Effect of Environmental Humidity on the Electrical Properties of Lead Halide Perovskites

In large quantities, water is detrimental to lead halide perovskite solar cells, mainly because of the decomposition of the perovskite layer. In contrast, the presence of small quantities of water has been observed to play a key role in the crystallization of the perovskite and in the performance of the corresponding devices. However, the exact role of water during the operation of perovskite solar cells is still under debate. In this paper, impedance spectroscopy is used to analyze the changes produced by environmental humidity in the electronic properties of methylammonium lead triiodide perovskite. Our results show that water absorbed from environmental humidity induces a huge increase in the capacitance of this material. This capacitance can reach values as large as the accumulation capacitance found in devices based on perovskite, which is responsible for the characteristic large hysteresis observed between forward and reverse J – V curves. In parallel to this outstanding rise of the capacitance, water absorption produces a significant rise of the conductivity, in agreement with previous reports in the literature. An activation energy of 0.52 eV is found for electronic transport, a value in line with the activation energy of ionic transport found in the literature, which suggests ambipolar diffusion as the transport mechanism that links these two phenomena.