the perovskite [6].
2.3 Working Mechanism of PSC
The working principle of the PSC can be understood by the working of the DSSC and organic photovoltaic. In PSC, the perovskite is used as a light sensitizer, the same as dye in DSSC. The three functions performed by PSC are formation of the exciton (electron-hole pair) by the adsorption of the photon, charge transport, and charge extraction. When light falls on the perovskite, excitons are formed. Because of the difference in the binding energy of exciton of perovskite material, free carriers (free electron and hole) are formed from exciton, which can either generate current or can recombine. Because of the low recombination rate of free carriers in perovskite material and high charge mobility, the diffusion distance and lifetime of the carrier are long. And as a result, PSC is able to achieve such a high efficiency. Highly crystalline CH3NH3PbI3 perovskite material is able to achieve charge carrier diffusion length as long as 10 micrometers [9]. As shown in Figure 2.4, the electron is generated in perovskite and then moves to the electron transporting layer (ETL) and finally to the anode. Simultaneously, holes generated in perovskite, move to the hole transporting layer (HTL) and finally to the cathode. The electron migrates to the anode which can be FTO or ITO. The hole migrates to the cathode, which is either Ag or Au [10].
Figure 2.4 Working of the perovskite solar cell [11].
2.4 Device Architecture
Device arrangement is definitely one of the most significant aspects for the performance of the PSC. Depending on the position of the transport layer in the exterior portion, PSC can be classified into two configurations like mesoporous structure and planar structure as depicted in Figure 2.5. These two configurations are further classified into regular (n-i-p) and inverted (p-i-n) as shown in Figure 2.6.
Other two configurations are also possible like ETM-free structure and HTM-free structure, where P-contact is ETM, whereas N-contact is HTM. The brief overview of mesoporous structure and planar structure is discussed further in the chapter.
2.4.1 Mesoporous Structure
A typical n-i-p mesoporous structure is shown in Figure 2.6.a, ETM is on the exterior portion. The first arrangement of the PSC to be tested was a conventional n-i-p mesoporous structure in which the light-harvesting dye was replaced by the lead halide perovskite materials (CH3NH3PbBr3 and CH3NH3PbI3 ) in a traditional DSSC [3]. Mesoporous structure of PSC is widely used because of its high porosity and high surface area. As shown in Figure 2.6.a, the mesoporous n-i-p PSC consists of FTO, ETM, a mesoporous layer of metal oxide, perovskite layer, HTM, and an electrode layer. In mesoporous p-i-n PSC, the HTM layer is on the exterior portion as shown in Figure 2.6.b. The perovskite material adheres to the mesoporous structure to increase the light-absorbing area of perovskite, which results in increased efficiency. The mesoporous structure results in efficient crystallization of perovskite and electron injection [9]. There is a depletion region at the TiO2 side of the TiO2/MAPbI3 interface, which provides contribution in separation of photogenerated charge carriers and hence it results in high performance of TiO2-based cells [14]. TiO2 mesoporous layer helps to transport the electrons, block holes, and inhibit the recombination of the electron and hole pair. The electron diffusion length and recombination lifetime is governed by the mesoporous layer of TiO2 and hence is not varied by perovskite composition [15].
Figure 2.5 Depiction of two main types of PSC [12].
Figure 2.6 Schematic diagram of four types of perovskite solar cell. (a) n-i-p mesoporous structure, (b) p-i-n mesoporous structure, (c) n-i-p planar structure, (d) p-i-n planar structure [13].
The most typical mesoporous framework material is TiO2. Bi et al. achieved PCE of 20.8%, which contains TiO2 as the mesoporous layer and Spiro-OMeTAD as the electron and hole specified contacts [16]. The limitation of using TiO2 as a mesoporous layer is comparatively low electron mobility [17]. In place of TiO2, Al2O3, and ZnO have also been used as a mesoporous layer [18]. ZnO has two orders of magnitude of electron mobility higher than TiO2 [19]. The recombination rate of charge is significantly slower in TiO2 as compared with Al2O3 [20]. Also, Zn2SnO4, BaSnO3, and SrTiO3 are tested as mesoporous layers. Zn2SnO4-based PSC showed fast electron transport (up to 10 times) and greater charge collection capability as compared with TiO2-based PSC with the same physical and chemical conditions [21]. BaSnO3 has a large bandgap and relatively higher electron mobility than TiO2. BaSnO3 was able to achieve 12.3% but has a recombination rate greater than TiO2 [22]. SrTiO3 had achieved 1.01 eV open-circuit voltage, which is 25% higher than TiO2 [23]. The most common hole transport material is Spiro-OMeTAD(2,2′,7,7′-Tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene). An alternative to Spiro-OMeTAD, spiro[fluorene-9,9′-xanthene], achieved the PCE of 19.8% with low fabrication cost [16]. KTM3 showed higher Voc (1.08V) and FF (78.3%) than Spiro-OMeTAD [24]. The commonly used counter electrode materials are Ag, Au, and Pt. For large-scale and continuous production of PSC containing the mesoporous layer of TiO2, the electrospray deposition method is more viable [25].
2.4.2 Planar Heterostructures
Planar heterostructure is an evolution of the mesoporous structure. The main difference in the planar heterostructure is the absence of the mesoporous layer. The perovskite material is sandwiched between ETM and HTM. Hence, it results in a simpler structure. The planar structure can achieve high PCE even in the absence of a mesoporous layer by carefully controlling the interfaces between layers. There are two interfaces present. First is in between ETM and perovskite and second in between perovskite and HTM. Therefore, electron and hole pairs are separated quickly and efficiently by these two interfaces. The planar structure gives better insight in the understanding of the mechanism of photon absorption and charge separation, which will be useful for better optimization of the PSC. The configuration of planar heterostructure is FTO, ETM, perovskite layer, HTM, electrode layer. FAPbI3 solution-processed planar heterostructure PSC showed PCE of 14.2% with large diffusion length and large tunable bandgap [26]. The planar heterostructure PSC constructed with vapor deposition showed PCE of 15% [27]. J-V hysteresis significantly depends on HTM in planar heterostructures. However, it became negligible when Spiro-OMeTAD was replaced with poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) or any other inorganic HTM [28]. Moreover, J-V hysteresis also depends on the voltage scan direction, scan rate, and range [29]. In the inverted planar heterostructure (p-i-n), HTM is deposited before ETM. The holes travel themselves in an inverted
