# Developments on Perovskite Solar Cells (PSCs): A Critical Review
## 4 Perovskite Solar Cell Device Architecture  
PSCs were first reported by [PERSON] (2009), whereby the perovskite ABX\({}_{3}\) organic-inorganic lead halide material was used as a light absorber in sensitized solar cells. In this study, the perovskite device structure preserved the same cell structure as DSSCs. Hence, it was assumed that PSCs operate similar to DSSCs. However, as knowledge developed in this field, it was found that the fundamental basis of operation in PSCs was distinctive from DSSCs [15; 37]. In a PSC device, the perovskite material (ABX\({}_{3}\)) is deposited onto a conductive glass substrate bearing an n-type (compact or mesoporous) electron transport material (ETM), which is followed by the deposition of a p-type hole transport material (HTM). When light is absorbed by the perovskite material, electrons are excited to the LUMO, leaving behind holes in the perovskite HOMO, generating excitons that diffuse within the perovskite layer towards the opposite ETM and HTM contacts. At the interface with the perovskite material, the ETM and HTM selectively transport charge carriers. Electrons travel from the anode electrode to the external circuit, generating electricity across an electrical load. The perovskite absorber regenerates as electrons recombine with holes at the cathode electrode (e.g., Au, Ag, Pt, and Al), minimizing charge imbalance at opposite electrodes [39]. A conventional PSC has a device architecture similar to a p-n junction solar cell with the perovskite intrinsic (i) absorber sandwiched between n-type and p-type semiconducting materials, as illustrated in Figure 4a; this device architecture is referred to as the mesoscopic-(n-i-p) device architecture. The realization that the perovskite material can, on its own, facilitate charge transportation and further the rearrangement of the device architecture  
Figure 3: Band–gap structure of a cubic (**a–c**) and tetragonal (**d–f**) MAPbI\({}_{3}\) crystal structure. Total and partial density of states of MAPbI\({}_{3}\) and ions (**g**) [35; 36]. has contributed significantly towards improving the device performance and stability [40]. Figure 4b-d illustrates such devices improvements. The planar-(n-i-p) architecture in Figure 4b results from PSC devices without the mesoscopic metal oxide required in DSSCs as a structural scaffold. A planar architecture is a promising architecture for large surface area manufacturing on flexible substrates because this architecture is fabricated from low-temperature preparation methods [41]. The planar-(n-i-p) devices are reported to reach high efficiencies of up to 23.6%, slowly approaching the 25.5% of mesoscopic-(n-i-p) architecture. Although the (n-i-p)-device architecture demonstrates high efficiencies, their stability still suffers due to the negative impacts (e.g., degradation) imposed on the perovskite layer by the acidic and hydrophilic nature of the traditional Spiro-MeOTAD and PEDOT:PSS HTMs [26; 42; 43]. The planar-(p-i-n) in Figure 4c and mesoscopic-(p-i-n) in Figure 4d are inverted structures of the conventional-(n-i-p) architecture, whereby the sequence of deposition is reversed. The inverted architecture partially overcomes the instability issues by reducing the corrosive nature of the HTMs on the device light-active layer. Optimizing the device thickness by removing the mesoporous ETM and the change in device architecture has shown to be efficient for high-performance PSCs. Another significant route for improving the device PCE and operational stability lies in designing the perovskite ABX\({}_{3}\) composition from simple chemical approaches. Figure 5 shows the basis of ABX\({}_{3}\) design. Figure 5a,b present the IPCE and I-V characteristic performance from MAPbI\({}_{3}\) and MAPbBr\({}_{3}\) single halide PSCs. The distinctive difference in the iodide and bromide performance presented sufficient motivation in achieving hybrid materials of organic-inorganic perovskite materials through ABX\({}_{3}\) designing [5].