CsPbX3 -ITO (X = Cl, Br, I) Nano-Heterojunctions: Voltage Tuned Positive to Negative Photoresponse.

All-Inorganic perovskite CsPbX3 (X = Cl, Br, I) quantum dots (QDs) have attracted tremendous attention in the past few years for their appealing performance in optoelectronic applications. Major properties of CsPbX3 QDs include the positive photoconductivity (PPC) and the defect tolerance of the in-band trap states. Here it is reported that when hybridizing CsPbX3 QDs with indium tin oxide (ITO) nanocrystals to form CsPbX3 -ITO nano-heterojunctions (NHJs), a voltage tuned photoresponse-from PPC to negative photoconductivity (NPC) transform-is achieved in lateral drain-source structured ITO/CsPbX3 -ITO-NHJs/ITO devices. A model combining exciton, charge separation, transport, and most critical the voltage driven electron filling of the in-band trap states with drain-source voltage (VDS ) above a threshold, is proposed to understand this unusual PPC-NPC transform mechanism, which is different from that of any known nanomaterial system. This finding exhibits potentials for developing devices such as photodetectors, optoelectronic switches, and memories.

Defect Engineering of MoTe2 via Thiol Treatment for Type III van der Waals Heterojunction Phototransistor.

Molybdenum ditelluride (MoTe2) nanosheets have displayed intriguing physicochemical properties and opto-electric characteristics as a result of their tunable and small band gap (Eg ∼ 1 eV), facilitating concurrent electron and hole transport. Despite the numerous efforts devoted to the development of p-type MoTe2 field-effect transistors (FETs), the presence of tellurium (Te) point vacancies has caused serious reliability issues. Here, we overcome this major limitation by treating the MoTe2 surface with thiolated molecules to heal Te vacancies. Comprehensive materials and electrical characterizations provided unambiguous evidence for the efficient chemisorption of butanethiol. Our thiol-treated MoTe2 FET exhibited a 10-fold increase in hole current and a positive threshold voltage shift of 25 V, indicative of efficient hole carrier doping. We demonstrated that our powerful molecular engineering strategy can be extended to the controlled formation of van der Waals heterostructures by developing an n-SnS2/thiol-MoTe2 junction FET (thiol-JFET). Notably, the thiol-JFET exhibited a significant negative photoresponse with a responsivity of 50 A W-1 and a fast response time of 80 ms based on band-to-band tunneling. More interestingly, the thiol-JFET displayed a gate tunable trimodal photodetection comprising two photoactive modes (positive and negative photoresponse) and one photoinactive mode. These findings underscore the potential of molecular engineering approaches in enhancing the performance and functionality of MoTe2-based nanodevices as key components in advanced 2D-based optoelectronics.

Negative Phototransistors with Ultrahigh Sensitivity and Weak-Light Detection Based on 1D/2D Molecular Crystal p-n Heterojunctions and their Application in Light Encoders.

Anomalous negative phototransistors in which the channel current decreases under light illumination hold potential to generate novel and multifunctional optoelectronic applications. Although a variety of design strategies have been developed to construct such devices, NPTs still suffer from far lower device performance compared to well-developed positive phototransistors (PPTs). In this work, a novel 1D/2D molecular crystal p-n heterojunction, in which p-type 1D molecular crystal (1DMC) arrays are embedded into n-type 2D molecular crystals (2DMCs), is developed to produce ultrasensitive NPTs. The p-type 1DMC arrays act as light-absorbing layers to induce p-doping of n-type 2DMCs through charge transfer under illumination, resulting in ineffective gate control and significant negative photoresponses. As a result, the NPTs show remarkable performances in photoresponsivity (P) (1.9 × 108 ) and detectivity (D*) (1.7 × 1017 Jones), greatly outperforming previously reported NPTs, which are one of the highest values among all organic phototransistors. Moreover, the device exhibits intriguing characteristics undiscovered in PPTs, including precise control of the threshold voltage by controlling light signals and ultrasensitive detection of weak light. As a proof-of-concept, the NTPs are demonstrated as light encoders that can encrypt electrical signals by light. These findings represent a milestone for negative phototransistors, and pave the way for the development of future novel optoelectronic applications.

Highly Tunable, Broadband, and Negative Photoresponse MoS2 Photodetector Driven by Ion-Gel Gate Dielectrics.

Revealing the light-matter interaction of molybdenum disulfide (MoS2) and further improving its tunability facilitate the construction of highly integrated optoelectronics in communication and wearable healthcare, but it still remains a significant challenge. Herein, polyvinylidene fluoride and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PVDF-EMIM-TFSI) ion-gel are employed to replace the oxide to fabricate a MoS2-based phototransistor. The high capacitance enables a large tunability of the carrier concentration that results in ambipolar transport of MoS2. It is found that the photoelectrical effect of the MoS2 ion-gel phototransistor can be greatly tuned by the gate voltage including its photoresponsivity, detectivity, and response wavelength. An abnormal negative photoelectrical effect in both the electron branch and the hole branch is observed which is due to the adsorption/desorption of the C2F6NO4S2- ion. By tuning the carrier concentration, the photoresponse can be extended from the visible region to the short infrared region. At 1200 nm, the photoresponse and detectivity can be tuned as large as 0.90 A/W and 1.88 × 1011 Jones, respectively. Ultimately, by combining the tunability of gate voltage and wavelength, it is demonstrated that the photoelectrical effect is dominated by the photogating effect in the hole carrier, while it is coregulated by a photogating and photothermal effect in electron carrier. This study provides new insights for developing a highly tunable broadband photodetector with low consumption.

Ultraviolet Light-Activated Charge Modulation Heterojunction for Versatile Organic Thin Film Transistors.

Organic thin film transistors (OTFTs) are a promising technology for the application of photosensors in smart wearable devices. Light-induced electrical behavior of OTFTs is explored to achieve diverse functional requirements. In most studies, OTFTs show an increased drain current (ID) under light irradiation. Here, we use an ultraviolet (UV) light absorption top layer, tris(8-hydroxyquinoline) aluminum (Alq3), to improve the UV light response of poly(3-hexylthiophene-2,5-diyl) (P3HT)-based OTFTs. Unexpectedly, the Alq3-covered device operated at the accumulation mode demonstrates a decreased ID during the UV light irradiation. N,N'-Ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI, electron acceptor), pentacene (electron donor), and lithium fluoride (LiF, insulator) as an interlayer were inserted between the P3HT and the Alq3 layers. The PTCDI/Alq3-covered device also shows an unusual decrease in ID under the UV light but an increase in ID under the green light. The pentacene/Alq3-covered device shows an increased ID during the UV light irradiation and, unexpectedly, a memory effect in ID after removing the UV light. The LiF/Alq3-covered device exhibits an electrical behavior similar to the bare P3HT-based device under the UV light. Results of spectroscopic analyses and theoretical calculations have shown that the occurrence of charge transfer at heterojunctions during the UV light irradiation causes charge modulation in the multilayered P3HT-based OTFTs and then results in an unusual decrease or memory effect in ID. In addition, the unexpected ID reduction can be observed in the Alq3-covered poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]-based OTFTs under UV light. The features, including opposite electrical responses to different wavelengths of light and optical memory effect, provide the multilayered P3HT-based OTFTs with potential for various optical applications, such as image recognition devices, optical logic gates, light dosimeters, and optical synapses.