Near-infrared Phototransistors Research Articles

Colorless Near-Infrared Absorbing Dyes Based on B-N Fused Donor-Acceptor-Donor π-Conjugated Molecules for Organic Phototransistors.

The introduction of a colorless function to organic electronic devices allows responses to light in the near-infrared (NIR) region and is expected to broaden the applications of these devices. However, the development of a colorless NIR dye remains a challenge due to the lack of a rational molecular design for controlling electronic transitions. In this study, to suppress the π-π* transitions in the visible region, polycyclic donor-acceptor-donor π-conjugated molecules with boron bridges (Py-FNTz-B and IP-FNTz-B) are designed and synthesized, which contain pyrrole or indenopyrrole as donor units with fluorinated naphthobisthiadiazole (FNTz) as an acceptor unit. The pyrrole end-capped Py-FNTz-B shows an absorption band in the NIR region without distinct visible-light absorption, which has led to the establishment of colorless characteristics. The indenopyrrole end-capped IP-FNTz-B shows a narrow optical energy gap of 0.87eV in films. Time-resolved microwave conductance and field-effect transistors demonstrate the semiconducting characteristics of these molecules, and Py-FNTz-B-based devices function as NIR phototransistors. Theoretical analyses indicate that the combination of a polyene-like electronic structure with orbital symmetry is important to obtain NIR wavelength-selective absorption. This study suggests that a molecular design based on electronic structures can be effective in the development of colorless NIR-absorbing dyes for organic electronics.

Ultralow Voltage Floating Film Transferred DPP-DTT-Based Near-Infrared Phototransistor

Ultralow Voltage Floating Film Transferred DPP-DTT-Based Near-Infrared Phototransistor

Multilayer WS2 for low-power visible and near-infrared phototransistors

Mechanically exfoliated multilayer WS2 flakes are used as the channel of field effect transistors for low-power photodetection in the visible and near-infrared (NIR) spectral range. The electrical characterization as a function of the temperature reveals devices with n-type conduction and slightly different Schottky barriers at the drain and source contacts. The WS2 phototransistors can be operated in self-powered mode, yielding both a current and a voltage when exposed to light. The spectral photoresponse in the visible and the NIR ranges shows a high responsivity (4.5 μA/W) around 1250 nm, making the devices promising for telecommunication applications.

Controlling Electron/Hole Recombination in Near-Infrared Polymer Phototransistors through an Insulation Medium: A Pathway to Ultrahigh Photosensitivity.

In near-infrared (NIR) polymer phototransistors, the photoresponse is proportional to the turn-on voltage shift (ΔVth). Due to the narrow band gap of NIR polymers, the ΔVth value is usually small. However, the use of a single bulk heterojunction (BHJ) layer has a minimal effect on increasing the value of ΔVth. This is because doping with high concentrations of acceptors results in strong current traps and accelerates electron/hole recombination. In this work, a new strategy is proposed to control the recombination of electrons/holes. By doping an insulating medium made of polystyrene (PS) into BHJs, PC61BM:PS:PDPP3T-based ternary NIR phototransistors with high acceptor concentrations were prepared by using a one-step film transfer method (FTM). Compared with a PC61BM:PDPP3T-based binary device (1:1), a ternary device (1:1:1) exhibited a significant performance improvement. The ΔVth value (∼29.5 ± 1.0 V) increased by approximately 4-fold, the Iph/Idark (∼4.4 × 106) increased by a factor of 3000 to 4000-fold, and the dark current decreased by 2-3 orders of magnitude (@ Vg = 0 V). Additionally, the ternary devices demonstrated excellent performance across a wide ternary ratio range (1:1:1 to 4:2:1).

