Photoactive Region Research Articles

Filterless Bandpass Photodetectors Enabled by 2D/3D Perovskite Heterojunctions

AbstractBandpass photodetectors have tremendous applications in colorimetric analysis, light communication, imaging and machine vision systems. Compared to broadband photodetectors combined with external optical filters, solution‐processed filterless bandpass photodetectors enjoy high integration density and low cost, but suffer from broadened response edges and insufficient spectral rejection ratios (SRRs). Here, prototype filterless bandpass photodetectors with near square‐shape photoresponse are developed based on (CmH2m+1NH3)2PbI4/MAPbI3 (m = 4–8) and (CmH2m+1NH3)2PbBr4/MAPbBr3 (m = 2–8) perovskite heterojunctions. The strong and sharp excitonic absorption of 2D perovskites defines the ultranarrow response onset widths of 10 ± 1 nm. Besides, the strong photoluminescence (PL) self‐absorption of 2D perovskites suppresses the PL leakage from the front absorber to the back photoactive region, which is crucial to the achieved record high SRR of >2000. By integrating Br‐based 2D/3D perovskite photodetectors with onset edge wavelength discrimination of ≈20 nm, a multiple‐channel optical communication system with low spectral crosstalk is demonstrated.

Large-Scale Direct Growth of Monolayer MoS2 on Patterned Graphene for van der Waals Ultrafast Photoactive Circuits.

Two-dimensional (2D) van der Waals heterostructures combine the distinct properties of individual 2D materials, resulting in metamaterials, ideal for emergent electronic, optoelectronic, and spintronic phenomena. A significant challenge in harnessing these properties for future hybrid circuits is their large-scale realization and integration into graphene interconnects. In this work, we demonstrate the direct growth of molybdenum disulfide (MoS2) crystals on patterned graphene channels. By enhancing control over vapor transport through a confined space chemical vapor deposition growth technique, we achieve the preferential deposition of monolayer MoS2 crystals on monolayer graphene. Atomic resolution scanning transmission electron microscopy reveals the high structural integrity of the heterostructures. Through in-depth spectroscopic characterization, we unveil charge transfer in Graphene/MoS2, with MoS2 introducing p-type doping to graphene, as confirmed by our electrical measurements. Photoconductivity characterization shows that photoactive regions can be locally created in graphene channels covered by MoS2 layers. Time-resolved ultrafast transient absorption (TA) spectroscopy reveals accelerated charge decay kinetics in Graphene/MoS2 heterostructures compared to standalone MoS2 and upconversion for below band gap excitation conditions. Our proof-of-concept results pave the way for the direct growth of van der Waals heterostructure circuits with significant implications for ultrafast photoactive nanoelectronics and optospintronic applications.

Two-Dimensional GeS/SnSe2 Tunneling Photodiode with Bidirectional Photoresponse and High Polarization Sensitivity.

A two-dimensional (2D) broken-gap (type-III) p-n heterojunction has a unique charge transport mechanism because of nonoverlapping energy bands. In light of this, type-III band alignment can be used in tunneling field-effect transistors (TFETs) and Esaki diodes with tunable operation and low consumption by highlighting the advantages of tunneling mechanisms. In recent years, 2D tunneling photodiodes have gradually attracted attention for novel optoelectronic performance with a combination of strong light-matter interaction and tunable band alignment. However, an in-depth understanding of the tunneling mechanisms should be further investigated, especially for developing electronic and optoelectronic applications. Here, we report a type-III tunneling photodiode based on a 2D multilayered p-GeS/n+-SnSe2 heterostructure, which is first fabricated by the mechanical exfoliation and dry transfer method. Through the Simmons approximation, its various tunneling transport mechanisms dependent on bias and light are demonstrated as the origin of excellent bidirectional photoresponse performance. Moreover, compared to the traditional p-n photodiode, the device enables bidirectional photoresponse capability, including maximum responsivity values of 43 and 8.7 A/W at Vds = 1 and -1 V, respectively, with distinctive photoactive regions from the scanning photocurrent mapping. Noticeably, benefiting from the in-plane anisotropic structure of GeS, the device exhibits an enhanced photocurrent anisotropic ratio of 9, driven by the broader depletion region at Vds = -3 V under 635 nm irradiation. Above all, the results suggest that our designed architecture can be potentially applied to CMOS imaging sensors and polarization-sensitive photodetectors.

