Transition metal dichalcogenides (TMDCs) have emerged as promising materials for photodetection due to their tunable bandgaps, strong light-matter interactions, and compatibility with flexible substrates. These layered materials, with the general formula MX2, where M is a transition metal (Mo, W, etc.) and X is a chalcogen (S, Se, Te), exhibit exceptional optoelectronic properties. Their direct bandgap in monolayer form and indirect bandgap in bulk allow for versatile applications in photodetectors across visible to near-infrared spectral ranges. Key performance metrics such as responsivity, detectivity, response time, and spectral range are critical in evaluating TMDC-based photodetectors. Hybrid architectures, particularly those integrating graphene with TMDCs, further enhance these metrics by leveraging the complementary properties of both materials.

Responsivity, defined as the photocurrent generated per unit power of incident light, is a fundamental parameter for photodetectors. Monolayer MoS2 photodetectors have demonstrated responsivities in the range of 10 to 1000 A/W under varying bias conditions and illumination intensities. The high responsivity arises from the material's strong excitonic effects and efficient carrier multiplication. For instance, defect-engineered MoS2 devices exhibit enhanced responsivity due to trap-assisted photoconductive gain, where trapped holes prolong the lifetime of photogenerated electrons. Hybrid graphene-MoS2 photodetectors achieve even higher responsivities, often exceeding 1000 A/W, by combining graphene's high carrier mobility with MoS2's strong light absorption. The built-in electric field at the graphene-TMDC interface facilitates efficient charge separation and collection, reducing recombination losses.

Detectivity, a measure of a detector's ability to discern weak signals from noise, is another critical performance metric. TMDC photodetectors typically achieve detectivities in the range of 10^10 to 10^13 Jones, depending on material quality and device architecture. Lower dark currents and higher photoconductive gains contribute to improved detectivity. For example, WSe2 photodetectors with hexagonal boron nitride (hBN) encapsulation exhibit reduced dark currents due to suppressed interfacial traps, resulting in detectivities approaching 10^13 Jones. Graphene-TMDC heterostructures further enhance detectivity by minimizing contact resistance and enabling fast carrier extraction, which reduces noise. The combination of low noise and high responsivity in these hybrid systems makes them suitable for low-light applications such as imaging and sensing.

Response time determines the speed at which a photodetector can follow varying light signals. TMDC-based devices exhibit response times ranging from microseconds to nanoseconds, influenced by factors such as carrier mobility, recombination rates, and device geometry. Monolayer WS2 photodetectors with metal contacts have shown response times as fast as 50 ns, attributed to the material's high carrier mobility and efficient charge collection. However, defects and trap states can prolong response times by introducing carrier trapping and release processes. Hybrid architectures address this limitation by leveraging graphene's ultrafast carrier transport. Graphene-MoS2 photodetectors demonstrate response times below 10 ns, as graphene rapidly extracts photogenerated carriers before they recombine or become trapped. Engineering the interface between graphene and TMDCs is crucial for optimizing response times while maintaining high responsivity.

The spectral range of TMDC photodetectors is determined by their bandgap, which can be tuned by varying composition and layer thickness. Monolayer MoS2 and WS2 absorb strongly in the visible range (1.8 to 2.1 eV), while WSe2 extends into the near-infrared (1.2 to 1.6 eV). Alloying TMDCs, such as MoS2(1-x)Se2x, further enables bandgap engineering for tailored spectral response. For instance, Mo0.5W0.5S2 alloys exhibit intermediate bandgaps, allowing photodetection across a broader wavelength range. Heterostructures combining multiple TMDCs can achieve multispectral detection by stacking layers with different bandgaps. Graphene-TMDC hybrids extend the spectral range into the mid-infrared, as graphene's zero bandgap allows for broadband absorption. However, the responsivity in these systems tends to decrease at longer wavelengths due to weaker light-matter interactions in graphene.

Hybrid architectures, particularly graphene-TMDC heterostructures, offer significant advantages over standalone TMDC photodetectors. The graphene-TMDC junction forms a Schottky barrier, which creates a built-in electric field for efficient charge separation. Graphene's high mobility ensures rapid carrier transport, while the TMDC layer provides strong light absorption. Vertical heterostructures, where graphene is stacked directly on TMDCs, minimize carrier scattering and enable ultrathin device geometries. Lateral heterostructures, with patterned graphene electrodes, allow for scalable fabrication and customizable active areas. Encapsulating these hybrids with hBN further improves performance by reducing environmental degradation and interfacial traps. For example, hBN-encapsulated graphene-MoS2 photodetectors exhibit stable operation with responsivities exceeding 2000 A/W and detectivities above 10^12 Jones.

Device engineering plays a crucial role in optimizing TMDC photodetectors. Electrode design, for instance, affects both responsivity and response time. Asymmetric metal contacts, such as Ti/Au for electrons and Pd/Au for holes, create a built-in electric field that enhances charge separation. Light trapping structures, such as plasmonic nanoparticles or photonic crystals, can enhance absorption in ultrathin TMDC layers. Strain engineering, achieved through flexible substrates or localized deformation, modifies the bandgap and carrier mobility, enabling tunable photoresponse. For example, strained MoS2 photodetectors on polyethylene terephthalate (PET) substrates show redshifted spectral response and enhanced responsivity due to strain-induced bandgap narrowing.

Environmental stability remains a challenge for TMDC photodetectors, as exposure to oxygen and moisture can degrade performance over time. Encapsulation with inert materials like hBN or Al2O3 mitigates this issue by preventing chemical reactions at the TMDC surface. Alternatively, chemical passivation with organic molecules can stabilize the material while preserving its optoelectronic properties. For instance, thiol-based passivation of MoS2 reduces trap states and improves air stability without compromising responsivity.

In summary, TMDCs offer a versatile platform for high-performance photodetectors with tunable spectral response, high responsivity, and fast response times. Hybrid architectures, particularly those integrating graphene, further enhance these metrics by combining complementary material properties. Advances in device engineering, including heterostructure design, electrode optimization, and environmental protection, continue to push the boundaries of TMDC-based photodetection. These developments position TMDCs as competitive candidates for next-generation optoelectronic applications, from imaging to communication systems.