# Developments on Perovskite Solar Cells (PSCs): A Critical Review
## 4 Perovskite Solar Cell Device Architecture
(b) Time-resolved photoluminescence spectrum and (c) photovoltaic performance in planar heterojunction solar cells of MAPbI\({}_{3}\) and MAPbI\({}_{3\times}\)Cl\({}_{x}\) at various wt% addition of MACI [20]. In a similar study, [PERSON] (2014) characterized the effect of halogen (x% = I, Br, and Cl) composition in mixed-halide PSCs' optical and PV properties. In Figure 7b, a shift towards lower wavelengths is observed in the absorption spectra as indicated by the arrow. This absorption behavior further confirms the successful bandgap tunability in Figure 7a reported above [47]. In Figure 7c*1, a significant reduction in J\({}_{\text{Sc}}\) and V\({}_{\text{oc}}\) is observed. This low device performance is attributed to the shift towards lower wavelengths reported from the absorption spectra in Figure 7b. Figure 7c*2 shows an appreciable increase in J\({}_{\text{Sc}}\) and V\({}_{\text{oc}}\) with the introduction of a third halogen (Cl) component. However, the shift in absorption further persists to decrease the device performance in the triple halide PSCs. The complex perovskite crystal structure ABX\({}_{3}\) offers the advantage of modelling the resulting absorber layer from stochiometric solution preparation methods [54, 55]. [PERSON] (2017) studied cell properties of a mixed-cation (Cs/FA/MA) perovskite device and obtained characteristic results of a PSC with the device configuration of FTO/TiO\({}_{2}\)/Meso-TiO\({}_{2}\)/Cs/FA/MA/Spiro-MeOTAD/Au as shown by the cross-sectional image in Figure 8d [48]. In previous studies of MA/FA perovskite thin films, it is reported that the MA+ cation at small ratios acts as a FA+ nucleating site, increasing the degree of crystallinity in the FA photoactive film, thus also enhancing the light sensitivity of the +FA thin films. However, the MA/FA systems showed traces of small phase impurities that eventually affect overall morphology and stability. It is reported that the incorporation of cesium (Cs) inorganic cation improves MA/FA intrinsic properties [56, 57]. Figure 7: (**a**) A plot of diffusion length (L nm) against bandgap energies of aged and non-aged MAPPb(I\({}_{1-x}\)Br\({}_{x}\))\({}_{3}\) perovskite thin films. (**b**) Absorption spectra of mixed-halide perovskite film. (**c**) I–V curve in a hybrid mixed-halide PSC [28, 47]. Figure 8a shows an SEM micrograph image of a neat MA/FA system with a bumpy distribution of grains and the presence of surface grooves, indicating a low level of adhesion between grain interfaces; in Figure 8b, at a 5 wt% addition of Cs, the crystal grains increase in size and the surface boundary between the grain becomes diluted, this might suggest an improved surface adhesion between grains and, hence, a high degree of crystallinity. However, at high additions of 10 wt% Cs, as indicated in Figure 8c, the surface adhesion disappears and grains are reduced to a small particle size, increasing the possibility of crystal defects. Figure 8e shows the characteristic absorption band of a perovskite absorber layer with absorption over the UV-vis spectrum. Figure 8f shows XRD patterns at 0, 5, and 10 wt% Cs addition, all showing a perovskite characteristic peak at a \(\sim\)14\({}^{\circ}\) diffraction angle, corresponding to the (110) plane. Defect peaks at diffraction angles lower than 14\({}^{\circ}\) are seen for the neat MA/FA system; this might be assigned to the surface boundaries between grains. At 5 wt% Cs addition, the defect peaks disappear and a there is a significant increase in perovskite peak intensity at a 14\({}^{\circ}\) diffraction angle, suggesting an increase in crystallinity. This behavior is however not favored for higher concentrations of Cs addition. The inorganic Cs metal encourages crystallization and increases the device stability, illustrated by the occurrence of little to no J-V hysteresis between the forward and reverse bias measurements, as shown in Figure 8h. Electricity is generated by the direct absorption of sunlight in a PV solar cell.