High external quantum efficiency monolayer MoS2(1−x)Se2x phototransistor with alloying-induced near-infrared absorption

Due to intriguing electrical and optical properties, two-dimensional MoS2 has gained significant attention and emerged as a promising material in photonic and optoelectronic fields. Nevertheless, the intrinsic optical absorption of monolayer MoS2 is limited in the visible region only, restricting applications toward near-infrared (NIR) photodetection. Herein, we engineered the optical properties of MoS2 via alloying with Se to extend its optical absorption to the NIR region, and the phototransistor was fabricated based on monolayer MoS2(1−x)Se2x (x = ∼0.1). When under 780 nm (∼1.59 eV) illumination, the device delivered a photoresponsivity of 75.38 A/W, a specific detectivity of ∼1012 Jones, and an external quantum efficiency up to 11 230%. Additionally, it was revealed by density functional theory calculations that NIR absorption originated from the transition of valence states of sulfur vacancy (Vs) interband energy states between +1 and 0, providing an interband energy level of 1.58 eV away from the conduction band minima. Moreover, alloying of Se can suppress deep-level defects formed via Vs, further boosting device performance. This work has demonstrated high-performance NIR phototransistors based on ternary monolayer MoS2(1−x)Se2x, providing both a viable solution and fundamental mechanisms for NIR-blind MoS2 with extended optical absorption.

Water‐Stable and Photo‐Patternable Siloxane‐Encapsulated Upconversion Nanoparticles toward Flexible Near‐Infrared Phototransistors

AbstractUpconversion nanoparticles (UCNPs), as near‐infrared (NIR) absorbers, are promising materials for use in flexible NIR photodetectors, which can be applied for wearable healthcare applications due to their advantages in a broad spectral range, high photostability, and biocompatibility. However, to apply UCNPs in wearable and large‐area integrated devices, water stability and micro‐patterning methods are required. In this work, the UCNPs are encapsulated with a siloxane polymer (UCNP@SiOx) via a sol–gel process to enable photo‐patternability and photo‐stabililty in water conditions. The UCNP@SiOx can be photo‐patterned down to micron‐scale feature sizes and exhibit no significant decrease in upconversion photoluminescence (PL) intensities and PL decay time after immersion in water for 2 h. Moreover, UCNP@SiOx is evaluated by an in vitro biocompatibility test and found to be non‐toxic. By integrating the UCNP@SiOx with MoS2 phototransistors (MoS2 + UCNP@SiOx), the devices exhibit enhanced responsivity (0.79 A W−1) and specific detectivity (2.22 × 107 Jones), which are 2.8 times higher than in the bare MoS2 phototransistors, and excellent mechanical durability over 1000 cycles of 20% compression and re‐stretch test. This work opens the way for the facile synthesis of water‐stable and photo‐patternable siloxane‐encapsulated UCNPs and a strategy for fabricating high‐performance flexible NIR phototransistors through wavelength conversion.

Electrochemically exfoliated phosphorene nanosheet thin films for wafer-scale near-infrared phototransistor array

Two-dimensional (2D) black phosphorus (BP), or phosphorene, has recently emerged as a promising 2D semiconductor because of its p-type charge transport behavior and near-infrared photoresponsivity. However, the application of BP in practical electronic and optoelectronic devices is hindered by challenges in producing high-quality BP films over large areas. In this manuscript, we present a facile solution-based process to create wafer-scale BP films for fabrication of p-channel field-effect transistors that are responsive to near infrared light. Few-layer BP nanosheets are first exfoliated from the bulk crystal via electrochemical intercalation of cationic molecules and then vacuum-filtered through an anodic aluminum oxide membrane. The resulting BP film can be transferred onto an SiO2-coated silicon substrate, thereby allowing for realization of field-effect transistors after electrode deposition and thermal annealing. The transistor array exhibits spatial uniformity in electrical performance with an average hole mobility of ~0.002 cm2 V−1 s−1 and on/off ratio of 130. Furthermore, gate-induced modulation of the BP channel allows for enhancement in the photoresponsivity for 1550-nm light illumination up to 24 mA W−1, which benefits the application of the phototransistor array for near infrared imaging.

