Role of spiro-OMeTAD in performance deterioration of perovskite solar cells at high temperature and reuse of the perovskite films to avoid Pb-waste

As the long-term stability and toxicity of Pb are two profound concerns for the commercialization of Pb-based perovskite solar cells, we have undertaken this study to understand the performance degradation of perovskite solar cells at high temperature (60, 80, 100 and 120 °C) and under a humid (30–50% relative humidity) environment, and then tried to reuse the perovskite films from the degraded cells to recover the cell efficiency so as to avoid dumping of Pb-waste into the environment. As found in this study, the performance of MAPbI₃ (MA = CH₃NH₃) cells using spiro-OMeTAD as the hole transport material (HTM) deteriorated mainly not due to the degradation of the perovskite but because of the modification of the interface between the perovskite and spiro-OMeTAD at high temperature. In addition, the spiro-OMeTAD layer underwent a severe morphological deformation at high temperature, showing large voids in it, which reduced the cell performance further. However, despite the complete reconversion of PbI₂ to perovskite and the replacement of the degraded spiro-OMeTAD film with a fresh layer in the MAPbI₃ cells, the cell performance was not recovered to the initial value because the modified perovskite/spiro-OMeTAD interface became worse after recycling. The thermal stability and performance recovery upon recycling were found to depend on the composition of the perovskite; faster degradation of MAPbI₃ cells with a slight excess of MAI and better stability of cells not containing MA (FA₀.₈₅Cs₀.₁₅PbI₃, FA = CH(NH₂)₂) indicated certain involvement of MA⁺ ions in the alteration of the perovskite/spiro-OMeTAD interface. Unlike the heat-treatment case, MAPbI₃ cells, when exposed to a humid environment, degraded significantly to PbI₂ and the reconversion of this PbI₂ to perovskite followed by deposition of a fresh spiro-OMeTAD layer increased the cell performance but it did not recover to the initial value. This poor recovery in these cells was due to inefficient carrier transport that resulted in a lower photocurrent in the recycled devices.