Improved Hybrid Solar Cells via in situ UV Polymerization

One approach for making inexpensive inorganic–organic hybrid photovoltaic (PV) cells is to fill highly ordered TiO₂ nanotube (NT) arrays with solid organic hole conductors such as conjugated polymers. Here, a new in situ UV polymerization method for growing polythiophene (UV‐PT) inside TiO₂ NTs is presented and compared to the conventional approach of infiltrating NTs with pre‐synthesized polymer. A nanotubular TiO₂ substrate is immersed in a 2,5‐diiodothiophene (DIT) monomer precursor solution and then irradiated with UV light. The selective UV photodissociation of the C? I bond produces monomer radicals with intact π‐ring structure that further produce longer oligothiophene/PT molecules. Complete photoluminescence quenching upon UV irradiation suggests coupling between radicals created from DIT and at the TiO₂ surface via a charge transfer complex. Coupling with the TiO₂ surface improves UV‐PT crystallinity and π–π stacking; flat photocurrent values show that charge recombination during hole transport through the polymer is negligible. A non‐ideal, backside‐illuminated setup under illumination of 620‐nm light yields a photocurrent density of ≈5 µA cm²—surprisingly much stronger than with comparable devices fabricated with polymer synthesized ex situ. Since in this backside architecture setup we illuminate the cell through the Ag top electrode, there is a possibility for Ag plasmon‐enhanced solar energy conversion. By using this simple in situ UV polymerization method that couples the conjugated polymer to the TiO₂ surface, the absorption of sunlight can be improved and the charge carrier mobility of the photoactive layer can be enhanced.     

Surface plasmonic effect of nanoparticle-like silver nanostructure on the high responsivity of visible/infrared silver-based heterojunction photodetector

The fabrication of a silver (Ag) based photodetector on a silicon dioxide/p-silicon (SiO₂/p-Si) substrate using direct current (DC) magnetron sputtering is demonstrated. The proposed method deposits a nanoparticle-like Ag thin film that favours the photoconduction mechanism under light illumination at 468?nm and laser illumination at 660 and 980?nm. The thin film is characterized using scanning electron microscope (SEM), energy dispersive x-ray (EDX), x-ray diffraction (XRD), Raman scattering, photoluminescence (PL) and ultraviolet–visible (UV–VIS) analysis. Current–voltage (I–V) analysis and the calculated rectifying ratio (RR) suggests the establishment of good Schottky contacts for incident light/laser at 468, 660 and 980?nm, with good responsivity towards light and laser illumination in the forward and reverse DC bias regions. The responsivity increases as the wavelength decreases from 980?nm → 660?nm → 468?nm, with the highest responsivity of 213.7?mAW^?1 at 468?nm indicating better photoconduction at low light powers.     

GaP/GaNP Heterojunctions for Efficient Solar‐Driven Water Oxidation

The growth and characterization of an n‐GaP/i‐GaNP/p⁺‐GaP thin film heterojunction synthesized using a gas‐source molecular beam epitaxy (MBE) method, and its application for efficient solar‐driven water oxidation is reported. The TiO₂/Ni passivated n‐GaP/i‐GaNP/p⁺‐GaP thin film heterojunction provides much higher photoanodic performance in 1 m KOH solution than the TiO₂/Ni‐coated n‐GaP substrate, leading to much lower onset potential and much higher photocurrent. There is a significant photoanodic potential shift of 764 mV at a photocurrent of 0.34 mA cm^?2, leading to an onset potential of ≈0.4 V versus reversible hydrogen electrode (RHE) at 0.34 mA cm^?2 for the heterojunction. The photocurrent at the water oxidation potential (1.23 V vs RHE) is 1.46 and 7.26 mA cm^?2 for the coated n‐GaP and n‐GaP/i‐GaNP/p⁺‐GaP photoanodes, respectively. The passivated heterojunction offers a maximum applied bias photon‐to‐current efficiency (ABPE) of 1.9% while the ABPE of the coated n‐GaP sample is almost zero. Furthermore, the coated n‐GaP/i‐GaNP/p⁺‐GaP heterojunction photoanode provides a broad absorption spectrum up to ≈620 nm with incident photon‐to‐current efficiencies (IPCEs) of over 40% from ≈400 to ≈560 nm. The high low‐bias performance and broad absorption of the wide‐bandgap GaP/GaNP heterojunctions render them as a promising photoanode material for tandem photoelectrochemical (PEC) cells to carry out overall solar water splitting.     

