Fabrication of p-n Heterostructured Photocatalysts with Triazine-Based Covalent Organic Framework and CuInS2 for High-Efficiency CO2 Reduction

The application of covalent organic frameworks (COFs) for the photocatalytic reduction of CO2 is mostly limited by severe charge recombination and low sunlight utilization. Herein, a triazine-based COF with an electron-rich and large π-conjugated system (TCOF) was employed as a building block and integrated with CuInS2 (CIS) to construct a noble-metal-free and high-efficiency photocatalyst for CO2 reduction. The in situ growth of CIS nanosheets on TCOF creates a p-n heterojunction, named CIS@TCOF. Compared with TCOF, the CIS@TCOF heterostructure exhibits a dramatically boosted photocatalytic performance in the reduction of CO2. The produced HCOOH yield over 10 wt % CIS@TCOF can be up to 171.2 μmol g-1 h-1 under visible light irradiation with good reproducibility, which is about 3 times as high as that over TCOF. Further in-depth studies indicate that the introduction of CIS not only enhances the visible light utilization but also restrains the recombination of photogenerated electron-hole pairs efficiently and facilitates the photoinduced charge transfer via the p-n heterojunction system due to the unique structural and compositional features. This research shows the great potential of COFs as efficient photocatalytic carbon fixation materials and provides a versatile route to construct semiconductor-COF heterostructures for photocatalysis.

A signal polarity conversion photoelectrochemical immunosensor for neuron-specific enolase detection based on MgIn2S4-sensitized CsPbBr3

A photoelectrochemical (PEC) immunosensor was designed based on MgIn2S4-decorated inorganic halide perovskite CsPbBr3 combined with the signal polarity conversion strategy for neuron-specific enolase (NSE) detection. CsPbBr3 was applied as the basic photoactive material owing to its excellent optical and electronic properties, which provide a good PEC performance for sensor construction. In order to improve the stability of this perovskite, the three-dimensional flower-like MgIn2S4 with a desirable direct band gap was applied to enhance the PEC response. Also, the excellent structure of MgIn2S4 provides large surface-active sites for CsPbBr3 loaded. For enhancing the detection sensitivity of PEC immunosensor, p-type CuInS2 was used as a signal probe which fixed on detection antibody (Ab2). When the target NSE was present, the photogenerated electrons produced by CuInS2 were transferred to the test solution, and the polarity of PEC signal changes. Based on the above photosensitive materials and signal conversion strategy, the proposed PEC immunosensor showed favorable detection performance, and the linear detection range is 0.0001 ~ 100 ng/mL with a 38 fg/mL of detection limit. The proposed strategy improved the adhibition of CsPbBr3 in the analytical chemistry field as well as provided a reference method for other protein detections.

Temperature- and Size-Dependent Photoluminescence of CuInS2 Quantum Dots

We present the results of a temperature-dependent photoluminescence (PL) spectroscopy study on CuInS2 quantum dots (QDs). In order to elucidate the influence of QD size on PL temperature dependence, size-selective precipitation was used to obtain several nanoparticle fractions. Additionally, the nanoparticles' morphology and chemical composition were studied using transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The obtained QDs showed luminescence in the visible-near infrared range. The PL energy, linewidth, and intensity were studied within an 11-300 K interval. For all fractions, a temperature decrease led to a shift in the emission maximum to higher energies and pronounced growth of the PL intensity down to 75-100 K. It was found that for large particle fractions, the PL intensity started to decrease, with temperature decreasing below 75 K, while the PL intensity of small nanoparticles remained stable.

Non-Toxic CuInS2 /ZnS Colloidal Quantum Dots for Near-Infrared Light-Emitting Diodes

