Solution-processed Photodetectors Research Articles

Ultrathin, High-Aspect-Ratio Bismuth Sulfohalide Nanowire Bundles for Solution-Processed Flexible Photodetectors.

In this study, a novel synthesis of ultrathin, highly uniform colloidal bismuth sulfohalide (BiSX where X = Cl, Br, I) nanowires (NWs) and NW bundles (NBs) for room-temperature and solution-processed flexible photodetectors are presented. High-aspect-ratio bismuth sulfobromide (BiSBr) NWs are synthesized via a heat-up method using bismuth bromide and elemental S as precursors and 1-dodecanethiol as a solvent. Bundling of the BiSBr NWs occurs upon the addition of 1-octadecene as a co-solvent. The morphologies of the BiSBr NBs are easily tailored from sheaf-like structures to spherulite nanostructures by changing the solvent ratio. The optical bandgaps are modulated from 1.91 (BiSCl) and 1.88eV (BiSBr) to 1.53eV (BiSI) by changing the halide compositions. The optical bandgap of the ultrathin BiSBr NWs and NBs exhibits blueshift, whose origin is investigated through density functional theory-based first-principles calculations. Visible-light photodetectors are fabricated using BiSBr NWs and NBs via solution-based deposition followed by solid-state ligand exchanges. High photo-responsivities and external quantum efficiencies (EQE) are obtained for BiSBr NW and NB films even under strain, which offer a unique opportunity for the application of the novel BiSX NWs and NBs in flexible and environmentally friendly optoelectronic devices.

Uniaxial alignment of perovskite nanowires via brush painting technique for efficient flexible polarized photodetectors

A polarization-sensitive and flexible photodetector was fabricated through the precise alignment of perovskite nanowires (NWs) using a brush coating technique. The alignment of the NWs was meticulously examined, considering various chemical properties of the solvent, such as boiling point, viscosity, and surface tension. Notably, when the NWs were brush-coated with toluene dispersion, the NWs were aligned in higher order than those processed from octane dispersion. The degree of alignment was correlated with the photodetector property. Especially, the well-aligned NW photodetector exhibited a two-fold disparity in current response contingent on the polarization direction. Furthermore, even after enduring 500 bending cycles, the device retained 80 % of its photodetector performance. This approach underscores the potential of solution-processed flexible photodetectors for advanced optical applications under dynamic operating conditions.

Lead-free perovskite Cs2AgBiBr6 photodetector detecting NIR light driven by titanium nitride plasmonic hot holes

Lead-free perovskite Cs2AgBiBr6 manifests great potential in developing high-performance, environmentally friendly, solution-processable photodetectors (PDs). However, due to the relatively large energy bandgap, the spectrum responses of Cs2AgBiBr6 PDs are limited to the ultraviolet and visible region with wavelengths shorter than 560 nm. In this work, a broadband Cs2AgBiBr6 PD covering the ultraviolet, visible, and near infrared (NIR) range is demonstrated by incorporating titanium nitride (TiN) nanoparticles that are prepared with the assistance of self-assembled polystyrene sphere array. In addition, an atomically thick Al2O3 layer is introduced at the interface between the Cs2AgBiBr6 film and TiN nanoparticles to alleviate the dark current deterioration caused by nanoparticle incorporation. As a result, beyond the spectrum range where Cs2AgBiBr6 absorbs light, the external quantum efficiency (EQE) of the TiN nanoparticle incorporated Cs2AgBiBr6 PD is enhanced significantly compared with that of the control, displaying enhancement factors as high as 2000 over a broadband NIR wavelength range. The demonstrated enhancement in EQE arises from the photocurrent contribution of plasmonic hot holes injected from TiN nanoparticles into Cs2AgBiBr6. This work promotes the development of broadband solution-processable perovskite PDs, providing a promising strategy for realizing photodetection in the NIR region.