ZnO Film UV Photodetector with Enhanced Performance: Heterojunction with CdMoO4 Microplates and the Hot Electron Injection Effect of Au Nanoparticles

A novel CdMoO₄–ZnO composite film is prepared by spin‐coating CdMoO₄ microplates on ZnO film and is constructed as a heterojunction photodetector (PD). With an optimized loading amount of CdMoO₄ microplates, this composite film PD achieves a ≈18‐fold higher responsivity than pure ZnO film PD at 5 V bias under 350 nm (0.15 mW cm⁻²) UV light illumination, and its decay time shortens to half of the original value. Furthermore, Au nanoparticles are then deposited to modify the CdMoO₄–ZnO composite film, and the as‐constructed photodetector with an optimized deposition time of Au nanoparticles yields an approximately two‐fold higher photocurrent under the same condition, and the decay time reduces by half. The introduced CdMoO₄ microplates form type‐II heterojunctions with ZnO film and improve the photoelectric performance. The hot electrons from Au nanoparticles are injected into the CdMoO₄–ZnO composite film, leading to the increased photocurrent. When the light is off, the Schottky barriers formed between Au nanoparticles and CdMoO₄–ZnO composite film block the carrier transportation and accelerate the decay process of current. The study on Au‐nanoparticle‐modified CdMoO₄–ZnO composite film provides a facile method to construct ZnO film based PD with novel structure and high photoelectric performance.     

Light Confinement Effect Induced Highly Sensitive,Self‐Driven Near‐Infrared Photodetector and Image Sensor Based on Multilayer PdSe2/Pyramid Si Heterojunction

In this study, a highly sensitive and self‐driven near‐infrared (NIR) light photodetector based on PdSe₂/pyramid Si heterojunction arrays, which are fabricated through simple selenization of predeposited Pd nanofilm on black Si, is demonstrated. The as‐fabricated hybrid device exhibits excellent photoresponse performance in terms of a large on/off ratio of 1.6 × 10⁵, a responsivity of 456 mA W^?1, and a high specific detectivity of up to 9.97 × 10¹³ Jones under 980 nm illumination at zero bias. Such a relatively high sensitivity can be ascribed to the light trapping effect of the pyramid microstructure, which is confirmed by numerical modeling based on finite‐difference time domain. On the other hand, thanks to the broad optical absorption properties of PdSe₂, the as‐fabricated device also exhibits obvious sensitivity to other NIR illuminations with wavelengths of 1300, 1550, and 1650 nm, which is beyond the photoresponse range of Si‐based devices. It is also found that the PdSe₂/pyramid Si heterojunction device can also function as an NIR light sensor, which can readily record both “tree” and “house” images produced by 980 and 1300 nm illumination, respectively.     

Au Nanoclusters Sensitized Black TiO2−x Nanotubes for Enhanced Photodynamic Therapy Driven by Near‐Infrared Light

The low reactive oxygen species production capability and the shallow tissue penetration of excited light (UV) are still two barriers in photodynamic therapy (PDT). Here, Au cluster anchored black anatase TiO_2?x nanotubes (abbreviated as Au₂₅/B‐TiO_2?x NTs) are synthesized by gaseous reduction of anatase TiO₂ NTs and subsequent deposition of noble metal. The Au₂₅/B‐TiO_2?x NTs with thickness of about 2 nm exhibit excellent PDT performance. The reduction process increased the density of Ti³⁺ on the surface of TiO₂, which effectively depresses the recombination of electron and hole. Furthermore, after modification of Au₂₅ nanoclusters, the PDT efficiency is further enhanced owing to the changed electrical distribution in the composite, which forms a shallow potential well on the metal–TiO₂ interface to further hamper the recombination of electron and hole. Especially, the reduction of anatase TiO₂ can expend the light response range (UV) of TiO₂ to the visible and even near infrared (NIR) light region with high tissue penetration depth. When excited by NIR light, the nanoplatform shows markedly improved therapeutic efficacy attributed to the photocatalytic synergistic effect, and promotes separation or restrained recombination of electron and hole, which is verified by experimental results in vitro and in vivo.     

