The meteoric rise of the photo-conversion efficiency (PCE) of organic-inorganic hybrid halide-based perovskite solar cells from 3.8% to 25.5% within less than a decade makes them a promising candidate for efficient solar energy harvesting. However, In the era of rapid increment of PCE based on advanced fabrication techniques, the instability of the perovskite is remained unresolved. The poor stability against moisture and heat of organic cations such as methylammonium lead triiodide (MaPbI₃) and formamidinium lead triiodide (FaPbI₃) limits device lifetime. Apart from these, the organic additives such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tert-butylpyridine (tBP) which are commonly used for the improvement of the conductivity of Spiro-MeOTAD hole transport layer (HTL) are also hygroscopic and promotes degradation. Moreover, the fabrication of the perovskite solar cells completely depends on the sophisticated glove box or dry rooms. Several approaches have been proposed to overcome such issues and the replacement of organic cations by inorganic cations such as Cs⁺, Rb⁺ etc. are one of them.

The complete substitution of organic cation by Cs⁺ inorganic cation which results in the formation of CsPbX₃ (X: I, Br, Cl) generic formula based all-inorganic perovskite material possess a dramatic improvement of thermal stability, photo-stability (under UV illumination) and moisture resistance. Nowadays all-inorganic perovskite solar cells shows an increasing trend in device performances and a series of breakthroughs have led to a PCE exceeding 20% for the mixed halide inorganic perovskites. However, due to the deviation of the Goldschmidt tolerance factor from the ideal range of cubic phase the stability CsPbI₃ black phase is always challenging. The partial incorporation of Br^- with I^- at the X-site can improve the black phase stability. Based on that, several mixed halides of all-inorganic perovskite materials were reported so far such as CsPbBr₂, CsPbI₂Br, CsPb(I_0.8Br_0.3)₃. Apart from the phase stability, the Br^- incorporation can also play a significant role in the enlargement of open-circuit voltage (V_oc) due to higher bandgap. Additionally, the suitable selection of the electron transport layer (ETL) is another approach to achieve a higher V_oc. Several conventional materials were used as ETL starting from TiO₂, SnO₂ to ZnO and for the SnO₂ ETL layer, a 1.41 V of highest V_oc was observed so far. Even though the all-inorganic perovskite solar cells perform dramatically well on the device stability in comparison with organic-inorganic mixed halide perovskite solar cells, still they are lagging in terms of the overall PCE. The low crystal quality of the inorganic perovskite is the main culprit behind this due to this poor carrier transportability and high charge carrier recombination was observed. Therefore, the improvement of the crystal quality by optimizing nucleation and growth mechanism can lead to the form of high-quality crystals. Additionally, the solubility of the Cs-based precursors limits the film thickness which leads to the formation of unfavorable film morphology and resulting insufficient light absorption and low current density. Similarly, due to the high moisture sensitivity of the I rich perovskites undergoes through phase transformation from black to non-perovskite yellow phase. The above problems can be minimized up to some extent by DMSO solvent incorporation which dramatically improves the solubility of the Cs-based precursors and helps to form a uniform and compact thin film. However, the DMSO incorporation can efficiently hinder the crystallization rate. In addition to that, the incorporation of the 2D organic compounds can efficiently improve the phase stability and moisture resistance. Based on the knowledge of solubility and phase transition, several new approaches on the fabrication techniques were suggested such as multi-step coating, vacuum deposition, non-traditional gradient thermal annealing method etc.

Undoubtedly, the single junction all-inorganic perovskite solar cells show promising performance, but in order to meet energy requirements, it is necessary to go beyond the SQ limits and which is only possible through multi-junction solar cells (i.e., Tandem cells). In tandem solar cells a wide-bandgap materials based solar cells are fabricated on top of the bottom cell (e.g., Si Solar cell) in such a way that the top cell absorbs visible light on the other hand infrared light is captured by the bottom cell. According to the bandgap values, the inorganic perovskites are somewhat inappropriate for single-junction solar cells eventually resulting in light absorption loss. However, the all-inorganic perovskite solar cells can potentially serve as top cells in the tandem solar cells due to their suitable higher bandgap.

Therefore, the all-inorganic perovskite materials have the potential to serve as next-generation photovoltaic materials due to their several intrinsic properties. However, a rigorous study on the crystal nucleation, growth, controlled stoichiometric ratio & incorporation of advanced fabrication strategies it is possible to make a highly efficient, stable inorganic perovskite solar cell that can challenge the Si photovoltaic industry. Additionally, it also has the potential to serve as an efficient top cell for the tandem devices which may break all the records of the photovoltaic community.

Possible ways to stabilize cubic black phase of CsPbI3