Transition metal carbides, nitrides, and carbonitrides, collectively known as MXenes, have emerged as promising candidates to replace indium tin oxide (ITO) in optoelectronic applications. Their unique combination of high electrical conductivity and optical transparency makes them suitable for transparent conductive electrodes (TCEs), a critical component in displays, touchscreens, and solar cells. Unlike ITO, which suffers from brittleness and the scarcity of indium, MXenes offer mechanical flexibility and abundant elemental constituents, though their adoption in rigid optoelectronics primarily hinges on their optoelectronic performance and deposition compatibility.

The optoelectronic performance of MXenes is characterized by two key metrics: transmittance and sheet resistance. For TCE applications, the figure of merit (FoM), defined as the ratio of electrical conductivity to optical conductivity, is often used to compare materials. Ti₃C₂Tₓ, the most studied MXene, exhibits a sheet resistance as low as 20 Ω/sq with a transmittance of 90% at 550 nm wavelength when prepared as ultrathin films. These values are competitive with ITO, which typically achieves 10-20 Ω/sq at 85-90% transmittance. The optical properties of MXenes are thickness-dependent, with transmittance decreasing linearly as the number of layers increases. For instance, a 5 nm Ti₃C₂Tₓ film may show 95% transmittance, while a 20 nm film drops to 80%. The sheet resistance, however, improves with thickness due to increased charge carrier pathways.

The electrical conductivity of MXenes stems from their metallic-like electron transport within the transition metal carbide layers. Surface terminations (Tₓ), such as -O, -F, or -OH groups, influence both conductivity and optical properties. Oxygen-terminated MXenes generally exhibit higher conductivity due to reduced electron scattering compared to fluorine or hydroxyl terminations. The carrier concentration in Ti₃C₂Tₓ can reach 10²² cm⁻³, comparable to doped metal oxides, while maintaining moderate carrier mobility of 1-10 cm²/V·s. These properties enable FoM values exceeding 50 for optimized MXene films, meeting industrial requirements for TCEs.

Deposition techniques play a crucial role in determining the optoelectronic performance of MXene films. Spin-coating is the most widely used method for laboratory-scale fabrication due to its simplicity and uniformity. A typical process involves dispersing MXene flakes in water or organic solvents, followed by multiple spin-coating cycles to achieve desired thickness. The spin speed and solution concentration directly affect film morphology; speeds of 1000-3000 rpm yield homogeneous films with minimal roughness. However, spin-coating suffers from material waste and limited scalability.

Langmuir-Blodgett (LB) assembly offers precise control over monolayer deposition, enabling ultra-thin films with uniform coverage. In this technique, MXene flakes are dispersed at the air-water interface and compressed to form a packed monolayer before transfer onto a substrate. LB-deposited Ti₃C₂Tₓ films demonstrate superior uniformity compared to spin-coated films, with roughness below 1 nm. This method is particularly advantageous for achieving high transmittance (>95%) while maintaining low sheet resistance (<100 Ω/sq). The main drawback of LB assembly is its slow throughput, making it less suitable for large-area applications.

Vacuum filtration is another common technique, producing films with excellent conductivity but limited optical performance due to higher thickness. Recent advances in spray-coating and inkjet printing have improved scalability without significant performance degradation. Spray-coated MXene films achieve sheet resistances of 30-50 Ω/sq at 80% transmittance, with the added benefit of patternability. Inkjet printing enables direct patterning of MXene electrodes, reducing post-processing steps. Both methods require optimization of ink rheology and substrate wettability to prevent coffee-ring effects or uneven drying.

Environmental stability remains a challenge for MXene-based TCEs. Oxidation in ambient conditions can degrade electrical performance over time, necessitating protective coatings or encapsulation. Al₂O₃ or SiO₂ thin films deposited by atomic layer deposition (ALD) have been shown to extend MXene film lifetimes without compromising optical transparency. Alternatively, chemical passivation with reducing agents can stabilize the MXene surface against oxidation.

In comparison to other ITO alternatives like graphene or silver nanowires, MXenes offer a balance between conductivity, transparency, and processability. Graphene suffers from higher sheet resistance (>100 Ω/sq at 90% transmittance) unless doped, while silver nanowire networks face issues with haze and poor adhesion. MXenes also outperform conductive polymers like PEDOT:PSS in both conductivity and environmental stability.

The optoelectronic performance of MXenes can be further enhanced through doping or hybridization. Incorporation of carbon nanotubes or metal nanoparticles improves conductivity without significant transmittance loss. Nitrogen doping of Ti₃C₂Tₓ has been shown to increase carrier density while reducing surface oxidation. Hybrid structures with graphene or metal grids combine the strengths of each material, achieving sub-10 Ω/sq sheet resistance at >90% transmittance.

Industrial adoption of MXenes as ITO replacements will depend on scaling up synthesis and deposition processes while maintaining performance. Current MXene production relies on selective etching of MAX phases, which can be optimized for higher yields and fewer defects. Roll-to-roll compatible deposition methods, such as slot-die coating, are under development to meet manufacturing demands. Standardization of characterization protocols will also be critical for benchmarking against established materials.

In summary, MXenes present a viable alternative to ITO for rigid optoelectronic applications, offering competitive optoelectronic performance and versatile deposition options. Continued research in material processing and stability will determine their commercial viability in displays, photovoltaics, and other transparent electrode applications. Their tunable properties and compatibility with existing fabrication methods position them as a leading candidate in the search for ITO-free optoelectronics.