In Situ Fabrication of Vertical Multilayered MoS2/Si Homotype Heterojunction for High‐Speed Visible–Near‐Infrared Photodetectors

c2D transition metal dichalcogenides (TMDCs)‐based heterostructures have been demonstrated to achieve superior light absorption and photovoltaic effects theoretically and experimentally, making them extremely attractive for realizing optoelectronic devices. In this work, a vertical multilayered n‐MoS₂/n‐silicon homotype heterojunction is fabricated, which takes advantage of multilayered MoS₂ grown in situ directly on plane silicon. Electrical characterization reveals that the resultant device exhibits high sensitivity to visible–near‐infrared light with responsivity up to 11.9 A W^–1. Notably, the photodetector shows high‐speed response time of ≈30.5 µs/71.6 µs and capability to work under higher pulsed light irradiation approaching 100 kHz. The high response speed could be attributed to a good quality of the multilayer MoS₂, as well as in situ device fabrication process. These findings suggest that the multilayered MoS₂/Si homotype heterojunction have great potential application in the field of visible–near‐infrared detection and might be used as elements for construction of high‐speed integrated optoelectronic sensor circuitry.     

Gate-Tunable van der Waals Photodiodes with an Ultrahigh Peak-to-Valley Current Ratio

Photodetectors and imagers based on 2D layered materials are currently subject to a rapidly expanding application space, with an increasing demand for cost-effective and lightweight devices. However, the underlying carrier transport across the 2D homo- or heterojunction channel driven by the external electric field, like a gate or drain bias, is still unclear. Here, a visible-near infrared photodetector based on van der Waals stacked molybdenum telluride (MoTe₂) and black phosphorus (BP) is reported. The type-I and type-II band alignment can be tuned by the gate and drain voltage combined showing a dynamic modulation of the conduction polarity and negative differential transconductance. The heterojunction devices show a good photoresponse to light illumination ranging from 520–2000 nm. The built-in potential at the MoTe₂/BP interface can efficiently separate photoexcited electron–hole pairs with a high responsivity of 290 mA W⁻¹, an external quantum efficiency of 70%, and a fast photoresponse of 78 µs under zero bias.     

Efficient Self‐Driven Photodetectors Featuring a Mixed‐Dimensional van der Waals Heterojunction Formed from a CdS Nanowire and a MoTe2 Flake

Heterojunctions formed from low‐dimensional materials can result in photovoltaic and photodetection devices displaying exceptional physical properties and excellent performance. Herein, a mixed‐dimensional van der Waals (vdW) heterojunction comprising a 1D n‐type Ga‐doped CdS nanowire and a 2D p‐type MoTe₂ flake is demonstrated; the corresponding photovoltaic device exhibits an outstanding conversion efficiency of 15.01% under illumination with white light at 650 µW cm^?2. A potential difference of 80 meV measured, using Kelvin probe force microscopy, at the CdS–MoTe₂ interface confirms the separation and accumulation of photoexcited carriers upon illumination. Moreover, the photodetection characteristics of the vdW heterojunction device at zero bias reveal a rapid response time (<50 ms) and a photoresponsivity that are linearly proportional to the power density of the light. Interestingly, the response of the vdW heterojunction device is negligible when illuminated at 580 nm; this exceptional behavior is presumably due to the rapid rate of recombination of the photoexcited carriers of MoTe₂. Such mixed‐dimensional vdW heterojunctions appear to be novel design elements for efficient photovoltaic and self‐driven photodetection devices.     

