Copper Indium Sulfide Nanocrystals Research Articles

Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots.

Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium- and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using low-temperature single quantum dot spectroscopy on core-shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features. The first class shows sharp resolution-limited emission lines that are attributed to zero-phonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.

Hydrothermal Synthesis of Aqueous-Soluble Copper Indium Sulfide Nanocrystals and Their Use in Quantum Dot Sensitized Solar Cells.

A facile hydrothermal method to synthesize water-soluble copper indium sulfide (CIS) nanocrystals (NCs) at 150 °C is presented. The obtained samples exhibited three distinct photoluminescence peaks in the red, green and blue spectral regions, corresponding to three size fractions, which could be separated by means of size-selective precipitation. While the red and green emitting fractions consist of 4.5 and 2.5 nm CIS NCs, the blue fraction was identified as in situ formed carbon nanodots showing excitation wavelength dependent emission. When used as light absorbers in quantum dot sensitized solar cells, the individual green and red fractions yielded power conversion efficiencies of 2.9% and 2.6%, respectively. With the unfractionated samples, the efficiency values approaching 5% were obtained. This improvement was mainly due to a significantly enhanced photocurrent arising from complementary panchromatic absorption.

Hot injection synthesis of CuInS2 nanocrystals using metal xanthates and their application in hybrid solar cells

Copper indium sulfide nanocrystals with sizes of 3–4 nm were synthesized from metal xanthates in a hot injection reaction. After ligand exchange, their performance as acceptors in polymer/nanocrystal hybrid solar cells was evaluated.

Ligand-free preparation of polymer/CuInS2 nanocrystal films and the influence of 1,3-benzenedithiol on their photovoltaic performance and charge recombination properties.

Bulk heterojunction solar cells based on conjugated polymer donors and fullerene-derivative acceptors have received much attention in the last decade. Alternative acceptors like organic non-fullerene acceptors or inorganic nanocrystals have been investigated to a lesser extent; however, they also show great potential. In this study, one focus is set on the investigation of the in situ growth of copper indium sulfide nanocrystals in a conjugated polymer matrix. This preparation method allows the fabrication of a hybrid active layer without long-chain ligands, which could hinder charge separation and transport. In contrast, surfactants for the passivation of the nanocrystal surface are missing. To tackle this problem, we modified the absorber layer with 1,3-benzenedithiol and investigated the influence on charge transfer and solar cell performance. Using ToF-SIMS measurements, we could show that 1,3-benzenedithiol is successfully incorporated and homogeneously distributed in the absorber layer, which significantly increases the power conversion efficiency of the corresponding solar cells. This can be correlated to an improved charge transfer between the nanocrystals and the conjugated polymer as revealed by transient absorption spectroscopy as well as prolonged carrier lifetimes as disclosed by transient photovoltage measurements.

Enhanced photocatalytic activity of ternary CuInS2 nanocrystals synthesized from the combination of a binary Cu(I)S precursor and InCl3

Ternary copper indium sulfide (CIS) nanocrystals (NCs) have been synthesized by mixing of binary precursor [CuI(bdpa)2][CuICl2] (1) and/or [CuI(mdpa)2][CuICl2] (2) (where, mdpa and bdpa represent methyl and benzyl ester of 3,5-dimethyl pyrazole-1-dithioic acid, respectively) with InCl3 in a low-temperature solvothermal process. The +1 oxidation state of copper and the atomic ratio Cu to S (1:2) is atomically maintained in the pyrazole-based Cu(I)–S precursor to synthesize phase pure CuInS2. Coordinating solvents like ethylene diamine (EN) and ethylene glycol (EG) have been used in the synthesis without any surfactants. No use of external surfactants in the synthesis of CIS nanoparticles reveals that precursor acts as stabilizing agent. The synthesized nanocrystals were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectroscopy (EDX) studies. The optical property of the nanocrystals shows a pronounced quantum confinement effect in the particles with band gap energy ca. 1.5 eV. The formation mechanism of ternary CIS has been proposed. The pore size distributions of the particles show the average pore diameters 13.1 nm from 1 and 5.3 nm from 2. The calculated values of the specific surface area are 8.123 and 9.577 m2/g for 1 and 2, respectively. The excellent photocatalytic degradation of rose bengal (RB) and rhodamine B (RhB) was demonstrated by the porous CIS nanocrystals.

