**2.1 Materials for habitats**

Constructing a habitat on the Moon can be done in two ways, by delivering materials from Earth and by using local resources. Although the latter option is more sustainable, the first one cannot be completely avoided. An important consideration that needs to be made when choosing materials is the type of habitat. NASA considers several types of habitat for different use, namely rigid (metals, alloys, and concrete) [18], inflatable (e.g., inflatable concrete [19]), or hybrid structures, as well as underground construction [20]. Depending on the type of habitat, different materials will be used [16, 21]. For example, unprocessed lunar regolith may be used for radiation shielding of habitat (e.g., lunar regolith geopolymer) [22–25], as well as for construction when converted into concrete [26, 27], 3D-printed [28–30], or processed into other construction material (e.g., bricks and glass) [16, 21]. For materials delivered from Earth, it is crucial to ensure their low weight, as well as resistance to very high and very low temperatures (which change from 127°C in the daytime to 173°C at night on the Moon surface) and radiation, durability, reusability, and structural reliability [16].

Metals and alloys are essential structural materials for construction given their compressive strength and good tensile properties and for other applications, such as energy carrier/storage (wires) [31] or equipment (e.g., excavation tools, molds, and rovers) [32]. Al, Ca, Fe, Ti, and Mg are the most abundant metals in the lunar regolith, which also contains smaller amounts of Ni, Cr, Mn, Zr, and V [5, 20]. These metals—together with Si, also abundant on the Moon—can be used to produce alloys. However, only Fe can be easily separated from regolith (using magnets). Other metals are present in the form of oxides and thus have to be obtained by manufacturing. Metal and alloy manufacturing will be extremely important for the exploration of the Moon as they represent an essential part of the construction and are critical ingredients for most technologies.

### **2.2 Materials for energy production**

One of the crucial steps toward the Moon exploration and settlement is a reliable energy technology for electricity generation and power storage [33, 34] that would withstand the temperature gradients, high levels of radiation, and impact. The primary energy sources considered for future crewed lunar missions are solar power [35, 36], nuclear power [37], and fuel cells [38, 39]. Other ways may include the production of electricity from the excess heat from the sunlight collected by an "evergreen" inflatable dome [40]. In this chapter, we focus on solar cells, a safe and reliable source of electricity in space.

In the past decades, solar cells for space applications have evolved from singlecrystalline Si-based cells to multi-junction (MJ) ones based on GaInP, GaAs, and Ge [41–43]. A promising class of materials for next-generation lightweight and highpower-conversion efficiency [44] solar cells are hybrid organic-inorganic perovskites (HOIPs) [45–47], which are considered as potential candidates for use on future lunar bases [34].

HOIPs possess a unique combination of properties, such as enhanced charge carrier mobility [48–51], diffusion length, and lifetime [48, 52, 53], high optical absorption [54, 55], and low production costs [56], representing a paradigm shift in solar cell technology [57] on Earth [58] and for space applications [59–62]. Given their flexibility [63], low weight, small dimensions (0.5 μm as compared to 200 μm for Si solar cells), the possibility of *in situ* manufacturing via 3D-printing techniques [60, 64, 65] at low temperature, and their high resistance to radiation [60, 66–71], HOIPs qualify as exceptional candidates for easily deployable and resilient solar cells in space missions.