An Electrically Modulated Single-Color/Dual-Color Imaging Photodetector

An electrically modulated single-/dual-color imaging photodetector with fast response speed is developed based on a small molecule (CO_i8DFIC)/perovskite (CH₃NH₃PbBr₃) hybrid film. Owing to the type-I heterojunction, the device can facilely transform dual-color images to single-color images by applying a small bias voltage. The photodetector exhibits two distinct cut-off wavelengths at ≈544 nm (visible region) and ≈920 nm (near-infrared region), respectively, without any power supply. Its two peak responsivities are 0.16 A W⁻¹ at ≈525 nm and 0.041 A W⁻¹ at ≈860 nm with a fast response speed (≈10² ns). Under 0.6 V bias, the photodetector can operate in a single-color mode with a peak responsivity of 0.09 A W⁻¹ at ≈475 nm, showing a fast response speed (≈10² ns). A physical model based on band energy theory is developed to illustrate the origin of the tunable single-/dual-color photodetection. This work will stimulate new approaches for developing solution-processed multifunctional photodetectors for imaging photodetection in complex circumstances.     

Ultrahigh Responsivity UV Photodetector Based on Cu Nanostructure/ZnO QD Hybrid Architectures

Strong near‐surface electromagnetic field formed by collective oscillation of electrons on Cu nanostructure a shows a strong dependence on geometry, offering a promising approach to boost the light absorption of ZnO photoactive layers with enhanced plasmon scattering. Here, a facile way to fabricate UV photodetectors with tunable configuration of the self‐assembled Cu nanostructures on ZnO thin films is reported. The incident lights are effectively confined in ZnO photoactive layers with the existence of the uplayer Cu nanostructures, and the interdiffusion of Cu atoms during fabrication of the Cu nanostructures can improve the carrier transfer in ZnO thin films. The optical properties of the hybrid architectures are successfully tailored over a control of the geometric evolution of the Cu nanostructures, resulting in significantly enhanced photocurrent and responsivity of 2.26 mA and 234 A W^?1 under a UV light illumination of 0.62 mW cm^?2 at 10 V, respectively. The photodetectors also exhibit excellent reproducibility, stability, and UV–visible rejection ratio (R_{370 nm}/R_{500 nm}) of ≈370, offering an approach of high‐performance UV photodetectors for practical applications.     

Novel construction of CdS-encapsulated TiO2 nano test tubes corked with ZnO nanorods

A novel TiO₂ nanotube array/CdS nanoparticle/ZnO nanorod (TiO₂ NT/CdS/ZnO NR) photocatalyst was constructed by chemical assembling CdS into the TiO₂ NTs, and then laying ZnO NRs on the surface. The SEM results showed that the TiO₂ NTs looked like many “nano test tubes” and the ZnO NRs served as the corks to seal the nozzle. This photocatalyst exhibited a wide absorption range (200-535 nm) in both ultraviolet and visible regions (UV-vis region), and maintained very high photoelectrocatalytic (PEC) activities. The maximum photoelectric conversion efficiencies (η) of TiO₂ NT/CdS/ZnO NRs are 31.8 and 5.98% under UV light (365 nm) and visible light (420-800 nm), respectively.     

Facile Synthesis of γ‐In2Se3 Nanoflowers toward High Performance Self‐Powered Broadband γ‐In2Se3/Si Heterojunction Photodiode

An effective colloidal process involving the hot‐injection method is developed to synthesize uniform nanoflowers consisting of 2D γ‐In₂Se₃ nanosheets. By exploiting the narrow direct bandgap and high absorption coefficient in the visible light range of In₂Se₃, a high‐quality γ‐In₂Se₃/Si heterojunction photodiode is fabricated. This photodiode shows a high photoresponse under light illumination, short response/recovery times, and long‐term durability. In addition, the γ‐In₂Se₃/Si heterojunction photodiode is self‐powered and displays a broadband spectral response ranging from UV to IR with a high responsivity and detectivity. These excellent performances make the γ‐In₂Se₃/Si heterojunction very interesting as highly efficient photodetectors.     

