Advancements in Inorganic Hole-Transport Materials for Perovskite Solar Cells: A Comparative Review

Abstract

Single-junction perovskite solar cells (PSCs) have been one of the most promising photovoltaic technologies owing to their high-power conversion efficiencies (PCEs) of ~27% and the low-cost fabrication processes involved, which pay off significantly given their distinct structural characteristics. Recently, inorganic hole-transport materials (HTMs) such as nickel oxide (NiO_x) have been developed and received considerable attention for use in OPVs due to their excellent thermal stability, low-cost materials, and compatibility with scalable deposition methods. Here, we summarize the recent progress on inorganic HTMs for PSCs, which can be divided into three categories: NiO_x, copper-based compounds, and emerging new alternatives. The deposition method (sputtering, atomic layer deposition, or a solution-based technique) is one of the most important factors affecting the performance and stability of PSCs. Finally, we review interfacial engineering strategies, such as surface modifications and doping, which can enhance charge transport and extend a device’s lifetime. We also balance the benefits of inorganic HTMs against the key challenges in advancing to commercialization, namely interior defects and environmental degradation. In this review, we summarize the recent progress and challenges toward developing cost-efficient and stable PSCs with inorganic HTMs and provide insights into the future development of these materials.

1. Introduction

With rising energy consumption, environmental degradation, and the limited supply of fossil fuels, the global demand for sustainable and renewable energy has progressed drastically in recent years [1,2,3]. There are many approaches to producing alternative energy, but among them, solar energy is the most abundant, renewable, and environmentally friendly [4,5,6]. Silicon-based solar cells, the market-dominating photovoltaic (PV) technology, with their intrinsic cost-effectiveness and power conversion efficiencies (PCEs) of 27.0% for single-junction cells and greater than 33% for tandem cells [7,8,9], have set the pace in photovoltaics. However, littered with high manufacturing costs, inflexible substrates, and lengthy fabrication procedures, these technologies, although mature, are usually hindered by technological implementations [10,11,12]. Perovskite solar cells (PSCs) provide a promising approach, especially with their comparable efficiencies and low-temperature solution-based fabrication capability [13,14,15]. To date, the certified PCE of PSCs has surpassed 26% as of early 2025, with further improvements found through novel material design, interfacial engineering, and device architecture [16]. Perovskite materials have extraordinary optoelectronic properties, including large absorption coefficients, extended carrier diffusion lengths, and tunable bandgaps, all of which make these compounds promising next-generation PV materials [17,18,19].

Yet, a prominent challenge in PSC commercialization is device stability, especially under ambient conditions [20,21,22]. Some of the major components causing instability comprise organic materials used in the hole-transport layer (HTL)—like spiro-OMeTAD—which is often hygroscopic and thermally unstable [23,24,25]. Under these circumstances, inorganic hole-transport materials (HTMs) have emerged as a priority topic in research because of their improved thermal stability, chemical durability, and possible compatibility with large-scale processing methods [26,27]. This review presents a comparative and updated insight into the recent progress of inorganic HTMs for PSCs. Specifically, we concentrate on well-established materials such as CuI, CuSCN, Cu₂O, and the broadly employed NiO_x in conjunction with developing candidates [24,26,28]. We emphasize their material properties, energy level alignment, deposition methods, and suitability for normal and inverted PSC architectures [24,28,29]. This review also addresses interfacial engineering strategies and recent attempts to alleviate commercialization hurdles [23,24,30]. This work builds on recent findings in the literature to help clarify the important role of inorganic HTMs in advancing PSC efficiency and operational lifetime and define encouraging avenues for future work [14,24,30].