Ternary CuInS₂ quantum dots (QDs) with photoluminescence that is tunable from the visible to the near-infrared (NIR) region are promising light-emitters for consumer electronics due to the absence of toxic elements such as Pb, Cd, or As. Despite the compelling performance of visible-light-emitting CuInS₂ QDs, reports on NIR emission remain limited, with modest efficiencies at wavelengths beyond 900 nm. In this work, the facile synthesis of NIR-emitting CuInS₂ /ZnS QDs is reported. A combination of two sulfur precursors w as used in the synthesis, comprising 1-dodecanethiol (DDT) and hexamethyldisilathiane (HMDS). The reactive HMDS facilitates faster nucleation and leads to a higher density of emissive Cu-deficiency sites. The resulting QDs exhibit high photoluminescence quantum efficiency (PLQE) of 65% at a long emission wavelength of 920 nm. Using these QDs, NIR light-emitting diodes (LED) are fabricated, which attain an external quantum efficiency (EQE) of 8.2%. This efficiency is comparable to the best reported PbS and InAs QD LEDs, and the emission wavelength exceeds that of lead iodide perovskites. This work thus marks one of the first reports of efficient NIR LEDs based on environmentally benign CuInS₂ QDs and may open up promising new applications in consumer electronic products.

Monitoring Transformations of Catalytic Active States in Photocathodes Based on MoSx Layers on CuInS2 Using In Operando Raman Spectroscopy

The electrochemical activation of CuInS₂ /MoS_x for photoelectrochemical (PEC) H₂ production was revealed for the first time through in operando Raman spectroscopy. During the activation process, the initial metallic MoS_x phase was transformed to semiconducting MoS_x , which facilitates charge carrier transfer between CuInS₂ and MoS_x . Ex situ X-ray photoelectron spectroscopy and Raman spectroscopy suggest the existence of MoO₃ after the activation process. However, apart from contradicting these results, in operando Raman spectroscopy revealed some of the intermediate steps of the activation process.

Optimization of PEC and photocathodic protection performance of TiO2/CuInS2heterojunction photoanodes

The establishment of heterojunction is a powerful strategy to enhance the photoresponse performance of photoanode. Here, TiO₂/CuInS₂(T/CIS) composites were prepared via a two-step hydrothermal method, and their morphologies were controlled by adjusting the reaction time. The absorption spectra show that CuInS₂can significantly improve the absorption of visible light. The T/CIS2 (2 h reaction time) photoanode exhibited the most outstanding photoelectrochemical (PEC) performance, with a photocurrent density of 168% that of the pure TiO₂photoanode. Under simulated sunlight, this photoanode can supply a protective photocurrent of 0.49 mA cm^-2and a protective voltage of 0.36 V to stainless steel (304ss), which are about 4 and 2 times those of the TiO₂sample. The enhancement in the photocathodic protection performance is attributed to enlarged visible light absorbance and higher charge separation rate. This study demonstrates that the TiO₂/CuInS₂photoanode is a promising candidate for application in photoinduced cathodic protection of metallic materials.

Laser ablative TiO2 and tremella-like CuInS2 nanocomposites for robust and ultrasensitive photoelectrochemical sensing of let-7a

A photoelectrochemical (PEC) biosensor based on a multiple signal amplification strategy was established for highly sensitive detection of microRNA (miRNA). TiO₂ was prepared on the surface of titanium sheet by laser etching to improve its stability and photoelectrical properties, and CuInS₂-sensitized TiO₂ was used to form a superior photoelectrical layer, which realized the initial signal amplification. The electron donor dopamine (DA) was modified to H2 as a signal regulator, which effectively increased the photocurrent signal. To further amplify the signal, an enzyme-free hybridization reaction was implemented. When target let-7a and fuel-DNA (F-DNA) were present, the base of H1 specifically recognized let-7a and forced dopamine@AuNPs-H2 away from the electrode surface. Subsequently, the end base of H1 specifically recognized F-DNA, and let-7a was replaced and recycled to participate in the next cycle. Enzyme-free circulation, as a multifunctional amplification method, ensured the recycling of target molecules. This PEC sensor for let-7a detection showed an excellent linear response from 0.5 to 1000 pM with a detection limit of 0.12 pM. The intra-batch RSD was 3.8% and the recovery was 87.74-108.1%. The sensor was further used for clinical biomolecular monitoring of miRNA, showing excellent quantitative detection capability.

