CuInS2 Nanocrystals Research Articles

Single Source Precursor Chemical Vapor Decomposition Method to Fabricate Stable, Bright Emissive Aluminum Hydroxide Phosphors for UV‐Pumped White Light‐Emitting Devices

AbstractAluminum hydroxides are considered to be a potential abundant, low‐cost blue emissive material to construct UV‐pumped white light‐emitting diodes (WLEDs). In this work, a single‐source precursor chemical vapor decomposition method is adapted to fabricate stable, strongly emissive aluminum hydroxide phosphors with photoluminescence quantum yield of up to 69% in solid state by thermal decomposing aluminum diacetate hydroxide (Al(OH)(Ac)2) at temperatures (260–360 °C) in N2 atmosphere. It exhibits layered, amorphous structure and its emission of aluminum hydroxide is enhanced by electron‐donating surface hydroxyl groups and originates from F+ centers and carbon‐related defects contributing to UV (388 nm) and blue (450–520 nm) spectral regions, respectively. Employing the as‐fabricated aluminum hydroxides as blue phosphor and CuInS2 nanocrystals as red phosphor, high color rendering and high‐efficiency WLEDs with luminous efficiencies up to 37 lm W−1 and high color rendering index of 91 are fabricated. This is the best performance reported so far for aluminum hydroxide based LEDs.

Formation of Colloidal Copper Indium Sulfide Nanosheets by Two-Dimensional Self-Organization.

Colloidal 2D semiconductor nanosheets (NSs) are an interesting new class of materials due to their unique properties. However, synthesis of these NSs is challenging, and synthesis procedures for materials other than the well-known Pb- and Cd-chalcogenides are still underdeveloped. In this paper, we present a new approach to make copper indium sulfide (CIS) NSs and study their structural and optical properties. The CIS NSs form via self-organization and oriented attachment of 2.5 nm chalcopyrite CuInS2 nanocrystals (NCs), yielding triangular- and hexagonal-shaped NSs with a thickness of ∼3 nm and lateral dimensions ranging from 20 to 1000 nm. The self-organization is induced by fast cation extraction, leading to attractive dipolar interactions between the NCs. Primary amines play a crucial role in the formation of the CIS NSs, both by forming in situ the cation extracting agent, and by preventing the attachment of NCs to the top and bottom facets of the NSs. Moreover, DFT calculations reveal that the amines are essential to stabilize the covellite crystal structure of the product CIS NSs. The NSs are indium-deficient and the off-stoichiometry gives rise to a plasmon resonance in the NIR spectral window.

Growth Mechanism and Surface State of CuInS2 Nanocrystals Synthesized with Dodecanethiol.

Ternary metal chalcogenide nanocrystals (NCs) offer exciting opportunities as novel materials to be explored on the nanoscale showing optoelectronic properties tunable with size and composition. CuInS2 (CIS) NCs are the most widely studied representatives of this family as they can be easily prepared with good size control and in high yield by reacting the metal precursors (copper iodide and indium acetate) in dodecanethiol (DDT). Despite the widespread use of this synthesis method, both the reaction mechanism and the surface state of the obtained NCs remain elusive. Here, we perform in situ X-ray diffraction using synchrotron radiation to monitor the pre- and postnucleation stages of the formation of CIS NCs. SAXS measurements show that the reaction intermediate formed at 100 °C presents a periodic lamellar structure with a characteristic spacing of 34.9 Å. WAXS measurements performed after nucleation of the CIS NCs at 230 °C demonstrate that their growth kinetics depend on the degree of precursor conversion achieved in the initial stage at 100 °C. NC formation requires the cleavage of S-C bonds. We reveal by means of combined 1D and 2D proton and carbon NMR analyses that the generated dodecyl radicals lead to the formation of a new thioether species R-S-R. The latter is part of a ligand double layer, which consists of dynamically bound dodecanethiolate ligands as well as of head-to-tail bound R-S-R molecules. This ligand double layer and a high ligand density (3.6 DDT molecules per nm2) are at the origin of the apparent difficulty to functionalize the surface of CIS NCs obtained with the DDT method.