Improved ultraviolet photodetector performances using solution-processed nitrogen-doped carbon quantum dots/ZnO hybrid thin films

We fabricated the solution-processed photodetectors using the nitrogen-doped carbon quantum dots/zinc oxide (NCQDs/ZnO) hybrid films and characterized using ultraviolet (UV) spot-light of 365 nm at 1 mW/cm2. The NCQDs adsorbed onto the surface and/or grain boundaries of ZnO crystallites control ZnO particle size (by capping) in the NCQDs/ZnO hybrid thin films, resulting in the improved film quality and decreased the dark current in the devices. The NCQDs to ZnO [%(w/v)] ratio of 1.2 × 10−3 in the hybrid film UV photodetector exhibits the stable and reproducible photoresponses with the highest responsivity & external quantum efficiency (which are ∼3 times higher), and detectivity (∼7.5 times higher) than the pristine ZnO based photodetector, attributed to the effective transfer of photogenerated electrons from NCQDs to ZnO nanoparticles in the hybrid film. The solution-processed NCQDs/ZnO hybrid films will be very much useful to improve the UV photodetector performances for the cost-effective commercial applications.

Solution-processed Self-driven Bulk-heterojunction Photodetectors by Passivating with Polymers for Ultrasensitive Upconverters

Solution-processed Self-driven Bulk-heterojunction Photodetectors by Passivating with Polymers for Ultrasensitive Upconverters

Engineering Band Gap of Ternary Ag2TexS1−x Quantum Dots for Solution-Processed Near-Infrared Photodetectors

Silver-based chalcogenide semiconductors exhibit low toxicity and near-infrared optical properties and are therefore extensively employed in the field of solar cells, photodetectors, and biological probes. Here, we report a facile mixture precursor hot-injection colloidal route to prepare Ag2TexS1−x ternary quantum dots (QDs) with tunable photoluminescence (PL) emissions from 950 nm to 1600 nm via alloying band gap engineering. As a proof-of-concept application, the Ag2TexS1−x QDs-based near-infrared photodetector (PD) was fabricated via solution-processes to explore their photoelectric properties. The ICP-OES results reveal the relationship between the compositions of the precursor and the samples, which is consistent with Vegard’s equation. Alloying broadened the absorption spectrum and narrowed the band gap of the Ag2S QDs. The UPS results demonstrate the energy band alignment of the Ag2Te0.53S0.47 QDs. The solution-processed Ag2TexS1−x QD-based PD exhibited a photoresponse to 1350 nm illumination. With an applied voltage of 0.5 V, the specific detectivity is 0.91 × 1010 Jones and the responsivity is 0.48 mA/W. The PD maintained a stable response under multiple optical switching cycles, with a rise time of 2.11 s and a fall time of 1.04 s, which indicate excellent optoelectronic performance.

Charge Management Enables Efficient Spontaneous Chromatic Adaptation Bipolar Photodetector.

Solution-processed photodetectors have emerged as promising candidates for next-generation of visible-near infrared (vis-NIR) photodetectors. This is attributed to their ease of processing, compatibility with flexible substrates, and the ability to tune their detection properties by integrating complementary photoresponsive semiconductors. However, the limited performance continues to hinder their further development, primarily influenced by the difference of charge transport properties between perovskite and organic semiconductors. In this work, a perovskite-organic bipolar photodetectors (PDs) is introduced with multispectral responsivity, achieved by effectively managing charges in perovskite and a ternary organic heterojunction. The ternary heterojunction, incorporating a designed NIR guest acceptor, exhibits a faster charge transfer rate and longer carrier diffusion length than the binary heterojunction. By achieving a more balanced carrier dynamic between the perovskite and organic components, the PD achieves a low dark current of 3.74nAcm-2 at -0.2V, a fast response speed of <10 µs, and a detectivity of exceeding 1012 Jones. Furthermore, a bioinspired retinotopic system for spontaneous chromatic adaptation is achieved without any optical filter. This charge management strategy opens up possibilities for surpassing the limitations of photodetection and enables the realization of high-purity, compact image sensors with exceptional spatial resolution and accurate color reproduction.