Origin of Interface Limitation in Zn(O,S)/CuInS2-Based Solar Cells

Copper indium disulfide (CuInS₂) grown under Cu-rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination, resulting in poor solar cell performance. An unfavorable "cliff"-like conduction band alignment at the buffer/CuInS₂ interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in chemical bath-deposited Zn(O,S)/co-evaporated CuInS₂-based devices. Temperature-dependent current-voltage analyses indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bulk band gap, confirming the dominant recombination channel to be present at the Zn(O,S)/CuInS₂ interface. However, photoelectron spectroscopy measurements indicate a small (0.1 eV) "spike"-like conduction band offset at the Zn(O,S)/CuInS₂ interface, excluding an unfavorable energy-level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also suggests Fermi-level pinning not to be the main reason, leaving near-interface defects (as recently observed in Cu-rich CuInSe₂) as the likely reason for the performance-limiting interface recombination.

CuInS2 Photocathodes with Atomic Gradation-Controlled (Ta,Mo)x(O,S)y Passivation Layers for Efficient Photoelectrochemical H2 Production

An atomic gradient passivation layer, (Ta,Mo)_x(O,S)_y, is designed to improve the charge transportation and photoelectrochemical activity of CuInS₂-based photoelectrodes. We found that Mo spontaneously diffused to the a-TaO_x layer during e-beam evaporation. This result indicates that the gradient profile of MoO_x/TaO_x is formed in the sublayer of (Ta,Mo)_x(O,S)_y. To understand the atomic-gradation effects of the (Ta,Mo)_x(O,S)_y passive layer, the composition and (photo)electrochemical properties have been characterized in detail. When this atomic gradient-passive layer is applied to CuInS₂-based photocathodes, promising photocurrent and onset potential are seen without using Pt cocatalysts. This is one of the highest activities among reported CuInS₂ photocathodes, which are not combined with noble metal cocatalysts. Excellent photoelectrochemical activity of the photoelectrode can be mainly achieved by (1) the electron transient time improved due to the conductive Mo-incorporated TaO_x layer and (2) the boosted electrocatalytic activity by Mo_x(O,S)_y formation.

Unusual Spectral Diffusion of Single CuInS2 Quantum Dots Sheds Light on the Mechanism of Radiative Decay

The luminescence of CuInS₂ quantum dots (QDs) is slower and spectrally broader than that of many other types of QDs. The origin of this anomalous behavior is still under debate. Single-QD experiments could help settle this debate, but studies by different groups have yielded conflicting results. Here, we study the photophysics of single core-only CuInS₂ and core/shell CuInS₂/CdS QDs. Both types of single QDs exhibit broad PL spectra with fluctuating peak position and single-exponential photoluminescence decay with a slow but fluctuating lifetime. Spectral diffusion of CuInS₂-based QDs is qualitatively and quantitatively different from CdSe-based QDs. The differences reflect the dipole moment of the CuInS₂ excited state and hole localization on a preferred site in the QD. Our results unravel the highly dynamic photophysics of CuInS₂ QDs and highlight the power of the analysis of single-QD property fluctuations.

p-CuInS2 /n-Polymer Semiconductor Heterojunction for Photoelectrochemical Hydrogen Evolution

An inorganic p-type CuInS₂ semiconductor was combined with the semiconducting polymer of PNDI3OT-Se1 and PNDI3OT-Se2 with different HOMO/LUMO levels for photoelectrochemical hydrogen production. Charge transfer behaviors at polymer/CuInS₂ junctions were investigated by electrochemical impedance spectroscopy. The heterojunction of p-CuInS₂ and n-type polymer (both PNDI3OT-Se1 and Se2) successfully made p-n junctions and showed improved charge transfer. However, we found that higher HOMO levels of polymer than valence band maximum (VBM) of CuInS₂ spurred charge recombination at interfaces. As a result, CuInS₂ /PNDI3OT-Se1/TiO₂ /Pt, which has suitable energy levels matched between PNDI3OT-Se1 and CuInS₂ , shows photocurrent (-15.67 mA cm^-2 ) improved concretely when compared to a CuInS₂ /TiO₂ /Pt photoelectrode (-7.11 mA cm^-2 ) at 0.0 V vs. RHE applied potential. Additionally, the photoelectrochemical stability of CuInS₂ /PNDI3OT-Se1/TiO₂ /Pt photoelectrode was also investigated.

