Maximum Photoresponsivity Research Articles

Ultrafast and Stable Organic Single‐Crystal Vertical Phototransistor for Self‐Powered Photodetection and High‐Speed Imaging

AbstractOrganic semiconductor crystals herald new opportunities for fabricating high‐performance optoelectronic devices, due to their intrinsic properties including outstanding charge transport properties, long exciton lifetime, and diffusion length. Despite remarkable progress in key figures of merit, the cut‐off frequency and responsivity are still lagging behind for most currently reported organic devices. Here, a high‐performance broadband vertical phototransistor based on rubrene single crystal, whose response covers the UV to visible range (300–650 nm) is reported. It shows a record‐high −3 dB bandwidth of ≈18 kHz and a maximum photoresponsivity of 105 A W−1. Due to the asymmetric Schottky contact and excellent light absorption of organic crystal, the device exhibits excellent zero‐bias photoresponse even under ultralow light illumination (≈114 fW). In particular, such organic vertical phototransistor holds good air stability and environmental robustness, and even without any form of encapsulation, no degradation of performance is observed for 2 months continuous operating. In addition, several single‐pixel imaging to show its potential for practical applications are demonstrated. The results provide a promising strategy to fabricate ultrafast and sensitive organic photodetectors for future video‐frame‐rate imaging.

Solution-processed ZnO nanoparticles (NPs)/CH3NH3PbI3/PTB7/MoO3/Ag inverted structure based UV–visible-Near Infrared (NIR) broadband photodetector

This paper reports a ZnO Nanoparticles (NPs)/CH3NH3PbI3/PTB7/MoO3/Ag inverted structure based UV–visible-Near Infrared (NIR) broadband photodetector fabricated on the fluorine-doped tin oxide (FTO) coated glass substrate. ZnO NPs layer acts as the electron transport layer (ETL), CH3NH3PbI3 acts as the active layer, PTB7 acts as the hole transport layer (HTL) and MoO3 is used to optimize the recombination current in the proposed photodetector structure. Spin coating method has been used for depositing the layers of ZnO NPs, CH3NH3PbI3 and PTB7 in the proposed structure while the MoO3 layer has been grown by the thermal evaporation method. The optical measurements over 350–850 nm wavelength range give the maximum photoresponsivity, detectivity and external quantum efficiency (EQE) of ∼0.36 A/W, ∼7.8x1012 Jones and ∼83.67% under −2 V, respectively. The proposed photodetector gives a rise time and fall time of 81 ms and 75 ms, respectively.

A Sensitive UV Photodetector Based on Non-Wide Bandgap MAPbBr3 Nanosheet

A sensitive ultraviolet photodetector (UVPD) based on an ultrathin MAPbBr3 nanosheet was developed in this study. A simulation based on technology computer-aided design showed that the optical absorption of MAPbBr3 can be tailored by changing the material thickness, which is reasonable considering the wavelength dependence of the absorption coefficient of MAPbBr3. In addition, an apparent blueshift of the position of the maximum photocurrent is observed upon decreasing the nanosheet thickness. These tunable optoelectronic properties resulted in a device fabricated from medium-bandgap 43-nm-thick MAPbBr3 nanosheets being sensitive to UV light illumination. The device has a maximum photoresponse at 300 nm, a responsivity of 27.2 mA <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot \text{W}$ </tex-math></inline-formula> −1, and a response speed of 0.103/0.087 s, respectively, which are comparable to conventional UVPDs based on low-dimensional wide bandgap materials (e.g., ZnO and TiO2). This novel UVPD has application potential to optoelectronic systems.