Dual-Photosensitizer Synergy Empowers Ambient Light Photoactivation of Indium Oxide for High-Performance NO2 Sensing.

Light-activated chemiresistors offer a powerful approach to achieving lower-temperature gas sensing with unprecedented sensitivities. However, an incomplete understanding of how photoexcited charge carriers enhance sensitivity obstructs the rational design of high-performance sensors, impeding the practical utilization under commonly accessible light sources instead of ultravioletor higher-energy sources. Here, a rational approach is presented to modulate the electronic properties of the parent metal oxide phase, exemplified by this model system of Bi-doped In2O3 nanofibers decorated with Au nanoparticles (NPs) that exhibit superior NO2 sensing performance. Bi doping introduces mid-gap energy levels into In2O3, promoting photoactivation even under visible blue light. Additionally, green-absorbing plasmonic Au NPs facilitate electron transfer across the heterojunction, extending the photoactive region toward the green light. It is revealed that the direct involvement of photogenerated charge carriers in gas adsorption and desorption processes is pivotal for enhancing gas sensing performance. Owing to the synergistic interplay between the Bi dopants and the Au NPs, the Au-BixIn2-xO3 (x=0.04) sensing layers attain impressive response values (Rg/Ra=104 at 0.6ppm NO2) under green light illumination and demonstrate practical viability through evaluation under simulated mixed-light conditions, all of which significantly outperforms previously reported visible light-activated NO2 sensors.

Surface Photovoltage Method for Photovoltaic Quality Control of GaAs-Based Solar Cells

In this paper, we demonstrate the potential of the contactless surface photovoltage (SPV) method for fast and reliable control of GaAs-based solar cells directly on epitaxial heterostructures before metallization and photolithography processes. The magnitude of the SPV corresponds to the generated photovoltage in the photoactive region, which is related to the open circuit voltage of the cell. The focus of this investigation is the potential of dilute nitride compounds grown by low-temperature liquid-phase epitaxy (LPE) for application as intermediate cells in multijunction solar cells. First, SPV spectroscopy is used to determine the photosensitivity spectral range and bandgap of the grown dilute nitride compound layers. Further, the photovoltaic quality of the grown solar cell heterostructures is evaluated by comparing the magnitude of their SPV signals with that of a reference GaAs solar cell. A drastic reduction in the measured SPV is observed for nitrogen-containing solar cell structures, which correlates with the lowering of solar cell open-circuit voltage values measured under standard test conditions. Finally, solar cell structures based on nitrogen-free GaAsSb compounds with the same long-wavelength photosensitivity limit as GaAsSbN are grown by LPE. They show one order of magnitude higher SPV signal and, therefore, have a great potential for solar cell application.

Self-Powered Wide-Narrow Bandgap-Laminated Perovskite Photodetector with Bipolar Photoresponse for Secure Optical Communication.

Perovskite photodetectors with bipolar photoresponse characteristics are expected to be applied in the field of secure optical communication (SOC). However, how to realize the perovskite photodetector with bipolar response remains challenging. Herein, by introducing bismuth iodide (BiI3 ) into Sn-Pb mixed perovskite precursor solution, 2D perovskite FA3 Bi2 I9 is spontaneously formed at the bottom to realize a wide-narrow bandgap-laminated perovskite film. Wavelength-dependent bipolar response is realized based on the absorption difference of the photoactive region with different bandgap combined with the carrier competition of the homotypic transport layer adopted in the as-fabricated photodetector. Under the visible/near-infrared (NIR) light irradiation, the bottom/top of the film generates a higher carrier concentration, where electrons are easier to be separated and transported by the SnO2 /PC61 BM to the bottom/top electrodes, respectively, resulting in a negative and positive bipolar response. Finally, based on positive NIR signal as the effective signal and negative visible signal as the interference signal, the SOC system is realized, where the positive NIR signal is well hidden by the negative visible signal. This work provides a simple and feasible strategy for fabrication of laminated perovskite films to achieve bipolar response.