Solution Processable 1D Fullerene C60 Crystals for Visible Spectrum Photodetectors

Visible spectrum photodetector devices fabricated using molecular crystals of carbon C₆₀ are reported. The devices operate efficiently, extending over and beyond the full visible light spectrum (300–710 nm) with a bias voltage tunable responsivity of 4 mA–0.5 mA W^?1. Across this range of wavelengths, the noise equivalent power of these devices remains below 10² nW Hz^?1/2, providing a detectivity of 10⁷ Jones. The noise current in these devices is found to have a strong dependence on both bias voltage and frequency, varying by 4 orders of magnitude from 1 nA Hz^?1/2 to 0.1 pA Hz^?1/2. The devices also display a near‐linear dependence of photocurrent on light intensity over 4 orders of magnitude, providing a dynamic range approaching 80 dB. The 3 dB bandwidth of the devices is found to be above 10² Hz, while the 18 dB bandwidth exceeds 1 kHz. The transient photocurrents of the devices have a rise time of ≈50 µs and a long fall time of ≈4 ms. The spectral photocurrent of the devices is found to quench gradually with a reduction in temperature from ≈300 K and is fully quenched at temperatures below T ≈ 100 K. Upon reheating, the device performance is fully recovered.     

ZnO Coated Anodic 1D TiO2 Nanotube Layers: Efficient Photo‐Electrochemical and Gas Sensing Heterojunction

The authors demonstrate, in this work, a fascinating synergism of a high surface area heterojunction between TiO₂ in the form of ordered 1D anodic nanotube layers of a high aspect ratio and ZnO coatings of different thicknesses, produced by atomic layer deposition. The ZnO coatings effectively passivate the defects within the TiO₂ nanotube walls and significantly improve their charge carrier separation. Upon the ultraviolet and visible light irradiation, with an increase of the ZnO coating thickness from 0.19 to 19 nm and an increase of the external potential from 0.4–2 V, yields up to 8‐fold enhancement of the photocurrent density. This enhancement translates into extremely high incident photon to current conversion efficiency of ≈95%, which is among the highest values reported in the literature for TiO₂ based nanostructures. In addition, the photoactive region is expanded to a broader range close to the visible spectral region, compared to the uncoated nanotube layers. Synergistic effect arising from ZnO coated TiO₂ nanotube layers also yields an improved ethanol sensing response, almost 11‐fold compared to the uncoated nanotube layers. The design of the high‐area 1D heterojunction, presented here, opens pathways for the light‐ and gas‐assisted applications in photocatalysis, water splitting, sensors, and so on.

     

Photoelectrochemical Water Splitting Using Dense and Aligned TiO2 Nanorod Arrays

Dense and aligned TiO₂ nanorod arrays are fabricated using oblique‐angle deposition on indium tin oxide (ITO) conducting substrates. The TiO₂ nanorods are measured to be 800–1100 nm in length and 45–400 nm in width with an anatase crystal phase. Coverage of the ITO is extremely high with 25 × 10⁶ mm^?2 of the TiO₂ nanorods. The first use of these dense TiO₂ nanorod arrays as working electrodes in photoelectrochemical (PEC) cells used for the generation of hydrogen by water splitting is demonstrated. A number of experimental techniques including UV/Vis absorption spectroscopy, X‐ray diffraction, high‐resolution scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and photoelectrochemistry are used to characterize their structural, optical, and electronic properties. Both UV/Vis and incident‐photon‐to‐current‐efficiency measurements show their photoresponse in the visible is limited but with a marked increase around ≈400 nm. Mott–Schottky measurements give a flat‐band potential (V_FB) of +0.20 V, a carrier density of 4.5 × 10¹⁷ cm^?3, and a space‐charge layer of 99 nm. Overall water splitting is observed with an applied overpotential at 1.0 V (versus Ag/AgCl) with a photo‐to‐hydrogen efficiency of 0.1%. The results suggest that these dense and aligned one‐dimensional TiO₂ nanostructures are promising for hydrogen generation from water splitting based on PEC cells.     

