivation degradation. Combining these findings with insights from indoor UV-acceleration and LiSo experiments, an on-site heat-assisted, recombination-driven defect-annihilation mechanism is proposed to describe this stability-governing process.