Perovskite, an organic–inorganic hybrid material, tends to be a promising light-harvesting material. In 2009, the Miyasaka group [1] reported a perovskite solar cell (PSC) of 3.8% using a DSSC system configuration with liquid electrolyte based on MAPbI₃ (MA = CH₃NH₃⁺). Park group [2] obtained a nearly doubled power conversion efficiency (PCE) of 6.5% in 2011 using a high concentration perovskite precursor solution. Since then, several efforts have been made to enhance PSC photovoltaic efficiency from various angles, including perovskite layer fabrication methods, interface engineering, cell architecture design, and development of the hole transporting materials (HTM), an electron transporting materials (ETM). A certified PCE of 22.7% has already been achieved via the above optimization [3]. Although the PCE currently available is appealing, the PSCs still have low stability (thermal, light, and moisture stability), which hinders their commercialization.

The discovery of high-efficiency and highly stable perovskite solar cells has sparked extensive research, which is still ongoing ^[4][5][4,5]. Particularly, organometallic semiconducting perovskite has a direct band gap with high absorption coefficients [6] that enables efficient light absorption in ultra-thin films. Furthermore, it has a long diffusion length ^[7][8][9][7,8,9], low exciton binding energy ^[10][11][10,11], high carrier mobility ^[12][13][12,13], and simple and easy preparation techniques [14] that help to get high efficiency and low-cost showing promising alternative to the conventional crystalline silicon-based solar cell. Moreover, perovskite materials can be implemented in two different cell structures, either as planer (n-i-p) or inverted (p-i-n) architecture. Moreover, both architectures could be (i) regular structures in which no mesoporous layer is employed, and (ii) mesoscopic structures where a mesoporous layer is needed. The significant improvement in efficiency already achieved in all kinds of architecture, and the stability of PSCs remain the key concerns for the researchers at present time. Many changes were made to the working electrode, the electron transport layer (ETL), and the hole transport layer (HTL) to improve their stability and charge transport properties. The hole transporting materials is a very much important factor in PSCs to achieve high efficiency and performance. It acts as the mediator to transfer positive charges (Holes) between the perovskite and counter electrode [15]. Particularly, highest efficiency (PSCs) are achieved with organic HTL such as 2,2,7,7-tetrakis-(N,N-di-pmethoxyphenylamine)-9,90-spiro-biuorene(spiro-MeOTAD) [16]. The other most commonly used organic HTMs are poly(3,4-ethylene dioxythiophene) (PEDOT) or poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) [17], poly-[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5 thiophenediyl] (PCDTBT) ^[18][19][18,19], poly-[3-hexylthiophene-2,5-diyl] (P3HT) ^[20][21][20,21], 4-(diethylamino)-benzaldehyde diphenylhydrazone (DEH) [22], poly-triarylamine (PTAA) ^[19][23][19,23], N,N-dialkyl perylene diimide (PDI) [24], polypyrrole (PPy), polyaniline (PANI) [25], etc. From a commercial standpoint, the production of solar cells utilizing an organic hole transport layer has encountered numerous challenges, the most significant of which are material cost and stability. Particularly, high purity spiro-OMeTAD is more costly than novel metals such as gold and platinum, which are commonly used as a counter electrodes. Commercially available spiro-OMeTAD is nearly ten times more expensive than platinum and gold. On the other hand, organic HTMs are typically hygroscopic in nature and that’s why it has an impact on the PSCs’ general stability.

In contrast, several low-cost inorganic HTLs were also proposed and implemented for enhancing the stability of PSCs, among them, some of the HTMs are CuSCN [26], NiO_x [27], Cu₂O or CuO [28], CuI [29], CuGaO₃ [30] and CuAlO₂ [31], MoO_x [32], CuS [33], MoS₂ [34], and polymer electrolyte [35]. The above-mentioned HTMs have shown potential as they offer suitable properties for application in PSCs including the suitable band-to-band alignment with the perovskite layer, low resistivity, and low-cost solution-process ability [2]. In the case of inorganic HTM, increased demand for inorganic HTM will certainly lower the cost of large-scale manufacturing, while organic HTM will likely stay expensive due to the preparation processes and materials with very high purity required for solar cell applications. These are the primary reason why researchers have concentrated their efforts on the development of an inorganic HTM. Consecutively, the quest for the perfect HTM is a great topic yet. There is a lot of literature on various HTMs, but only a few of them show promise in terms of improving the overall efficiency and stability of the PSCs. Several approaches have evolved to utilize inorganic p-type semiconductor materials, such as NiO_x, CuO_x, etc., focusing on developing non-hygroscopic and highly conductive HTMs [36]. Moreover, carbon-based materials, including graphene, activated carbon, carbon black, graphite powder, carbon nanotube (CNT), etc., have been employed in the case of HTM-free PSC structures ^{[37][38][39][40]}[37,38,39,40]. In particular, current approaches to HTMs are low cost, high mobility, low absorption in the visible region, ease of synthesis, and good chemical stability that could ensure high efficiency and stable PSCs. In recent years, several review works have been published on inorganic metal oxide hole-transporting materials for perovskite solar cells in different formats and among them, some are focused on fabrication way, some are on efficiency and some are focused on stability. A list of some important review articles on inorganic metal oxide-based PSCs for the year 2015–2021 is shown in Table 1.