Role of Hole-Transport Materials (HTMs) in Perovskite Solar Cells

Hole-transport materials (HTMs), which serve as selective contact layers enabling the efficient extraction and transport of photogenerated holes from the perovskite absorber to an electrode, are crucial for PSCs [31]. Efficient wide-bandgap perovskite solar cells, high hole mobility, proper energy level alignment with the perovskite valence band, optical transparency in the visible range, and long-term operational stability are critical features of HTMs to be combined with high-power conversion efficiencies (PCE) [32]. The HTM is sandwiched between the perovskite layer and the back electrode in conventional PSC architecture, as shown in Figure 1. It performs the following three key roles: (i) facilitating fast hole extraction from the perovskite; (ii) preventing electron transport to the anode, which suppresses recombination losses; and (iii) ensuring that the interface remains stable, preventing perovskite degradation under environmental stressors [14,26]. For example, traditional organic HTMs such as spiro-OMeTAD have been most frequently employed in PSCs because of their simple solution processing route and better energetics. Nevertheless, these materials suffer from moisture sensitivity, low intrinsic conductivity, and thermal instability. Furthermore, they need expensive additives such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP) that negatively influence the durability of the devices [28,33]. To address these disadvantages, inorganic HTMs are receiving major attention owing to their better thermal stability, inherent chemical stability, and suitability for high-throughput deposition methods [14,23]. Such materials provide additional advantages, non-doping operations, and stability under ambient processing conditions, which are critical for industrial applications.

One of the most promising inorganic HTMs is CuSCN, a wide bandgap (~3.6 eV), high-mobility (~10⁻² cm²/V·s) material processible by solution techniques. Utilizing CuSCN as the HTM, PSCs have reported efficiencies of >15% with improved stability over time [34,35]. One other extensively researched material is CuI, displaying high conductivity and can be easily made at low temperatures. In addition, CuI-based devices suffer from interfacial recombination and low open-circuit voltage (Voc) from ineffective interface passivation [27,35]. Nickel oxide (NiO_x) is among the most widely used inorganic HTMs in inverted (p–i–n) device architectures. It possesses good optical transparency, high valence band edge energy (~5.0 eV), and high chemical stability. Therefore, several recent studies have also examined PSCs incorporating NiO_x layers that are either sputtered or physically deposited via atomic layer deposition (ALD), which were found to yield PCEs greater than 20%, highlighting that such material is highly useful for stable device architectures [36]. To achieve the highest efficiency, the HTM-HOMO level should be close to the perovskite valence band, allowing efficient hole extraction and minimal energetic losses. Moreover, HTMs should be smooth in surface morphology and adhere well to adjacent layers with fewer interfacial trap states, which contribute to device reproducibility [24,37]. For these reasons, transparency in the visible region and thermal stability well above 100 °C are critically important for all types of devices, especially multi-junction cells and outdoor environments. The rational design and choice of inorganic HTMs provide great opportunities to promote PSCs with high efficiency, great stability, and easy fabrication. We compare several established and emerging inorganic HTMs in terms of their structural, electronic, and interfacial properties and discuss their implementation in standard and inverted device architectures in the following sections.

2. Comparative Analysis of Inorganic HTMs for Perovskite Solar Cells

Inorganic hole-transport materials (HTMs) have many benefits compared to organic HTMs, such as thermal stability, long-term endurance, and compatibility with various fabrication methods. The following section outlines a series of the most commonly researched inorganic HTMs from the recent developments in PSCs, such as NiO_x, CuSCN, CuI, Cu₂O, V₂O₅, and CoOx, comparing their physical properties, device performance, and structural compatibility tabulated in

Table 1. A quantitative comparison of inorganic HTMs in Table 1 shows their strengths and weaknesses in various key properties.

HTM	Bandgap (eV)	Hole Mobility (cm2/V·s)	Work Function (eV)	Suitable for PSC Type	Stability	Notable Feature
NiOx	+/−3.6	10−3–10−1	~5.0	Inverted (p–i–n)	High	Widely used; ALD/sputtered films offer high stability [14,23,36]
CuSCN	+/−3.6	~10−2	~5.3	Normal (n–i–p)	Moderate	Good mobility and transparency; sensitive to humidity [34,38]
CuI	~3.0–3.1	~10−3	~5.1	Normal (n–i–p)	Low–Mod.	Easy processing but suffers from recombination and instability [35]
Cu2O	~2.1	Up to 256	~5.0	Normal (n–i–p)	Moderate	High hole mobility; lower absorption in the visible range [39]
V2O5	~2.3–2.8	~10−⁴	~5.3	Inverted (p–i–n)	High	Wide bandgap; low conductivity; often used with dopants [40]
CoOx	~2.4–2.8	~10−⁴	~5.2	Inverted (p–i–n)	High	Emerging material; promising electrochemical performance [41]

.