Ultraflexible and Ultrasensitive Near‐Infrared Organic Phototransistors for Hemispherical Biomimetic Eyes

AbstractThe diverse biological eyes vision systems found in nature can provide attractive design inspiration for near‐infrared (NIR) imaging devices. Outstanding features of the retina are its concave hemisphere geometry and high‐sensitivity image acquisition, which simplify the optical complexity of the eye and improve the imaging quality of visual perception. However, developing high‐performance NIR biomimetic imaging systems with these characteristics is extremely challenging due to the limitations of their hemisphere‐like structures, constituent materials, and conventional imaging modules. Here, a hemispherical biomimetic eyes imaging system based on ultraflexible all‐polymer heterojunction NIR phototransistors is presented. The device features a self‐supporting ultrathin (≈659 nm) NIR imaging structure that can maintain stable photoelectric performance while adhering to the human body or arbitrary‐shaped objects and have extraordinary photosensitivity (≈106) under dim light conditions (0.038 mW cm−2, 808 nm). Based on the versatile NIR device array, hemispherical ultrasensitive NIR biomimetic eyes are successfully achieved and higher‐resolution imaging is realized. Results demonstrated in this work provide a new strategy for constructing ultraflexible and ultrasensitive NIR photodetectors, showing remarkable application potential in next‐generation visual prosthetics and intelligent bioimaging systems.

Organic Single-Crystalline Microwire Arrays toward High-Performance Flexible Near-Infrared Phototransistors.

Flexible organic near-infrared (NIR) phototransistors hold promising prospects for potential applications such as noninvasive bioimaging, health monitoring, and biometric authentication. For integrated circuits of high-performance devices, organic single-crystalline micro-/nanostructures with precise positioning are prominently anticipated. However, the manufacturing of organic single-crystalline arrays remains a conundrum due to difficulties encountered in patterning arrays of dewetting processes at micron-scale confined space and modulating the dewetting dynamics. Herein, weutilize a capillary-bridge lithography strategy to fabricate organic 1Darrayswith high quality, homogeneous size, and deterministic location toward high-performance flexible organic NIR phototransistors. Regular micro-liquid stripes and unidirectional dewetting are synchronously achieved by adapting micropillar templates with asymmetric wettability. As a result, high-throughput 1D arrays based organic field-effect transistors exhibit high electron mobility up to 9.82cm2 V-1 s-1 . Impressively, flexible NIR phototransistors also show outstanding photoelectronic performances with a photosensitivity of 9.87×105 , a responsivity of 1.79×104 AW-1 , and a specific detectivity of 3.92×1014 Jones. This work paves a novel way to pattern high-throughput organic single-crystalline microarrays toward flexible NIR organic optoelectronics.

An Irradiance-Adaptable Near-Infrared Vertical Heterojunction Phototransistor.

Bioinspired artificial visual perception devices with the optical environment-adaptable function have attracted significant attention for their promising potential in applications like robotics and machine vision. In this regard, a photodetector with in-sensor adaptability is longed for in terms of complexity, efficiency, and cost. Here, a near-infrared phototransistor with a benign light irradiance-adaptability is presented. The phototransistor uses a vertically stacking graphene/lead sulfide quantum dots/graphene heterojunction as the conductive channel. Compared with ordinary lead sulfide quantum dots-decorated graphene phototransistors, the present device demonstrates a faster photoresponse speed and an abnormal transfer characteristic. The latter characteristic is induced by the gate voltage-tunable Fermi level in the heterojunction and the abundant electron trap states in the quantum dot film, which jointly results in an intense dependence of the photoresponse on the gate voltage. The dynamic trapping and de-trapping processes in the quantum dot film enable the inhibition or potentiation of the photoresponse, based on which the photopic or scotopic adaptation behavior of the human retina is successfully mimicked, respectively. By providing an irradiance-adaptable photodetector with a spectral response beyond visible light, this work should inspire future research on artificial environment-adaptable perception devices.

Near‐Infrared Phototransistor Based on Graphene/C60/PbPc Heterojunction with Tunable Bidirectional Photoresponse

AbstractGraphene–organic heterojunction phototransistors have great potential to achieve sensitive photoresponse owing to the excellent absorption of organic layers and fast charge transport in graphene. However, the photoresponse of most graphene‐based phototransistors is limited within visible light region with narrow bandwidth and poor sensitivity in the near‐infrared (NIR) region. Herein, a graphene–organic NIR phototransistor is fabricated by integrating an organic heterojunction layer composing of phthalocyanine molecules and fullerene C60 onto the graphene channel. The phototransistor exhibits a high photoresponsivity of 2.2 × 103 A W−1 under 850 nm irradiation with the power density of 35.4 mW cm−2 (Vds = 1 V). Meanwhile, a bidirectional photoresponse (both positive and negative) is obtained at different wavelength regions. Further studies indicate that the gate voltage enables to effectively tune the photoresponse in the wavelength range where PbPc has a significant optical absorption. Such a tunability of photoresponse is especially beneficial for the exploration of new functionalities in logic function devices. This work presents a feasible strategy to prepare graphene–organic heterojunction phototransistor with broadband photoresponse as well as sensitive and tunable bidirectional signals at NIR region.