Hetero-radial MgO capped TiO2 nanowire arrays as a deep UV and self-powered photodetector

Nanowires acts as a potential component in photodetectors (PDs) owing to its superior capacity to generate morphology-dependent broad absorption spectrum. In this study, we have demonstrated an efficient self-powered PD based on the radial heterostructure of the MgO capping layer on top of TiO2 nanowires (RHNWs). RHNWs are highly sensitive under deep ultraviolet (UV) illumination when compared to TiO2-MgO axial heterostructure nanowires (AHNWs) PD. The finite-difference time-domain (FDTD) method is used to simulate electric field distribution, absorption spectra and generation rate profile of the structures. The RHNWs device possesses two times lower leakage current (∼ 1.78 × 10−9 A) than AHNWs (∼ 3.84 ×10−9 A) at – 1.5 V. Theoretical analysis shows a higher photoconductive gain for RHNWs (∼ 12) than the corresponding AHNWs device (∼ 9.9). Maximum photoresponsivity of ∼ 18 A/W has been observed for RHNWs with a sharp rise time (Tr) ∼ 0.12 s and fall time (Tf) ∼ 0.13 s at 200 nm (– 9 V) and the device also behaves like a self-powered PD under white light illumination.

A Weyl semimetal WTe2/GaAs 2D/3D Schottky diode with high rectification ratio and unique photocurrent behavior

Since the discovery of Dirac semimetal graphene, two-dimensional (2D) Weyl semimetals (WSMs) have been widely used in low-energy photon detection, polarization imaging, and other systems due to their rich physical characteristics, such as unique nonlinear optical structure, topological nontrivial electronic structure, thickness-tunable bandgap, high electric conductivity, and so on. However, it is difficult to detect the photocurrent signal at room temperature because of its large intrinsic background current. Fortunately, the fabrication of a van der Waals (vdW) heterojunction based on WSM can effectively suppress the background current, greatly extend the detection range, improve the light absorption efficiency, and increase the response speed. Herein, the 2D type-II WSM 1T′-WTe2/bulk GaAs vdW vertical Schottky diode is investigated. Benefiting from the lateral built-in electric field of 260 meV and zero-bandgap structure of 52 nm 1T′-WTe2, it delivers a rectifying ratio over 103 and can respond to the wavelength range of 400–1100 nm. Particularly, when the light power density is 0.02 mW/cm2, the maximum photoresponsivity (R) and specific detectivity (D*) under 808 nm are 298 mA/W and 1.70 × 1012 Jones, respectively. Meanwhile, the Ilight/Idark ratio and response time are 103 and 520/540 μs, respectively. Moreover, an abnormal negative response behavior can be observed with thin WTe2 (11 nm) under 1064 nm illumination because of the open surface bandgap. It is suggested that such 2D WTe2/GaAs mixed-dimensional vdW structure can be extended to other WSM/3D semiconductor junctions and used in fast response and wide broadband spectrum photodetectors' arrays.

Enhanced photogalvanic effect in a 2D ferroelectric ZrI2 by interlayer sliding

Two-dimensional ferroelectrics with moderate band gap enable unprecedented applications in optoelectronics. Here, we report that the photogalvanic effect (PGE) of narrow-band-gap semiconductor bilayer ZrI2 is significantly enhanced by the interlayer sliding which turns out to be ferroelectric with both in-plane and out-of-plane polarizations. The intrinsic ferroelectric field promotes the separation efficiency of photogenerated carriers and reduces the recombination rate of electron–hole pairs, thus improving the photoelectric conversion efficiency and resulting in a larger photoresponse. The magnitude of the maximum photoresponse in the bilayer β-ZrI2 (ferroelectric) is enhanced by about 5 times than in the bilayer s-ZrI2 (paraelectric). A robust broadband photoresponse from mid-infrared to visible can be found from its photodetector. Our results provide great insights to engineering novel optoelectronic applications by ferroelectric slidetronics.

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

An organic semiconductor with high carrier mobility and efficient light absorption over a wide spectral range is of the most important yet challenging material for constructing a broadband responsive organic photodetector. However, the development of such organic semiconductors, especially for air-stable n-type organic small molecule semiconductors, is still at an early stage. Here we report the fabrication of high-performance n-type semiconducting crystalline nanosheets and the development of air-stable field-effect transistors, phototransistors, with high response over a broad spectrum. The n-type small molecule semiconductor is assembled into a crystalline nanosheet based on the solvent-phase interfacial self-assembly method. N-type field-effect transistors with high electron mobility are fabricated and their electrical performances exhibit excellent air stability. Impressively, the demonstrated phototransistors exhibit an ultrahigh responsivity over a wide spectral range from 365 to 940 nm, with a maximum photoresponsivity of 9.2 × 105 A W−1 and specific detectivity of 5.26 × 1013 Jones, which is the best performance among the reported n-type organic small molecule-based phototransistors.