A Hybrid Perovskite Solar Cell Modified With Copper Indium Sulfide Nanocrystals to Enhance Hole Transport and Moisture Stability

Lead halide perovskite solar cells have attracted tremendous attention for their impressive power conversion efficiencies. However, stability is still one of the most significant concerns for perovskite solar cells. Here, a layer of copper indium sulfide nanocrystals (CIS NCs) has been introduced into the perovskite solar cell, giving rise to enhanced charge transfer property as well as longer lived films and devices. Ultrafast photoluminescence studies on the charge transfer behavior have revealed that the CIS NC layer can modify the interface between perovskite and hole transporting material, suppressing charge recombination pathways. The degradation mechanism in this system is also studied under various simulated environmental conditions. Devices without CIS NCs show significant degradation after exposure to high humidity (over 80%) without device encapsulation. In contrast, the perovskite thin films with a CIS layer show superior resistance to humidity, while perovskite-CIS NC solar cells exhibit significantly improved photovoltaic stability.

Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS2/ZnS Core/Shell Colloidal Quantum Dots.

Copper indium sulfide (CIS) quantum dots (QDs) are attractive as labels for biomedical imaging, since they have large absorption coefficients across a broad spectral range, size- and composition-tunable photoluminescence from the visible to the near-infrared, and low toxicity. However, the application of NIR-emitting CIS QDs is still hindered by large size and shape dispersions and low photoluminescence quantum yields (PLQYs). In this work, we develop an efficient pathway to synthesize highly luminescent NIR-emitting wurtzite CIS/ZnS QDs, starting from template Cu2-xS nanocrystals (NCs), which are converted by topotactic partial Cu+ for In3+ exchange into CIS NCs. These NCs are subsequently used as cores for the overgrowth of ZnS shells (≤1 nm thick). The CIS/ZnS core/shell QDs exhibit PL tunability from the first to the second NIR window (750–1100 nm), with PLQYs ranging from 75% (at 820 nm) to 25% (at 1050 nm), and can be readily transferred to water upon exchange of the native ligands for mercaptoundecanoic acid. The resulting water-dispersible CIS/ZnS QDs possess good colloidal stability over at least 6 months and PLQYs ranging from 39% (at 820 nm) to 6% (at 1050 nm). These PLQYs are superior to those of commonly available water-soluble NIR-fluorophores (dyes and QDs), making the hydrophilic CIS/ZnS QDs developed in this work promising candidates for further application as NIR emitters in bioimaging. The hydrophobic CIS/ZnS QDs obtained immediately after the ZnS shelling are also attractive as fluorophores in luminescent solar concentrators.

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.

A comparison of copper indium sulfide-polymer nanocomposite solar cells in inverted and regular device architecture

In this study, the influence of device architecture (regular versus inverted) and different interlayers on performance and lifetime of polymer-nanocrystal hybrid solar cells is explored. The absorber layers of the investigated solar cells consist of the low band gap conjugated polymer poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and copper indium sulfide (CIS) nanocrystals. These are prepared directly in the polymer matrix via an in situ approach using copper and indium xanthates, which are converted to CIS nanocrystals by mild thermal annealing. Different interlayers – poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and inorganic metal oxides (V2O5, MoO3) – are used as hole extraction layers between absorber and anode. All three layers were successfully applied in solar cells in regular architecture. Using an inverted device design, a PEDOT:PSS interlayer and Ag electrodes, open circuit voltages of 550mV could be obtained, which are comparable to values obtained with solar cells having easily oxidizing low work function metal electrodes. However, inverted solar cells with a V2O5 hole extraction layer could not be obtained due to possible incompatibility of the V2O5 coating process with the underlying absorber layer. In the case of MoO3 interlayers, the introduction of additional layers led to working inverted solar cells.

Room temperature synthesis of CuInS2 nanocrystals

Herein, we investigate a synthetic approach to prepare copper indium sulfide nanocrystals at room temperature.

Thiol-Free Synthesized Copper Indium Sulfide Nanocrystals as Optoelectronic Quantum Dot Solids

Colloidal semiconductor nanocrystals are a platform for developing applications in optoelectronics but are mostly based on materials with toxic elements. Copper indium sulfide nanocrystals (CIS NCs) are an alternative but have been investigated in only a few reports as a functional optoelectronic film. We synthesized various sizes of emitting copper-poor CIS NCs in the absence of dodecanethiol. We employed a Zn-shelling as a surface stabilizing agent that yielded high quantum yield and stable colloids in both organic and aqueous solvents. We found that these nanocrystals possess high exciton binding energies and exhibit a decrease in photoluminescence lifetimes with distance under the Forster relationship. We then deposited CIS QD films by solid-state ligand exchange and characterized their field effect transistor and photoconductive characteristics.

From Binary Cu2S to ternary Cu-In-S and quaternary Cu-In-Zn-S nanocrystals with tunable composition via partial cation exchange.

