Perovskite solar cells have emerged as a promising photovoltaic technology due to their high power conversion efficiencies and relatively low fabrication costs. A critical component in these devices is the hole transport layer, which plays a vital role in extracting and transporting holes from the perovskite absorber while minimizing recombination losses. Both organic and inorganic nanomaterials have been explored as HTLs, each offering distinct advantages and challenges.

Organic hole transport materials such as Spiro-OMeTAD and PTAA are widely used due to their favorable energy level alignment with perovskite materials. Spiro-OMeTAD, in particular, has been instrumental in achieving high-efficiency perovskite solar cells, with devices exceeding 25% efficiency in laboratory settings. However, its widespread adoption is hindered by high costs, the need for hygroscopic dopants such as lithium bis(trifluoromethanesulfonyl)imide, and susceptibility to moisture-induced degradation. PTAA offers better hydrophobicity and thermal stability but suffers from high synthetic complexity and expense. Deposition of these organic HTLs is typically done via spin-coating, which is suitable for lab-scale production but poses challenges for large-area uniformity and scalability.

Inorganic nanomaterials present an attractive alternative due to their inherent stability, lower cost, and potential for dopant-free operation. Nickel oxide (NiOx) is a prominent inorganic HTL, offering high hole mobility, excellent chemical stability, and suitable energy levels for efficient hole extraction. Solution-processed NiOx nanoparticles can be deposited through spin-coating, spray pyrolysis, or slot-die coating, making them more amenable to scalable manufacturing. Copper thiocyanate (CuSCN) is another inorganic candidate with high transparency and hole conductivity. Its low-temperature processability allows compatibility with flexible substrates, though issues such as interfacial recombination and film morphology control remain challenges.

The primary function of the HTL is to facilitate charge extraction while suppressing non-radiative recombination at the perovskite/HTL interface. Nanostructured HTLs enhance this process by improving interfacial contact and reducing charge trapping. For instance, mesoporous NiOx layers provide a larger interfacial area for hole collection, while optimized nanoparticle sizes in CuSCN films minimize resistive losses. Additionally, inorganic HTLs often exhibit better stability under thermal and light stress compared to their organic counterparts, contributing to prolonged device lifetimes.

Despite these advantages, inorganic HTLs face limitations such as lower hole mobility compared to doped organic materials and interfacial defects that can lead to voltage losses. Surface modification strategies, including the use of ultrathin passivation layers or molecular linkers, have been employed to mitigate these issues. For example, a thin layer of graphene oxide between the perovskite and NiOx has been shown to reduce recombination and improve charge extraction.

A significant innovation in HTL development is the emergence of dopant-free materials, which eliminate the instability associated with hygroscopic dopants. Dopant-free organic polymers and small molecules, such as triarylamine derivatives, have demonstrated competitive performance while simplifying device fabrication. Similarly, inorganic HTLs like vanadium oxide and copper iodide operate efficiently without dopants, offering improved environmental stability. These materials reduce moisture ingress and ion migration, key degradation mechanisms in perovskite solar cells.

Scalability remains a critical challenge for HTL integration in commercial perovskite photovoltaics. While spin-coating is prevalent in research, techniques such as blade coating, inkjet printing, and vapor deposition are being explored for roll-to-roll manufacturing. Inorganic nanomaterials are particularly promising in this regard due to their compatibility with solution-based deposition methods. However, achieving uniform film coverage over large areas without pinholes or thickness variations requires further optimization.

Moisture sensitivity is another persistent issue, especially for organic HTLs. Encapsulation techniques help, but intrinsically stable HTLs are preferable for long-term operation. Inorganic alternatives like NiOx and CuSCN exhibit better moisture resistance, but their processing conditions must be carefully controlled to prevent interfacial defects that could compromise performance.

Recent advancements in HTL design include hybrid approaches combining organic and inorganic nanomaterials to leverage the benefits of both. For instance, composite HTLs incorporating conductive polymers with metal oxide nanoparticles have shown enhanced conductivity and stability. Another promising direction is the use of two-dimensional materials such as transition metal dichalcogenides, which offer tunable electronic properties and excellent environmental stability.

The impact of HTL selection on device longevity cannot be overstated. Degradation mechanisms such as ion migration, interfacial delamination, and UV-induced damage are influenced by the HTL’s chemical and electronic properties. Inorganic HTLs generally exhibit superior thermal and photostability, but their integration must be carefully engineered to avoid introducing new failure modes. Accelerated aging tests under controlled humidity and temperature conditions provide valuable insights into long-term performance.

In summary, the choice of hole transport material significantly influences the efficiency, stability, and scalability of perovskite solar cells. Organic HTLs like Spiro-OMeTAD and PTAA have driven early efficiency records but face cost and stability limitations. Inorganic nanomaterials such as NiOx and CuSCN offer improved robustness and scalability, though their electronic properties require further refinement. Innovations in dopant-free materials and hybrid nanostructures are paving the way for more durable and commercially viable perovskite photovoltaics. Continued research into deposition techniques, interfacial engineering, and material design will be essential to overcoming existing challenges and unlocking the full potential of this technology.