A high mobility air-stable n-type organic small molecule semiconductor with high UV-visible-to-NIR photoresponse

Broadband light detection spanning ultraviolet, visible, and near-infrared wavelengths is vital for environmental monitoring, imaging, biomedical uses, and industrial control. Achieving such broadband photodetectors is challenging because the active semiconductor must combine high carrier mobility with efficient absorption across a wide spectral range. Organic semiconductors are attractive due to low-cost processing, tunable optoelectronic properties, and solution processability. Although recent advances have improved organic phototransistor performance, most broadband devices rely on blends (e.g., donor/acceptor bulk heterojunctions) that combine multiple absorbers, introducing complexity and often ambipolar transport that can reduce detectivity. A single-component phototransistor with broadband response and high photoresponsivity is highly desirable, but has been limited by the scarcity of organic semiconductors that simultaneously offer wide spectral absorption and high mobility. This work addresses that gap by demonstrating an air-stable, solution-processed, n-type small-molecule phototransistor based on thiophene-diketo-pyrrolopyrrole-based quinoidal (TDPPQ) crystalline nanosheets, delivering high electron mobility and ultrabroadband photoresponse from UV to NIR.

Prior efforts to realize broadband organic phototransistors typically employ multi-component active layers such as donor/acceptor bilayers or bulk heterojunctions to extend spectral coverage because single organic materials often have limited absorption windows. While these blends can broaden response, they complicate fabrication and frequently exhibit ambipolar transport that degrades detectivity. Single-component phototransistors are considered ideal for simpler, lower-cost processing and potentially improved device metrics, but have been constrained by the absence of materials with both high mobility and broad absorption. Diketopyrrolopyrrole (DPP)-based building blocks are known to yield high-performance organic semiconductors due to their planar, electron-deficient cores, hydrogen-bonding capability, strong π–π stacking, and intramolecular charge transfer, which together can enhance charge transport.

Material and self-assembly: The n-type small molecule TDPPQ was dissolved in chloroform in a screw-cap vial. Methanol (a poor solvent for TDPPQ) was added dropwise onto the chloroform surface using a syringe, forming a two-phase system (methanol on top, chloroform below due to density difference). Over several hours, methanol slowly diffused into the TDPPQ solution, inducing supramolecular self-assembly at the interface. TDPPQ crystallized into well-ordered nanosheets through strong π–π stacking interactions. Concentration of TDPPQ solution was varied; lower concentrations favored isolated single nanosheets suitable for device fabrication. The resulting hexagonal nanosheets (several to tens of micrometers) were transferred onto substrates. Characterization: SEM and optical microscopy assessed morphology and lamellar growth; TEM and SAED confirmed crystallinity and hexagonal shape. XRD exhibited multiple high-order reflections at 2θ = 4.17°, 8.34°, 12.55°, 16.75°, 20.98°, and 25.23° corresponding to (001)–(006) planes, evidencing layer-by-layer packing. UV–Vis–NIR absorption of nanosheets dispersed in methanol showed broadened absorption versus spin-coated thin films, with a red-shifted 0–0 peak at 760 nm and broadband coverage from 250 to 1100 nm. Transistor fabrication: Bottom-gate, top-contact organic field-effect transistors (OFETs) were prepared on n-doped Si/SiO2 substrates. The Si served as the gate; the SiO2 dielectric was OTS-modified to promote interface quality. Gold source/drain electrodes were deposited using an organic ribbon soft shadow mask to form channels beneath TDPPQ nanosheets. Electrical measurement: Devices were measured under ambient conditions. Transfer and output characteristics were recorded; mobility was extracted from the saturation regime. A set of 30 devices was characterized to obtain mobility statistics and assess size dependence. Air stability was evaluated by storing devices in air and remeasuring after up to two months. Phototransistor measurement: Devices were illuminated with monochromatic light of tunable wavelength and intensity (e.g., 365, 550, 670, 765, 940 nm). Transfer curves under dark and illumination were collected to evaluate photoresponse, responsivity, and specific detectivity across the spectral range.

• Self-assembly produced single-crystalline, hexagonal TDPPQ nanosheets with lamellar, highly ordered molecular packing verified by XRD (distinct (001)–(006) reflections at 2θ = 4.17°, 8.34°, 12.55°, 16.75°, 20.98°, 25.23°) and SAED. • The nanosheets displayed exceptionally broad absorption from 250 to 1100 nm with a red-shifted 0–0 peak at 760 nm, indicating strong intermolecular coupling. • OFET performance: unipolar n-type transport with average electron mobility μFE = 1.5 cm² V⁻¹ s⁻¹ across 30 devices; best device μFE = 2.1 cm² V⁻¹ s⁻¹. Output characteristics saturated for VD > 30 V. • Air stability: devices stored under ambient conditions showed only slight mobility decrease after 2 months; stability attributed to the TDPPQ LUMO level (−4.51 eV). • Phototransistor performance: ultrahigh photoresponsivity over a wide spectral range from 365 to 940 nm. Maximum responsivity reached 9.2 × 10⁵ A W⁻¹ and specific detectivity 5.26 × 10¹³ Jones (at 760 nm). • Device-to-geometry effects: smaller-width nanosheets yielded higher mobilities, suggesting fewer grain boundaries and improved charge transport pathways compared to spin-coated films.

The results demonstrate that carefully engineered molecular self-assembly of an n-type small molecule (TDPPQ) into crystalline nanosheets can simultaneously achieve high electron mobility and broadband optical absorption, addressing two key bottlenecks in single-component organic phototransistors. The highly ordered, lamellar packing facilitates strong π–π overlap and long-range order, enabling efficient electron transport and yielding mobilities up to 2.1 cm² V⁻¹ s⁻¹. The collective molecular ordering also broadens and red-shifts absorption into the NIR, enabling detection from UV through NIR within a single semiconductor. Compared to multi-component blends that often suffer from ambipolar transport and fabrication complexity, this single-component system provides high responsivity and detectivity with simple processing and good air stability. The slight mobility decay over two months suggests practical robustness. Together, the structural order and energy level alignment (LUMO at −4.51 eV) underpin the observed device stability and performance, positioning TDPPQ nanosheets as strong candidates for broadband, high-sensitivity organic photodetectors.

This work introduces an air-stable, solution-processed, single-component n-type organic phototransistor based on self-assembled TDPPQ crystalline nanosheets. The devices combine high electron mobility (up to 2.1 cm² V⁻¹ s⁻¹), robust ambient stability, and ultrabroadband photoresponse (365–940 nm) with record-high responsivity (9.2 × 10⁵ A W⁻¹) and detectivity (5.26 × 10¹³ Jones) among n-type small-molecule-based phototransistors. The findings highlight the power of solvent-phase interfacial self-assembly to create highly ordered organic nanostructures with superior optoelectronic properties. Future work should focus on scalable fabrication of 2D crystalline TDPPQ thin films (e.g., solution shearing, screen printing, drop casting) to enable high-throughput transistor arrays and integrated broadband photodetector systems.

The interfacial self-assembly yields high-quality nanosheets but presents challenges for scalable, uniform large-area fabrication and integration into transistor arrays. Device performance shows size dependence, implying potential variability in large-scale production. Although air stability is good, a slight mobility decrease after two months indicates some residual environmental sensitivity.