One-Pot Synthesis of Zincblende CuInSe2 Nanocrystals via a Green Solution Reaction Route

The facile and ecofriendly method for the synthesis of a suitable colloidal nanocrystal ink plays an important role in the field of solar cells. Here, we describe our recent efforts toward this direction by a simple one-pot colloidal method to engineer CuInSe2 (CISe) nanocrystals with cubic zincblende (ZB) structure. The suitable band gap value and obvious photoresponse of the as-synthesized CISe nanocrystals indicate their potential application in the field of thin film solar cells. In addition, a possible crystal growth mechanism has been suggested for the formation of ZB CISe.

Preparation of metal selenide nanocrystals with hot-injection method

Nanomaterials have received extensive and growing research interests due to their unique properties and potential applications in various fields. They have contributed immensely to modern materials science and technology during the last three decades. Among various types of functional nanomaterials, metal selenide nanocrystals have attracted considerable attention because of their special properties. These nanomaterials have been applied in a wide range, such as medicine, electronics, photonics, energy conversion and storage devices, etc. Interestingly, when metals are chemically combined with selenide in the form of nanocrystals, they exhibit semiconductor-like electronic properties. These metal selenide nanocrystals have attracted significant attention in recent years due to their potential applications in solar energy conversion and storage, which are the most important challenges to be addressed to meet the world’s rising demand for clean energy in the future. Hot-injection method is an effective method for the synthesis of metal selenide nanocrystals. The success of this approach lies in the use of non-ionic precursors in high-boiling organic solvents. This makes it possible to grow the nanocrystals relatively slowly at a high temperature, which yields defect-free, well-passivated nanocrystals. The anther important aspect of this method is the separation of the nucleation and growth stages. Due to this, a high degree of monodispersity can be achieved. The fundamental principle of hot-injection method with concrete steps has been introduced. This review surveys the recent progress of the preparation of metal selenide nanocrystals by hot-injection method in terms of binary metal selenide nanocrystals, ternary metal selenide nanocrystals, quaternary, and multinary metal selenide nanocrystals. For binary metal selenide nanocrystals, four important binary metal selenide nanocrystals including CdSe, ZnSe, PbSe, and Sb2Se3 nanocrystals are discussed. Then the achievement of synthesis for CuInSe2 and CuGaSe2 nanocrystals by hot-injection method and their application in photovoltaic field are systematically cataloged as well. And the progress in preparation for CuInGaSe2 and Cu2ZnSnSe4 nanocrystals by hot-injection method has been clarified clearly. Organoselenide chemicals are generally toxic and unstable. The replacement of phosphine containing compounds in the synthesis process of metal selenide nanocrystals via hot-injection method has received considerable attention in recent years. Many attempts have been made to replace trioctylphosphine (TOP) as a medium for delivering selenium in these reactions, but each alternative possesses has significant limitations. This review mainly emphasizes on the recent advances toward the development of methods to achieve “green Se precursor” with high activity and “facile green synthesis” of metal selenide nanocrystals involved in various approaches. Two kinds of different schemes for synthesis and utilization “green Se precursor” are described. Finally, technical challenges and future trends are presented for synthesis of high quality metal selenide nanocrystals, in terms of composition-tunable and structure-controllable multinary metal selenide nanocrystals, green synthesis, industrial production and large-scale application.

Spectro-electrochemical Probing of Intrinsic and Extrinsic Processes in Exciton Recombination in I-III-VI2 Nanocrystals.