2.1. Key Inorganic HTMs

Importantly, Table 1 offers a comparison of the key physical and electrical attributes of inorganic hole-transport materials (HTMs) that have been widely investigated to be adapted in PSCs, paving a path toward device architecture and processing conditions suitable for successful commercialization [14,23].

Of these materials, nickel oxide (NiO_x) has attracted attention as a widely used HTM for inverted (p–i–n) PSCs due to its large bandgap (3.6–4.0 eV), deep valence band (~5.0 eV) and decent hole-selective property. Ideal thermal and chemical stability, together with the facility for using vacuum-based deposition techniques like sputtering and atomic layer deposition (ALD), enable devices with high reproducibility and scalability to be used [34,36]. NiO_x HTLs have shown PCEs > 21% over large areas, along with device stability under continuous irradiation [14,38]. In contrast, common cationic Cu-based HTMs, such as CuSCN, CuI, and Cu₂O, are often used in normal (n–i–p) structures, directly contacting with the metal electrode and requiring the efficient extraction of the holes generated on the upper perovskite surface [34,35]. These materials are appealing due to their high conductivity and compatibility with solution-processable techniques at low temperatures for the development of flexible and printable solar cells [39,40]. However, Cu-based materials have their limitations, especially for ambient stability and interfacial quality. For example, CuSCN is transparent and chemically stable, yet it is moisture-sensitive, leading to non-uniform film formation, which affects its fill factor (FF) and long-term stability unfavorably [34,41]. Despite being ultra-conductive, the CuI sometimes triggers interfacial recombination as well as lower open-circuit voltage (Voc), mainly attributed to poor band alignment and interface traps with certain perovskites [13,35].

Cu₂O features high hole mobility, with some reports approaching >100 cm²/V·s, which is one order of magnitude higher than that of most organic HTMs [27,39]. Its relatively narrow bandgap (~2.0–2.2 eV), however, causes parasitic absorption and necessitates careful energy-level engineering to minimize Voc losses and ensure selective carrier extraction [38,39]. The properties of coordinate metal show high environmental tolerance, and the position of their wide valence band is ideal for hole extraction in inverted PSCs, which has led to the exploration of transition metal oxides like vanadium pentoxide (V₂O₅) and cobalt oxide (CoOx) as HTM candidates for solution-processed hole-transport layers [40,41]. However, these materials usually possess an inherently low conductivity and limited film-forming capabilities, making them suitable mostly as buffer layers or bilayer HTM structures, particularly with the addition of interfacial dopants or surface modifiers [13]. This survey study indicates that there is no ideal inorganic HTM that can outperform in all important metrics (i.e., energy-level alignment, conductivity, transparency, and stability) [14,27]. Thus, the choice of materials has to rely on specific device configurations, processing techniques, and applications. Often, trades are unavoidable, and the best HTM might rely more on interface fabrication and deposition control than the inherent material properties [13,36].

2.2. HTMs for Specific Device Architectures

To date, the introduction of inorganic hole-transport materials (HTMs) into perovskite solar cells (PSCs) must be flawlessly matched to the demands generated by chemistry and device architecture, both structurally and functionally [34,35,39]. There are two general device architectures for PSCs: normal (n–i–p) and inverted (p–i–n), and for each architecture, the optoelectronic properties of HTM should differ depending on the PSC structure [14,28,40].

