An A-D-A'-D-A-Type Narrow Bandgap Electron Acceptor Based on Selenophene-Flanked Diketopyrrolopyrrole for Sensitive Near-Infrared Photodetection.

Near-infrared organic photodetectors possess great application potential in night vision, optical communication, and image sensing, but their development is limited by the lack of narrow bandgap organic semiconductors. A-D-A'-D-A-type molecules, featuring multiple intramolecular charge transfer effects, offer a robust framework for achieving near-infrared light absorption. Herein, we report a novel A-D-A'-D-A-type narrow bandgap electron acceptor named DPPSe-4Cl, which incorporates a selenophene-flanked diketopyrrolopyrrole (Se-DPP) unit as its central A' component. This molecule demonstrates exceptional near-infrared absorption properties with an absorption onset reaching 1120 nm and a low optical bandgap of 1.11 eV, owing to the strong electron-withdrawing ability and quinoidal resonance effect induced by the Se-DPP unit. By implementing a doping compensation strategy assisted by Y6 to reduce the trap density in the photoactive layer, the optimized organic photodetector based on DPPSe-4Cl exhibited efficient spectral response and remarkable sensitivity in the range of 300-1100 nm. Particularly, a specific detectivity surpassing 1012 Jones in the wavelength range of 410-1030 nm is achieved. This work offers a promising approach for developing highly sensitive visible to near-infrared broadband photodetection technology using organic semiconductors.

Chiral-Achiral Cations Intercalation Induced Lead-Free Chiral-Polar Hybrid Perovskites Enable Self-Powered X-Ray and Ultraviolet-Visible-Near-Infrared Photo Detection.

Lead halide hybrid perovskites have made great progress in direct X-ray detection and broadband photodetection, but the existence of toxic Pb and the demand for external operating voltage have severely limited their further applications and operational stability improvements. Therefore, exploring "green" lead-free hybrid perovskite that can both achieve X-ray detection and broadband photodetection without external voltage is of great importance, but remains severely challenging. Herein, using centrosymmetric (BZA)3BiI6 (1, BZA = benzylamine) as a template, a pair of chiral-polar lead-free perovskites, (BZA)2(R/S-PPA)BiI6 (2-R/S, R/S-PPA = (R/S)-1-Phenylpropylamine) are successfully obtained by introducing chiral aryl cations of (R/S)-1-Phenylpropylamine. Compared to 1, chiral-polar 2-R presents a significant irradiation-responsive bulk photovoltaic effect (BPVE) with an open circuit photovoltage of 0.4 V, which enables it with self-powered X-ray, UV-vis-NIR broadband photodetection. Specifically, 2-R device exhibits an ultralow detection limit of 18.5 nGy s-1 and excellent operational stability. Furthermore, 2-R as the first lead-free perovskite achieves significant broad-spectrum (377-940 nm) photodetection via light-induced pyroelectric effect. This work sheds light on the rational crystal reconstruction engineering and design of "green" hybrid perovskite toward high-demanded self-powered radiation detection and broadband photodetection.

Tunable Contacts of Bi2 O2 Se Nanosheets MSM Photodetectors by Metal-Assisted Transfer Approach for Self-Powered Near-Infrared Photodetection.

Owing to the Fermi pinning effect arose in the metal electrodes deposition process, metal-semiconductor contact is always independent on the work function, which challenges the next-generation optoelectronic devices. In this work, a metal-assisted transfer approach is developed to transfer Bi2 O2 Se nanosheets onto the pre-deposited metal electrodes, benefiting to the tunable metal-semiconductor contact. The success in Bi2 O2 Se nanosheets transfer is contributed to the stronger van der Waals adhesion of metal electrodes than that of growth substrates. With the pre-deposited asymmetric electrodes, the self-powered near-infrared photodetectors are realized, demonstrating low dark current of 0.04 pA, high Ilight /Idark ratio of 380, fast rise and decay times of 4 and 6 ms, respectively, under the illumination of 1310 nm laser. By pre-depositing the metal electrodes on polyimide and glass, high-performance flexible and omnidirectional self-powered near-infrared photodetectors are achieved successfully. This study opens up new opportunities for low-dimensional semiconductors in next-generation high-performance optoelectronic devices.

Ultrasensitive Near-Infrared InAs Colloidal Quantum Dot-ZnON Hybrid Phototransistor Based on a Gradated Band Structure.