Enhanced Optical Management in Organic Solar Cells by Virtue of Square-Lattice Triple Core-Shell Nanostructures.

This research focuses on enhancing the optical efficacy of organic photovoltaic cells, specifically their optical absorbance and electrical parameters. The absorbance of photons in organic solar cells (OSCs) was studied by incorporating an optical space layer and triple core-shell square-lattice nanostructures. For better chemical and thermal stability, a dielectric-metal-dielectric nanoparticle can be replaced for embedded metallic nanoparticles in the absorption layer. The 3D (finite-difference time-domain) FDTD method was used to analyze the absorption and field distribution in OSCs using 3D model morphology. Firstly, an optimization of thickness of the optical spacer layer was analyzed and secondly, the impact of adding triple core-shell nanostructures at different levels of an OSC were studied. The photovoltaic properties such as short circuit current density, power conversion efficiency, fill factor, Voc were investigated. The proposed design has demonstrated an improvement of up to 80% in the absorption of light radiation in the photoactive region (donor or acceptor) of OSCs in the wavelength range of 400 nm to 900 nm when compared with that of nanostructures proposed at various layers of OSC.

In-Situ Piezoelectric Effect for Augmenting Performance of Self-Powered ZnO-Based Photodetector

In this study, an in-situ piezoelectric effect is integrated into a photoactive region to develop a self-powered ultraviolet photodetector based on a p-n junction of ZnO@Polyvinylidene fluoride (PVDF) and poly [9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB). A ZnO@β-PVDF nanocomposite is fabricated using PVDF with the β-phase as the polymer matrix and ZnO nanoparticles as fillers. The strong piezoelectricity of β-PVDF can facilitate the separation and transport of photogenerated electrons in the depletion area and considerably reduce the dark current when the device is polarized with an external bias, resulting in an improvement in the on/off ratio and detectivity. Under 365-nm UV illumination, the as-fabricated device exhibits a high detectivity of 4.99 × 1011 Jones, an excellent on/off ratio (up to 2.75 × 104), and a fast response speed of 46/53 ms (rise/fall times). The device functions stably over approximately 1000 continuous on/off cycles and exhibits extremely long-lasting photostability when exposed to UV light. The findings demonstrate a promising strategy for enhancing the performance of photodetectors for industrial applications.

Modelling the Structure and Optical Properties of Reduced Graphene Oxide Produced by Laser Ablation: Insights from XPS and Time-Dependent DFT

Graphene derivatives such as reduced graphene oxide and graphene-based composites are regarded as highly promising materials for optoelectronics and photodetection applications. Recently, considerable interest has arisen in using facile and environmentally friendly methods of graphene production. Despite significant progress in experimental studies of graphene and graphene-based composites, some fundamental questions about their structures, and the interaction between components in these systems, remain open. In the present work, several atomistic models of oxidized graphene fragments have been proposed based on XPS compositional analysis and DFT calculations, representing reduced graphene oxide produced by laser irradiation. The composition of the oxygen-containing functional groups, their equilibrium configuration and influence on the electronic and optical properties of graphene sheets were determined. The nature of the low-lying excited states, as well as the photoactive regions, has also been studied for the proposed models. The calculated absorption spectra of the graphene sheets were compared with experimental UV-Vis spectrum of rGO produced by laser ablation.