2.2.1. HTMs for Normal (n–i–p) Architectures

In the standard (n–i–p) device architecture, the HTM is deposited onto the perovskite layer, which is then in direct contact with the metallic back electrode, typically gold (Au) or silver (Ag). In this regard, HTMs should possess superior film-forming characteristics, be upward-aligned on HOMO energy with respect to the perovskite valence band, and provide low interfacial recombination at the perovskite/metal interface [27,41,42]. Of these HTMs, copper(I) thiocyanate (CuSCN) is one of the most widely investigated elements and has shown considerable potential due to its large bandgap (~3.6 eV), high optical transparency, and compatibility with low-temperature, solution-based processing methods, in particular for substrates that may require flexible treatment [24,34,36]. Glabau et al. were the first to report on CuSCN-based PSCs that exceeded PCEs by over 16% with improved environmental stability [28,34,38]. Copper(I) iodide (CuI), despite also providing high intrinsic conductivity and rapid deposition, is subject to interface shunting and recrystallization losses, mainly in thin perovskite layers, and pinhole creation can enable direct contact with the electrode [23,35,43]. Hence, due to its use, it frequently needs extra passivation or a multilayer style for performance stability. Another compelling candidate in this regard is copper(I) oxide (Cu₂O) due to its remarkably high hole mobility and significant light absorption in the visible region. On the other hand, its relatively narrower bandgap (~2.0–2.2 eV) increases the risk of parasitic absorption and requires stringent energy-level alignment strategies to mitigate Voc losses [39,40]. In addition, its surface chemistry and compatibility with perovskites require meticulous design interface engineering. Doped Cu-based HTMs (e.g., CuSCN:Ag, CuI:Na) with core–shell structures have been proposed to effectively mitigate interfacial losses due to improved coverage, less recombination, and tunable energy alignment [28,38,44].

2.2.2. HTMs for Inverted (p–i–n) Architectures

In inverted (p–i–n) PSCs, the HTM is directly deposited as the first active layer onto the transparent conducting oxide (TCO) layer (e.g., indium tin oxide (ITO) and fluorine-doped tin oxide (FTO)) [14,45,46]. The HTM in this setup must exhibit thermal and chemical stability with regard to perovskite deposition, low-temperature processability, and effective, selective hole extraction at the TCO interface [27,37,47]. Nickel oxide (NiO_x) is the most popular inorganic HTM for this context due to its deep valence band (~5.0 eV), wide bandgap, and excellent optical transparency [14,48,49]. It can be deposited through solution processing, sputtering, or atomic layer deposition (ALD) [42], and recent advancements in terms of surface passivation and doping (e.g., Mg, Cu, Li) have led to improved conductivity and compatibility with perovskite films [24,50]. Besides NiO_x, vanadium pentoxide (V₂O₅) and cobalt oxide (CoOx) are also being investigated due to their chemical stability and high work functions, facilitating hole extraction and promoting interface stability [40,41,51]. However, these materials typically have low intrinsic hole mobility, requiring interfacial dopants or bilayer buffer strategies to enhance charge transport and reduce recombination [27,41,42].

2.2.3. Current Trends in HTMs–Device Integration

Recently, new strategies of HTM design have been developed with encouragement toward the efficiency, scalability, and reliability of devices [24,36,49]. To improve the interfacial selectivity and minimize energy loss in charge extraction layers, graded or gradient-layer HTMs with spatially varied band alignments are also under development [16,52,53]. The key physical, electrical, and thermal properties of prominent inorganic HTMs are shown in

Table 2. Physical, electrical, and thermal properties of prominent inorganic HTMs.

Material	Bandgap (eV)	Hole Mobility (cm2/V·s)	Work Function (eV)	Conductivity (S/cm)	Processability	Thermal Stability	References
NiOx	3.6–4.0	10−3–10−1	~5.0	10−⁴–10−2	ALD; Sputtering; Sol–Gel	Excellent	[13,14,23]
CuSCN	~3.6	~10−2	~5.3	10−2–10−3	Spin-coating; Electrodeposition	Good	[34,38]
CuI	~3.1	~10−3	~5.1	~1	Spin-coating; Drop-casting	Moderate	[35]
Cu2O	2.0–2.2	Up to 256	~5.0	10−⁴–10−2	ALD; Oxidation; Sputtering	Moderate	[27,39]
V2O5	2.3–2.8	~10−⁴	~5.3	Low	Thermal evaporation; Sol–Gel	High	[40]
CoOx	2.4–2.8	~10−⁴	~5.2	Moderate	Sputtering; Sol–Gel	High	[41]

. Synergistic advantages such as energy-level alignment, film morphology, and chemical passivation are associated with bilayer HTL systems that consist of an inorganic HTM and ultra-thin interlayers (for TiO₂, MoO₃, or organic modifiers) [27,37,54]. For commercialization, HTMs that are compatible with scalable deposition techniques, i.e., blade coating, slot-die coating, spray pyrolysis, and ALD, are increasingly being focused on, owing to their manufacturable nature for large-area and roll-to-roll technological applications [24,49,55].