Towards high performance near infrared sensitive multilayer organic phototransistors: effects of the acceptor and its position

Due to its strong optical absorption in the near infrared (NIR) region, lead phthalocyanine (PbPc) is frequently used as the photoactive material in NIR sensitive organic phototransistors (OPTs). Limited by the low charge carrier mobility of NIR sensitive organic semiconductors, the active part of NIR OPTs adopts generally multilayer or multicomponent structure consisting of electron donor, electron acceptor and charge transporting molecules. Here, we investigate the effect of acceptor locations in the active structure on the performances of NIR phototransistors with PbPc as the photoactive electron donor. The performances of OPTs with C60 layer lying on the top of PbPc (Si/SiO2/pentacene/PbPc/C60/Au (source and drain electrode, S&D)), above PbPc and Au S&D (Si/SiO2/pentacene/PbPc/Au (S&D)/C60)), and in the form of bulk-heterojunction with PbPc (Si/SiO2/pentacene/PbPc:C60/Au (S&D)) were comparatively studied. The results show that the device with pentacene/PbPc:C60 exhibits the best performance among them. At an incident light intensity of 0.08 mW cm−2, a high photoresponsivity of 44.35 A W−1, and specific detectivity of 1.08 × 1012 Jones are achieved.

Diffusion interface layer controlling the acceptor phase of bilayer near-infrared polymer phototransistors with ultrahigh photosensitivity

The narrow bandgap of near-infrared (NIR) polymers is a major barrier to improving the performance of NIR phototransistors. The existing technique for overcoming this barrier is to construct a bilayer device (channel layer/bulk heterojunction (BHJ) layer). However, acceptor phases of the BHJ dissolve into the channel layer and are randomly distributed by the spin-coating method, resulting in turn-on voltages (Vo) and off-state dark currents remaining at a high level. In this work, a diffusion interface layer is formed between the channel layer and BHJ layer after treating the film transfer method (FTM)-based NIR phototransistors with solvent vapor annealing (SVA). The newly formed diffusion interface layer makes it possible to control the acceptor phase distribution. The performance of the FTM-based device improves after SVA. Vo decreases from 26 V to zero, and the dark currents decrease by one order of magnitude. The photosensitivity (Iph/Idark) increases from 22 to 1.7 × 107.

The Impact of Benzothiadiazole on the Optoelectronic Performance of Polymer/PC71BM Blend Films and Their Application in NIR Phototransistors

AbstractD‐A conjugated polymers P1 and P2 are synthesized by copolymerizing bisthiophene‐fused diketopyrrolopyrrole (4,11‐bis(2‐decyltetradecyl)‐7H,14H‐thieno[3',2':7,8]‐indolizino[2,1‐a]thieno[3,2‐g]indolizine‐7,14‐dione, BTI) with terthiophene and thiophene‐benzothiadiazole‐thiophene, respectively. Molecular packing is characterized by X‐ray diffraction. The film morphology is characterized by atomic force and scanning electron microscopy. The vertical phase separation is characterized by XPS. The mechanism of film morphology formation is explained from intermolecular interaction, kinetics, and surface free energy. The spin‐coated films of polymers blended with PC71BM are served as semiconductor layers in near‐infrared (NIR) phototransistors. The impact of morphology on optoelectronic performance is analyzed. The mobility of organic field effect transistors based on P1 or P2 thin films remains constant with the addition of PC71BM with weight ratio being less than 50%. The main reason is that π–π stacking in both P1 and P2 thin films is at all directions and PC71BM does not disturb the charge percolation path. Benzothiadiazole in P2 enables stronger mutual attraction between P2 and PC71BM, and therefore large contact area between P2 and PC71BM forms in the heterojunction. The photoresponsivity improves by 2 times for NIR phototransistors based on P2/PC71BM when decreasing light intensity from 63 to 1.8 µW cm‐2. The above results suggest a feasible conjugated polymer design rule for high sensitivity NIR phototransistors.

