# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 4 Impact of Thermal Stress on Various Layers and Possible Solutions
### Hole Transport Layers  
The most commonly used hole transport layer in inverted perovskite device structure is poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). However, the prime disadvantage of the PEDOT:PSS based hole transport layer (HTL) is that it leads to the chemical instability of perovskite solar devices due to its acidity and highly hygroscopic nature [54; 55; 56]. [PERSON] et al. [57] investigated the thermal stability of PEDOT:PSS and found a decrease in the electrical conductivity of perovskites with aging due to the shrinkage of PEDOT conductive grains. Moreover, the XPS and UPS measurements suggested a broken conformational PEDOT:PSS structure, which results in the thermal instability of this material [57]. Despite this, a recent study [58] showed the potential for improved efficiency and stability of perovskite solar cells with enhanced PEDOT: PSS HTLs. This was done by fabricating PEDOT:PSS monolayers, using spin coating and ultrapure water rinse, and constructing ultra-thin layer of PEDOT:PSS. The water rinsed device sustained more than 50% of its stability, attributed to the less hygroscopic nature of the thin PEDOT:PSS layer [58]. Another improvement approach is to use an oxide-based hole transporting material such as NiO, which exhibits better air [59; 60; 61] and thermal stability [62]. [PERSON] et al. [63] synthesized cross-linked films using an arylamine derivative (N4,N4\({}^{\prime}\)-Di(naphthalene-1-yl)-N4,N4\({}^{\prime}\)-bis(4-vinylphenyl)biphenyl-4,4\({}^{\prime}\)-diamine) (VNBP) [63]. A two-step process was employed to construct a thermally cross-linked hole extracting layer, as shown in Figure 8a. In the first step, a layer of VNBP is deposited and then thermally cross-linked; in the second step, an interface doping layer of MoO\({}_{3}\) is simply deposited on top of the HTL. The device architecture consisted of a double layer of VNBP-MoO\({}_{3}\), and showed a steady-state efficiency of 16.5% and improved thermal stability compared to the control device. The device performance shown in Figure 8b,c indicates that the devices with cross-linked VNBP-MoO\({}_{3}\) HTL retained almost 90% of their initial performance with low  
Figure 7: Steady state PL measured over time with (**a**) compact TiO\({}_{2}\) or (**b**) C\({}_{60}\) acceptor electron transport layers (ETLs) at short circuit and different forward biases. (**c**) Time resolved PL decays for the devices before and after applying 1 V forward bias. The pre-biased devices are measured within 3 s after applying the forward bias. (**d**) Photovoltaic parameters extracted from J-V curves for both cells after heat treatment at 60 \({}^{\circ}\)C for 500 h. J-V curves were measured every 65 min. Reprinted with permission from [53]. Copyright (2015) American Chemical Society. hysteresis for 1 h of annealing at 110 \({}^{\circ}\)C. Figure 8d illustrates the optical microscopy images of the control and cross-linked device before and after annealing. No morphological deformation was found for the cross-linked device, while the control device with spiro-OMeTAD HTL exhibited the chain-like structures which lead to irreversible degradation. The thermally cross-linked devices were found to be thermally stable for 30 days during thermal annealing at 110 \({}^{\circ}\)C for 1 h and high humidity exposure at 70% relative humidity, as evidenced by the XRD patterns (Figure 8e). The authors suggested that the exceptional barrier properties of cross-linked organic hole transport material with metal oxide allowed the device to maintain high performance under thermal stress for a longer period than that found in previous studies. [PERSON] et al. investigated the effect of the HTL on the thermal degradation of perovskite solar cells [64]. A schematic illustration of the investigated device structure is shown in Figure 9a. The authors investigated thermal stability by employing the three most common and highly efficient HTLs, Spiro-OMeTAD, P3 HT, and PTAA.