Abstract
Near-infrared (NIR) absorbing materials, capable of harvesting light in the 700-2500 nm wavelength range that constitutes over 50% of the solar spectrum, have emerged as pivotal components in advanced optoelectronic and biomedical technologies. This review provides a comprehensive overview of recent advancements in NIR-absorbing materials, encompassing organic semiconductors, inorganic nanostructures, metal-organic frameworks, and metamaterials. We discuss their underlying absorption mechanisms, synthetic strategies, and highlight breakthrough applications in solar energy conversion, deep-tissue bioimaging, photocatalysis, and optical sensing. The review concludes with current challenges and future prospects for developing high-performance, stable, and cost-effective NIR-absorbing materials.
1. Introduction
The near-infrared region of the electromagnetic spectrum represents a vast, underutilized resource with tremendous potential for technological innovation. While visible light (400-700 nm) has been extensively exploited in conventional optical technologies, the NIR range remains relatively untapped despite comprising approximately half of the total solar irradiance reaching Earth’s surface. Recent breakthroughs in material science have enabled the design and synthesis of specialized materials that efficiently absorb NIR radiation, opening new frontiers in energy harvesting, medical diagnostics, environmental monitoring, and optical communication.
NIR-absorbing materials (NIRAMs) exhibit unique advantages stemming from the intrinsic properties of NIR light: deeper tissue penetration with minimal photodamage for biomedical applications, reduced solar glare in optical devices, and the ability to utilize otherwise wasted solar energy in photovoltaic systems. This article presents a systematic analysis of the most promising NIRAM classes, their working mechanisms, and cutting-edge applications that are transforming multiple industries.
2. Classification of NIR-Absorbing Materials
2.1 Organic NIR Absorbers
Organic NIR-absorbing materials have gained significant attention due to their synthetic tunability, low cost, and solution processability. This category includes small-molecule acceptors (SMAs), conjugated polymers, and dye molecules specifically engineered for NIR absorption. A notable example is Y-Senf, a selenophene-fused SMA that exhibits a 65 nm red-shifted absorption compared to its thiophene-based counterpart, enabling efficient harvesting of longer NIR wavelengths. When incorporated as a guest acceptor in ternary polymer solar cells (PSCs), Y-Senf contributes to an impressive power conversion efficiency (PCE) of 19.28% with excellent stability exceeding 200 hours under maximum-power-point tracking.
Another important class of organic NIR absorbers includes radical anions of stacked aromatic imides, fused porphyrin arrays, and polythiophenes, which show promise for photonic and telecommunications applications due to their electrochromic properties and efficient NIR absorption in the 1000-2000 nm range. These materials leverage intramolecular charge transfer mechanisms to achieve strong absorption in the desired wavelength range.
2.2 Inorganic Nanostructures
Inorganic materials offer superior stability and tunable optical properties through quantum confinement effects. Titanium nitride (TiN) heterostructures deposited onto porous anodized aluminum oxide templates have demonstrated ultrabroadband absorption covering both visible (400-700 nm) and NIR (700-2500 nm) regions with average absorption efficiencies of 99.1% and 80.1%, respectively. The incorporation of a titanium dioxide (TiO₂) layer extends hot carrier lifetimes by 2.7 times, significantly enhancing photocatalytic performance.
Lead-free tin-based halide perovskites (CsSnI₃) represent another breakthrough in inorganic NIR materials, enabling the fabrication of NIR light-emitting diodes (LEDs) with peak emission at 948 nm, high radiance of 226 W sr⁻¹ m⁻², and operational half-lifetimes of 39.5 hours. These materials address critical limitations in toxicity and stability compared to traditional lead-based perovskites.
2.3 Metamaterials and Hybrid Structures
Metamaterials engineered at the nanoscale provide unprecedented control over light-matter interactions. A recently developed metal-dielectric-metal-dielectric-metal (MDMDM) structure utilizing a titanium nano-cross layer achieves near-perfect absorption from 2.05 to 6.08 μm, with average absorption reaching 97.41%. This ultra-broadband performance stems from combined surface plasmon polariton resonance and magnetic resonance cavity effects, making it suitable for astronomical imaging and remote sensing applications.
Core-shell metallo-dielectric nanoparticles exhibit tunable surface plasmon resonance in the NIR regime (1400-3000 nm) through precise control of core-shell radii. These structures find applications in integrated photonics due to their wavelength-selective absorption properties.
3. Synthesis Strategies
The development of effective synthetic methodologies has been crucial for advancing NIR-absorbing materials. For organic systems, rational molecular design strategies such as selenophene substitution in SMAs have proven effective in red-shifting absorption spectra while enhancing crystallinity and intermolecular interactions. This molecular engineering approach suppresses non-radiative energy loss and improves charge transport in photovoltaic devices.