First-Principles Study of Half Metallic Ferromagnetic and Optical Properties of Nb Doped Cubic ZnS using TB-mBJ Approximation

Structural, electronic and optical properties of niobium doped ZnS are studied by using full-potential linearized augmented plane wave plus local orbital (FPLAPW+lo) method within the density functional theory (DFT). Computed results of Nb doped ZnS are compared with that of the pristine zinc blende. Tran-Blaha approach of modified Becke and Johnson local spin density approximation (TB-mBJ) is used to study electronic and optical properties. Estimated result shows that Nb reduces the bandgap of ZnS due to hybridization of Nb-4d orbital with S-3p orbital near the Fermi level. Niobium dopant provides half metallic nature to ZnS with 100% spin polarization. Maximum photo-response is noticed in the ultraviolet range for Zn1-xNbxS (x = 25, 12.5, 6.25 %). Highest peaks are shifted toward the lower energy range for higher dopant percentage. All these suggest that Nb doped ZnS solid solutions are suitable candidate for both energy filter of UV spectrum and spintronic device. Dhaka Univ. J. Sci. 69(3): 194-201, 2022 (June)

Ultrafast-response and broad-spectrum polarization sensitive photodetector based on Bi1.85In0.15S3 nanowire

Alloying of semiconductors is a good strategy to manipulate their electronic band structures, which can broaden the photoresponse range of the corresponding optoelectronic devices. In addition, building a Schottky diode and improving the crystal quality of the channel semiconductor can improve the photoresponse speed of the optoelectronic device. Here, we report the design and preparation of Bi1.85In0.15S3 nanowires by a facile chemical vapor transport method. The individual Bi1.85In0.15S3 nanowire photodetectors realize excellent photoresponse in a broadband range from solar-blind deep ultraviolet (266 nm) to near-infrared (830 nm), and the obtained maximum external photoresponsivity of 95.99 A/W and detectivity of about 3.52×1011 Jones at 638 nm. Furthermore, the photodetectors also exhibit the ultrafast photoresponse speed with the rise time of 190 ns and the fall time of 180 ns, owing to the high crystal quality and the Schottky contacts between the Au electrodes and nanowires. In addition, the photoresponse of photodetectors is polarization angle sensitive in a broadband range from 266 to 808 nm, and the obtained maximum dichroic ratio is 3.54 at 808 nm, which results from the structural anisotropy of the Bi1.85In0.15S3 crystal. These performances are superior to the reported Bi2S3, In2S3, and other Bi or In sulfide nanowire photodetectors. The results render (BixIn1−x)2S3 photodetectors have significant application potentials in multifunctional optoelectronics and electronics.

Bandgap Engineering of Ternary ε‐InSe1−xSx and ε‐InSe1−yTey Single Crystals for High‐Performance Electronics and Optoelectronics

AbstractAlloying offers an efficient strategy to tune the bandgap of two‐dimensional (2D) layered materials, enabling them to tailor the optical and electronic attributes without compromising the structural integrity. Here the authors report the synthesis of a series of ternary InSe1−xSx and InSe1−yTey alloys possessing ε‐polymorph and single crystalline structure. Both the photoluminescence and Raman spectra of multilayer InSe1−xSx and InSe1−yTey demonstrate that an effective modulation of bandgap and concomitant optical properties is achieved by tuning the alloy compositions, consistent with density functional theory calculations. Field‐effect transistors fabricated from the multilayer alloys on SiO2 dielectric substrates display electron field‐effect mobilities of up to ≈127 cm2 V−1 s−1. All the multilayer alloy devices show a high current on/off ratio of ≈108. When fabricated into photodetectors, multilayer InSe0.9S0.1 and InSe0.9Te0.1 exhibit maximum photoresponsivities of 5.4 × 105 and 7.7 × 104 A W−1, respectively. Moreover, the InSe1−yTey alloys are able to expand the photoresponse range into 1250 nm due to the bandgap narrowing upon Te alloying. This work sheds light on rationally designing 2D layered InSe with tunable bandgaps via alloying, and demonstrates their promising applications in electronics and optoelectronics.

