Perovskite photovoltaic technology has attracted significant attention in recent years due to its rapid efficiency progress and strong potential for low-cost, large-area manufacturing. However, long-term stability remains a major challenge for industrial deployment, making encapsulation a critical process step for protecting devices from environmental stressors while preserving performance [1].

In this work, we investigate the origin of performance losses observed after high-temperature encapsulation of large-area perovskite solar modules. Damp heat testing under IEC standard conditions has demonstrated that this encapsulation method provides excellent protection against ambient atmosphere and moisture ingress. However, a significant Voc reduction is observed for the investigated device architecture. To decouple encapsulation-related thermal effects from other degradation pathways, a controlled thermal annealing on non-encapsulated devices at different temperatures is applied. A direct correlation is established between high-temperature encapsulation and Voc degradation in perovskite modules.

First, thermal annealing at different stages of the fabrication process helps identify the temperature sensitive layer. Device performance is monitored using current–voltage measurements combined with photoluminescence (PL) imaging, providing direct insight into how radiative recombination and charge extraction are affected. A strong correlation between PL quenching and Voc loss confirms the optoelectronic origin of the degradation. SnO₂/ITO electron transport layers are identified as particularly sensitive to thermal treatment, likely due to thermally induced changes leading to energy band misalignment and increased non-radiative recombination. High-temperature processing of SnO₂ transport layers has been shown in literature [2] to affect film morphology and increase non-radiative recombination, supporting our identification of SnO₂/ITO layers as thermally sensitive.

Second, the introduction of neutral interfacial buffer layers aims to distinguish between bulk and interfacial degradation mechanisms. The losses are not mitigated with this strategy indicating that degradation is dominated by bulk layer effects.

As a potential mitigation strategy, light preconditioning is shown to partially recover Voc and overall device performance, suggesting the presence of reversible degradation processes.

Overall, this study provides a layer-resolved understanding of thermal annealing losses and demonstrates the value of photoluminescence as a non-destructive diagnostic tool. Interface engineering, low-temperature encapsulation approaches, and light preconditioning emerge as promising pathways to improve module stability. These findings contribute directly to the industrialization and reliable large-scale deployment of perovskite photovoltaic modules.