High-speed flexible near-infrared organic photodiode for optical communication.

Optical communication is a particularly compelling technology for tackling the speed and capacity bottlenecks in data communication in modern society. Currently, the silicon photodetector plays a dominant role in high-speed optical communication across the visible-near-infrared spectrum. However, its intrinsic rigid structure, high working bias and low responsivity essentially limit its application in next-generation flexible optoelectronic devices. Herein, we report a narrow-bandgap non-fullerene acceptor (NFA) with a remarkable π-extension in the direction of both central and end units (CH17) with respect to the Y6 series, which demonstrates a more effective and compact 3D molecular packing, leading to lower trap states and energetic disorders in the photoactive film. Consequently, the optimized solution-processed organic photodetector (OPD) with CH17 exhibits a remarkable response time of 91ns (λ = 880nm) due to the high charge mobility and low parasitic capacitance, exceeding the values of most commercial Si photodiodes and all NFA-based OPDs operating in self-powered mode. More significantly, the flexible OPD exhibits negligible performance attenuation (<1%) after bending for 500 cycles, and maintains 96% of its initial performance even after 550h of indoor exposure. Furthermore, the high-speed OPD demonstrates a high data transmission rate of 80MHz with a bit error rate of 3.5 [Formula: see text] 10-4, meaning it has great potential in next-generation high-speed flexible optical communication systems.

Fast Near-Infrared Photodetectors Based on Nontoxic and Solution-Processable AgBiS2.

Solution-processable near-infrared (NIR) photodetectors are urgently needed for a wide range of next-generation electronics, including sensors, optical communications and bioimaging. However, it is rare to find photodetectors with >300kHz cut-off frequencies, especially in the NIR region, and many of the emerging inorganic materials explored are comprised of toxic elements, such as lead. Herein, solution-processed AgBiS2 photodetectors with high cut-off frequencies under both white light (>1MHz) and NIR (approaching 500kHz) illumination are developed. These high cut-off frequencies are due to the short transit distances of charge-carriers in the ultrathin photoactive layer of AgBiS2 photodetectors, which arise from the strong light absorption of this material, such that film thicknesses well below 120nm are sufficient to absorb >65% of NIR to visible light. It is also revealed that ion migration plays a critical role in the photo-response speed of these devices, and its detrimental effects can be mitigated by finely tuning the thickness of the photoactive layer, which is important for achieving low dark current densities as well. These outstanding characteristics enable the realization of air-stable, real-time heartbeat sensors based on NIR AgBiS2 photodetectors, which strongly motivates their future integration in high-throughput systems.

Self-powered solution-processed photodetectors utilizing FAPbBr3:C60 bulk heterojunction

Hybrid halide perovskites have emerged as pivotal optoelectronic materials due to their easy and cost-effective solution-based manufacturing processes. In this study, we have combined formamidinium lead bromide (FAPbBr3) with buckminsterfullerene (C60) to create the bulk heterojunction (BHJ) photoactive layer for solution-processed photodetectors. Remarkably, the FAPbBr3:C60 BHJ-based photodetectors exhibited a substantial enhancement in optoelectronic performance, with the responsivity increasing from 0.87 A/W to 1.47 A/W compared to the pristine FAPbBr3. This improvement can be attributed to the superior surface morphology and crystallinity of the FAPbBr3:C60 BHJ thin film, which facilitated enhanced charge transport properties. Furthermore, the fabricated photodetectors demonstrated remarkable detectivity even at 0V, enabling self-powered operation. By employing the space charge limited conduction (SCLC) model, we were able to elucidate the dominant charge transport mechanism. At lower voltages, trap-free SCLC was identified as the primary mechanism, while traps governed charge transport at higher voltages. Additionally, we conducted impedance analysis to investigate the underlying factors contributing to the distinctive optoelectronic performance observed in the photodetectors.

