II.1.      The electron transport material

 
As indicated in Figure 2, the role of ETM is usually played by the TiO2. This material is transparent to visible light, has low absorption and high refractive index (e.g. at λ = 550 nm the refractive index and the extinction coefficient are respectively n = 2.54 and k < 10−4 ) [21] and it mainly occurs in three crystalline polymorphs: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic). The rutile bulk phase is thermodynamically the most stable while anatase is the most interesting for use in high surface area photocatalytic and PV devices [22]. The anatase TiO2 is an indirect bandgap semiconductor that presents intrinsic n-type conductivity [23]. The nature of this conductivity comes from oxygen vacancies and/or titanium interstitials and can be improved by the incorporation of shallow donor impurities (e.g., Nb, F, and H) [24]. For the donor carrier density of the order of 1018 cm−3 has been reported [25, 26] and, interestingly, it has being pointed that the distribution of these donors levels in the TiO2 follows an exponential density of states (DOS) bellow the conduction band [27, 28].
Importantly, the layers of TiO2 for PV applications can be grown by many low cost techniques (e.g. spin coating and spray pyrolysis [23]) obtaining crystalline films as well as nanostructured coatings (e.g. nanotubes, nanosheets, nanorods and nanofibers [29,30]) on top of the FTO. The work by He et al. [31] comments on several TiO2 nanostructures for PSCs, despite that in regular structure it is presented in two main configurations: (i) as a flat compact layer and (ii) as a mesoporous scaffold deposited on top of a planar compact film.
Among the different alternatives as ETM some examples can be mentioned: Al2O3 [3234], SnO2 [3537], ZnO [3841], ZrO2 [42,43] and carbon/graphene derivatives [4446]. About the latter, the works by Acik & Darling [44] and Covallini & Delgado [46] are illustrative. Nonetheless, particular attention is paid here to the fullerene derivative PCBM, extensively used in organic solar cells (OSCs) [47] and typically employed as n-type electrode for the devices with inverted configuration. In its two flavours, the [6,6]-phenyl-C-61-butyric acid methyl ester (PC60BM) and the [6,6]-phenyl-C-71-butyric acid methyl ester (PC70BM), the PCBM film is deposited on top of the perovskite layer constituting the interface with the counter electrode. For this materials it is known that the relatively low electron mobility and relatively big size molecules influences the charge transporting and phase separation. Anyway, the optimization of the electron’s selective extraction is currently a priority via the development of new materials and/or by modifying and/or mixing the already known ones. In connection to it, we list the review by Yang et al. [48].
 

II.2.      The hole transport material

 
The characteristic HTM that is deposited above the perovskite in the regular configured PSCs is the 2,2’(7,7’)-tetrakis-(N,N-di-p-methoxyphenyl - amine)9,9’ - spiro- bifluorene, earlier referred as spiro-OMeTAD (see Figure 2). This organic semiconductor has been extensively studied due to its applications as HTM in solid-state DSSCs [49]. In its pristine state, spiro-OMeTAD presents low intrinsic hole–mobility and –conductivity that has been found to increment the cell series resistance. Consequently the material needs to be p-doped to increase the charge carrier density. This occurs naturally during exposure to oxygen and light (so-called photodoping), nevertheless, several chemical dopants have been investigated to controllably oxidize the material [50, 51].
Alternatively, among those used as hole-selective contacts, most of the reported materials are organics or hybrid compounds, as summarized by Calió et al. [52]. As inorganic HTMs we list here CuSCN [5354], CuOx [55], NiOx [4156] MoOx [57] and VOx [58], being CuOx the one with best reported PCE, as highlighted in the survey by Rajeswari et al. [59].
At this point we once more emphasize on the predominant HTM in the PSCs with inverted structure: the poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(4-styrenesulfonate) (PSS). The PEDOT:PSS is an oxidized electro-chemically stable conjugated polymer that has been extensively investigated given its applications in OSCs and light-emitting devices (p-LEDs) [60]. Its characteristic moderate transparency [6062], that increases with polarization [63], allows to deposit it on top of the surface of indium tin oxide (ITO) letting the light pass for being absorbed at the perovskite, that is the next layer to be deposited in the inverted configuration.
 

II.3.      The light harvesting material

 
Among perovskites it seems that the presence of halides is required when seeking PV applications, as pointed by Li et al. [64], and until now it is CH3NH3PbI3 (referred as MAPbI3 in the next) the most representative in this field. As systematically described by Stoumpos et al. [65], in its high temperature α cubic phase, the methyl ammonium organic cation CH3NH3+ (MA) is A in the perovskite general formula while the lead and the halogen are B and X, respectively (see Figure 1).
The MAPbI3 has been found to be a direct band-gap semiconductor [19, 66, 67] with high absorption coefficient (104 − 105 cm−1 in the visible range) [68, 69] and large carrier mobility (µ = 102 − 103 cm2V−1s−1) [65, 69]. The deposition of the material follows easy solution-based fabrication processes, e.g. dip and spin coating [68, 70], and the layers can be obtained with good crystalline quality and at relatively high reaction rates, even when processed at low  temperatures. Importantly, the electrical intrinsic conductivity of MAPbI3 can be modified from p-type to n-type by controlling growth conditions, i.e. by managing the concentration of donor or acceptor shallow defects [71]. Calculated transition energy levels of MAPbI3 point defects in the literature [72, 74] have shown as dominant shallow levels close the valence band edge: the vacancies of lead (VPb) and methyl ammonium (VMA) and the antisite substitution of MA in a lead site (MAPb). On the other hand, the shallow levels near the conduction band bottom are the iodine vacancies (VI ) and interstitial methyl ammoniums (MAi). Other deep energy levels in the bandgap stand as Shockley-Read-Hall non-radiative recombination centers, which reduce minority carrier lifetimes, and therefore the open circuit voltage (VOC) [75]; e.g. the donors interstitial leads (Pbi) and leads in iodine sites (PbI), and the acceptors iodines in methyl ammonium (MA) and lead (IPb) sites.
The tunable conductivity character of MAPbI3 has produced a significant scattering in the representation of the energy band diagram of PSCs. In this context many studies have determined the work function of the different constituent layers via photoelectron spectroscopy and/or the measurement of the contact potential difference at the interfaces by using Kelvin probe force microscopy [27]. As result, and only citing a few examples, several works [71, 7678] report that a p-n-n+ junction is formed by the p-HTM and the both n-type ETM and MAPbI3, considering the depletion region towards the perovskite/HTM interface. Some other authors [26,71,79,80] have found a p-p-n junction considering a p-type MAPbI3 with the consequent space charge region towards the perovskite/ETM interface. And a third group of publications [6, 81, 82] supports that a p-i-n junction, with intrinsic MAPbI3, takes place in some PSCs. Anyway, it is clear that the fabrication procedure and the interface engineering directly affects the conductivity, e.g. in the study by Wang et al. [83] a quantification of this effect with respect to the precursors concentration and the thermal annealing is made. The three possible situations for the band diagram in equilibrium short circuit condition are presented in the Figure 3.