We present an approach for the synthesis of ternary copper indium sulfide (CIS) and quaternary copper indium zinc sulfide (CIZS) nanocrystals (NCs) by means of partial cation exchange with In(3+) and Zn(2+). The approach consists of a sequential three-step synthesis: first, binary Cu2S NCs were synthesized, followed by the homogeneous incorporation of In(3+) by an in situ partial cation-exchange reaction, leading to CIS NCs. In the last step, a second partial exchange was performed where Zn(2+) partially replaced the Cu(+) and In(3+) cations at the surface, creating a ZnS-rich shell with the preservation of the size and shape. By careful tuning reaction parameters (growth and exchange times as well as the initial Cu(+):In(3+):Zn(2+) ratios), control over both the size and composition was achieved. This led to a broad tuning of photoluminescence of the final CIZS NCs, ranging from 880 to 1030 nm without altering the NCs size. Cytotoxicity tests confirmed the biocompatibility of the synthesized CIZS NCs, which opens up opportunities for their application as near-infrared fluorescent markers in the biomedical field.

Roles of Nucleation, Denucleation, Coarsening, and Aggregation Kinetics in Nanoparticle Preparations and Neurological Disease

Kinetic models for nucleation, denucleation, Ostwald ripening (OR), and nanoparticle (NP) aggregation are presented and discussed from a physicochemical standpoint, in terms of their role in current NP preparations. Each of the four solid-state mechanisms discussed predict a distinct time dependence for the evolution of the mean particle radius over time. Additionally, they each predict visually different particle size distributions (PSDs) under limiting steady-state (time-independent) conditions. While nucleation and denucleation represent phase transformation mechanisms, OR and NP aggregation do not. Thus, when modeling solid-state kinetics relevant to NP processing, either the time evolution of the mean particle radius or the fractional conversion data should be fit using appropriate models (discussed herein), without confusing/combining the two classes of models. Experimental data taken from the recent literature are used to demonstrate the usefulness of the models in real-world applications. Specifically, the following examples are discussed: the preparation of bismuth NPs, the synthesis of copper indium sulfide nanocrystals, and the aggregation of neurological proteins. Because the last process is found to obey reaction-limited colloid aggregation (RLCA) kinetics, potential connections between protein aggregation rates, the onset of neurological disease, and population lifespan dynamics are suggested by drawing a parallel between RLCA kinetics and Gompertz kinetics. The physical chemistry underpinning NP aggregation is investigated, and a detailed definition of the rate constant of aggregation, k(a), is put forth that provides insight into the origin of the activation energy barrier of aggregation. For the two nanocrystal preparations investigated, the initial kinetics are found to be well-described by the author's dispersive kinetic model for nucleation-and-growth, while the late-stage NP size evolution is dominated by OR. At intermediate times, it is thought that the two mechanisms both contribute to the NP growth, resulting in PSD focusing as discussed in a previous work [Skrdla, P. J. J. Phys. Chem. C2012, 116, 214-225]. On the basis of these two mechanisms, a synthetic procedure for obtaining monodisperse NP PSDs, of small and/or systematically targeted mean sizes, is proposed.

Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission

We report an efficient synthesis of copper indium sulfide nanocrystals with strong photoluminescence in the visible to near-infrared. This method can produce gram quantities of material with a chemical yield in excess of 90% with minimal solvent waste. The overgrowth of as-prepared nanocrystals with a few monolayers of CdS or ZnS increases the photoluminescence quantum efficiency to > 80%. On the basis of time-resolved spectroscopic studies of core/shell particles, we conclude that the emission is due to an optical transition that couples a quantized electron state to a localized hole state, which is most likely associated with an internal defect.

Phase-Selective Synthesis of CuInS2 Nanocrystals

Single-source precursor, [(Ph3P)CuIn(SC{O}Ph)4] (1), and dual-source precursors, [Cu(SC{O}Ph)] (2) and [In(bipy)(SC{O}Ph)3] (3), have been used to obtain the monodispersed wurtzite (hexagonal) and zincblende (also called sphalerite, cubic) phases of copper indium sulfide nanocrystals (CIS NCs). The NCs have been characterized by X-ray powder diffraction patterns, transmission electron microscopy, selected area electron diffraction, and energy-dispersive X-ray analysis. It is shown that the relative ratios of surfactants have influence on the formation of the wurtzite or zincblende phase of CIS. Moreover, the reaction temperature plays a role in stabilizing the high-temperature metastable zincblende cubic phase at room temperature. In the presence of trioctylphosphine oxide (TOPO) and dodecanethiol (DT), 1 generates the wurtzite phase of CIS when the reaction temperature is below 275 °C, but above this temperature the obtained product belongs to zincblende (cubic). The morphology of the CIS also changes from nanoplates to nanoparticles when it undergoes phase transformation. In the wurtzite phase, monodispersed nanoplates are formed at 175 °C and nanorods (NRs) produced at 250 °C along with the plates. Wurtzite and zincblende CIS nanocrystals exhibit intense emission in the ultraviolet region and weak emission in the visible region. The nonlinear optical (NLO) properties of the CIS NCs have also been characterized with femtosecond laser pulses at a wavelength of 780 nm.