Triarylamine-based small molecules, such as Spiro-OMeTAD, have emerged as critical hole transport materials (HTMs) in optoelectronic devices, particularly perovskite solar cells (PSCs) and organic light-emitting diodes (OLEDs). Their success stems from a combination of favorable electronic properties, tunable molecular structures, and compatibility with solution processing. However, challenges related to conductivity and long-term stability persist, driving ongoing research into molecular engineering and doping strategies.

The molecular design of triarylamine derivatives is centered around a core aromatic amine structure, where three aryl groups are attached to a nitrogen atom. This configuration enables efficient hole transport due to the delocalization of electrons across the conjugated system. Spiro-OMeTAD, a prominent example, features a spirobifluorene core with methoxy-substituted triphenylamine side groups. The spiro-core introduces a three-dimensional geometry that suppresses crystallization and enhances film-forming properties, while the methoxy groups improve solubility and hole mobility. The highest occupied molecular orbital (HOMO) level of Spiro-OMeTAD aligns well with the valence band of perovskite materials like methylammonium lead iodide (MAPbI3), facilitating efficient hole extraction.

Conductivity in triarylamine HTMs is inherently low in their pristine state, necessitating chemical doping to achieve practical device performance. Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP) are commonly used dopants for Spiro-OMeTAD. Li-TFSI acts as an oxidizing agent, generating hole carriers by extracting electrons from the triarylamine core, while tBP prevents aggregation and improves film morphology. Doped Spiro-OMeTAD typically achieves a hole mobility in the range of 10^-4 to 10^-3 cm^2 V^-1 s^-1, which is sufficient for efficient charge extraction in PSCs. However, the hygroscopic nature of Li-TFSI introduces instability, as moisture ingress accelerates device degradation. Alternative dopants, such as cobalt or copper complexes, have been explored to mitigate this issue, though trade-offs in conductivity and processing complexity remain.

In perovskite solar cells, Spiro-OMeTAD serves as a benchmark HTM, enabling power conversion efficiencies exceeding 25% in state-of-the-art devices. Its effectiveness arises from the combination of high hole mobility, appropriate energy level alignment, and uniform film formation. However, the thermal and environmental instability of doped Spiro-OMeTAD limits the operational lifetime of PSCs. Under prolonged illumination or elevated temperatures, dopants migrate, and the HTM layer undergoes phase separation, leading to increased series resistance and reduced fill factor. Encapsulation helps but does not fully resolve these degradation pathways. Recent efforts focus on modifying the triarylamine structure to enhance intrinsic stability, such as incorporating fluorinated aryl groups or crosslinkable moieties that resist dopant diffusion.

OLEDs also benefit from triarylamine-based HTMs, where they function as hole injection and transport layers. In these devices, the low ionization potential of triarylamines ensures efficient hole injection from the anode, while their amorphous nature prevents exciton quenching at grain boundaries. Spiro-OMeTAD and its derivatives exhibit high glass transition temperatures, which are crucial for thermal stability during device operation. However, the same doping-related instability issues observed in PSCs persist in OLEDs. Dopant-induced degradation can lead to voltage rise and luminance loss over time, particularly in high-brightness applications. Research into non-doping strategies, such as molecular design to enhance intrinsic conductivity, is an active area of investigation.

Stability improvements in triarylamine HTMs often involve structural modifications that enhance environmental resistance without compromising electronic properties. For instance, replacing methoxy groups with more electron-donating substituents can lower the HOMO level, improving oxidative stability. Incorporating bulky substituents or sterically hindered linkages reduces molecular mobility, mitigating dopant migration. Another approach involves blending triarylamine small molecules with inert matrices to improve mechanical robustness while maintaining charge transport pathways. Despite these advances, achieving a perfect balance between conductivity, stability, and processability remains a challenge.

Comparative studies of triarylamine derivatives reveal key structure-property relationships. For example, increasing the conjugation length of the aryl groups tends to improve hole mobility but may reduce solubility. Introducing asymmetric substituents can suppress crystallization but may also introduce energetic disorder, broadening the density of states. The choice of dopants and additives further complicates optimization, as their interactions with the host material are often non-trivial. Systematic molecular engineering, supported by computational screening, has identified several promising candidates beyond Spiro-OMeTAD, though none have yet surpassed its overall performance in commercial applications.

The future of triarylamine-based HTMs lies in addressing the stability-conductivity trade-off through innovative molecular design and alternative doping mechanisms. Crosslinkable triarylamines, for instance, offer a route to insoluble networks that resist dopant migration while maintaining high mobility. In situ doping methods, where oxidizing agents are generated during film formation, could provide more uniform charge carrier distribution. Additionally, the development of dopant-free triarylamines with intrinsically high conductivity would eliminate a major degradation pathway, though achieving this without sacrificing other properties remains difficult.

In summary, triarylamine small molecules like Spiro-OMeTAD play a pivotal role in advancing perovskite solar cells and OLEDs due to their tunable electronic properties and solution-processability. Their molecular design enables efficient hole transport, but reliance on chemical doping introduces stability challenges that hinder long-term performance. Ongoing research focuses on structural modifications and doping alternatives to enhance both conductivity and durability. While significant progress has been made, further innovation is needed to meet the demands of next-generation optoelectronic devices.