Ternary CuInS2 nanocrystals (CIS NCs) are attracting attention as nontoxic alternatives to heavy metal-based chalcogenides for many technologically relevant applications. The photophysical processes underlying their emission mechanism are, however, still under debate. Here we address this problem by applying, for the first time, spectro-electrochemical methods to core-only CIS and core/shell CIS/ZnS NCs. The application of an electrochemical potential enables us to reversibly tune the NC Fermi energy and thereby control the occupancy of intragap defects involved in exciton decay. The results indicate that, in analogy to copper-doped II-VI NCs, emission occurs via radiative capture of a conduction-band electron by a hole localized on an intragap state likely associated with a Cu-related defect. We observe the increase in the emission efficiency under reductive electrochemical potential, which corresponds to raising the Fermi level, leading to progressive filling of intragap states with electrons. This indicates that the factor limiting the emission efficiency in these NCs is nonradiative electron trapping, while hole trapping is of lesser importance. This observation also suggests that the centers for radiative recombination are Cu2+ defects (preexisting and/or accumulated as a result of photoconversion of Cu1+ ions) as these species contain a pre-existing hole without the need for capturing a valence-band hole generated by photoexcitation. Temperature-controlled photoluminescence experiments indicate that the intrinsic limit on the emission efficiency is imposed by multiphonon nonradiative recombination of a band-edge electron and a localized hole. This process affects both shelled and unshelled CIS NCs to a similar degree, and it can be suppressed by cooling samples to below 100 K. Finally, using experimentally measured decay rates, we formulate a model that describes the electrochemical modulation of the PL efficiency in terms of the availability of intragap electron traps as well as direct injection of electrons into the NC conduction band, which activates nonradiative Auger recombination, or electrochemical conversion of the Cu2+ states into the Cu1+ species that are less emissive due to the need for their "activation" by the capture of photogenerated holes.

Exciton Splitting in Thin Copper Indium Disulfide Nanosheets

Nanosheets of Cu2–xS having discrete thicknesses of five, six, and eight crystallographic monolayers are prepared by adaptation of literature methods. These are converted to CuInS2 nanosheets having the same discrete monolayer thicknesses by cation-exchange reactions. The six- and eight-monolayer CuInS2 nanosheets exhibit broad, nearly featureless extinction (absorption) spectra that are generally characteristic of CuInS2 nanocrystals. However, the thinnest, five-monolayer CuInS2 nanosheets exhibit structured extinction spectra having a well-resolved doublet feature with peaks at 2.74 and 2.54 eV. An exciton-splitting (valence-band-splitting) model assigns the transitions to excitons derived from inequivalent A, B, and C holes from the bulk band structure of CuInS2. This splitting is analogous to the heavy-hole, light-hole splitting observed in the absorption spectra of III–V quantum wells and pseudo-two-dimensional II–VI nanocrystals. The model semiquantitatively accounts for the magnitude of the observed splitting, which is largely due to quantum confinement and the differing A, B, and C hole effective masses. The lack of resolved splitting in CuInS2 nanocrystals having larger dimensions is also explained.

Synthesis of a conjugated pyrrolopyridazinedione\u2013benzodithiophene (PPD\u2013BDT) copolymer and its application in organic and hybrid solar cells

Herein, we describe the synthesis and characterization of a conjugated donor–acceptor copolymer consisting of a pyrrolopyridazinedione (PPD) acceptor unit, and a benzodithiophene (BDT) donor unit. The polymerization was done via a Stille cross-coupling polycondensation. The resulting PPD–BDT copolymer revealed an optical bandgap of 1.8 eV and good processability from chlorobenzene solutions. In an organic solar cell in combination with PC70BM, the polymer led to a power conversion efficiency of 4.5%. Moreover, the performance of the copolymer was evaluated in polymer/nanocrystal hybrid solar cells using non-toxic CuInS2 nanocrystals as inorganic phase, which were prepared from precursors directly in the polymer matrix without using additional capping ligands. The PPD–BDT/CuInS2 hybrid solar cells showed comparably high photovoltages and a power conversion efficiency of 2.2%.Graphical abstract

Temperature induced CuInSe2 nanocrystal formation in the Cu2Se-In3Se2 multilayer thin films

The paper deals with the impact of annealing on Cu2Se-In3Se2 multilayer structure and discusses the quantum confinements. Thermal evaporation technique was used to prepare multilayer films over the glass substrates. The films were annealed at different temperatures (150 °C–350 °C) under vacuum atmosphere. The XRD pattern reveals that the films exhibit (112) peaks with CuInSe2 Chalcopyrite structure and upon annealing crystallinity improved. The grain size comes around 13–19 nm. The optical band gap value was found to be 2.21 to 2.09 eV and band gap splitting was observed for higher annealing temperatures. The increase in the band gap is related to quantum confinement effect. SEM image shows nano crystals spread over the entire surface for higher annealing temperatures. Optical absorption and PL spectra shows the blue shift during annealing. The HR-TEM shows the particle size in the nano range and which confirms the CuInSe2 nanocrystal formation. AFM image shows the rough surface with homogenous grains for the as deposited films and smooth surface for annealed films.