The State-of-the-Art Solution-Processed Single Component Organic Photodetectors Achieved by Strong Quenching of Intermolecular Emissive State and High Quadrupole Moment in Non-Fullerene Acceptors.

A bulk-heterojunction (BHJ) blend is commonly used as the photoactive layer in organic photodetectors (OPDs) to utilize the donor (D)/acceptor (A) interfacial energetic offset for exciton dissociation. However, this strategy often complicates optimization procedures, raising serious concerns over device processability, reproducibility and stability. Herein, we demonstrate highly efficient OPDs fabricated with single-component organic semiconductors via solution-processing. The non-fullerene acceptors (NFAs) with strong intrinsic D/A character are used as the photoactive layer, where the emissive intermolecular charge transfer excitonic (CTE) states are formed within <1ps, and efficient photocurrent generation is achieved via strong quenching of these CTE states by reverse bias. Y6 and IT-4F based OPDs show excellent OPD performances; low dark current density (∼10-9 A cm-2 ), high responsivity (≥ 0.15 A W-1 ), high specific detectivity (> 1012 Jones) and fast photo-response time (< 10 µs), comparable to the state-of-the-art BHJ OPDs. Together with strong CTE state quenching by electric field, these excellent OPD performances are also attributed to the high quadrupole moments of NFA molecules, which can lead to large interfacial energetic offset for efficient CTE dissociation. Our work opens a new way to realize efficient OPDs using single-component systems via solution-processing and provides important molecular design rules. This article is protected by copyright. All rights reserved.

Controllable Colloidal Synthesis of MAPbI3 Perovskite Nanocrystals for Dual-Mode Optoelectronic Applications.

This study demonstrates an acetate ligand (AcO-)-assisted strategy for the controllable and tunable synthesis of colloidal methylammonium lead iodide (MAPbI3) perovskite nanocrystals (PNCs) for efficient photovoltaic and photodetector devices. The size of colloidal MAPbI3 PNCs can be tuned from 9 to 20 nm by changing the AcO-/MA ratio in the reaction precursor. In situ observations and detailed characterization results show that the incorporation of the AcO- ligand alters the formation of PbI6 octahedral cages, which controls PNC growth. A well-optimized AcO-/MA ratio affords MAPbI3 PNCs with a low defect density, a long carrier lifetime, and unique solid-state isotropic properties, which can be used to fabricate solution-processed dual-mode photovoltaic and photodetector devices with a conversion efficiency of 13.34% and a detectivity of 2 × 1011 Jones, respectively. This study provides an avenue to further the precisely controllable synthesis of hybrid PNCs for multifunctional optoelectronic applications.

Solution-Processed Heterojunction Photodiodes Based on WSe2 Nanosheet Networks.

Solution-processed photodetectors incorporating liquid-phase-exfoliated transition metal dichalcogenide nanosheets are widely reported. However, previous studies mainly focus on the fabrication of photoconductors, rather than photodiodes which tend to be based on heterojunctions and are harder to fabricate. Especially, there are rare reports on introducing commonly used transport layers into heterojunctions based on nanosheet networks. In this study, a reliable solution-processing method is reported to fabricate heterojunction diodes with tungsten selenide (WSe2 ) nanosheets as the optical absorbing material and PEDOT: PSS and ZnO as injection/transport-layer materials. By varying the transport layer combinations, the obtained heterojunctions show rectification ratios of up to ≈104 at ±1V in the dark, without relying on heavily doped silicon substrates. Upon illumination, the heterojunction can be operated in both photoconductor and photodiode modes and displays self-powered behaviors at zero bias.