Enhanced photoresponse performance in GaN based symmetric type MSM ultraviolet-A and MIS ultraviolet-A to C photodetectors

To realize an improved sustainable future, additional developments in the energy consumption is a prerequisite important aspect for the operation of any electronic device. This is correspondingly true and intended for all the commercially existing photodetectors (PDs) which functions using large external bias. So far, thin films have been commonly used in the photodetection of several bands of electromagnetic spectrum. Recently, rare earth oxides have received tremendous attention as a gate dielectric in the photodetection filed on account of their superior characteristics such as large bandgap, high dielectric constant accompanied by thermodynamical stability. Besides, the earlier reported GaN PDs mostly functioned in UV-A region only and there are no pertinent reports pointed at the deep UV region. In this manuscript, we have fabricated and reported Au/GaN MSM UV-A (300–400 nm) and Au/Nd2O3/GaN MIS UV-A to C/VIS (200–600 nm) BB PDs using cost-effective electron beam evaporation technique at room temperature. The reported Au/Nd2O3/GaN MIS BB PD device is sensitive to wavelengths of light from 200 nm to 600 nm (UV to VIS region). This improvement is correlated with the intentionally deposited Nd2O3 visible light absorber thin film at the Au/GaN interface as evidenced from the optical absorption study. The direct optical bandgap, crystal structure, chemical composition/oxidation states and surface morphology of the Nd2O3 material deposited on GaN were demonstrated using UV-Visible, XRD, XPS and AFM techniques. The Au/Nd2O3/GaN MIS BB PD device exhibited an outstanding photodetection performance with maximum photoresponsivity of 6382 mA/W, detectivity of 9.4 × 1014 Jones, external quantum efficiency of 3.67 × 103% and response speed of 770 ms/2320 ms (τFall/τrise) compared to Au/GaN MSM PD device. Thus, these results suggest that the Au/Nd2O3/GaN MIS type BB PD device structure could be employed as a potential candidate to tune the wavelength selectivity from UV to VIS region of the electromagnetic spectrum in the future.

Low Power Consumption Gate-Tunable WSe2/SnSe2 van der Waals Tunnel Field-Effect Transistor

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have attracted attention as promising next-generation electronic devices and sensors. In this study, we fabricated a novel nanoelectronic device based on a black-phosphorus-gated WSe2/SnSe2 van der Waals (vdW) tunnel field-effect transistor (TFET), where hexagonal boron nitride (h-BN) was used as the gate insulator. We performed morphological, electrical, and optoelectronic characterizations. The p-WSe2/n-SnSe2 heterostructure-based TFET exhibited p-type behavior with a good dependence on the gate voltage. The TFET device showed a trend toward negative differential resistance (NDR) originating from band-to-band tunneling, which can be tuned by applying a gate voltage. The optoelectronic performance of the TFET device was low, with a maximum photoresponsivity of 11 mA W−1, owing to the large device length. The results obtained herein promote the integration of black phosphorus into low-energy-consumption 2D vdW TFETs.

Electrophoretic deposition of MoS2 nanosheets for photoelectrochemical type photodetector

Herein, we demonstrate the electrophoretic deposition of MoS2 nanosheets on ITO coated glass substrate for preparation of working electrode for photoelectrochemical type photodetector. Vertical alignment of MoS2 nanosheets on substrate offers high surface-to-volume ratio, enhancing the electrochemical charge transport. MoS2 nanosheets based working electrodes shows obvious photovoltaic effect with photocurrent of 0.5 μA/cm2 at zero bias under 670 nm light of power intensity 100 mW/cm2. Maximum photoresponsivity of 170 μA/W, IPCE of 3 × 10−2% is realised owing optimized conditions. Overall, present finding advocates the development of large area, transparent and photosensitive electrode for PEC type photodetection application for visible light.

