OUTDOOR STABILITY OF Cs_1-z(MA_xFA_1-x)_zPb(I_yBr_1-y)₃ PEROVSKITE SOLAR CELLS

While record perovskite solar cells (PSCs) efficiency already approaches that of conventional silicon solar cells, their long-term stability remains a major issue. Some of the performance losses were reported to be reversible, thus degradation during the day is expected to be partially compensated by night-time recovery in realistic operational conditions. Here we present the indoor continuous light stressing and outdoor stability studies of the perovskite cells, with the structure of glass/ITO/SnO₂/Cs_1-z(MA_xFA_1-x)_zPb(I_yBr_1-y)₃/Spiro-OMeTAD/Au.

The results revealed the presence of both reversible and irreversible degradation mechanisms in the studied PSCs, whose relative contribution vary at different ageing stages. To study reversibility of photo-induced losses, the cells were stressed under continuous illumination and then allowed to recover in the dark. We found full restoration of the cells efficiency in the dark at early degradation stages. Later some irreversible losses occurring in parallel with reversible ones were revealed. Further cell degradation resulted in very different dynamics: J_SC and FF exhibited a pronounced drop in the dark but strong improvement under the light. The dark recovery of the cell performance at this degradation stage was shown to be possible but requiring much longer than one night, so it appears as irreversible degradation in real operational conditions. We coined term “apparently irreversible degradation” for such type of behavior. The interplay of degradation mechanisms with different dynamics results in two types of outdoor behavior: fresh cells degrade under illumination and partially recover during the night, while significantly aged cells degrade at night and partially recover under sunlight. Possible underlying mechanisms responsible for reversible, irreversible and “apparently irreversible” processes are discussed on the basis of the analysis of evolution of the cell current-voltage curves, photoluminescence kinetics and Raman spectra.