The essential physical and electrical properties of leading inorganic hole-transport materials (HTMs) used in PSCs are listed in Table 2 as a comparative summary. Of these, NiO_x is favored for its broad bandgap (3.6–4.0 eV), moderate hole mobility, and exceptional thermal and chemical stability, which renders it advantageous in inverted (p–i–n) perovskite topologies [13,14,23]. On the other hand, owing to their favorable conductivity and solution processability, Cu-based materials, including CuSCN, CuI, and Cu₂O, are used in normal (n–i–p) architectures [34,35,38]. However, their performance is usually influenced by their surface roughness, susceptibility to ambient environments, and insufficient long-term stability if not surface-passivated or embedded in composite structures. Cu₂O has an incredibly high hole mobility (as high as 256 cm²/V·s), but its smaller bandgap could induce parasitic absorption, and its misalignment with common perovskite absorbers would require interface tuning [27,39]. While V₂O₅ and CoOx exhibit stability and environmental tolerance, their poor conductivity requires doping elements or building layered architecture on electrodes to achieve enhanced performance [40,41]. This comparative perspective illustrates that no single inorganic HTM is universally superior and that the material choice must integrate performance parameters as well as processing and structural characterizations.

2.2.4. Tabular Comparisons: HTM–Device Architecture Matching

Table 3. Suitability of HTMs in device architectures.

HTM	Compatible Architecture	Typical Deposition Method	Peak PCE Reported (%)	Notes	References
NiOx	Inverted (p–i–n)	Sputtering; ALD; Sol–gel	21.50%	Most used inorganic HTM	[14,23,27]
CuSCN	Normal (n–i–p)	Solution; Electrodeposition	16.60%	Transparent and cost-effective	[34,38]
CuI	Normal (n–i–p)	Spin-coating; Drop-casting	~6–12%	Prone to recombination	[35]
Cu2O	Normal (n–i–p)	ALD; Oxidation	13.30%	High mobility and poor long-wavelength absorption	[27,39]
V2O5	Inverted (p–i–n)	Evaporation; Sol–Gel	~19%	Good chemical barrier	[40]
CoOx	Inverted (p–i–n)	Sputtering; Sol–Gel	~18%	Still under active study	[41]

aligns the HTMs to their compatible device architectures, as well as various deposition techniques and their peak PCEs. NiO_x, particularly in inverted PSCs, provides large PCEs (>21%) and has strong thermal stability, particularly when deposited via sputtering or atomic layer deposition (ALD) [14,27,36]. CuSCN has shown itself to be effective in normal structures due to its wide bandgap (~3.6 eV), proper energy-level alignment, and transparency [34,38].

CuI has good processability and is cost-effective but may have poor interfacial passivation, potentially leading to recombination losses in the absence of extra interlayers [35]. Cu₂O, when used in normal architecture, provides high mobility, but stable band alignment would need to be achieved through tuning surface states and interfacial layers to suppress chemical reactivity [27,39]. Other emerging HTMs (e.g., V₂O₅ and CoOx) are compatible with flexible inverted devices and may enable tandem cell integration, but they usually have rather low hole mobility, requiring further optimization [40,41]. These results underline the necessity of architecture-specific material-engineering approaches aimed at both maximizing charge extraction and reducing interfacial losses in a hybrid–organic system that is strongly hierarchy-driven.

2.2.5. Tabular Comparisons: Deposition Techniques and Scalability

Deposition Techniques: Owing to the different functionalities of 2D materials, various deposition techniques are important.

Table 4. Deposition techniques for HTMs.