Mg-doped beta-Ga2O3 films deposited by plasma-enhanced atomic layer deposition system for metal-semiconductor-metal ultraviolet C photodetectors

In this study, magnesium-doped gallium oxide (Ga2O3:Mg) films with various Mg contents were deposited by a plasma-enhanced atomic layer deposition (PE-ALD) system and were used as an active layer of metal-semiconductor-metal ultraviolet C photodetectors (MSM UVC-PDs). Compared with Ga element, owing to the lower electronegativity of Mg element, Mg–O bonds were more easily formed than Ga–O bonds. Consequently, oxygen vacancies could be reduced to improve properties. By using the measurement of X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL), the results verified that Ga2O3:Mg films with Mg content of 3.28 at.% had the lowest oxygen vacancy density. The cut-off wavelength of the devices was blue-shifted from 250 nm to 220 nm by increasing Mg content from 0 at.% to 5.22 at.%. The maximum photoresponsivity of 22.06, 10.24, 6.89, and 1.67 A/W was obtained for the devices with the Mg content of 0 at.%, 1.91 at.%, 3.28 at.%, and 5.22 at.%, respectively. The corresponding UV–visible rejection ratio was 3.56 × 104, 3.86 × 104, 4.42 × 104, and 3.48 × 103, respectively. The flicker noise was the dominant noise. By appropriately doping the Mg content in Ga2O3 films, the detectivity of the MSM UVC-PDs was effectively improved.

Gate‐Tunable Photovoltaic Effect in MoTe2 Lateral Homojunction

AbstractSince it is urgent to develop flexibly tunable photosensors in artificial vision network, atomically thin 2D materials are promising candidates for susceptible gate modulation and thickness‐dependent bandgaps. Here, a gate‐tunable MoTe2 lateral homojunction is reported with asymmetric thickness. Through gate modulation, a tunable and abrupt built‐in field can form at the interface and realize the broadband self‐driven photodetection ranging from visible (405 nm) to near‐wavelength infrared (1550 nm) region. The photocarriers are effectively separated by the tunable electric‐field, leading to an obvious photovoltaic behavior with maximum Voc of 0.27 V (0.20 V) and maximum PCE of 6% (1.85%) under light illumination of 635 nm (1064 nm). It also demonstrates an ultra‐low noise power density at the positive gate voltage, revealing a hypersensitized photodetection with a maximum photoresponsivity of 1200 mA W−1 (560 mA W−1) and specific detectivity up to 1012 (1011) Jones under 635 nm (1064 nm). Meanwhile, the underlying mechanism of the gate control effect on the electrical, optoelectrical properties and noise level is revealed. By utilizing the tunable photovoltages as logic optoelectronic application, the lateral homojunction can convert a light signal to different electric signals. This photovoltage‐tunable homojunction hosts promising innovation for potential development of computational sensors and logic optoelectronics.

Modification of defects in SnO2 nanowire arrays by gallium doping for enhanced photodetection

Glancing angle deposition technique was used to grow as-deposited SnO2 and also Gallium doped SnO2 (Ga: SnO2) Nanowire (NW) arrays over Si substrate. The relative amount of Ga dopants was varied from 5% to 20% and tested for various structural and optical characteristics. A blue shift in bandgap and enhanced photo absorption was observed for Ga: SnO2 NWs as compared to the as-deposited SnO2 NWs. Photoluminescence study indicates the suppression of emission due to modification in defects after Ga doping with the least emission in case of 5% Ga: SnO2 NWs which was also supported by the X-ray photoelectron spectroscopy results. Photodetectors (PD) were fabricated using all samples and among them the based-on Au/5% Ga: SnO2 NW/Si device exhibited the lowest dark current (~2.67 nA) along with a maximum photoresponse of ~27. Furthermore, a superior transient response with a rise time and fall time of 1.3 ms and 2 ms respectively was observed for 5% Ga: SnO2 NWs PDs. Thus, large e-h pair generation and reduced defect states in case of 5% Ga: SnO2 NWs sample signifies that gallium doping level of 5% in SnO2 is the optimum concentration for enhanced optoelectronic applications.