A panchromatic graphite-derived-carbon-dot TiO2 photocatalyst: Broad visible light absorption via ligand-to-metal charge transfer

In recent decades, solar-to-chemical conversion catalysts for energy and environmental applications have drawn significant attention. Catalysts composed of nano-size TiO2 and various dye-based photosensitizers can absorb solar light efficiently and produce promising photocatalysts, but their photoactive region is limited. In this study, we demonstrate that nanocomposites composed of graphite-derived carbon dots (GCDs) and TiO2 are capable of ligand-to-metal charge transfer (LMCT), which was active across the entire visible-light spectrum. The effect of GCDs on the photocatalytic activity of TiO2 was determined through the production of hydrogen peroxide and the degradation of pollutants. On the basis of chemical and optical investigation, we demonstrate the formation of an LMCT band in the TiO2/GCD nanocomposites, which are responsible for photoactivation below the bandgap of each component. Our research provides conclusive evidence for the presence of the LMCT band in TiO2/GCD and reveals a novel method for expanding the light-absorption spectrum of the photocatalyst.

High-efficiency (over 33 %) indoor organic photovoltaics with band-aligned and defect-suppressed interlayers

In recent years, the emergence of the Internet of Things (IoT) has led to growing interest in the use of organic-based artificial light cells (OALCs; indoor organic photovoltaics) to harvest ambient light energy. This study employs atomic layer deposition (ALD)-processed vanadium oxide (V2O5) hole-transport-layers (HTLs) in non-fullerene acceptor-based OALCs to achieve record power conversion efficiency (PCE) exceeding 33 % under a light-emitting diode lamp (19500 lx; light intensity IL = 4.5 mW/cm2). The material most widely used for HTLs is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). OALCs with ALD-processed V2O5 HTLs substantially outperform those with PEDOT:PSS HTLs because they have superior hole selectivity, excellent electron-blocking, a smaller interfacial area, and substantially enhanced step coverage which significantly suppresses charge recombination. Moreover, the proposed V2O5-based OALCs have diffusion-free characteristics into the photoactive region and demonstrate excellent performance stability, maintaining a PCE of 92 % after 1000 h under ambient conditions. The results provide insights that enable simultaneous improvements in the efficiency and ambient stability of OALCs for low-powered IoT applications.

Optical simulation and design of high-absorption thin-film perovskite halide solar cells based on embedded quadrilateral cluster nanoparticles

Recently, thin-film organic–inorganic hybrid perovskite solar cells have a high efficiency-to-cost ratio. However, due to the thickness of the absorber layer, photon management in the absorber layer is still not optimized. This research suggests an embedded side for the plasmonic perovskite solar cells. Using Maxwell equations solver (finite-difference time-domain method), numerical optimization of photocurrent for gold, copper, silver, and aluminum cluster of nanoparticles in different geometries including quadrilateral, hexagonal, octagonal, and dodecagonal was investigated at a wavelength range from 300 nm to 800 nm. Moreover, the basic factors of optical simulation such as absorption coefficient, generation rate, and electric field distribution are shown in the optimal case, and the important parasitic absorption challenge for the cluster of nanoparticles was considered in the calculation of the net absorption. The reference cell uses a perovskite absorber (CH3NH3PbI3) with a thickness of 250 nm, which can significantly reduce the amount of lead, and the solar cell's toxicity. The photocurrent density for the optimized case utilizing spherical cluster of nanoparticles is obtained 21.74mAcm−2, which is enhanced by approximately 25.6% compared to perovskite solar cells without nanoparticles. Finally, the best-case cluster nanoparticle is coated with an ultra-thin-film SiO2 layer, to help the chemical and thermal stability of metallic nanoparticles in the photoactive region. According to the final proposed model, the photocurrent of 22.28mAcm−2 and enhancement of 28.72% were obtained. Besides, the proposed solar cell's open-circuit voltage, fill factor, and conversion efficiency are calculated at around 1.003 V, 0.88, and 19.71%, respectively.