Technique	Type	Cost	Scalability	Suitable HTMs	Key Notes	References
Spin-coating	Solution-based	Low	Moderate	CuI, CuSCN	Lab-scale standard	[34,35]
Electrodeposition	Solution-based	Low	High	CuSCN, Cu2O	Low-temperature and scalable	[38,39]
Atomic Layer Deposition (ALD)	Vapor-phase	High	High	NiOx, Cu2O	Uniform films; precise control	[14,23]
Sputtering	Physical vapor	Medium	High	NiOx, CoOx, Cu2O	High reproducibility	[27,41]
Slot-Die Coating	Solution-based	Low	Very High	NiOx (sol–gel)	Roll-to-roll compatible	[23]
Blade Coating	Solution-based	Low	Very High	NiOx, CuSCN	Simple; scalable	[38]

compares the deposition techniques along with their scalability, compatibility with other materials, and the quality of the films.

We demonstrate that vacuum-based methods like sputtering and ALD are being broadly adopted to obtain high purity and defect-free HTM films for NiO_x and Cu₂O, promoting the high efficiency of PSCs and tandem structures [14,27,36]. These techniques afford a tight control of thickness and uniformity but at a higher expense and lower throughput. Solution-based methods, such as blade coating, slot-die coating, etc., and related methods, such as electrodeposition, have drawn much attention for their low cost and compatibility with yield and flexible die. The electrodeposited CuSCN and sol–gel NiO_x films made by these methods have performed very well, and reproducibility has improved significantly [24,34,38]. Moreover, both blade coating and slot-die coating allow the fabrication of HTM layers at high speeds and continuously under ambient conditions, which is well-suited for roll-to-roll processing, which is a main requirement for commercialization [23,24]. More recently, blade-coated NiO_x HTLs have been shown to achieve >18% PCEs when combined with passivation strategies and to be highly air-stable [24]. These results show that the scalability of HTMs is attributable as much to the method of deposition as to the material itself, and hybrid processing strategies may provide a route to balance performance and manufacturability.

2.3. Deposition Techniques for Synthesizing Inorganic HTMs

The radar plot demonstrates the intrinsic trade-offs that guide the selection of inorganic hole-transport materials (HTMs) in perovskite solar cells (PSCs), which are determined by parameters like charge mobility, environmental stability, transparency, and process compatibility [14,23]. The visualization of trade-offs across six key criteria of the radar plot are stability, hole mobility, conductivity, band alignment, scalability, and transparency, which are displayed in Figure 2. Out of the studied candidates, Cu₂O demonstrates excellent charge carrier mobility, with hole mobility exceeding the 100 cm²/V·s reported, which allows for fast hole extraction and lower resistive losses, although its inefficient transparency and small area of fabrication limits its potential [39,52].

The values of pairs agree well (on a relative scale ranging from 1 to 10, established according to a hybrid of experimental data and literature reports) and allow for a fair comparison to be made across the six fundamental material properties that we specifically examine in this work: stability, hole mobility, conductivity, band alignment, scalability, and transparency. This feature facilitates an understanding of the images presented and, thus, aids in the comparative study of inorganic HTMs in PSCs. NiO_x demonstrates exceptional stability and energy alignment, while Cu₂O offers superior hole mobility. CuSCN and CuI provide high conductivity and processability, whereas V₂O₅ and CoOx exhibit environmental robustness but require the enhancement of conductivity. Nickel oxide (NiO_x) is a well-investigated HTM utilized in several inverted (p–i–n) configurations that is known for its chemical and thermal stability as well as favorable band alignment, with the perovskite valence band contributing to the low-voltage drop and high-efficiency of devices as well as long-term operational stability [27,36]. Moreover, its tunable nature via doping (e.g., Cu or Li) and compatibility with scalable deposition techniques (e.g., atomic layer deposition (ALD) and sputtering) further boost its attractiveness for commercialization [28,41]. Copper(I) thiocyanate (CuSCN), which possesses a wide bandgap (~3.6 eV) coupled with a high degree of hole selectivity, presents an advantageous balance between optical transparency and transport properties. Notably, this results in a favorable property that makes them suitable for ambient-condition low-temperature solution-based processes such as spin-coating and electrodeposition [34,38]. CuSCN-based devices have also exhibited high power conversion efficiencies and improved shelf stability over conventional organic hole-transporting materials (HTMs) [35,37].

