Remarkably, an early work by Noh et al. [84] demonstrated the fabrication of colorful PSCs via band-gap tuning. Subsequently, new investigations on tunable structural color [85, 86] and neutral-colored devices [87, 88] have come out. These are important contributions towards building applications, such as in replacing windows, roofs, and even walls. Regarding the general performance, and as pointed in the early work by Yin et al. [89], the two other most characteristic perovskites are the hybrid halide CH3NH3PbI3-xClx and the formamidinium (FA) cation composed HC(NH2)2PbI3 [90]. About the former, the chlorine incorporation has been found to mainly improve the carrier transport across the heterojunction interfaces [91] while in FAPbI3-based devices a broader absorption toward the infrared region [92] has been obtained. In this sense, the bandgap engineering is a clear pathway for augmenting PCE; for instance, band-gaps of have been reported [93, 94] by using tin compounds like MASnxPb1-xI3 and MASnI3, which is within the ideally optimal band-bap range (1.1 − 1.4 eV) for a single-junction device [95].
Furthermore, the development of lead-free PSCs is another crucial issue aiming at avoiding the risks due to the toxicity of Pb. Here three important alternatives can be highlighted as the most promising: (i) tin-based perovskites and (ii) antimony- and bismuth-based perovskites [96]. The use of ASnX3 compounds has demonstrated improvement in stability and theoretical studies point out that an absorption coefficient similar to that of MAPbI3 can be obtained. The Sb and Rb-based perovskites, on the other hand, has been proposed for high-bandgap PV application. A recent review by Shi et al. [97] deals with these subject.
Supplementing the above discussions, and as a handy tool for practical use in the modeling and general comprehension of PSCs, the Table 1 presents a summary of experimental parameters for the set of materials often used in this kind of devices.
Closing this section we recommend the work of Habibi et al. [134] for considering the fabrication and optimization of the absorber materials. Also the paper by Xinzhe et al. [135], that summarizes the recent progress in the synthesis of low-dimensional perovskites.
  
Table 1. Some experimental reports from literature on bandgap energy Eg, work function Φ, electron affinity χn and room temperature dielectric constant ar for several materials typically used in PSCs. Here Φ and χn are given in absolute values with respect to the vacuum level.
 
Materials Role Eg Φ χn ar
eV ref. eV ref. eV ref. ref.
FTO TCO 4.0 − 4.5 [98, 99] 4.4 − 5.0 [100, 101] 5.6 [100] 3.7 [99]
ITO 3.5 − 4.0 [102, 103] 4.4 − 4.8 [101, 102, 104] 4.1-4.5 [103] 4.0 [105]
TiO2 ETM 3.2 [23] 3.7 − 4.2 [76, 106] 3.6-4.1 [23, 76] 18-22 [26]
PCBM 2.1 [107] 4.4 − 5.0 [108, 109] 2.7-4.2 [110, 111]∗ 3.4-3.9 [112, 113]
Spiro-OMeTAD HTM 3.0 − 3.6 [4, 50] 3.9 − 5.2 [114, 115] 2.11 [4]∗ 3.0 [116]
PEDOT 1.5 − 2.1 [61, 117] 4.9-5.3 [115, 118, 119] 2.7-3.0 [117, 120]∗ 3.5 [121]
MAPbI3 Light Absorber 1.51 − 1.61 [65, 67] - - 3.9-4.8 [122, 123] 22-35 [19, 124, 125]
MAPbI1-xClx 1.57 − 1.74 [126, 127] - - 3.9 [123] 18-29 [125, 128]
FAPbI3 1.48 − 1.52 [19, 65, 127] - - 4.2 [129, 130] 47-49 [130, 131]
Ag Metal Contacts - - 4.8 − 5.2 [81, 132, 133] - - - -
Au - - 4.3-4.4 [81, 132, 133] - - - -
∗Measurements made via cyclic voltammetry where a correction of LUMO relative to the vacuum level (−4.44 eV) is considered for the electrochemical scale.

III.      DEGRADATION AND STABILITY OF PSCs

 Early structural studies by Stoumpos et al. [65] stated that although MAPbI3 is stable in air for months, meaning that its bulk properties are retained, an important surface effect take place given that it is affected by humidity and lose their crystalline luster after a couple of weeks. MAPbI3 degradation in humid air proceeds by two competing reactions: (i) the generation of a MAPbI3 hydrate phase by H2O incorporation and (ii) the PbI2 formation by the desorption of CH3NH3I species [136]. Subsequently, loss of Nevertheless, provided the material is properly encapsulated (or measured in lab conditions under inert atmosphere) devices are still unstable. In particular, it has been shown that ionic transport induced by the electrical field can lead to the chemical reactivity of the external contacts with iodide ions [145, 146]. In addition, it is still not totally clear as whether MAPbI3 is photostable [147].
Furthermore, the role of selective contacts on stability seems to be serious. For instance, an earlier study by T. Leijtens et al. [148] identified a critical instability in mesoporous TiO /MAPbI  Cl  arising from light-induced desorption CH3NH3  and I species and decomposition into PbCO3, Pb(OH)2, and PbO take place [137]. Illustratively, Noh et al. [84] reported an exposition to relative humidity of 55 % during 24 hours at room temperature as critical for the MAPbI3 stability, which could be observed by an abrupt drop of more than the half of the performance efficiency in devices and a remarkable color change from dark brown to yellow. This feature can be observed in Figure 4.