# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 4 Impact of Thermal Stress on Various Layers and Possible Solutions
### Metal Contacts  
The most commonly used electrodes for high efficiency perovskite solar devices are gold (Au) or silver (Ag) [65; 66; 67]. Gold is expensive and can be typically replaced by Ag as an electrode in perovskite  
Figure 9: (**a**) Schematic of a solar cell where carbon nanotube/polymer composite is used as a hole-transport layer. (**b**) Photographs illustrating the visible degradation of the perovskite layer for various HTLs. (**c**) Temperature dependent power conversion efficiency for perovskite solar cells as a function of temperature for different HTLs. (**d**) Absorption measurements and (**e**) XRD patterns of sample with different HTLs before and after annealing for 96 h. Reprinted with permission from [64]. Copyright (2014) American Chemical Society. PMMA: poly (methyl methacrylate); SWNT: single-walled nanotubes.  
solar cells [18; 68], but both Ag and Au layers show degradation when exposed to thermal stress [69; 70]. [PERSON] et al. reported the migration of gold through HTL into the perovskite materials at 70 \({}^{\circ}\)C [70]. [PERSON] et al. showed, in an inverted perovskite solar cell, that ions migrating from MAPbJ\({}_{3}\) thin film diffused through the Phenyl-C61-butyric acid methyl ester (PCBM) ETL and accumulated at the Ag surface in N\({}_{2}\) at 85 \({}^{\circ}\)C [71]. The proposed mechanism of the Ag-electrode induced thermal degradation of the inverted PSCs is shown in Figure 10a. The loss of methylammonium iodide (MAI) occurred at the grain boundaries (GBs), and upon thermal annealing, the AgI, MA\({}^{+}\), and I\({}^{-}\) ions migrate to reconstruct the GBs, creating more defects in the bulk grains and at the MAPbJ\({}_{3}\)/PCBM interface. Multiple reports have investigated an alternative to Au and Ag based electrodes by employing carbon as a back-contact material [16; 72]. [PERSON] et al. [16] fabricated a hole-conductor-free PSC based on a triple layer architecture with printed carbon electrode. The triple layer consists of mesoporous TiO\({}_{2}\) and ZrO\({}_{2}\), as well as a 10 \(\upmu\)m thick carbon layer. A schematic is shown in Figure 10b. The perovskite precursor solution was infiltrated by drop casting using a carbon counter electrode. The fabricated perovskite was (5-AVA)\({}_{\mathrm{x}}\)(MA)\({}_{1-\mathrm{x}}\)PbI\({}_{3}\), where 5-AVA is 5-ammoniumvaleric acid iodide. The device structure displayed excellent stability and an encapsulated device showed stable performance during outdoor operation for seven consecutive days (Figure 10c) in Jeddah, Saudi Arabia [16]. Moreover, the encapsulated device remained thermally stable when annealed at 80-85 \({}^{\circ}\)C for 90 days, in the dark. Hence, the use of carbon as a back-contact can be beneficial to make stable PSCs at a temperature of 100 \({}^{\circ}\)C, representing progress toward the commercialization of PSCs [73]. [PERSON] et al. [73] also fabricated a three-layer printable HTL-free MAPbJ\({}_{3}\) PSC with a mesoporous carbon back contact and UV-curable sealant and performed thermal stability tests of over 1500 h at 100 \({}^{\circ}\)C. Interestingly, they found that the position of the UV-curing glue considerably affected device stability. The PCE of side-sealed devices (Figure 10e) remained almost constant during the thermal test over 1500 h (Figure 10f), while for the over-sealed device (Figure 10d), the PCE deteriorated to 20% of the initial value during the 100 \({}^{\circ}\)C thermal stress test after only 30 h.