Electrochemical Infilling of CuInSe2 within TiO2 Nanotube Layers and Subsequent Photoelectrochemical Studies.

Anodic self‐organized TiO2 nanotube layers (with different aspect ratios) were electrochemically infilled with CuInSe2 nanocrystals with the aim to prepare heterostructures with a photoelectrochemical response in the visible light. The resulting heterostructure assembly was confirmed by field‐emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X‐ray diffraction (XRD). High incident photon‐to‐electron conversion efficiency values exceeding 55% were obtained in the visible‐light region. The resulting heterostructures show promise as a candidate for solid‐state solar cells.

Flexible CuInSe2 Nanocrystal Solar Cells on Paper

Solar cells on paper have the potential to be inexpensive and portable due to several unique features of the substrate: paper is cheap, flexible, lightweight, biodegradable, and manufactured by roll-to-roll processing. Here, we report the first nanocrystal photovoltaic devices (PVs) made on paper. Using spray-deposited CuInSe2 nanocrystals as the absorber material on substrates composed of bacterial cellulose nanofibers synthesized by the microorganism Gluconacetobacter hansenii, these devices demonstrate exceptional electrical and mechanical integrity. There is no significant loss in PV device performance after more than 100 flexes to 5 mm radius, and the devices continue to perform when folded into a crease. The practical use of these paper PVs is demonstrated with a prototype device powering liquid crystal displays (LCDs) mounted to various kinds of surfaces.

Growth kinetics and mechanisms of multinary copper-based metal sulfide nanocrystals.

Multinary copper-based metal sulfide (Cu-M-S) nanocrystals (NCs) usually have high absorption coefficients and near-optimum direct band gaps, which have been considered as novel photo-absorption materials for quantum dot-sensitized solar cells (QDSCs) and hole-transport materials for perovskite solar cells (PSCs). However, the formation and phase transformation mechanisms of Cu-M-S NCs during the solution-based preparing approaches are complicated. Herein, Cu-M-S NCs, including Cu2ZnSnS4 (CZTS), Cu2SnS3 (CTS), CuInS2 (CIS), and CuSbS2 (CAS), have been synthesized through solution-based hot-injection methods. Their formation and phase transformation mechanisms have been studied in terms of the growth kinetics. An effective method has been proposed to investigate the formation mechanisms of Cu-M-S NCs. The results suggest that CZTS, CTS, and CIS NCs are formed through an inter-reaction between metal sulfides rather than the classical cation exchange reactions, and CAS NCs are formed based on the CuxS structure; these findings provide new insights into the formation of Cu-M-S NCs. In addition, the anisotropic or isotropic growth processes during the growth stage have been found to be the key issues in the formation of a zinc blende or wurtzite structure NCs, respectively, which can be controlled by tuning the relative reactivity of metal precursors.

Synthesis and characterization of colloidal nanocrystals of ternary chalcogenide compounds

Colloidal nanocrystals of CuInS2 and CuInSe2 were synthesized in an apolar noncoordinating medium using 1-dodecanethiol as a ligand. A semiconductor shell of ZnS was formed for CuInS2 nanocrystals obtained by the injection method. The obtained samples were characterized by absorption spectroscopy and photoluminescence.