No.	Title	Journal	Year	References
01	Recent progress of inorganic hole transport materials for efficient and stable perovskite solar cells	Nano Select	2021	[41]
02	A brief review of hole transporting materials commonly used in perovskite solar cells	Rare Metals	2021	[42]
03	Nickel Oxide for Perovskite Photovoltaic Cells	Advanced Photonics Research	2021	[43]
04	Toward efficient and stable operation of perovskite solar cells: Impact of sputtered metal oxide interlayers	Nano Select	2021	[44]
05	Inorganic hole transport layers in inverted perovskite solar cells: A review	Nano Select	2021	[45]
06	Progress, highlights, and perspectives on NiO in perovskite photovoltaics	Chemical Science	2020	[46]
07	A review on the classification of organic/inorganic/carbonaceous hole-transporting materials for perovskite solar cell application	Arabian Journal of Chemistry	2020	[47]
08	Review of current progress in inorganic hole-transport materials for perovskite solar cells	Applied Materials Today	2019	[48]
09	Recent progress of inorganic perovskite solar cells	Energy & Environmental Science	2019	[49]
10	Inorganic hole transporting materials for stable and high efficiency perovskite solar cells	The Journal of Physical Chemistry C	2018	[50]
11	Analysing the prospects of perovskite solar cells within the purview of recent scientific advancements	Crystals	2018	[51]
12	Recent progress in stability of perovskite solar cells	Journal of Semiconductors	2017	[52]
13	Emerging of inorganic hole transporting materials for perovskite solar cells	The Chemical Record	2017	[53]
14	Recent advances in the inverted planar structure of perovskite solar cells	Accounts of chemical research	2016	[54]
15	The progress of interface design in perovskite-based solar cells	Advanced Energy Materials	2016	[55]
16	Recent progress on hole-transporting materials for emerging organometal halide perovskite solar cells	Advanced Energy Materials	2015	[36]