Improved electrical ideality and photoresponse in near-infrared phototransistors realized by bulk heterojunction channels

SummaryThe factors that affect the electrical ideality and photoresponse in near-infrared (NIR) organic phototransistors (OPTs) are still nebulous. Here, simultaneous increase in electrical ideality and NIR response in the OPTs is realized by applying a bulk heterojunction (BHJ) channel. The acceptor in the channel helps to trap the undesirable injected electrons, avoiding the accumulation of the electrons at the active channel/dielectric interface, and thereby improving the hole transporting. Use of a BHJ channel also helps reducing the contact resistance in the OPTs. The electrical stability is then improved with mitigated dependence of charge mobility on gate voltage in the saturation region. The BHJ channel also offers an improved photoresponse through enhanced exciton dissociation, leading to more than one order of magnitude increase in responsivity than that in a control OPT. The results are encouraging, which pave the way for the development of high-performing NIR OPTs.

Photomemory and Pulse Monitoring Featured Solution‐Processed Near‐Infrared Graphene/Organic Phototransistor with Detectivity of 2.4 × 1013 Jones

AbstractEmerging graphene/organic phototransistors are eye‐catching technologies owing to their unique merits including easy/low‐cost fabrication, temperature independent, and achieving various functions. However, their development in the near‐infrared (NIR) region is experiencing a bottleneck of inferior sensitivity due to low exciton dissociation efficiency and inefficient charge extraction rate. Here, a novel‐design solution‐processed graphene/organic NIR phototransistor is reported, that is, creatively introducing electron extraction layer of ZnO on graphene channel and employing organic ternary bulk heterojunction as photosensitive layer, successfully breaking that bottleneck. The phototransistor exhibits a high responsivity of 6.1 × 106 A W−1, a superior detectivity of 2.4 × 1013 Jones, and a remarkable minimum detection power of 1.75 nW cm−2 under 850 nm radiation. Considering its excellent NIR detection performance, a noncontact transmission‐type pulse monitoring is carried out with no external circuit support, from which human pulse signal and heart rate can be displayed in real time. The phototransistor, interestingly, can be switched into a photomemory function with a retention time of 1000 s in the atmosphere through a gate voltage of −20 V. The design takes the characteristics of graphene/organic phototransistors to a higher level, beyond the limit of sensitivity, and opens up a novel approach for developing multifunction devices.

High performance near-infrared phototransistors via enhanced electron trapping effect.

A high performance near-infrared organic phototransistor is achieved via introducing a small molecule acceptor as an electron trapping site into the narrow-bandgap conjugated polymer films. With only 10% (wt) addition of the acceptor molecule, the photoresponse to light of 850 nm has been significantly improved with a best photoresponsivity up to 2000 A W-1, high detectivity of 1016 Jones and fairly good photosensitivity in the order of 106.

High-performance near-infrared polymeric phototransistors realized by combining cross-linked polymeric semiconductors and bulk heterojunction bilayer structures

Flexible polymeric phototransistors highly responsive to near-infrared (NIR) light are crucial for the room temperature biological imaging due to some unique properties of organic semiconductors, such as light weight, low cost, and good biocompatibility. However, the low mobility and/or the high dark off-current (DOC) of low-bandgap polymeric phototransistors limit their NIR light detectability. This work proposes the development of a solution-processable bilayer heterostructure (BHS) for polymeric phototransistors, which uses cross-linked polymeric semiconductors (CLPS) as the bottom conducting channel and a bulk heterojunction (BHJ) as the photoactive layer. The BHS architecture ensures fast carrier transport and efficient exciton separation. Besides, the electron-trapping effect generated between the CLPS and BHJ results in a significantly lower DOC compared with other controlled devices. Thus, the BHS phototransistor based on low-bandgap polymer can detect NIR light with wavelengths of 808 nm, 1064 nm, and even 1550 nm, going from NIR-I (750–1000 nm) to NIR-II (1000–1700 nm). Meanwhile, its responsivity (R) is two or three orders of magnitude higher than the controlled device. Furthermore, an ultra-flexible NIR phototransistor array (~5 μm thick) using this strategy could detect image barcodes with different gray levels, demonstrating its potential application in flexible NIR imaging.

