Heterojunction Nanostructures Research Articles

SnO2@TiO2 Heterojunction Nanostructures for Lithium‐Ion Batteries and Self‐Powered UV Photodetectors with Improved Performances

AbstractTo overcome the issue of inferior cycling stability and rate capacity for SnO2 anode materials in lithium‐ion batteries, an effective strategy is explored to prepare a hybrid material consisting of rutile SnO2 nanoparticles and rutile TiO2 nanorods, considering not only the small lattice mismatch to achieve a better composited lattice structure but also their superior synergistic effect in electrochemical performances. The as‐prepared SnO2@TiO2 material, directly formed on a carbon cloth as a binder‐free anode, exhibits a reversible capacity of 700 mAh g−1 after 100 discharge/charge cycles at 200 mA g−1, as well as excellent cycling stability and rate capacity. After being calcinated at high temperature, the produced hollow SnO2@TiO2 hybrid microtubes were directly used to fabricate photoelectrochemical (PEC) UV detectors for future devices with self‐powered function. A high photocurrent response of 0.1 mA cm−2 was observed, together with an excellent self‐powered and fast response and “visible blind” characteristics. Such a hybrid material could achieve a complementary effect in lithium‐ion batteries and a superior band gap match in photovoltaic devices, and could consequently be extended to applications such as dye‐sensitized solar cells and supercapacitors.

Self-Organizing Mesomorphic Diketopyrrolopyrrole Derivatives for Efficient Solution-Processed Organic Solar Cells

We describe the synthesis, self-organization, and photovoltaic properties of diketopyrrolopyrrole (DPP)-based mesomorphic small-molecule materials, 3,6-bis{5-(4-alkylphenyl)thiophen-2-yl}-2,5-di(2-ethylhexyl)-pyrrolo[3,4-c]pyrrole-1,4-diones (DPP-TP6 and DPP-TP12), which have strong visible absorption characteristics. The effects of varying terminal alkyl chains on the self-assembling properties and photovoltaic device performances have been studied. With the appropriate ratio of the lengths of the alkyl chains to its rigid DPP core, DPP-TP6 exhibits liquid-crystalline properties and forms well-developed nanostructured bulk heterojunction (BHJ) architectures with a fullerene derivative (PC71BM). Organic solar cells (OSCs) employing BHJ DPP-TP6:PC71BM films show a power conversion efficiency as high as 4.3% with a high open-circuit voltage of 0.93 V and a fill factor of 0.55. These results demonstrate that liquid-crystalline organization to direct molecular self-assembly is a promising strategy for fabrica...

High‐Performance Photoelectrochemical‐Type Self‐Powered UV Photodetector Using Epitaxial TiO2/SnO2 Branched Heterojunction Nanostructure

TiO₂/SnO₂ branched heterojunction nanostructure with TiO₂ branches on electrospun SnO2 nanofiber (B-SnO₂ NF) networks serves as a model architecture for efficient self-powered UV photodetector based on a photoelectrochemical cell (PECC). The nanostructure simultaneously offers a low degree of charge recombination and a direct pathway for electron transport. Without correcting 64.5% loss of incident photons through light absorption and scattering by the F-doped tin oxide (FTO) glass, the incident power conversion efficiency reaches 14.7% at 330 nm, more than twice as large as the nanocrystalline TiO₂ (TiO₂ NC, 6.4%)-film based PECC. By connecting a PECC to an ammeter, the intensity of UV light is quantified using the output short-circuit photocurrent density (J(sc)) without a power source. Under UV irradiation, the self-powered UV photodetector exhibits a high responsivity of 0.6 A/W, a high on/off ratio of 4550, a rise time of 0.03 s and a decay time of 0.01 s for J(sc) signal. The excellent performance of the B-SnO₂ NF-based PECC type self-powered photodetector will enable significant advancements for next-generation photodetection and photosensing applications.

