Organic photodetectors represent a rapidly advancing field in optoelectronics, leveraging the unique properties of organic semiconductors to enable flexible, lightweight, and low-cost devices. Unlike their inorganic counterparts, organic photodetectors can be fabricated using solution-based processes, making them suitable for large-area applications and integration into wearable systems. Their tunable optical and electronic properties, achieved through molecular design, allow for tailored responses across the visible and near-infrared spectrum. Key advantages include mechanical flexibility, compatibility with unconventional substrates, and the potential for low-temperature processing, which are critical for emerging applications in health monitoring, biomedical imaging, and portable diagnostics.

Material selection plays a pivotal role in determining the performance of organic photodetectors. Among the most widely studied systems are blends of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), which serve as the donor and acceptor materials, respectively. P3HT exhibits strong absorption in the visible range, while PCBM facilitates efficient charge separation and transport. Other promising donor-acceptor pairs include low-bandgap polymers such as PTB7-Th and non-fullerene acceptors like ITIC, which extend spectral sensitivity and improve device efficiency. The choice of materials impacts critical parameters such as responsivity, detectivity, and response speed, with optimized systems achieving external quantum efficiencies exceeding 50% in specific wavelength ranges.

Device architectures for organic photodetectors typically follow planar or bulk heterojunction designs. Planar heterojunctions consist of stacked donor and acceptor layers, offering controlled interfaces but limited charge generation efficiency due to reduced interfacial area. In contrast, bulk heterojunctions blend donor and acceptor materials into a single active layer, creating a nanoscale network that enhances exciton dissociation. The bulk heterojunction architecture is more common due to its superior performance, though it requires careful optimization of film morphology to balance charge transport and recombination. Additional layers, such as hole-transporting materials (e.g., PEDOT:PSS) and electron-transporting materials (e.g., ZnO), are often incorporated to improve charge extraction and reduce interfacial losses.

Flexibility is a defining feature of organic photodetectors, enabling conformal integration onto curved or dynamic surfaces. This property is particularly advantageous for wearable sensors, where devices must withstand bending, stretching, and mechanical deformation. Substrates like polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) provide the necessary mechanical robustness while maintaining optical transparency. Recent advances in stretchable conductive electrodes, such as silver nanowires or conductive polymers, further enhance device durability under strain. Wearable applications include real-time monitoring of physiological signals, such as heart rate and blood oxygen levels, where organic photodetectors serve as the sensing element in optical plethysmography systems.

Biomedical imaging is another promising application, leveraging the biocompatibility and tunable spectral response of organic photodetectors. Their ability to detect low-intensity light makes them suitable for fluorescence and bioluminescence imaging, particularly in minimally invasive diagnostics. For instance, organic photodetectors have been integrated into compact endoscopes for high-resolution imaging of tissues. Their compatibility with flexible substrates allows for direct attachment to biological surfaces, enabling localized measurements with minimal discomfort.

Despite these advantages, stability remains a significant challenge for organic photodetectors. Environmental factors such as oxygen, moisture, and UV exposure can degrade organic materials, leading to performance deterioration over time. Encapsulation techniques, including thin-film barriers and inert coatings, are essential to prolong device lifetimes. Additionally, photochemical degradation under prolonged illumination can cause irreversible damage to the active layer, necessitating the development of more robust material systems. Research into stable non-fullerene acceptors and cross-linked polymers has shown promise in addressing these issues.

Fabrication costs are markedly lower for organic photodetectors compared to inorganic devices, primarily due to solution-processable techniques like spin-coating, inkjet printing, and roll-to-roll processing. These methods reduce material waste and enable high-throughput production, making organic photodetectors economically viable for large-scale applications. However, achieving uniformity and reproducibility over large areas remains a technical hurdle, particularly for multilayer devices requiring precise alignment.

In summary, organic photodetectors offer a compelling combination of flexibility, low-cost fabrication, and tunable optoelectronic properties, making them ideal for wearable sensors and biomedical imaging. Advances in material design, device engineering, and encapsulation strategies continue to address stability and performance limitations, paving the way for broader adoption in consumer electronics and healthcare technologies. Future research will likely focus on improving environmental resilience, expanding spectral coverage, and integrating these devices into multifunctional systems for real-world applications.