The emergence of perovskite solar cells (PSCs) recently has brought new hope to the solar cell industry due to their incredible improvement of the power conversion efficiency (PCE), which can now exceed 20.0% within seven years of tremendous research. The efficiency and stability of PSCs depend strongly on the morphology and type of materials selected as the electron transport layer (ETL) in the device. In this review, the functions of the ETL based on titania (TiO₂) in n–i–p architecture PSCs, including planar heterojunction and mesoporous-structured devices, are reviewed in terms of the device performance and stability. Studies found that the application of suitable fabrication techniques and manipulation of the nanostructural properties of TiO₂ are crucial factors in ameliorating the short-circuit current density, J_SC, and fill factor, FF, of PSCs. On top of that, the effect of substituting TiO₂ with other potential inorganic materials like zinc oxide (ZnO), tin oxide (SnO₂), ternary metal oxides, and metal sulphides, as well as organic semiconductors including fullerene, graphene, and ionic liquids, towards the photovoltaic properties and stability of the devices are also elaborated and discussed. Meanwhile, the utilization of non-electron transport layers (non-ETLs), such as alumina (Al₂O₃) and zirconia (ZrO₂), as the mesoporous scaffold in PSCs is found to enhance the open-circuit voltage, V_OC, of the devices.

This review summarizes the current designs and development of new types of organic small molecules as a hole-transporting material (HTM) in a meso-structured perovskite solar cell (PSC). The roles of each layer in the meso-structured perovskite device architecture are elaborated and the employment of new types of organic HTMs in the device is compared with the commercially available HTM spiro-OMeTAD in terms of the properties, device performance and stability. The studies found that nearly half of the new synthesized and pristine HTMs have comparable or better photovoltaic properties than those of doped spiro-OMeTAD. These HTMs have the characteristics of a fused planar core structure with extended π-conjugated lengths and electron-donating functional groups, which are believed to contribute to their high intrinsic conductivity and help make them an alternative to spiro-OMeTAD as a better HTM in meso-structured PSCs. Some of the devices based on the new synthesized HTMs even have longer device lifetimes than their spiro-OMeTAD-based PSC counterparts. Moreover, studies found that the cost per gram (C_g) and cost-per-peak Watt (C_w) of synthesized HTMs can be reduced via minimizing the number of synthesis steps and by optimization of the starting materials in order to yield low-cost HTMs for meso-structured PSC applications.