Interface formation of nanostructured heterojunction SnO2:Eu/GaAs and electronic transport properties

Thin films of tin dioxide (SnO2) are deposited by the sol–gel-dip-coating technique, along with GaAs layers, deposited by the resistive evaporation technique. The as-built heterojunction has potential application in optoelectronic devices, combining the emission from the rare-earth doped transparent oxide (Eu3+-doped SnO2 presents very efficient red emission) with a high mobility semiconductor. The advantage of this structure is the possibility of separation of the rare-earth emission centers from the electron scattering, leading to a strongly indicated combination for electroluminescence. Electrical characterization of the heterojunction SnO2:Eu/GaAs shows a significant conductivity increase when compared to the conductivity of the individual films, and the monochromatic light irradiation (266nm) at low temperature of the heterojunction GaAs/SnO2:Eu leads to intense conductivity increase. Scanning electron microscopy (SEM) of the heterojunction cross section shows high adherence and good morphological quality of the interfaces substrate/SnO2 and SnO2/GaAs, even though the atomic force microscopy (AFM) image of the GaAs surface shows disordered particles, which increases with sample thickness. On the other hand, the good morphology of the SnO2:Eu surface, shown by AFM, assures the good electrical performance of the heterojunction. The observed improvement on the electrical transport properties is probably related to the formation of short conduction channels at the semiconductors interface, which may exhibit two-dimensional electron gas (2DEG) behavior.

Effect of annealing on conversion efficiency of nanostructured CdS/CuInSe2 heterojunction thin film solar cell prepared by chemical ion exchange route at room temperature

We report, the effect of air annealing on solar conversion efficiency of chemically grown nanostructured heterojunction thin films of CdS/CuInSe2, such 100, 200 and 300°C air annealed thin films characterized for physicochemical and optoelectronic properties. XRD pattern obtained from annealed thin films confirms tetragonal crystal geometry of CuInSe2 and an increase in average crystallite size from 16 to 32nm. An EDAX spectrum confirms expected and observed elemental composition in thin films. AFM represents high energy induced grain growth and agglomeration due to polygonization process. Increase in optical absorbance strength and decrease in energy band gap from 1.36 to 1.25eV is observed. Increase in charge carrier concentration from 2×1016 to 8×1017cm−3 is observed as calculated from Hall effect measurements and an enhancement in solar conversion efficiency from 0.26 to 0.47% is observed upon annealing.

Nanostructure designs for effective solar-to-hydrogen conversion

Abstract Conversion of energy from photons in sunlight to hydrogen through solar splitting of water is an important technology. The rising significance of producing hydrogen from solar light via water splitting has motivated a surge of developing semiconductor solar-active nanostructures as photocatalysts and photoelectrodes. Traditional strategies have been developed to enhance solar light absorption (e.g., ion doping, solid solution, narrow-band-gap semiconductor or dye sensitization) and improve charge separation/transport to prompt surface reaction kinetics (e.g., semiconductor combination, co-catalyst loading, nanostructure design) for better utilizing solar energy. However, the solar-to-hydrogen efficiency is still limited. This article provides an overview of recently demonstrated novel concepts of nanostructure designs for efficient solar hydrogen conversion, which include surface engineering, novel nanostructured heterojunctions, and photonic crystals. Those first results outlined in the main text encouragingly point out the prominence and promise of these new concepts principled for designing high-efficiency electronic and photonic nanostructures that could serve for sustainable solar hydrogen production.

CuInSe2 nanocrystals/CdS quantum dots/ZnO nanowire arrays heterojunction for photovoltaic applications

A low-cost and novel nanostructure heterojunction, which consists of the CuInSe2 nanocrystals (NCs)/CdS quantum dots (QDs)/ZnO nanowire (NW) arrays, is fabricated for photovoltaic applications. The ZnO NW arrays with high transmittance are used as a highly efficient channel for rapid collection of the carriers to reduce the charge recombination, as well as to increase the contact area of the heterojunction. The p-type CuInSe2 NCs are synthesized by a solvothermal method and then deposited on the CdS QDs coated ZnO NW arrays by an electrophoretic deposition technique. Results indicate that such a heterojunction configuration not only has high absorption for the incident light in the visible region but also can reduce the leakage current. A photovoltaic efficiency of the heterojunction is doubled as compared with that of the CuInSe2 NCs/CdS QDs/ZnO thin film photovoltaic devices.

Nanoimprinted Polymer Solar Cell

Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. A vertically bicontinuous and interdigitized heterojunction between donor and acceptor has been regarded as one of the ideal structures to enable both efficient charge separation and transport. Synergistic control of polymer orientation in the nanostructured heterojunction is also critical to improve the performance of polymer solar cells. Nanoimprint lithography has emerged as a new approach to simultaneously control both the heterojunction morphology and polymer chains in organic photovoltaics. Currently, in the area of nanoimprinted polymer solar cells, much progress has been achieved in the fabrication of nanostructured morphology, control of molecular orientation/crystallinity, deposition of acceptor materials, patterned electrodes, understanding of structure-property correlations, and device performance. This review article summarizes the recent studies on nanoimprinted polymer solar cells and discusses the outstanding challenges and opportunities for future work.