Carbon as a hole extracting layer for CdS based self-powered photodetector devices

In this work, self-powered photodetectors were fabricated by a simple, cost-effective method using cadmium sulfide (CdS) as photoactive layer and carbon as a hole transport layer. A study on the performance of photodetectors with varied thickness of CdS layers deposited on an FTO coated glass substrate via the Successive Ionic Layer Adsorption and Reaction (SILAR) method is studied in detail. X-Ray diffraction (XRD) data confirms the formation of the CdS layer. Field emission scanning electron microscopy (FESEM) and elemental mapping shows the spherical clustered formation with a uniform distribution of CdS nanoparticles. The band gap of the CdS layers in the range of 2.25 to 2.38 eV is closer to the desired band gap of visible light photodetector. Though solution-processed photodetectors show a very weak photoresponse, the fabricated device shows a responsivity of 0.11✗10−3 A/W and a rapid rise and fall time as fast as 0.16 s and 0.12 s.

Green-Solvent-Processed High-Performance Broadband Organic Photodetectors.

Solution-processed organic photodetectors with broadband activity have been demonstrated with an environmentally benign solvent, ortho-xylene (o-xylene), as the processing solvent. The organic photodetectors employ a wide band gap polymer donor PBDB-T and a narrow band gap small-molecule non-fullerene acceptor CO1-4F, both dissolvable in o-xylene at a controlled temperature. The o-xylene-processed devices have shown external quantum efficiency of up to 70%, surpassing the counterpart processed with chlorobenzene. With a well-suppressed dark current, the device can also present a high specific detectivity of over 1012 Jones at -2 V within practical operation frequencies and is applicable for photoplethysmography with its fast response. These results further highlight the potential of green-solvent-processed organic photodetectors as a high-performing alternative to their counterparts processed in toxic chlorinated solvents without compromising the excellent photosensing performance.

To improve device performance of self-driven heterojunction photodetectors by inserting a thin layer of silver nanoparticles into the electron-transporting layer

Lead selenide colloidal quantum dots (CQDs) are widely used in infrared photodetectors due to their size-dependent bandgap tunability, facile solution-processing techniques and low cost. Heterojunctions (HJs) are usually used to construct device to facilitate the separation of excitons and the transportation of photogenerated carriers. Based on the solution-processed HJ photodetector ITO/ZnO/PbSe/Ag, in which PbSe CQDs layer acts as the active layer and ZnO nanoparticles (NPs) layer as the electron-transporting layer, a greatly enhanced-performance was obtained after inserting 10 nm Ag NPs layer within the ZnO NPs layer close to the HJ ZnO/PbSe interface. By optimizing the concentration of Ag NPs solutions and the location of Ag NPs thin layer in ZnO film, the maximum responsivity of the self-driven HJ photodetector ITO/ZnO(50 nm):Ag-NPs(10 nm):ZnO(50 nm)/PbSe(300 nm)/Ag reaches to 6.25 mA/W with a specific detectivity D* of 5.17 ×1011 Jones under 8.5 μW/cm² 1550 nm illumination at zero bias. Further, the underlain physical mechanisms for the enhanced-performance were discussed in details. In this way, it provides a very efficient method for enhanced-performance self-driven HJ photodetectors by inserting a thin layer of metal NPs into the electron-transporting layer close to the heterojunction interface.

Atomic-level chemical reaction promoting external quantum efficiency of organic photomultiplication photodetector exceeding 108% for weak-light detection

Low-cost, solution-processed photomultiplication organic photodetectors (PM-OPDs) with external quantum efficiency (EQE) above unity have attracted enormous attention. However, their weak-light detection is unpleasant because the anode Ohmic contact causes exacerbation in dark current. Here, we introduce atomic-level chemical reaction in PM-OPDs which can simultaneously suppress dark current and increase EQE via depositing a 0.8 nm thick Al2O3 by the atomic layer deposition. Suppression in dark current mainly originates from the built-in anode Schottky junction as a result of work function decrease of hole-transporting layer of which the chemical groups can react chemically with the bottom surface of Al2O3 layer at the atomic-level. Such strategy of suppressing dark current is not adverse to charge injection under illumination; instead, responsivity enhancement is realized because charge injection can shift from cathode to anode, of which the neighborhood possesses increased photogenerated carriers. Consequently, weak-light detection limit of the forwardly-biased PM-OPD with Al2O3 treatment reaches a remarkable level of 2.5 nW cm−2, while that of the reversely-biased control is 25 times inferior. Meanwhile, the PM-OPD yields a record high EQE and responsivity of 4.31 × 108% and 1.85 × 106 A W−1, respectively, outperforming all other polymer-based PM-OPDs.