For inorganic nanostructures, solution-phase methods like the one-step double-emulsion technique have enabled the fabrication of porous biocompatible particles composed of polylactic acid matrices and polypyrrole nanoparticles. This approach enhances NIR light absorption through structural engineering, creating ultra-NIR-sensitive materials for sensing applications.
Vapor deposition techniques have been instrumental in creating heterostructured NIR absorbers like the TiN/TiO₂ systems, where precise control over layer thickness and morphology is achieved through atomic layer deposition on porous templates. This method ensures uniform coverage and optimal interface engineering for enhanced carrier lifetime.
4. Key Applications
4.1 Solar Energy Conversion
NIR-absorbing materials have revolutionized photovoltaic technology by extending light harvesting beyond the visible spectrum. Tandem organic solar cells incorporating ultra-narrow band gap acceptors like BTP-SEV-4F (1.17 eV band gap) have achieved PCEs of 14.2% in single-junction devices and 19% in tandem configurations. This breakthrough stems from suppressed triplet exciton formation and reduced energy loss (0.55 eV), enabling efficient utilization of NIR photons.
Figure 1: Schematic illustration of a ternary polymer solar cell architecture incorporating NIR-absorbing small-molecule acceptors (Y-Senf) showing enhanced light absorption in the 700-1000 nm range compared to binary systems.
4.2 Biomedical Imaging and Therapy
The NIR-II window (>900 nm) has emerged as particularly valuable for deep-tissue imaging due to reduced light scattering and tissue autofluorescence. Biomimetic NIR-II fluorescent proteins created by conjugating synthetic dyes to tag proteins (e.g., human serum albumin) have enabled high-performance lymphography and angiography. These materials exhibit enhanced brightness, photostability, and biocompatibility compared to conventional fluorophores.
Figure 2: NIR-II bioimaging of murine lymphatic system using biomimetic fluorescent proteins, demonstrating deep-tissue visualization with high spatial resolution.
4.3 Photocatalysis
Ultrabroadband NIR absorbers are transforming photocatalytic applications such as hydrogen evolution reactions (HER). TiN/TiO₂ heterostructures have shown a 1.9-fold improvement in hydrogen production efficiency compared to TiN alone, attributed to extended hot carrier lifetimes and efficient electron transfer. This development provides a sustainable route for solar-to-hydrogen energy conversion utilizing previously untapped NIR photons.
Figure 3: Schematic representation of photocatalytic hydrogen evolution using TiN/TiO₂ heterostructure absorbers under full-spectrum solar irradiation, highlighting the role of NIR absorption in enhancing efficiency.
4.4 Optical Sensing and Communication
MDMDM metamaterial absorbers are enabling next-generation optical sensors with applications in astronomical imaging, volcano detection, and biological monitoring. Their polarization-sensitive characteristics and ultra-broadband absorption (2.05-6.08 μm) make them ideal for detecting subtle wavelength shifts associated with chemical and biological analytes.
In telecommunications, organic NIR-absorbing materials are being developed for electrochromic variable optical attenuators, leveraging their reversible absorption changes under electrical stimulation for signal modulation in fiber optic networks.
5. Challenges and Future Perspectives
Despite significant progress, several challenges remain in the field of NIR-absorbing materials. Organic systems suffer from limited long-term stability under operational conditions, while inorganic nanostructures often face issues with toxicity (e.g., lead-based perovskites) and high production costs. Both material classes exhibit low external quantum yields in certain configurations, limiting their practical application.
Future research directions should focus on:
Developing stable, lead-free inorganic alternatives with tunable NIR absorption
Engineering organic-inorganic hybrids to combine the best properties of both material classes
Implementing nanostructured designs to enhance light-matter interactions and carrier lifetimes
Exploring multifunctional NIR absorbers for integrated applications in energy harvesting and sensing
Scaling up synthetic processes for cost-effective mass production
Advancements in computational modeling and machine learning will play increasingly important roles in accelerating the discovery and optimization of new NIR-absorbing materials with tailored properties for specific applications.
6. Conclusion
Near-infrared absorbing materials have evolved from laboratory curiosities to essential components in cutting-edge technologies. The diverse range of materials discussed—from organic semiconductors to metamaterials—offers unique advantages for efficiently harvesting and manipulating NIR radiation. Breakthroughs in solar energy conversion (19.28% efficiency), deep-tissue imaging (NIR-II window), and photocatalysis (1.9× hydrogen production enhancement) demonstrate the transformative potential of these materials.
As research continues to address current limitations in stability, efficiency, and cost, NIR-absorbing materials are poised to play an increasingly significant role in addressing global challenges in renewable energy, healthcare, and environmental monitoring. The ability to unlock the full potential of the NIR spectrum will undoubtedly lead to innovative technologies that harness this abundant, yet underutilized, natural resource.