All inorganic iron pyrite nano-heterojunction solar cells

The large absorption coefficient of iron pyrite (FeS(2)) nanocrystals coupled with their low-cost and vast-abundance shows great promise as a potential photovoltaic absorber. Here, we demonstrate that bulk heterojunction (BHJ) nanostructures consisting of 80 nm FeS(2) nanocubes (NCs) and 4 nm CdS quantum dot (QD) matrix, lead to a well-defined percolation network, which significantly improved open-circuit voltage (V(oc)) to 0.79 V and power conversion efficiency of 1.1% under AM 1.5 solar illumination. The localized surface plasmon resonances (LSPRs) arising from p-type colloidal FeS(2) NCs exhibit plasmonic photoelectron conversion. Our approach can be applied to a wide range of colloidal nanocrystals exhibiting the LSPRs effect and is compatible with solution processing, thereby offering a general tactic to enhancing the efficiency of all inorganic BHJ solar cells and LSPRs-based NIR photodetectors.

Improvement of photocatalytic activity of TiO2 nanoparticles on selectively reconstructed layered double hydroxide

Supported TiO2 nanoparticles have been successfully fabricated by selective reconstruction of a Cu2+, Mg2+, Al3+, Ti4+-containing layered double hydroxide (CuMgAlTi-LDH) precursor, synthesized by coprecipitation, through calcination and rehydration process. A systematic investigation of the structural characterization and the photodegradation tests of methylene blue (MB) dye molecules from solution under both UV and visible light irradiation for the resulting TiO2/CuMgAl-RLDH sample were carried out. Anatase-type TiO2 nanoparticles are found to be homogeneously distributed on the surface of the selectively reconstructed CuMgAl-RLDH support. And the direct evidence for the surface TiO2/LDH heterojunction formed on CuMgAl-RLDH is presented. For MB photodegradation under UV light or visible light illumination, TiO2/CuMgAl-RLDH sample has superior photocatalytic properties to the rehydrated single phase R-TiO2, the physical mixture of R-TiO2 and CuMgAl-RLDH, the composite CuTi/MgAl-RLDH synthesized by the rehydration of mixture of MgAl-MMO and CuTi-MMO, and the TiO2/MgAl-RLDH prepared under the same procedure as TiO2/CuMgAl-RLDH without containing Cu ions. The skeleton Cu2+ ions dispersed in the mainlayer of CuMgAl-RLDH support can enable the photocatalytic activity for MB photodegradation. The TiO2/LDH heterojunction nanostructure is proposed to contribute the efficient spatial separation between the photogenerated electrons and holes, which can concomitantly improve the photocatalytic activity. Our method provides a novel approach to fabricate new modes of load-type doped semiconductor photocatalysts which are both active under illumination by UV and visible light.

Effect of the frequency and temperature on the complex impedance spectroscopy (C–V and G–V) of p-ZnGa2Se4/n-Si nanostructure heterojunction diode

X-ray diffraction pattern and AFM results confirm the nanostructure of p-ZnGa2Se4/n-Si. The unit cell lattice parameters, the crystallite size L, the dislocation density δ, and the main internal strain e were calculated. The temperature and frequency-dependent electrical characteristics of the Al/p-ZnGa2Se4/n-Si/Al heterojunction diode (HJD) have been investigated to determine the interface states which are responsible for the non-ideal behavior of the characteristics of the diode. The capacitance–voltage (C–V), conductance–voltage (G–V), and series resistance–voltage (Rs–V) characteristics of the diode have been analyzed in the frequency range of 5 kHz–1 MHz and temperature range of 303–423 K. The interfaces states of the diode were determined using conductance–voltage technique. The interface state density profile for the diode was obtained as a function of temperature and frequency. The values of the built-in potential Vbi, the doping concentration Nd and the barrier height φb(C–V) of the diode were calculated at different temperatures and frequencies. Our experimental results revealed that both the series resistance and interface state density values must be taken into account in studying the impedance spectroscopy of HJD to stand up their performance for electronic applications characteristics.