Solution-processed near-infrared organic photodetector based on a liquid crystalline phthalocyanine derivative for vital signal monitoring

In this study, near-infrared (NIR) organic photodetectors (OPDs) based on a liquid crystalline phthalocyanine derivative, 1,4,8,11,15,18,22,25-octaoctyl-phthalocyanine (8H2Pc), and phenyl-C61-butyric acid methyl ester (PC61BM) were realized. The best device had a blend ratio of 1:1 by weight and exhibited responsivity of 0.2 A W−1, external quantum efficiency of 29% and shot-noise-limited specific detectivity of 1.3 × 1012 Jones at 740 nm with −1 V reverse bias. This notable performance was attributed to the uniformity and smooth surface morphology of the spin-coated active layer and the intermixed condition of the liquid crystalline 8H2Pc and PC61BM, resulting in smaller domain sizes and better separation of photogenerated exciton pairs. Finally, the future prospects of the realized NIR OPD in practical applications were demonstrated by monitoring the vital signals of a human subject with a very simple experimental setup.

Phosphorous-doped graphene as an efficient interfacial layer material for application in solution-processed photodetectors

This work demonstrates solution-processed photodetectors (PDs) with perovskite as the active layer material, reduced graphene oxide as the hole transport layer and phosphorus-doped graphene (P-G) as the interfacial layer. High detectivity of over 1012 Jones was attained with PDs fabricated with the P-G layer, suggesting the great potential of the P-G layer as an additional dark current blocking layer. Analysis of the diode parameters indicated that this improved optoelectronic performance was attributed to decreased recombination, which resulted from a decrease in interface and trap defects and improved surface morphology, as evident from the field emission scanning electron microscope images. Recombination resistance, carrier lifetime and charge transport resistance were estimated using impedance spectroscopy analysis. It was observed that suppressed recombination due to improved surface morphology, including large grain size, is the main reason for the enhanced optoelectronic performance of PDs fabricated with the P-G layer. This work demonstrates that the use of an additional blocking/interfacial layer can be an effective strategy to improve the performance of solution-processed optoelectronic devices.

Vertical Architecture Solution-Processed Quantum Dot Photodetectors with Amorphous Selenium Hole Transport Layer

Colloidal quantum dots (CQDs) provide wide spectral tunability and high absorption coefficients owing to quantum confinement and large oscillator strengths, which along with solution processability, allow a facile, low-cost, and room-temperature deposition technique for the fabrication of photonic devices. However, many solution-processed CQD photodetector devices demonstrate low specific-detectivity and slow temporal response. To achieve improved photodetector characteristics, limiting carrier recombination and enhancing photogenerated carrier separation are crucial. In this study, we develop and present an alternate vertical-stack photodetector wherein we use a solution-processed quantum dot photoconversion layer coupled to an amorphous selenium (a-Se) wide-bandgap charge transport layer that is capable of exhibiting single-carrier hole impact ionization and is compatible with active-matrix readout circuitry. This a-Se chalcogenide transport layer enables the fabrication of high-performance and reliable solution-processed quantum dot photodetectors, with enhanced charge extraction capabilities, high specific detectivity (D* ∼ 0.5–5 × 1012 Jones), fast 3 dB electrical bandwidth (3 dB BW ∼ 22 MHz), low dark current density (JD ∼ 5–10 pA/cm2), low noise current (in ∼ 20–25 fW/Hz1/2), and high linear dynamic range (LDR ∼ 130–150 dB) across the measured visible electromagnetic spectrum (∼405–656 nm).