Amorphous metal oxide semiconductor phototransistors (MOTPs) integrated with colloidal quantum dots (QDs) (QD-MOTPs) are promising infrared photodetectors owing to their high photoconductive gain, low off-current level, and high compatibility with pixel circuits. However, to date, the poor mobility of conventional MOTPs, such as indium gallium zinc oxide (IGZO), and the toxicity of lead (Pb)-based QDs, such as lead sulfide and lead selenide, has limited the commercial applications of QD-MOTPs. Herein, an ultrasensitive QD-MOTP fabricated by integrating a high-mobility zinc oxynitride (ZnON)-based MOTP and lead-free indium arsenide (InAs) QDs is demonstrated. A new gradated bandgap structure is introduced in the InAs QD layer that absorbs infrared light, which prevents carriers from moving backward and effectively reduces electron-hole recombination. Chemical, optical, and structural analyses confirm the movement of the photoexcited carriers in the graded band structure. The novel QD-MOTP exhibits an outstanding performance with a responsivity of 1.15 × 105 A W-1 and detectivity of 5.32 × 1016 Jones at a light power density of 2 µW cm-2 under illumination at 905 nm.

Light-Trapping-Enhanced Photodetection in Ge/Si Quantum Dot Photodiodes Containing Microhole Arrays with Different Hole Depths.

Photodetection based on assemblies of quantum dots (QDs) is able to tie the advantages of both the conventional photodetector and unique electronic properties of zero-dimensional structures in an unprecedented way. However, the biggest drawback of QDs is the small absorbance of infrared radiation due to the low density of the states coupled to the dots. In this paper, we report on the Ge/Si QD pin photodiodes integrated with photon-trapping hole array structures of various thicknesses. The aim of this study was to search for the hole array thickness that provided the maximum optical response of the light-trapping Ge/Si QD detectors. With this purpose, the embedded hole arrays were etched to different depths ranging from 100 to 550 nm. By micropatterning Ge/Si QD photodiodes, we were able to redirect normal incident light laterally along the plane of the dots, therefore facilitating the optical conversion of the near-infrared photodetectors due to elongation of the effective absorption length. Compared with the conventional flat photodetector, the responsivity of all microstructured devices had a polarization-independent improvement in the 1.0-1.8-µm wavelength range. The maximum photocurrent enhancement factor (≈50× at 1.7 µm) was achieved when the thickness of the photon-trapping structure reached the depth of the buried QD layers.

Direct Detection of Near-Infrared Circularly Polarized Light via Precisely Designed Chiral Perovskite Heterostructures.

Chiral metal halide perovskites (CMHPs) have recently shown great potential for direct circularly polarized light (CPL) detection. However, owing to the limited cutoff wavelength edge of these CMHPs, most of the detectors presented thus far are characterized only in the ultraviolet and visible range; CMHPs that target at the near-infrared (NIR) region are still greatly desired. Here, we design a novel CMHP heterostructure, synthesized via solution-processed epitaxial growth of crystalline 3D MAPbI3 on a 2D chiral (R-BPEA)2PbI4 (R-BPEA = (R)-1-(4-bromophenyl)ethylammonium) crystal, and provide the first demonstration of self-powered direct NIR-CPL detection. Compared with individual chiral (R-BPEA)2PbI4, the heterostructure not only retains the spin selectivity but also allows much broader absorbance, especially beyond 780 nm, where the (R-BPEA)2PbI4 cannot absorb. Furthermore, the built-in electric potential in the heterojunction forces spontaneous separation/transport of photogenerated carriers, enabling the fabrication of devices operating without external energy supply. By making use of the abovementioned advantages, the self-powered CPL detectors of the (R-BPEA)2PbI4/MAPbI3 heterostructures hence show competitive circular polarization sensitivity at 785 nm with a high anisotropy factor of up to 0.25. In addition, a large on/off switching ratio of ∼105 and an impressive detectivity of ∼1010 Jones are also achieved. As a pioneer study, our results may broaden the material scope for future chiroptical devices based on CMHPs.

Near-Infrared Photoresponse in Ge/Si Quantum Dots Enhanced by Photon-Trapping Hole Arrays.