Hybrid nanostructure heterojunction solar cells fabricated using vertically aligned ZnOnanotubes grown on reduced graphene oxide

Using reduced graphene oxide (rGO) films as the transparent conductive coating,inorganic/organic hybrid nanostructure heterojunction photovoltaic devices have beenfabricated through hydrothermal synthesis of vertically aligned ZnO nanorods(ZnO-NRs) and nanotubes (ZnO-NTs) on rGO films followed by the spin casting of apoly(3-hexylthiophene) (P3HT) film. The data show that larger interfacial area inZnO-NT/P3HT composites improves the exciton dissociation and the higher electrodeconductance of rGO films helps the power output. This study offers an alternative tomanufacturing nanostructure heterojunction solar cells at low temperatures usingpotentially low cost materials.

Effect of annealing on photovoltaic characteristics of nanostructured p-Cu 2S/n-CdS thin film

Nanostructured heterojunction solar cells of CdS/Cu 2S were grown by simple chemical ion exchange reaction by immersing the CdS thin films in Cu ion solution at room temperature. The as-grown nanostructured thin films were annealed at 250 °C in air for improving the interface and crystallinity of the heterojunction. These as grown and annealed nanostructured heterojunction were characterized for photovoltaic and other optoelectronic characteristics. Increase in conversion efficiency is obtained from annealed ( η = 0.24%) than the as grown heterojunction ( η = 0.09%), on illuminating to 100 mW/cm 2 light source. The X-ray diffraction (XRD) pattern confirms improvement in crystallinity with increase in crystallite size from 29 to 32 nm on annealing. The optical absorbance strength observed to be increased along with red shift in energy band gap value from E g = 2.02–2.20 eV after annealing.

ZnO nanorod array/CuAlO2 nanofiber heterojunction on Ni substrate: synthesis and photoelectrochemical properties

A novel ZnO nanorod array (NR)/CuAlO2 nanofiber (NF) heterojunction nanostructure was grown on a substrate of Ni plates using sol–gelsynthesis for the NFs and hydrothermal reaction for the NRs. Compared with a traditionalZnO/CuAlO2 laminar film nanostructure, the photocurrent of this fibrous network heterojunction issignificantly increased. A significant blue-shift of the absorption edge anda favorable forward current to reverse current ratio at applied voltages of − 2 to + 2 V were observed in this heterojunction with the increase ofZn2 + ion concentration in the hydrothermal reaction. Furthermore, thephotoelectrochemical properties were investigated and the highest photocurrent of 3.1 mA cm − 2 was obtained under AM 1.5 illumination with 100 mW cm − 2 light intensity at 0.71 V (versus Ag/AgCl). This novel 3D fibrous network nanostructureplays an important role in the optoelectronic field and can be extended to other binary orternary oxide compositions for various applications.

Synthesis of High-Activity TiO2-Based Photocatalysts by Compounding a Small Amount of Porous Nanosized LaFeO3 and the Activity-Enhanced Mechanisms

In this work, a feasible strategy has been developed to fabricate highly active TiO2-based photocatalysts for degrading rhodamine B solution under ultraviolet or visible irradiation. The key of this strategy is to compound a small amount of porous LaFeO3 to nanocrystalline TiO2, leading to the delay of anatase-rutile phase tansformation and then to the formation of the anatase TiO2/rutile TiO2/LaFeO3 three-phase nanostructured heterojunctions after thermal treatment at a high temperature of 800 °C. The resulting three-phase heterojunctions greatly favor the photoinduced charge separation, especially promoting the visible induced electrons of LaFeO3 captured by the adsorbed O2, based on their surface photovoltage responses, which is well responsible for the ultraviolet or visible activity enhancement. This work would provide a feasible route to fabricate high-performance TiO2-based nanomaterials.

Stoichiometry controlled conversion efficiency in nanostructured heterojunction solar cell of CdS/CuInSXSe2−X grown by chemical ion exchange method at room temperature

