Black phosphorus humidity sensors have gained attention due to their unique layer-dependent hygroscopicity and electrical response to moisture. Unlike many conventional materials, BP exhibits a strong thickness-dependent interaction with water molecules, making it particularly sensitive to changes in environmental humidity. The sensing mechanism primarily relies on the adsorption of water molecules on the BP surface, which alters its electrical conductivity. Monolayer and few-layer BP demonstrate higher sensitivity compared to bulk BP due to their larger surface-to-volume ratio and higher carrier mobility. The adsorption of water molecules introduces charge carriers, leading to a measurable change in resistance. Studies have shown that few-layer BP sensors can achieve response times as fast as a few seconds, with recovery times dependent on environmental conditions and material thickness.

However, BP humidity sensors face significant stability challenges, primarily due to oxidation. Black phosphorus degrades rapidly in ambient conditions, forming phosphorus oxides that degrade device performance. The degradation rate depends on factors such as layer thickness, exposure time, and environmental humidity. Few-layer BP degrades faster than bulk BP due to greater surface exposure. Encapsulation strategies have been developed to mitigate this issue, including atomic layer deposition of Al2O3, hexagonal boron nitride (hBN) passivation, and polymer coatings such as PMMA. These methods can extend the operational lifetime of BP sensors from hours to several weeks, depending on the encapsulation quality and environmental conditions. Inert gas environments or vacuum packaging further enhance stability but increase system complexity.

Comparisons with graphene and MoS2 humidity sensors highlight the advantages and limitations of BP. Graphene exhibits fast response times but suffers from low sensitivity due to its hydrophobic nature and weak interaction with water molecules. Functionalization with hydrophilic groups can improve graphene's humidity response but complicates fabrication. MoS2 shows better sensitivity than graphene but still lags behind BP in terms of response magnitude. BP's anisotropic structure and strong hygroscopicity contribute to its superior performance in humidity sensing. However, MoS2 outperforms BP in long-term stability, as it does not oxidize as readily under ambient conditions. The trade-off between sensitivity and stability must be considered when selecting a material for specific applications.

Quantitative studies reveal that BP humidity sensors can achieve sensitivity values exceeding 100% resistance change per relative humidity (RH) unit in the range of 10-90% RH. The exact sensitivity depends on the number of layers, with thinner films showing higher responses. For example, a five-layer BP sensor may exhibit a 50% resistance change at 60% RH, while a monolayer sensor could show over 80% change under the same conditions. The hysteresis effect, a common issue in humidity sensors, is more pronounced in BP than in MoS2 but can be mitigated through proper encapsulation and operational protocols.

The fabrication of BP humidity sensors involves mechanical or liquid-phase exfoliation to obtain thin flakes, followed by transfer onto substrates with pre-patterned electrodes. Electrode material selection influences performance, with gold being the most common due to its inertness and low contact resistance. Alternative electrode materials such as graphene or conductive polymers have been explored to improve flexibility and reduce fabrication costs. Device architectures range from simple two-terminal resistors to more complex field-effect transistor configurations, with the latter offering better control over the sensing response through gate modulation.

Future research directions for BP humidity sensors include improving encapsulation techniques to achieve long-term stability without sacrificing sensitivity. Hybrid structures combining BP with other 2D materials or polymers may offer a balance between performance and durability. Scalable fabrication methods are needed to transition from lab-scale devices to industrial applications. Additionally, understanding the fundamental interactions between water molecules and BP at the atomic level could lead to optimized sensor designs. While challenges remain, BP humidity sensors represent a promising technology for applications requiring high sensitivity and fast response, provided stability issues are adequately addressed.