Group-IV photonic devices that contain Si and Ge are very attractive due to their compatibility with integrated silicon photonics platforms. Despite the recent progress in fabrication of Ge/Si quantum dot (QD) photodetectors, their low quantum efficiency still remains a major challenge and different approaches to improve the QD photoresponse are under investigation. In this paper, we report on the fabrication and optical characterization of Ge/Si QD pin photodiodes integrated with photon-trapping microstructures for near-infrared photodetection. The photon traps represent vertical holes having 2D periodicity with a feature size of about 1 µm on the diode surface, which significantly increase the normal incidence light absorption of Ge/Si QDs due to generation of lateral optical modes in the wide telecommunication wavelength range. For a hole array periodicity of 1700 nm and hole diameter of 1130 nm, the responsivity of the photon-trapping device is found to be enhanced by about 25 times at λ=1.2 µm and by 34 times at λ≈1.6 µm relative to a bare detector without holes. These results make the micro/nanohole Ge/Si QD photodiodes promising to cover the operation wavelength range from the telecom O-band (1260-1360 nm) up to the L-band (1565-1625 nm).

Efficient Organic Upconversion Devices for Low Energy Consumption and High-Quality Noninvasive Imaging.

Infrared upconversion devices (UCDs) enable low-cost visualization of infrared optical signals without utilizing a readout circuit, which is of great significance for biological recognition and noninvasive dynamic monitoring. However, UCDs suffer from inferior photon to photon (p-p) efficiency and high turn-on voltage (Von ) for upconversion operation, hindering a further expansion in highly resolved infrared imaging. Herein, an efficient organic UCD integrating an interfacial exciplex emitter and a well-designed near-infrared (NIR) detector reveals a high efficiency up to 12.92% and a low Von down to 1.56 V. The low Von gives the capacity for detecting weak NIR light down to 3.2 µW cm-2 , significantly expanding the detection power scale of UCDs. Thus, the imaging linear dynamic range (I-LDR) is highly bias-tunable, ranging from 13.23 to 84.4 dB. The high I-LDR enables highly resolved and strong-penetration bioimaging especially for thick biological sections, indicating great potential in noninvasive defect and pathological detection.

Multilayer Black Phosphorus Near-Infrared Photodetectors.

Black phosphorus (BP), owing to its distinguished properties, has become one of the most competitive candidates for photodetectors. However, there has been little attention paid on photo-response performance of multilayer BP nanoflakes with large layer thickness. In fact, multilayer BP nanoflakes with large layer thickness have greater potential from the fabrication viewpoint as well as due to the physical properties than single or few layer ones. In this report, the thickness-dependence of the intrinsic property of BP photodetectors in the dark was initially investigated. Then the photo-response performance (including responsivity, photo-gain, photo-switching time, noise equivalent power, and specific detectivity) of BP photodetectors with relative thicker thickness was explored under a near-infrared laser beam (λIR = 830 nm). Our experimental results reveal the impact of BP's thickness on the current intensity of the channel and show degenerated p-type BP is beneficial for larger current intensity. More importantly, the photo-response of our thicker BP photodetectors exhibited a larger responsivity up to 2.42 A/W than the few-layer ones and a fast response photo-switching speed (response time is ~2.5 ms) comparable to thinner BP nanoflakes was obtained, indicating BP nanoflakes with larger layer thickness are also promising for application for ultra-fast and ultra-high near-infrared photodetectors.

Hot Electron-Based Near-Infrared Photodetection Using Bilayer MoS2.

Recently, there has been much interest in the extraction of hot electrons generated from surface plasmon decay, as this process can be used to achieve additional bandwidth for both photodetectors and photovoltaics. Hot electrons are typically injected into semiconductors over a Schottky barrier between the metal and semiconductor, enabling generation of photocurrent with below bandgap photon illumination. As a two-dimensional semiconductor single and few layer molybdenum disulfide (MoS2) has been demonstrated to exhibit internal photogain and therefore becomes an attractive hot electron acceptor. Here, we investigate hot electron-based photodetection in a device consisting of bilayer MoS2 integrated with a plasmonic antenna array. We demonstrate sub-bandgap photocurrent originating from the injection of hot electrons into MoS2 as well as photoamplification that yields a photogain of 10(5). The large photogain results in a photoresponsivity of 5.2 A/W at 1070 nm, which is far above similar silicon-based hot electron photodetectors in which no photoamplification is present. This technique is expected to have potential use in future ultracompact near-infrared photodetection and optical memory devices.