Not All That Glitters Is Gold: Metal-Migration-Induced Degradation in Perovskite Solar Cells

Abstract

Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is known about their long-term stability under thermal stress. So far, stability reports have hinted at the importance of substituting the organic components, but little attention has been given to the metal contact. We investigated the stability of state-of-the-art PSCs with efficiencies exceeding 20%. Remarkably, we found that exposing PSCs to a temperature of 70 °C is enough to induce gold migration through the hole-transporting layer (HTL), spiro-MeOTAD, and into the perovskite material, which in turn severely affects the device performance metrics under working conditions. Importantly, we found that the main cause of irreversible degradation is not due to decomposition of the organic and hybrid perovskite layers. By introducing a Cr metal interlayer between the HTL and gold electrode, high-temperature-induced irreversible long-term losses are avoided. This key finding is essential in the quest for achieving high efficiency, long-term stable PSCs which, in order to be commercially viable, need to withstand hard thermal stress tests.

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode

“…The low formation energies and the low diffusion barriers for ${\mathrm{Cu}}_i^{+}$, ${\mathrm{Ag}}_i^{+}$, and ${\mathrm{Au}}_i^{+}$ suggest that Cu, Ag, and Au can diffuse into MAPbI 3 when they are used as electrodes; however, only Au occupying the Pb site can induce deep levels inside the band gap, resulting in efficient nonradiative carrier recombination and solar cell degradation. This result explains the experimentally observed Au‐diffusion‐induced solar cell degradation 31. Thus, when Au is the electrode, care should be taken to prevent Au diffusion into MAPbI 3 .…”

“…${\mathrm{Au}}_{\mathrm{Pb}}^{}$ can trap both electrons and holes and the trapping levels are very deep, which should render ${\mathrm{Au}}_{\mathrm{Pb}}^{}$ a highly efficient nonradiative recombination center. This may explain the observed significant solar cell degradation upon Au diffusion from the Au electrode into MAPbI 3 31. On the contrary, the performance of MAPbI 3 solar cells with Cu electrodes was found to be stable even after prolonged annealing at elevated temperatures 34.…”

“…The unintentional impurity incorporation in MAPbI 3 could lead to significant device degradation. Many noble and transition metals (e.g., Au, Ag, Cu, Cr, Mo, Ni) have been used as electrodes in MAPbI 3 solar cells,31, 32, 33, 34, 35, 36, 37, 38, 39 which are potential sources of impurity contamination for the MAPbI 3 light absorption layer. The large dielectric constant in MAPbI 3 should promote the formation and diffusion of not only native defects but also impurities.…”

“…Even if there is a HTL separating the MAPbI 3 layer and the metal electrode, the metal atoms may still diffuse through the HTL causing contamination of the MAPbI 3 layer 31. A recent experiment based on the solar cell in the FTO/TiO 2 /MAPbI 3 /HTL/Au configuration showed that Au can diffuse into MAPbI 3 at 70 °C, leading to significant solar cell degradation 31.…”

“…A recent experiment based on the solar cell in the FTO/TiO 2 /MAPbI 3 /HTL/Au configuration showed that Au can diffuse into MAPbI 3 at 70 °C, leading to significant solar cell degradation 31. Coating the Au electrode with a thin Cr layer provided a stable but substantially lower PCE (13% with the Cr layer vs > 20% without Cr).…”