# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 4 Impact of Thermal Stress on Various Layers and Possible Solutions
### Electron Transport Layers  
The most commonly used ETL in perovskite solar devices is titanium dioxide (TiO\({}_{2}\)). However, non-stoichiometric defects, such as oxygen vacancies and titanium interstitials, can also form in this layer [50] and cause deep sub-band gap trap states. Perovskite devices with TiO\({}_{2}\) ETL show rapid degradation under illumination. Recently, [PERSON] et al. reported the degradation of the photo conversion efficiency of perovskite solar cells due to the decomposition of the perovskite film at the interface with TiO\({}_{2}\)[51]. As shown by the SEM images, degradation is initiated at the perovskite/TiO\({}_{2}\) interface in a TiO\({}_{2}\)-based device. The authors postulated that the trapped charges at the interface were responsible for the irreversible degradation of perovskites along grain boundaries [51]. To enhance stability, compact TiO\({}_{2}\) ETL can be replaced with C\({}_{60}\), which shows much more stable performance when deployed in the device structure.  
[PERSON] et al. used C\({}_{60}\) as interface modification layer for TiO\({}_{2}\) and as an electron-accepting layer [52; 53]. They performed steady-state PL measurements over time at the PL peak of 775 nm in the devices with TiO\({}_{2}\) and C\({}_{60}\) as ETLs (See Figure 7a,b) [53]. The C\({}_{60}\) layer was shown to increase the photoluminescence decay and exhibited more stable device performance under full-spectrum illumination for 500 h, as shown in Figure 7c [53]. The control device with TiO\({}_{2}\) as an n-type layer showed a notable reduction in the photovoltaic performance with time as shown in Figure 7d, while the C\({}_{60}\) based device exhibited much more stable performance when annealed at 60 \({}^{\circ}\)C for 500 h. To overcome the non-stoichiometric defects of TiO\({}_{2}\), [PERSON] et al. [50] used Al-doping for their TiO\({}_{2}\), and developed a sol-gel deposition process with an Al-containing precursor. Photo-thermal deflection spectroscopy (PDS) measurements indicated that Al-doping in TiO\({}_{2}\) reduced the number of trap states  
Figure 6: Temperature dependent (**a**) absorbance and (**b**) photoluminescence spectra of MAPbI\({}_{3}\) when the temperature is increased from 28 \({}^{\circ}\)C to 85 \({}^{\circ}\)C. Reprinted with permission from [47]. (**c**) photoluminescence (PL) intensity plotted as a function of temperature showing three transitional PL peaks during the heating process. (**d**) PL measurements as a function of temperature during the heating and cooling processes (room temperature to 52 \({}^{\circ}\)C). (**e**) Best-fit of lifetime decay using trapping and detrapping model. (**f**) Evolution of the lifetime of MAPbI\({}_{3-x}\)Cl\({}_{x}\) during heating and cooling processes. The trap depths of 145 meV for temperatures in the range 40–70 \({}^{\circ}\)C and 1.1 eV above 70 \({}^{\circ}\)C were obtained from a quantitative analysis of the Arrhenius plots. Reprinted with permission from [48].  
and they found that Al-doping not only passivated non-stoichiometric defects but also improved the stability of encapsulated devices.