Broadband light detection (UV-Vis-NIR) is essential for various applications, including environmental monitoring, imaging, biomedical applications, and industrial process control. High-performance broadband photodetectors require photoactive semiconductors with high carrier mobility and efficient broad-spectrum light absorption. Organic semiconductors offer advantages such as low-cost fabrication, tunable optoelectronic properties, and solution processability, making them promising candidates. While significant progress has been made in organic phototransistors (OPTs), broadband response, particularly from a single active component, remains challenging. Most broadband OPTs rely on mixing multiple photoactive materials with different absorption regions, such as donor/acceptor bulk heterojunction (BHJ) structures. These BHJ structures often exhibit ambipolar charge transport, reducing detectivity. Single-component OPTs are preferred for their simplicity and cost-effectiveness. This paper aims to address the challenges of creating a single-component broadband OPT by introducing a novel n-type organic small molecule semiconductor that exhibits both high carrier mobility and efficient light absorption across a wide spectral range.

The literature extensively covers organic semiconductors for light detection, highlighting the challenges in achieving broadband response from a single active layer. Many studies focus on organic small molecules and polymers for UV-Vis light detection, but achieving NIR response and high performance remains limited. The use of donor/acceptor blends and bulk heterojunction structures is a common approach to broaden the spectral response, but this often compromises the device's performance due to the inherent limitations of the active layer. The lack of suitable single-component organic semiconductors with both high mobility and broad absorption is the primary obstacle to creating high-performance single-component broadband photodetectors. This research addresses this gap.

This study utilizes a thiophene-diketo-pyrrolopyrrole-based quinoidal (TDPPQ) molecule as the n-type organic small molecule semiconductor. Crystalline TDPPQ nanosheets were fabricated using a solvent-phase interfacial self-assembly method. This method involves dissolving TDPPQ in chloroform and slowly adding a less dense, immiscible solvent (methanol) to create an interface where the TDPPQ molecules self-assemble into crystalline nanosheets through π-π stacking interactions. The morphology and structure of the nanosheets were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD). Bottom-gate, top-contact field-effect transistors (FETs) were fabricated using the TDPPQ nanosheets as the active layer, n-doped silicon as the gate electrode, OTS-modified SiO₂ as the dielectric layer, and gold as the source and drain electrodes. The electrical properties of the transistors, including electron mobility and air stability, were evaluated. The photoresponse of the devices was characterized by measuring the photocurrent under illumination with monochromatic light of varying wavelengths and intensities. The photoresponsivity and specific detectivity were calculated and compared to reported values for similar devices.

The TDPPQ nanosheets exhibit a highly ordered molecular packing structure, confirmed by XRD analysis, which leads to a high field-effect electron mobility (up to 2.1 cm² V⁻¹ s⁻¹). The transistors show excellent air stability, with minimal change in mobility after two months of storage in ambient conditions. Importantly, the TDPPQ-based phototransistors demonstrate a broad spectral response from 365 nm to 940 nm, showcasing a superior photoresponsivity of up to 9.2 × 10⁵ A W⁻¹ and a specific detectivity of 5.26 × 10¹³ Jones at 760 nm. These values significantly outperform previously reported n-type organic small molecule-based phototransistors. The broad absorption spectrum of the TDPPQ nanosheets (UV to NIR) is attributed to the highly ordered molecular packing within the nanosheets. The high electron mobility stems from the enhanced π-orbital overlap facilitated by the ordered structure, minimizing grain boundary effects observed in spin-coated thin films.

The results demonstrate the successful fabrication of a high-performance, air-stable, broadband organic phototransistor using a single n-type organic small molecule semiconductor. The superior performance is directly attributable to the highly ordered crystalline structure of the TDPPQ nanosheets, which significantly enhances both carrier mobility and light absorption. This approach addresses a critical challenge in the field, offering a pathway towards the development of cost-effective, high-performance broadband photodetectors based on single-component organic semiconductors. The findings highlight the potential of rationally designed organic molecules and advanced fabrication techniques for achieving improved optoelectronic device performance.

This work successfully demonstrates a high-performance, air-stable n-type organic small molecule phototransistor with exceptional broadband photoresponse. The use of a solvent-phase interfacial self-assembly method to create highly ordered TDPPQ nanosheets is key to achieving both high electron mobility and broad spectral absorption. The superior performance of the device opens up new possibilities for the design and fabrication of high-performance, cost-effective organic photodetectors for a range of applications. Future research will focus on exploring scalable fabrication methods to enable wider implementation of this technology and investigate the application of this material in other optoelectronic devices.

While the reported TDPPQ nanosheet-based phototransistors show excellent performance, the self-assembly method used for nanosheet fabrication may pose limitations for large-scale production. Further research is needed to explore alternative methods, such as solution shearing, screen printing, or drop casting, to develop scalable fabrication techniques for the creation of two-dimensional crystalline thin films of organic semiconductors. This will improve the manufacturability and widespread applicability of this promising material.