Elucidating the relationship between crystallo-chemistry and optical properties of CIGS nanocrystals

The performance of solar cells fabricated using Cu(In,Ga)(S,Se)2 nanocrystal (NC) inks synthesized using the hot injection method has yielded efficiencies up to 12% recently. The efficiency of these devices is highly dependent on the chemical composition and crystallographic quality of the NCs. The former has been extensively discussed as it can be easily correlated to the optical properties of the film, but detailed crystallographic structure of these NCs has scarcely been discussed and it can influence both the optical and electrical properties. Hence both chemical composition and crystal structure should be explored for these NCs in order for this material to be further developed for application in thin film solar cells. In this work, a thorough investigation of the composition and crystal structure of CuInxGa1−xSe2 NCs synthesized using the hot injection method over the entire composition range (0 ≤ x ≤ 1) has been conducted. Raman spectroscopy of the NCs complements the information derived from x-ray diffraction (XRD) and electron probe microanalysis (EPMA). EPMA, which was carried out for the first time, indicates good controllability of the NC Ga/(In + Ga) ratio using this synthesis method. Raman spectroscopy reveals that CuInSe2 NCs are a mixture of chalcopyrite and sphalerite with disordered cations, whereas CuGaSe2 NCs are purely chalcopyrite. The lattice parameters determined from XRD were found to deviate from those calculated using Vegard’s law for all compositions. Hence, it can be deduced that the lattice is distorted in the crystal. The optical and electrochemical band gap of CuInxGa1−xSe2 NCs increases as the Ga content increases. The energy band gap deviates from the theoretical values, which could be related to the contribution from cation disordering and strain. These results help to tailor the opto-electrical properties of semiconductors, which inherently depend on the crystalline quality, strain and composition.

Tunneling in the Delayed Luminescence of Colloidal CdSe, Cu+-Doped CdSe, and CuInS2 Semiconductor Nanocrystals and Relationship to Blinking

The photoluminescence decay dynamics of colloidal CdSe, Cu+:CdSe, and CuInS2 nanocrystals have been examined as a function of temperature and magnetic field. All three materials show photoluminescence decay on time scales significantly longer than the intrinsic lifetimes of their luminescent excited states, i.e., delayed luminescence, involving formation of a metastable trapped excited state followed by detrapping to re-form the emissive excited state. Surprisingly, the delayed luminescence decay kinetics are nearly identical for these three very different materials, suggesting they reflect universal properties of the delayed luminescence phenomenon in semiconductor nanocrystals. By measuring luminescence decay over 8 decades in time and 6 decades in intensity, we observe for the first time a clear deviation from power-law dynamics in delayed luminescence. Furthermore, for all three materials, the delayed luminescence decay dynamics are observed to be nearly independent of temperature between 20 K and roo...

Mesoporous Aluminum Hydroxide Synthesized by a Single-Source Precursor-Decomposition Approach as a High-Quantum-Yield Blue Phosphor for UV-Pumped White-Light-Emitting Diodes.

Strongly emissive (photoluminescence quantum yield up to 65%), thermally stable aluminum hydroxide blue phosphors are synthesized by a single-source precursor-decomposition approach. Blue-emitting UV-pumped light-emitting diodes (LEDs) based on the aluminum hydroxide phosphor reach luminous efficiency of 27.5 lm W-1 , while UV-white-LEDs integrating blue-emitting aluminum hydroxide and red-emitting CuInS2 nanocrystals achieve high color-rendering-index values of 94.3 and luminous efficiency of 23.5 lm W-1 .

Plastic Microgroove Solar Cells Using CuInSe2 Nanocrystals

Plastic photovoltaic devices (PVs) were fabricated by spray-depositing copper indium diselenide (CuInSe2) nanocrystals into micrometer-scale groove features patterned into polyethylene terephthalate (PET) substrates. Each groove has sidewall coatings of Al/CdS and Au and performs as an individual solar cell. These PV groove features can be linked electrically in series to achieve high voltages. For example, cascades of up to 15 grooves have been made with open-circuit voltages of up to 5.8 V. On the basis of the groove geometry, the power conversion efficiencies (PCEs) of the devices reached as high as 2.2%. Using the active area and photovoltaic response of devices determined from light-beam-induced current (LBIC) and photoreflectivity measurements gave PCE values as high as 4.4%.

