The demand for wearable and implantable dosimeters is high in medical diagnosis and radiotherapy, where real-time radiation monitoring is crucial for patient safety and treatment optimization. Existing inorganic semiconductor detectors, while sensitive, often lack biocompatibility or are expensive to produce. Organic single crystalline semiconductors (OSCS) offer a potential solution, but their slow charge transport often limits their effectiveness for real-time applications. Previous OSCS detectors, like 4-hydroxycyanobenzene (4HCB), showed some promise for charged particle detection, but their long measurement times (around 30 minutes) were a significant drawback. This research aims to address this limitation by investigating a novel biocompatible OSCS, 4-Hydroxyphenylacetic acid (4HPA), for real-time spectral detection of charged particles. The slow charge transport, often characterized by a fast and slow-rising component in the electric pulse, is a major challenge in organic detectors, hindering real-time applications. This study investigates the underlying mechanisms of this two-stage rising pulse and explores potential solutions for improved charge transport within the context of 4HPA.

Current state-of-the-art radiation detectors, primarily based on inorganic semiconductors like CdZnTe, Si, and CsPbBr3, suffer from either biocompatibility issues (due to toxic metal elements) or high fabrication costs. While solution-grown OSCS such as 4HCB and Rubrene have shown promise as detectors for various types of radiation, their performance in spectral detection mode (which provides both energy and count information) is limited by slow charge transport. The slow response times observed in organic detectors, manifesting as a slow-rising component in the pulse signal, are attributed to exciton diffusion and dissociation processes. However, the exact mechanisms and how to improve charge transport have not been fully understood. This review highlights the need for biocompatible, fast-response, low-cost semiconductors for real-time, position-sensitive radiation monitoring for applications such as in-vivo dosimetry in radiotherapy, personal dosimetry for astronauts and radiologists, and safety controls in medical imaging procedures.

This study utilized 4-Hydroxyphenylacetic acid (4HPA), a biocompatible organic compound found in olive oil, as a novel material for OSCS. 4HPA single crystals were grown using a controlled solvent evaporation method. The chemical composition and crystallographic structure were characterized using various techniques, including molecular formula analysis, X-ray diffraction (XRD), and atomic force microscopy (AFM). Density Functional Theory (DFT) calculations were employed to investigate the band structure, effective masses, and electron cloud densities of 4HPA. Time-of-flight (TOF) measurements were conducted using alpha particle irradiation to analyze charge transport properties along both in-plane (a-axis) and out-of-plane (c-axis) directions. Monte Carlo (MC) simulations were used to model the energy distribution of radiation-generated electrons. Photocurrent measurements under X-ray irradiation were performed to assess the exciton dissociation behavior and the role of surface layer edges. Finally, real-time spectral detection of charged particles and X-rays was performed using detectors with different crystallographic orientations. The Hecht function was used to extract the mobility-lifetime product (μτ) from the experimental data.

The research revealed that solution-grown 4HPA OSCS possesses a 2D anisotropic crystal structure, exhibiting superior electron transport properties along the in-plane (a-axis) direction. DFT calculations predicted a radiation-stimulated heavy-to-light electron transition, with electrons transitioning from a high-energy L1 level to a lower energy LUMO level, consistent with experimental TOF results. The TOF measurements showed a record electron drift velocity of 5 × 10⁵ cm s⁻¹ along the a-axis under radiation, while maintaining a high resistivity of (1.28 ± 0.003) × 10¹² Ω cm in the dark. A significant slow-rising component in the alpha particle-induced pulses was observed, attributed primarily to exciton dissociation processes assisted by surface layer edges along the a-axis. 4HPA detectors demonstrated real-time spectral detection of charged particles with a remarkable energy resolution of 36%, (μτ)e of (4.91 ± 0.07) × 10⁻⁵ cm² V⁻¹, and a detection time of just 3 ms. In addition to their performance in charged particle detection, 4HPA detectors also exhibited high X-ray detection sensitivity (16,612 µC Gy⁻¹air cm⁻³) and a low detection limit of 20 nGy⁻¹air s⁻¹, as well as excellent long-term stability under high doses of radiation. The observed high photoconductive gain (up to 15,000%) is likely linked to the charge injection induced by the surface layer edge states.

The findings demonstrate that 4HPA's superior performance stems from a combination of factors: a radiation-stimulated heavy-to-light electron transition that facilitates fast charge transport, and surface layer edges that promote exciton dissociation, thus reducing the slow-rising component of the pulse. The exceptional biocompatibility of 4HPA, coupled with its excellent radiation detection capabilities, addresses the key limitations of current radiation detectors. The real-time spectral detection of charged particles is a significant advancement for organic semiconductors. The ability to detect alpha particles, x-rays, and neutrons opens doors to various applications. The high sensitivity achieved in X-ray detection is promising for applications in medical imaging.

This study successfully demonstrated that biocompatible 4HPA OSCS is a highly effective material for real-time spectral detection of charged particles, X-rays, and fast neutrons, exhibiting superior performance compared to other organic semiconductor detectors. The novel mechanism of radiation-stimulated heavy-to-light electron transitions, along with surface layer edge-assisted exciton dissociation, contributes to its high detection efficiency and fast response. The compact, tissue-equivalent, and low-cost nature of 4HPA detectors makes them particularly attractive for wearable/implantable personal dosimeters and in-vivo radiation monitoring. Future research could explore modifications to 4HPA to further enhance energy resolution and explore other potential applications.

While this study demonstrates significant advancements in biocompatible radiation detectors, several limitations should be acknowledged. The energy resolution of 4HPA detectors, while superior to existing organic counterparts, is still lower than that of state-of-the-art inorganic detectors. Further optimization of crystal quality, device structure, and measurement electronics is necessary to improve energy resolution. The study primarily focuses on the detection of alpha particles and X-rays. Although detection of neutrons is demonstrated, more extensive studies are needed to fully characterize the neutron detection capabilities of 4HPA detectors.