Flexible photodetectors represent a rapidly advancing field in optoelectronics, driven by the demand for lightweight, conformable, and durable sensing systems. Unlike traditional rigid photodetectors based on silicon or other inorganic semiconductors, flexible variants leverage organic semiconductors, two-dimensional materials, and innovative fabrication techniques to achieve performance metrics suitable for emerging applications. These devices are particularly relevant for wearable health monitors and foldable displays, where mechanical flexibility and optical sensitivity are critical.

The choice of materials is central to the performance of flexible photodetectors. Organic semiconductors, such as conjugated polymers and small molecules, offer inherent flexibility, tunable optoelectronic properties, and compatibility with low-temperature processing. For example, polymers like poly(3-hexylthiophene) (P3HT) and small molecules such as pentacene exhibit strong light absorption and charge transport characteristics. These materials can be engineered to detect specific wavelengths, from ultraviolet to near-infrared, by modifying their chemical structure or blending them with other organic compounds. Additionally, organic semiconductors enable solution-based processing, which is essential for scalable and cost-effective manufacturing.

Two-dimensional materials, including graphene, transition metal dichalcogenides (TMDCs), and black phosphorus, are another promising class of materials for flexible photodetectors. Graphene offers ultra-broadband absorption, high carrier mobility, and exceptional mechanical strength, making it suitable for high-speed photodetection. TMDCs like molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) provide layer-dependent bandgaps, allowing precise control over spectral response. Black phosphorus, with its anisotropic properties and tunable bandgap, is particularly effective for mid-infrared detection. These materials can be integrated into heterostructures to enhance performance, combining the advantages of multiple 2D systems while maintaining flexibility.

Fabrication techniques for flexible photodetectors must accommodate the delicate nature of organic and 2D materials while ensuring scalability. Roll-to-roll printing has emerged as a leading method, enabling continuous, high-throughput production of devices on flexible substrates such as polyethylene terephthalate (PET) or polyimide. This technique involves depositing functional layers—conductive electrodes, active semiconductor layers, and encapsulation—using solution-based inks. Inkjet printing and screen printing are also employed for precise patterning of materials, offering resolutions suitable for high-density arrays. These methods reduce material waste and lower production costs compared to conventional lithography-based processes.

The performance of flexible photodetectors is evaluated based on responsivity, detectivity, response time, and mechanical robustness. Responsivity, measured in amperes per watt (A/W), indicates the efficiency of converting light into electrical signals. Detectivity, expressed in Jones, reflects the ability to detect weak signals amid noise. Organic photodetectors typically achieve responsivities in the range of 0.1 to 1 A/W, while 2D material-based devices can exceed 10 A/W due to their superior carrier mobility. Response times vary from microseconds in organic devices to nanoseconds in graphene-based detectors, depending on the recombination dynamics of photoexcited carriers. Mechanical robustness is assessed through bending tests, with many devices maintaining functionality after thousands of bending cycles at radii as small as 1 millimeter.

Wearable health monitors are a primary application for flexible photodetectors, leveraging their ability to conform to skin and other irregular surfaces. These devices are integrated into patches or textiles to monitor physiological signals such as heart rate, blood oxygen saturation, and UV exposure. For example, a photoplethysmography (PPG) sensor based on organic semiconductors can detect blood volume changes by measuring reflected light from the skin. Similarly, UV sensors using ZnO nanoparticles or organic dyes provide real-time alerts for sun exposure. The flexibility of these detectors ensures comfort during prolonged wear, while their low power consumption aligns with the energy constraints of wearable systems.

Foldable displays represent another major application, where flexible photodetectors serve as ambient light sensors or touchless input interfaces. In these systems, the photodetectors must withstand repeated folding and unfolding without degradation in performance. Organic photodetectors are particularly suited for this role due to their compatibility with display manufacturing processes. For instance, they can be integrated into the backplane of organic light-emitting diode (OLED) displays to enable adaptive brightness control. The ability to detect gestures or proximity without physical contact enhances user interaction in foldable smartphones and tablets.

Challenges remain in improving the environmental stability and scalability of flexible photodetectors. Organic semiconductors are susceptible to degradation from moisture and oxygen, necessitating robust encapsulation strategies such as thin-film barriers or atomic layer deposition. 2D materials, while more stable, face difficulties in large-area synthesis and transfer. Advances in material engineering and fabrication techniques are addressing these issues, with hybrid approaches combining organic and 2D materials showing particular promise.

The future of flexible photodetectors lies in the development of multifunctional systems that integrate sensing, energy harvesting, and data processing. For example, self-powered photodetectors incorporating perovskite solar cells or triboelectric nanogenerators could eliminate the need for external power sources. Similarly, the integration of artificial intelligence algorithms could enable real-time analysis of optical signals for advanced health diagnostics or environmental monitoring.

In summary, flexible photodetectors based on organic and 2D materials offer unique advantages for wearable and foldable applications. Their compatibility with roll-to-roll printing and other scalable techniques positions them as key components in the next generation of optoelectronic systems. Continued research into material properties, device architectures, and fabrication methods will further enhance their performance and expand their applicability across diverse fields.