Ultrahigh‐Performance Self‐Powered Flexible Double‐Twisted Fibrous Broadband Perovskite Photodetector

Self‐powered flexible photodetectors without an external power source can meet the demands of next‐generation portable and wearable nanodevices; however, the performance is far from satisfactory becuase of the limited match of flexible substrates and light‐sensitive materials with proper energy levels. Herein, a novel self‐powered flexible fiber‐shaped photodetector based on double‐twisted perovskite–TiO₂–carbon fiber and CuO–Cu₂O–Cu wire is designed and fabricated. The device shows an ultrahigh detectivity of 2.15 × 10¹³ Jones under the illumination of 800 nm light at zero bias. CuO–Cu₂O electron block bilayer extends response range of perovskite from 850 to 1050 nm and suppresses dark current down to 10^?11 A. The fast response speed of less than 200 ms is nearly invariable after dozens of cycles of bending at the extremely 90 bending angle, demonstrating excellent flexibility and bending stability. These parameters are comparable and even better than reported flexible and even rigid photodetectors. The present results suggest a promising strategy to design photodetectors with integrated function of self‐power, flexibility, and broadband response.     

Nanostructured mesoporous Au/TiO2 for photocatalytic degradation of a textile dye: the effect of size similarity of the deposited Au with that of TiO2 pores

Mesoporous Au/TiO₂ nanocomposites with different Au particle size (7.3–11.8 nm) were synthesized via deposition–precipitation method. The synthesized nanocomposites have been characterized by XRD, TEM, XPS, DLS, ICP-OES, N₂ sorpometry, and UV–Vis spectroscopy. Au/TiO₂ showed higher quantum yield and greater photocatalytic efficiency compared to pure TiO₂ under both ultraviolet and sunlight illumination. The increase of the photocatalytic efficiency of TiO₂ upon deposition with gold nanoparticles, Au NPs, is due to the interface electron transfer from Au nanoparticles to TiO₂ under visible light illumination and from TiO₂ to Au nanoparticles under UV illumination. For the first time, the effect of Au particle sizes when it is very similar to the interparticles pores of TiO₂ has been investigated. The highest reaction rate (5.7 × 10^?2 min^?1) and degradation efficiency of Safranin-O (SO) dye (97 %) were observed when the deposited gold nanoparticles are the smallest among the studied samples (sAu/TiO₂). In spite of blocking a high percentage of the TiO₂ pores, the sAu/TiO₂ sample demonstrated a complete degradation of SO dye in 50 min which is more efficient than any other reported catalysts in the literature.     

Liquid‐Alloy‐Assisted Growth of 2D Ternary Ga2In4S9 toward High‐Performance UV Photodetection

2D ternary systems provide another degree of freedom of tuning physical properties through stoichiometry variation. However, the controllable growth of 2D ternary materials remains a huge challenge that hinders their practical applications. Here, for the first time, by using a gallium/indium liquid alloy as the precursor, the synthesis of high‐quality 2D ternary Ga₂In₄S₉ flakes of only a few atomic layers thick (≈2.4 nm for the thinnest samples) through chemical vapor deposition is realized. Their UV‐light‐sensing applications are explored systematically. Photodetectors based on the Ga₂In₄S₉ flakes display outstanding UV detection ability (R _λ = 111.9 A W^?1, external quantum efficiency = 3.85 × 10⁴%, and D* = 2.25 × 10¹¹ Jones@360 nm) with a fast response speed (τ_ring ≈ 40 ms and τ_decay ≈ 50 ms). In addition, Ga₂In₄S₉‐based phototransistors exhibit a responsivity of ≈10⁴ A W^?1@360 nm above the critical back‐gate bias of ≈0 V. The use of the liquid alloy for synthesizing ultrathin 2D Ga₂In₄S₉ nanostructures may offer great opportunities for designing novel 2D optoelectronic materials to achieve optimal device performance.     

Synthesis of TiO2 nanotubes coupled with CdS nanoparticles and production of hydrogen by photocatalytic water decomposition

The CdS/TiO₂NTs composite was prepared by a simple two-step chemical solution routes to directly transfer trititanate nanotubes to TiO₂NTs and simultaneously coupled with CdS nanoparticles. The results of XRD, TEM, Diffuse reflectance UV-Visible absorption spectra revealed that the CdS nanoparticles were homogeneously embedded on the surface of TiO₂NTs and the absorption spectrum of TiO₂NTs was extended to visible region. The activity of hydrogen production by photocatalytic water decomposition for the CdS/TiO₂NTs composite was examined under visible light irradiation (λ > 400 nm) and the quantity of H₂ evolution was ca. 1708 μL/g for 6 h.