On the other hand, V₂O₅ and CoOx have been praised for their deep valence band positions and inherent environmental stability; however, their shortcomings in terms of low intrinsic conductivity and limited charge mobility do not allow them to stand on their own as effective HTMs or rather require more attention toward interfacial modifications or dopants to increase transport features [40,51]. Consequently, these oxides are commonly used in hybrid or bilayer architectures to enhance interface passivation and extend the device lifetime [42,50]. Together, these insights validate the notion that no single HTM is optimal in general; rather, the material selection should be driven by the constraints in the PSC architecture and the processing requirements of the PSC such that a trade-off among performance, compatibility, and manufacturability is established [16,55].

3. Recent Literature and Comparative Contextualization

A few recent reviews have covered the subject of inorganic HTMs for PSCs; however, our manuscript attempts to provide a more holistic perspective focused on comparative aspects across studies. For instance, Sajid et al. [56], who emphasized vacuum-free processing strategies, focused primarily on the structural and compositional tuning of NiO_x films. Instead, our review offers a wider comparative perspective of a diverse group of inorganic HTMs, ranging from NiO_xs to Cu-based materials to transition metal oxides with diverse energy alignment and processing capabilities. Likewise, the study by Huang et al. [57] also investigated various HTMs but focused more on device stability, while our manuscript addresses both performance and scalability aspects. The work by Chen and Park [58] studied interfacial charge dynamics in PSCs incorporating dopant-free HTMs, which is a theme we build upon here by discussing interfacial engineering strategies that encompass different classes of inorganic HTMs and surface modifications. Additionally, Yu and Sun [59] put forth an initial roadmap for organic HTM replacement, and our work extends this to a more up-to-date comparative treatment of material classes, deposition methods, and the compatibility of each with device architectures. The work by Wang et al. [60] and Park [61] prompts interface passivation and layered HTM designs. While these studies have primarily focused on interfacial design, our manuscript contributes further by considering wider commercial factors such as cost-effective synthetic routes, environmental stability, and compatibility with both n–i–p and p–i–n architectures. Overall, while the reported reviewed works above significantly contribute to specific elements of [56,57,58,59,60,61] inorganic HTM development, our review stands apart due to its broader and more inclusive comparative investigation of the aspects of material properties, interfacial engineering, fabrication approaches, and commercialization potential with the intent to guide future research and technology translation in PSCs.

4. Conclusions

This article reviews recent progress made on inorganic hole-transport materials (HTMs) as an alternative for perovskite solar cells (PSCs), highlighting their increasing importance in the context of widely recognized issues such as device stability, efficiency, and scalability. Compared with their organic counterparts (e.g., spiro-OMeTAD), inorganic hole-transport materials (HTMs) like NiO_x, CuSCN, CuI, Cu₂O, V₂O₅, and CoOx present remarkable advantages, including dopant-free processing, improved thermal and environmental stability, and device compatibility in both normal (n–i–p) and inverted (p–i–n) orientations. For inverted PSCs, in particular, NiO_x deposited by sputtering or atomic layer deposition (ALD) has become one of the leading candidates, with PCEs over 21% reported as well as excellent long-term operational stability. Cu-based materials (CuSCN and Cu₂O) provide a low-cost pathway for direct, scalable fabrication, given that techniques like electrodeposition and blade coating have been used to prepare devices with CuSCN that show a PCE greater than 16%. Materials such as CuI, whilst having a low bandgap and high conductivity, are known to face interfacial recombination and shunting issues requiring sophisticated surface passivation and device engineering. Also, since new-generation transition metal oxides, including V₂O₅ and CoOx, exhibit high chemical durability and good compatibility with flexible substrates, other features like lower hole mobility and conductivity exist as obstacles that need adjustment. In addition to this focus on material selection, this review further demonstrates that processing approaches—such as slot-die coating, sputtering, atomic layer deposition (ALD), and blade coating—need to be efficient in generating fairly homogenous, defect-less HTM layers while also being contiguous with the requirements for large-area, scalable production. Future work should focus on concepts of interfacial engineering to further suppress charge recombination, various doping strategies to enhance conductivity, the development of green and lead-free HTMs, and the integration of roll-to-roll compatible fabrication processes. More comprehensive information and insights on inorganic HTM structure property performance relationships, combined with advances in processing and device architecture, will be crucial for the realization of high-efficiency, stable, and commercially scaled perovskite solar technologies.