ZnO nanoparticles enhanced near-infrared photodetection in organic phototransistors based on narrow bandgap polymer

Highly sensitive near-infrared (NIR) photodetectors have attracted extensive attention due to their wide applications in thermal imaging, night vision and human health monitoring. Organic phototransistors (OPTs) are an important type of photodetector with three-terminal contact, which provides high sensitivity and tunable gain with an external quantum efficiency (EQE)>100% by controlling unbalanced charge transport through an optically controlled gate terminal. Thus, NIR OPTs are considered as a promising alternative NIR light detecting technology to conventional inorganic NIR phototransistors. The design and fabrication flexibility provided by the organic semiconductors and solution processes also have significant cost benefit, creating next-generation large-area, low-cost and flexible NIR photodetectors. However, now the performance of NIR-OPT is not high enough, which is still far from practical applications. Low field-effect mobility, limited exciton dissociation, and poor NIR light absorption are the main reasons to hamper the performance of NIR OPTs. A key technical development to circumvent this hurdle is to create an organic channel layer with ordered microstructure to provide high field-effect mobility and a smart device design to enhance exciton dissociation. In this work, an air stable conjugated semiconducting polymer of diketopyrrolopyrrole-dithienylthieno[3,2- b ]thiophene (DPP-DTT) with narrow band gap was selected as the NIR light response medium. The DPP-DTT nanowire network was obtained by a solution-processed and scalable approach called polymer-matrix-mediated molecular self-assembly. The matrix polymer of polystyrene (PS) was selectively removed from the resulted DPP-DTT:PS blend film, leaving the pristine DPP-DTT nanowire network thin film as the active channel layer for the NIR OPTs. The electrical properties and NIR response of the OPTs based on DPP-DTT nanowire network were studied. It is found that the pristine DPP-DTT nanowire network layer exhibited a typical p-type field-effect transport, showing the hole mobility of >1 cm2/(V s) and on/off ratio >106. These devices also showed sensitive NIR response with photoresponsivity ( R ) of 336 A/W and specific detectivity ( D *) of 5.6×1012 Jones under NIR light intensity of 8 μW/cm2. To further improve the NIR photodetection of the DPP-DTT nanowire network OPTs, zinc oxide (ZnO) nanoparticles were introduced into the devices as electron acceptor material. A small amount of ZnO nanoparticles (1.0 mg/mL) were spin coated onto the DPP-DTT nanowire network to increase the separation efficiency of photodenerated excitons and then capture the photogenerated electrons in trap sites, thereby minimizing the negative impact of photogenerated electrons on hole transport in the NIR OPTs. As a result, the NIR photodetection of the ZnO-modified OPTs was significantly improved. The photosensitivity of ZnO-modified devices reaches as high as 107, which is increased by about 4 to 5 orders of magnitude compared with the pristine DPP-DTT nanowire network NIR OPTs. Correspondingly, the R and D * of ZnO-modified NIR OPTs have been also enhanced. The maximum R value was increased to 721 A/W, which is more than twice the R value of the pristine DPP-DTT nanowire network NIR OPTs. It is noted that such a large R value has achieved the performance level of conventional inorganic NIR phototransistors. Moreover, the maximum D * of the ZnO-modified NIR OPTs reaches 1.3×1013 Jones under the NIR intensity of 8 μW/cm2. Therefore, this work provides a good idea for the structural design and device performance optimization of high-performance NIR OPTs.

Advances in solution-processable near-infrared phototransistors

Solution processable near infrared (NIR) photodetectors provide a promising alternative due to their low cost, flexible design, adaptability to various fabrications, and large area manufacturability, removing the limitations of traditional wafer-based inorganic semiconductor techniques. High performing NIR photodetectors offer attractive options for applications in visualizing NIR light, health and safety monitoring.