Zero power infrared sensing in 2D/3D-assembled heterogeneous graphene/In/InSe/Au.

Low- or self-powered infrared sensors can be used in a broad range of applications, including networking mobile edge devices and image recognition for autonomous driving technology. Here, we show state-of-the-art self-powered near-infrared (NIR) sensors using graphene/In/InSe/Au as a photoactive region. The self-powered NIR sensors show outstanding performance, achieving a photoresponsivity of ∼8.5 A W-1 and a detectivity of ∼1012 Jones at 850 nm light. Multiple self-powered InSe photodetectors with different device structures and contacts were systematically investigated. In particular, the asymmetrically assembled graphene/In/InSe/Au vertical heterostructure offers a high built-in field, which gives rise to efficient electron-hole pair separation and transit time that is shorter than the photocarrier lifetime. The built-in potential across the InSe was estimated using the Schottky barrier height at each metal contact with InSe, obtained using density functional theory calculations. We also demonstrate InSe vertical field-effect transistors and provide an out-of-plane carrier mobility of InSe. Using the out-of-plane mobility and structural parameters of each device, the built-in field, drift velocity, and corresponding transit time are estimated.

Temperature modes and mechanical stresses in photovoltaic converters of concentrated sunlight

In photovoltaic converters of concentrated sunlight, the thermal flow is directed from the photoactive region (p-n junction) to a heat-spreading basement through the substrate. The heat sink transfers the excess thermal to the environment by convection or cooled by a liquid carrier. Reducing the thickness of the substrate makes it possible to reduce the thermal resistance of the crystal and lower the operating temperature of the photoactive region. However, in this case, the mechanical stresses in it increase. This work discusses the balance between the mechanical strength of the sample and the decrease in its operating temperature. Keywords: photovoltaic converters, heat sink, temperature regime, mechanical stress.

Температурный режим и механические напряжения в корпусированных фотоэлектрических преобразователях концентрированного солнечного излучения

In photovoltaic converters of concentrated sunlight, the thermal flow is directed from the photoactive region (p-n junction) to a heat-spreading basement through the substrate. The heat sink transfers the excess thermal to the environment by convection or cooled by a liquid carrier. Reducing the thickness of the substrate makes it possible to reduce the thermal resistance of the crystal and lower the operating temperature of the photoactive region. However, in this case, the mechanical stresses in it increase. This work discusses the balance between the mechanical strength of the sample and the decrease in its operating temperature.

Replacing tunnel junctions in InP with conduction channels with GaP crystallites

The results of investigations by the method of Electron beam-induced current of p-n-junctions based on InP with GaP crystallites in the space charge region are presented. It is shown that the introduction of crystallites into the space charge region leads to short-circuiting of the p-n-junction. The quality of the material grown on top of the crystallites allows to create of photoactive regions, as evidenced by measurements of the photoluminescence spectra. Keywords: crystallites, tunnel junction, connecting element.

Probing the properties of polymer/non-fullerene/fullerene bulk heterojunction ternary blend solar cells, study of varied blend ratios of PBDB-T:ITIC-Th:PC71BM

A major noble way to realizing high-efficiency organic solar cell device is the use of ternary blends with non-fullerene acceptors. Although offset in the band gap (E-gap) of the donor/acceptor material is negligible, it still exhibits ultra-fast and efficient charge separation. Regardless of the progress made, device optimization and fundamental understanding of the role of structural configurations in charge transport mechanisms is still on the way. The performances and stability of such devices largely depend on the nanoscale solid-state interpenetrating morphology of their donor/acceptor components within the photoactive region. Here, we analyze the effects of the second acceptor material PC71BM which is fullerene derivative on the nano-morphology within the bulk-heterojunction active layer made of PBDB-T (as donor) and non-fullerene derivative ITIC-Th (as acceptor) as well as the photovoltaic performance. Upon varying the blend ratio, 2:1:1 ratio showed a remarkable performance with more photo-generated current resulting from the favorable nano-scaled interpenetrating network with less traps as can be visualized in Fig. 1.