Tunable optical properties of ternary non‐stoichiometric Cu–In–S nanocrystal emitters

Low toxic, non-stoichiometric colloidal copper–indium–sulphur [Cu–In–S (CIS)] ternary quantum dots with different Cu:In molar ratios by a hot-injection method in octadecene was synthesised. The Cu:In precursor molar ratios were 1/10, 1/20 and 1/40. Varying the fraction of cationic and capping agents, the compositions of CIS nanocrystals were precisely controlled. The photoluminescence (PL) results reveal that, reducing the Cu:In ratio in the CIS is the feasible strategy for bandgap engineering. Tunable PL emissions have been observed. X-ray powder diffraction and high-resolution transmission electron microscopy results indicate that as-prepared CIS nanocrystals are compositional homogeneity. The single-crystal nature of CIS nanocrystals improves the relative PL quantum yield up to 12%, which exhibits substantial enhancement comparing with the stoichiometric CuInS2 and CIS-based semiconductor core QDs. The PL decay curve of CIS has a triexponential feature. The value of τ 1, τ 2 and τ 3 of CIS sample agree well with CuInS2 core QDs. The average PL lifetime of CIS is reduced to 146.9 ns, which can be attributed to the reduction of surface-related trap states.

Electrospinning in Situ Synthesis of Graphene-Doped Porous Copper Indium Disulfide/Carbon Composite Nanofibers for Highly Efficient Counter Electrode in Dye-Sensitized Solar Cells

Porous graphene-doped copper indium disulfide/carbon (p-GN@CuInS2/C) composite nanofibers were fabricated via electrospinning, in situ synthesis, and carbonization. A polyacrylonitrile (PAN) solution containing graphene oxide nanosheets, copper dichloride (CuCl2), indium trichloride (InCl3), and thiourea (Tu.) in a mixed solvent of N,N-dimethylformamide/trichloromethane (DMF/CF) was used as the precursor solution for electrospinning. The resulting porous GN@CuInS2/C nanofibers were 107±24nm in diameter, and graphene nanosheets anchored with chalcopyrite CuInS2 nanocrystals 7–12nm in diameter were overlapped and embedded in the carbon matrix, aligning along the fiber axial direction. The Brunauer–Emmett–Teller (BET) surface area of the p-GN@CuInS2/C composite nanofibers was 795m2/g, with a total pore volume of 0.71cm3/g. These values were significantly larger than those of the sample without graphene and CuInS2/C nanofibers. A dye-sensitized solar cell (DSSC) assembled using the p-GN@CuInS2/C nanofibers as the counter electrode (CE) delivered a photoelectric conversion efficiency of 7.23%, which was higher than the efficiencies of DSSCs assembled using the samples without graphene (6.48%) and with the CuInS2/C nanofibers (5.45%). It was also much higher than that of the DSSC with a Pt CE (6.34%). The excellent photoelectric performance of the p-GN@CuInS2/C CE was attributed to its special hierarchical porous structure, which facilitated permeation of the liquid electrolytes and provided additional active catalytic sites for the oxidation reaction of the electrolytic (I−/I3−). The doping of reduced graphene oxide (RGO) resulted in the well-dispersed growth of CuInS2 nanocrystals in the carbon nanofibers, which further increased the number of active catalytic sites and promoted electron and ion transfer.

Radiative and Nonradiative Recombination in CuInS2 Nanocrystals and CuInS2-Based Core/Shell Nanocrystals.

Luminescent copper indium sulfide (CIS) nanocrystals are a potential solution to the toxicity issues associated with Cd- and Pb-based nanocrystals. However, the development of high-quality CIS nanocrystals has been complicated by insufficient knowledge of the electronic structure and of the factors that lead to luminescence quenching. Here we investigate the exciton decay pathways in CIS nanocrystals using time-resolved photoluminescence and transient absorption spectroscopy. Core-only CIS nanocrystals with low quantum yield are compared to core/shell nanocrystals (CIS/ZnS and CIS/CdS) with higher quantum yield. Our measurements support the model of photoluminescence by radiative recombination of a conduction band electron with a localized hole. Moreover, we find that photoluminescence quenching in low-quantum-yield nanocrystals involves initially uncoupled decay pathways for the electron and hole. The electron decay pathway determines whether the exciton recombines radiatively or nonradiatively. The development of high-quality CIS nanocrystals should therefore focus on the elimination of electron traps.