Critical failure of flexible perovskite solar mini-module by water vapor ingress under prolonged damp-heat test over 4000 h

The understanding of moisture-induced degradation is one of the major issues towards long-term stability of perovskite solar cells (PSCs) which severely limit their practical use. Herein, the degradation behavior of photovoltaic performance on water ingress of flexible methylammonium lead halide (MAPbI 3 ) PSC mini-modules with 3 × 3 cm 2 in size was discussed. For this purpose, the modules proceed the damp-heat (DH) test up to 4650 h at 85 °C/85 % relative humidity and water ingress was systematically controlled by varying water vapor transmission rates (WVTRs) of the barrier film from 0.005 to 0.44 g/m 2 /day. Upon DH testing, the evolution of solar cell parameters revealed that the power conversion efficiency (PCE) was gradually reduced over DH time. Remarkably, encapsulation with barrier films of high WVTRs (0.019–0.44 g/m 2 /day) caused ‘critical failure’ of module efficiency, evidently observed at the same level of accumulated water vapor. The phase decomposition emphasized that the critical failure mainly stemmed from the partial formation of intermediate MAPbI 3 monohydrate due to the water vapor permeated into encapsulated module, particularly in cases with high WVTRs. This hydrate intermediate accelerated the perovskite decomposition and suddenly reduced its module lifetime. On the other hand, the module encapsulated using a proper barrier film with low WVTR of 0.005 g/m 2 /day exhibited outstanding module stability by nearly 80 % of PCE upon 2700 h of DH test and no critical failure. This finding demonstrated significant improvement in module durability through the selection of barrier films, thereby extending the device lifetime and enhancing its long-term suitability for the potential application in photovoltaic industry. • The degradation mechanism of flexible perovskite solar cell (PSC) mini-modules was systematically studied under damp-heat testing, representing degradation behaviors of modules under commercial-scale production. • The ‘critical failure’ of module efficiency, caused by the decomposition of absorber layer was obviously detected in encapsulated modules with low-qualitied barrier film. • The proper encapsulation by high-qualitied barrier film effectively mitigated moisture-induced degradation, leading to an outstanding long-term stability up to 4650 h of DH testing.