Замена туннельных переходов в InP на каналы проводимости с кристаллитами GaP

The results of investigations by the method of Electron beam-induced current of p-n junctions based on InP with GaP crystallites in the space charge region are presented. It is shown that the introduction of crystallites into the space charge region leads to short-circuiting of the p-n junction. The quality of the material grown on top of the crystallites allows to create of photoactive regions, as evidenced by measurements of the photoluminescence spectra.

High-Throughput Exploration of Metal Vanadate Thin-Film Systems (M-V-O, M = Cu, Ag, W, Cr, Co, Fe) for Solar Water Splitting: Composition, Structure, Stability, and Photoelectrochemical Properties.

Combinatorial synthesis and high-throughput characterization of thin-film materials libraries enable to efficiently identify both photoelectrochemically active and inactive, as well as stable and instable systems for solar water splitting. This is shown on six ternary metal vanadate (M-V-O, M = Cu, Ag, W, Cr, Co, Fe) thin-film materials libraries, fabricated using combinatorial reactive magnetron cosputtering with subsequent annealing in air. By means of high-throughput characterization of these libraries correlations between composition, crystal structure, photocurrent density, and stability of the M-V-O systems in different electrolytes such as acidic, neutral and alkaline media were identified. The systems Cu-V-O and Ag-V-O are stable in alkaline electrolyte and exhibited photocurrents of 170 and 554 μA/cm2, respectively, whereas the systems W-V-O, Cr-V-O, and Co-V-O are not stable in alkaline electrolyte. However, the Cr-V-O and Co-V-O systems showed an enlarged photoactive region in acidic electrolyte, albeit with very low photocurrents (<10 μA/cm2). Complete data sets obtained from these different screening sets, including information on nonpromising systems, lays groundwork for their use to predict new systems for solar water splitting, for example, by machine learning.

Structural and photoelectrochemical properties in the thin film system Cu-Fe-V-O and its ternary subsystems Fe-V-O and Cu-V-O.

Thin-film material libraries in the ternary and quaternary metal oxide systems Fe-V-O, Cu-V-O, and Cu-Fe-V-O were synthesized using combinatorial reactive co-sputtering with subsequent annealing in air. Their compositional, structural, and functional properties were assessed using high-throughput characterization methods. Prior to the investigation of the quaternary system Cu-Fe-V-O, the compositions (Fe61V39)Ox and (Cu52V48)Ox with promising photoactivity were identified from their ternary subsystems Fe-V-O and Cu-V-O, respectively. Two Cu-Fe-V-O material libraries with (Cu29-72Fe4-27V22-57)Ox and (Cu11-55Fe27-73V12-34)Ox composition spread were investigated. Seven mixed ternary and quaternary phase regions were identified: I (α-Cu3FeV6O26/FeVO4), II (Cu5V2O10/FeVO4/α-Cu3Fe4V6O26), III (Cu5V2O10), IV (Cu5V2O10/FeVO4, V (FeVO4/γ-Cu2V2O7/α-Cu3Fe4V6O26), VI (β-Cu2V2O7/α-Cu3Fe4V6O26/FeVO4), and VII (β-Cu3Fe4V6O26/FeVO4). In the investigated composition range, two photoactive regions, (Cu53Fe7V40)Ox and (Cu45Fe21V34)Ox, were identified, exhibiting 103 μA/cm2 and 108 μA/cm2 photocurrent density for the oxygen evolution reaction at 1.63 V vs reversible hydrogen electrode, respectively. The highest photoactive region (Cu45Fe21V34)Ox comprises the dominant α-Cu3Fe4V6O24 phase and minor FeVO4 phase. This photoactive region corresponds to having an indirect bandgap of 1.87 eV and a direct bandgap of 2.58 eV with an incident photon-to-current efficiency of 30% at a wavelength of 310 nm.