Abstract Here in the present paper, we report on growth of stoichiometric and nonstoichiometric nanostructured heterojunction solar cell of CdS/CuInS X Se 2– X varying X from 0 to 2 in the interval of 0.5 using cost effective, simple, chemical ion exchange method at room temperature on ITO glass substrate. The as-grown varying composition solar cells annealed at 200 °C in air and characterized for structural, compositional, optical and illumination studies. The X-ray diffraction pattern obtained from CdS/CuInS X Se 2– X solar cell confirms the formation of CuInSe 2 , CuInS 0.5 Se 1.5 , CuInS 1 Se 1 , CuInS 1.5 Se 0.5 and CuInS 2 phases having tetragonal structure with varying crystallite size from 19, 19.37, 28, 33 and 20 nm respectively. The energy dispersive X-ray analysis (EDAX) confirms the expected elemental composition in the heterojunction solar cell. Optical absorbance analysis confirms composition controlled electronic transitions in the thin films while energy band gap observed to be red shifted with increase the value of X . The solar energy conversion efficiency achieved upon illuminating to 100 mW/cm 2 observed to be 0.27%, 0.06%, 0.17%, 0.02% and 0.23% for CuInSe 2 , CuInS 0.5 Se 1.5 , CuInS 1 Se 1 , CuInS 1.5 Se 0.5 and CuInS 2 respectively, which correspond for stoichiometric dependent electron–hole pair generation and separation phenomenon.

Poly(3-hexylthiophene)-block-poly(pyridinium phenylene)s: Block Polymers of p- and n-Type Semiconductors

Conjugated crystalline−crystalline donor−acceptor−donor block copolymer semiconductors, with regioregular poly(3-hexylthiophene) as a donor (p-type) block and poly(pyridinium pheneylene) as an acceptor (n-type) block within the backbone, were produced by sequential Grignard metathesis synthesis of poly(3-hexylthiophene), a Yamamoto-type cross-coupling polymerization−cyclization sequence. These conjugated block copolymers are soluble in organic solvents and display broad optical absorption bands extending close to the near-infrared region. They show reversible ambipolar redox properties with high electron affinities of 3.8−4.0 eV as well as useful ionization potentials of 5.1 eV that are characteristic of the respective blocks. Block copolymers from p- and n-type organic semiconductors are of interest for the formation of nanostructured bulk heterojunctions in photovoltaic devices.

Fabrication and photocatalytic properties of novel ZnO/ZnAl2O4 nanocomposite with ZnAl2O4 dispersed inside ZnO network

AbstractNovel ZnO/ZnAl2O4 nanocomposites with ZnAl2O4 nanoparticles homogeneously dispersed inside a network of ZnO are fabricated by thermal treatment of a single‐source precursor of ZnAl‐layered double hydroxides (ZnAl‐LDHs) at 800°C. The effects of the Zn/Al molar ratio of the LDH precursors on the structure, composition, morphology, textural as well as UV‐absorbing properties and photocatalytic activities of the nanocomposites are investigated in detail. The results show that the ZnO/ZnAl2O4 nanocomposites derived from the ZnAl‐LDHs precursors have superior photocatalytic performances to either single phase ZnO or similar ZnO/ZnAl2O4 samples fabricated by chemical coprecipitation or physical mixing method. The heterojunction nanostructure and the strong coupling between the ZnO and ZnAl2O4 phase derived from ZnAl‐LDHs precursors are proposed to contribute the efficient spatial separation between the photo‐generated electrons and holes, which can concomitantly improve the photocatalytic activities. © 2011 American Institute of Chemical Engineers AIChE J, 2012

Solution-phase synthesis of metal and/or semiconductor homojunction/heterojunction nanomaterials

The design and architecture of programmable metal-semiconductor nanostructures with excellent optoelectronic properties from metal and semiconductor building blocks with nanoscale dimensions have been a key aim of material scientists due to their central roles in the fabrication of electronic, optical, and optoelectronic nanodevices. This review focuses on the latest advances in the solution-phase synthesis of metal and/or semiconductor homojunction/heterojunction nanomaterials. It begins with the simplest construction of metal/metal and semiconductor/semiconductor homojunctions, and then highlights the synthetic design of metal/metal and semiconductor/semiconductor heterojunction nanostructures with different building blocks. Special emphasis is placed on metal/semiconductor heterojunction nanomaterials, which are the most challenging and promising nanomaterials for future applications in optoelectronic nanodevices. Finally, this review concludes with personal perspectives on the directions for future research in this field.

Strong Efficiency Improvements in Ultra‐low‐Cost Inorganic Nanowire Solar Cells

Cu2O-ZnO nanowire solar cells are synthesized by electrodeposition from solutions near room temperature. A high-quality, self-assembling, nanostructured heterojunction is produced, which is found to dramatically improve the charge collection efficiency in these ultra-low-cost devices. Incident-photon-to-electron conversion efficiencies approaching unity are measured and considerable efficiency improvements are observed.