Estimation of Maximum Photoresponsivity of n‐SnO2/p‐Si Heterojunction‐Based UV Photodetectors

Photoresponsivity (PR) in photodetectors is of profound importance in evaluation of the performance of the devices. A detailed study of PR for n‐SnO2/p‐Si heterojunction‐based photodetectors is performed. The PR is analyzed within the framework of its relationship with incident power intensity (Pin). Experimental results clearly indicate that the PR reaches its maximum value at low Pin values, where it almost becomes independent of Pin. This follows primarily from the experimentally observed incident power intensity and dark current dependences of PR. The detailed analyses in terms of incident power intensity and dark current in the investigated power intensity regime are utilized to extract the values for maximum attainable PR limit of the devices, which is estimated as 46.3 A W−1. Evidently, the knowledge gained this work can be applicable to extract the maximum PR value for other heterojunction‐based UV photodetectors.

Chemo-phototronic effect induced electricity for enhanced self-powered photodetector system based on ZnO nanowires

Self-powered ultraviolet (UV) photodetectors based on ZnO nanowires have attracted considerable interest due to low-cost and ease of fabrication. Effective strategies to enhance their photoresponse still remain challenging. Here we report a 4 × 4 matrix of self-powered ZnO UV photodetector system with optimized ZnO nanowire photoelectrode/electrolyte heterostructures. Modulating pH-dependent chemo-phototronic effect can regulate optoelectric and chemoelectric processes, resulting in significantly improved photodetection performance. The maximum photocurrent and photoresponsivity up to 290.02 μA and 4.74 × 10−2 A W−1 can be achieved by using NaOH aqueous electrolyte (pH 13), which have been enhanced by 2056% as compared with that of utilizing deionized water electrolyte (pH 7). The enhanced performance is related to the pH-dependent chemical reaction, Schottky-like barrier height and depletion region width modulation at ZnO/electrolyte interface. This work provides guidelines to fabricate high-performance self-powered photodetector system of using semiconductor/electrolyte heterostructures, pushing forward the advancement of high-sensitivity and low-cost photodetector array system.

Interband transition in plasmonic titanium nitride and its contribution towards ZnO based pyro-phototronic application

Interband transition generated charge carriers in plasmonic TiN have been utilized for optoelectronic device application by fabricating a highly responsive ultrafast pyro-phototronic self-powered UV photodetector. The excitonic-pyroelectric effect of the non-centrosymmetric wurtzite ZnO crystal combined with the interband transition effect of TiN helps in achieving high performance and ultrafast response of the fabricated photodetector. The maximum photoresponsivity of the device is calculated to be 50 mA/W in self-powered condition, which is one of the best values reported in case of ZnO based self-powered UV photodetectors. Apart from this high response, the device also registers an ultrafast response time of 21 μs which is attributed to the inherent pyroelectric effect of the ZnO crystal. Finite difference time domain simulation also reveals the contribution of interband transition in the device operation. This study thus provides an insight about the unexplored area of interband transition in plasmonic materials and its efficient use in combination with excitonic-pyroelectric effect of the base semiconductor.

Optoelectronic performance characterization of MoS2 photodetectors for low frequency sensing applications

The specific advantages of implementing MoS2 and other layered semiconductors for optoelectronic biosensing and other relevant photodetection applications remain unclear. In this work, we investigate the photoresponsivity and noise characteristics of in-plane MoS2 photodetectors. This work indicates that MoS2 photodetectors exhibit lower noise equivalent power (NEP) and detectivity (D*) in comparison with commercial CdS photodetectors. In addition, the low-frequency NEP and D* values of MoS2 photodetectors exhibit a prominent dependence on the MoS2 photoactive layer thickness. We have identified the optimal MoS2 thickness in the range of 8–30 nm. We also study the photoresponse characteristics of optimized MoS2 photodetectors at several different wavelengths that are important for clinical colorimetry assays. Such an optimized photodetector shows a maximum photoresponsivity of 164.3 A/W and a minimum NEP of 3.99 × 10−17 W/Hz1/2 (and a D* of 5.01 × 1010 J) with relative variance less than 14%. This work provides a useful guideline for optimizing the photoresponse characteristics of MoS2-based optoelectronic devices, which is critical to practical low-frequency optoelectronic biosensing applications.