In only few years, perovskite solar cells (PSC) have emerged as a new family of photovoltaic devices presenting a surprising power conversion efficiency (PCE) evolution from 3.8 % in 2009 to 23.7 % in 2018.^[1] Although the astonishing PCE evolution, these cells suffer of serious stability issues when submitted to high temperatures, oxygen and moisture.^[2] It is important to understand the impact of these factors, since in real operation conditions the device is exposed to a whole range of humidity values and can easily reach temperatures up to 85 ºC under direct sunlight; this may happen even in countries with mild climate.^[3]

This work focuses on the impact of oxygen and humidity in the triple-cation perovskite solar cells – Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃. For the humidity tests, three different scenarios were analyzed: i) water content in the solvents used for preparing the devices, resulting from the contact with a humidified atmosphere; ii) different levels of ambient humidity during preparation of devices and; iii) effect of ambient relative humidity over time (stability). An in-house assembled glove box that allows controlling ambient humidity and carrier gas (N₂ or air) was developed for studying the PSC stability to the humidity. The humidity effect was assessed based on the photovoltaic performance of several batches of the cells; the morphology and crystallinity were also analyzed by SEM and XRD, respectively. This relation between photovoltaic performance, morphology and structural analysis allowed to concluded about the main degradation pathways in devices under different humid environments.

The use of humidified solvents to prepare the precursor solutions of perovskite displayed a minor effect on the cell PCE for relative humidities between 0 – 60 %. However, as expected, when the cells were prepared in a humidified environment the PCE of the cells decreased with the humidity, mainly due to the loss of V_oc and J_sc. Surprisingly, in an inert atmosphere (nitrogen) and below 10 %RH the cells displayed no PCE losses, while at 30 % RH they displayed half of the expected PCE. In an air atmosphere, the relative humidity value for no PCE losses drops to 5 % RH. Since, only the perovskite layer was prepared inside the humidified glove box and the other layers were prepared in a dry nitrogen glove box, it is possible to conclude that the humidity is affecting the crystallization step of the perovskite. Although by XRD it was not noticeable the PbI₂ peak, indicating a complete conversion of FA perovskite into the black phase; from the SEM images it was possible to observe the formation of a non-uniform perovskite layer, enabling a direct contact between hole extraction and mesoporous layers. For devices kept in the laboratory bench, with RH between 30 % - 50 %, it was observed an inevitable loss of performance, over time. The most affected performance parameter was J_sc, while the V_oc was stable for more than 1000 h. This work reveals that the restricted conditions normally used to prepare high efficiency PSCs, like dry inert gases and low concentrations of oxygen,^[4,5] are not actually needed if triple-cation formulation is used. However, the encapsulation of the PSCs remains very important to protect the devices from extreme ambient conditions and for preventing the leakage of lead compounds from the perovskite layer.