2. Perovskite Solar Cell

PSCs (organic-inorganic perovskite solar cells) are considered a significant recent breakthrough in photovoltaics and have recently received great attention [49]. The power conversion efficiency (PCE) of PSCs has already enhanced from 3.8 percent to 25.8 percent through the system engineering and materials design regarding the correct optoelectronic aspects in just 10 years [56]. Thus, PSCs are recognized as the best alternative approach for replacing the costly and market-dominant crystalline silicon solar cells ^{[51][57][58][59][60]}[51,57,58,59,60]. Moreover, PSCs are more cost-effective than conventional inorganic semiconductor thin-film solar cells, such as CIGS and CdTe [52]. The real obstacle to commercialization, however, is maintaining long-term stability. PSCs are particularly susceptible to deterioration when exposed to moisture, oxygen, heat, and light, and they must address before they can use in practical applications. Perovskite is itself very reactive due to the presence of vacancies in its structure. This is the defect of perovskite and it can encourage ion migration through the perovskite layer. Furthermore, the organic cations which are used in PSCs are hygroscopic in nature. When the PSCs are contacted with moisture, the water molecule reacts with it and forms a weak hydrogen bond with the cation which results in the formation of a hydrated perovskite phase [52]. Oxygen, heat, and UV influence this chemical reaction and favor the instability of PSCs. For commercialization, PSCs must be able to operate without major degradation for almost 25 years in outdoor conditions [61]. PSCs have so far been claimed to have one-year stability, which is considerably less than the PV systems that are already on the market. Thus, it is evident that the stability and limited longevity of PSC PV are the main factors impeding its commercialization [62]. The basic building block of the perovskite structure, ABX₃, is shown in Figure 1, where A and B are cations with different sizes (A being larger than B) and X is an anion [63]. Figure 1 represents the simplest structure made up of cubic symmetry of corner-sharing BX₆ octahedra, where the B cations are in the middle of the octahedron and the X anions are at the corners ^[64][65][64,65]. In the gap of cuboctahedra, the A cations are located at interstices, surrounded by eight octahedral, and form a cubic Pm₃m crystal structure [66]. In the case of frequently used perovskites in solar cells are organo-metal halide perovskite materials, where ‘A’ may be an organic or inorganic cation (i.e., MA⁺, FA⁺, Cs⁺, K^+, and Rb⁺), while ‘B’ is a metal cation (i.e., Pb²⁺ or Sn²⁺), and ‘X’ is a halide anion (i.e., Cl, Br, I, etc.) ^[67][68][67,68]. It should be mentioned that the A, B, and X ions must satisfy this formula, t = (RA + RX)/2 (RB + RX), where RA, RB, and RX are the corresponding ionic radii and t = 1, is the tolerance factor. For most cubic perovskite structures, 0.8 t 0.9 is found quantitatively. In the case of lower symmetry, the value of “t” is very small and then the film structure will be tetragonal or orthorhombic. Alternatively, if t ≥ 1, hexagonal structures are formed, and layers of face-sharing octahedra are added to the structure ^[67][68][67,68]. Moreover, organometal halide perovskites have already been proven several outstanding optoelectronic properties, such as a large absorption coefficient, direct bandgap, small exciton-binding energy, ambipolar semiconducting characteristics, long charge-carrier diffusion length with high charge-carrier mobility ^[67][68][67,68]. Furthermore, the researcher proposed hybrid organometal perovskite material with structure ABX_3−xY_x, for example, MAPbI_3−xCl_x and MAPbI_3−xBr_x, which has tunable optical properties. The tunable optical properties make it easier to experiment with device performance and improve PSCs’ overall performance [68]. On the other hand, perovskite films can be prepared by versatile low cost and simple film deposition methods, such as spin-coating ^[69][70][69,70], sequential deposition ^[71][72][71,72], and evaporation ^[73][74][73,74] techniques. Low-temperature spin coating is the simplest method to fabricate low-cost and high-efficiency PVSC devices. However, it is very challenging to form continuous perovskite films means non-fully covered perovskite films by spin-coating via the direct methyl ammonium halide and lead iodide (PbI₂) mixed precursor solution ^[75][76][75,76]. All the above process has their own limitation and commercial viability. Miyasaka and co-workers first reported the liquid-electrolyte-based dye-sensitized solar cells (DSCs) of PCE as a maximum of 3.8% using MAPbI₃ and MAPbBr₃ perovskites as light absorbers [1]. However, due to the dissolution of the perovskites in the liquid electrolyte, the system was found to be very unstable. In 2012, a significant advance was made independently by Grätzel et al. [60] and Snaith et al. [77] where the liquid electrolyte was replaced with a small-molecule-based hole-transporting material (HTM), 2,2′,7,7′-tetrakis(N,N-di-p methoxyphenylamine)-9,9′-spirobifluorene(spiro-OMeTAD). The perovskite is penetrated the mesoporous TiO₂ (mp-TiO₂) scaffold with an additional capping layer as shown in Figure 2a, which is covered with a thin layer of the HTM in a typical mesoscopic PSC. Finally, a metal electrode, preferably gold (Au), is deposited on the top of the HTM ^[61][77][61,77]. Instead of TiO₂, Al₂O₃ insulating scaffold can also be used in this mesoscopic structure [77]. The device has been found to work well, signifying that the perovskite could serve as a light harvester as well as an electron transporter (ETM). This finding led to a planar PSC configuration without the mesoporous scaffold as shown in Figure 2b. Particularly, in planar PSCs, the perovskite is simply sandwiched between a thin layer of HTM and a compact ETM, such as TiO₂, ZnO, SnO₂, etc. [78]. Moreover, HTM-free PSC also reported where the perovskite works as a hole transporter as well as a light absorber [79]. Moreover, ambipolar semiconducting characteristics of the perovskite support fabricating PSC in an inverted fashion, which is typically known as inverted PSCs. Figure 2c represents the mesoscopic inverted PSCs where a p-type mesoporous matrix (such as NiO) is used to deposit the perovskite, and then, a thin layer of ETM is deposited on top of the perovskite [80]. Finally, fabrication has been completed by depositing a metal electrode, such as silver (Ag), by the thermal evaporation technique. Analogous to usual architectures, the PSCs in inverted structure can be fabricated as shown in Figure 2d, where the perovskite layer is sandwiched by an ETM, such as PCBM, and a thin HTM, such as poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) [54]. As previously mentioned, PSCs use primarily two types of system structures (normal and inverted) and obviously, transparent conductive oxide (TCO) (such as ITO or FTO), HTM, perovskite layer, ETM, and contact electrodes (like Au and Ag) are the main components of both structures as shown in Figure 2a,b. The energy band diagram of a normal configuration, shown schematically in Figure 3, depicts the transporting trajectory of electrons and holes during the action. Excitons are produced and then separated into free carriers when sunlight illuminates the perovskite active layer. The generated electrons and holes can then be transported to each interface and injected into ETM and HTM, respectively. Finally, counter electrodes capture electrons and holes in ETM and HTM, respectively, transport them to an external circuit, and generate current ^[55][81][55,81]. Charge separation between MAPbI₃ and HTM such as spiro-MeOTAD was observed in transient absorption spectroscopy, but electron injection at open-circuit conditions was not detected yet [61]. It has already been confirmed that HTM plays a crucial role in carrier separation and transport in PSCs [50] which will be discussed in their s study for most of the inorganic HTMs used in PSCs. Particularly, there are primarily four types of interfaces in the inverted and/or normal structure of PSCs as shown in Figure 3. Each of the interfaces is methodically related to interfacial carrier dynamics including charge separation, charge injection, charge transport, charge collection, and recombination processes, and consequently affects how well the device functions in the end. Charge transport, extraction, and collection in real-time operation of PSCs are usually accompanied by charge recombination, which is closely related to PCE, stability, and hysteresis. It clearly shows that interface engineering is essential for developing effective and reliable PSCs