Photoelectrochemical aptasensor for thrombin based on Au-rGO-CuS as signal amplification elements

A photoelectrochemical platform for thrombin determination was developed based on Au-rGO-CuS as multiple signal amplification elements. CuInS₂ QDs was used to sensitize burr-shape TiO₂ (b-TiO₂) to obtain a strong photocurrent. Under the specific recognition between aptamer and thrombin, a sandwichlike structure was formed and the Au-rGO-CuS-labeled aptamer (S2@Au-rGO-CuS) was immobilized on the electrode surface. This induced a sharp decrease in photocurrent. The phenomenon is mainly due to the fact that CuS NPs can competitively consume the light energy and electron donor with CuInS₂/b-TiO₂. The rGO can increase the amount of CuS NPs and the Au NPs can accelerate charge transferring which depress the recombination of photogenerated electrons and holes in CuS to further enhance the competitive capacity of CuS. The sandwichlike structure has a steric hindrance effect. Therefore, the S2@Au-rGO-CuS has a multiple signal amplification function for thrombin determination. Under optimal conditions, the PEC aptasensor exhibited a wide linear concentration range from 0.1 pM to 10 nM with a low detection limit of 30 fM (S/N = 3) for thrombin. Besides, the designed aptasensor performed well in the assay of human serum sample, indicating good potential for the determination of thrombin in real samples. Graphical abstract.

Shell-Free Copper Indium Sulfide Quantum Dots Induce Toxicity in Vitro and in Vivo

Semiconductor quantum dots (QDs) are attractive fluorescent contrast agents for in vivo imaging due to their superior photophysical properties, but traditional QDs comprise toxic materials such as cadmium or lead. Copper indium sulfide (CuInS₂, CIS) QDs have been posited as a nontoxic and potentially clinically translatable alternative; however, previous in vivo studies utilized particles with a passivating zinc sulfide (ZnS) shell, limiting direct evidence of the biocompatibility of the underlying CIS. For the first time, we assess the biodistribution and toxicity of unshelled CIS and partially zinc-alloyed CISZ QDs in a murine model. We show that bare CIS QDs breakdown quickly, inducing significant toxicity as seen in organ weight, blood chemistry, and histology. CISZ demonstrates significant, but lower, toxicity compared to bare CIS, while our measurements of core/shell CIS/ZnS are consistent with literature reports of general biocompatibility. In vitro cytotoxicity is dose-dependent on the amount of metal released due to particle degradation, linking degradation to toxicity. These results challenge the assumption that removing heavy metals necessarily reduces toxicity: indeed, we find comparable in vitro cytotoxicity between CIS and CdSe QDs, while CIS caused severe toxicity in vivo compared to CdSe. In addition to highlighting the complexity of nanotoxicity and the differences between the in vitro and in vivo outcomes, these unexpected results serve as a reminder of the importance of assessing the biocompatibility of core QDs absent the protective ZnS shell when making specific claims of compositional biocompatibility.

Shape-Controlled Synthesis of Copper Indium Sulfide Nanostructures: Flowers, Platelets and Spheres

Colloidal semiconductor nanostructures have been widely investigated for several applications, which rely not only on their size but also on shape control. CuInS₂ (often abbreviated as CIS) nanostructures have been considered as candidates for solar energy conversion. In this work, three-dimensional (3D) colloidal CIS nanoflowers and nanospheres and two-dimensional (2D) nanoplatelets were selectively synthesized by changing the amount of a sulfur precursor (tert-dodecanethiol) serving both as a sulfur source and as a co-ligand. Monodisperse CIS nanoflowers (~15 nm) were formed via the aggregation of smaller CIS nanoparticles when the amount of tert-dodecanethiol used in reaction was low enough, which changed towards the formation of larger (70 nm) CIS nanospheres when it significantly increased. Both of these structures crystallized in a chalcopyrite CIS phase. Using an intermediate amount of tert-dodecanethiol, 2D nanoplatelets were obtained, 90 nm in length, 25 nm in width and the thickness of a few nanometers along the a-axis of the wurtzite CIS phase. Based on a series of experiments which employed mixtures of tert-dodecanethiol and 1-dodecanethiol, a ligand-controlled mechanism is proposed to explain the manifold range of the resulting shapes and crystal phases of CIS nanostructures.

Facile and Robust Solvothermal Synthesis of Nanocrystalline CuInS₂ Thin Films

This work demonstrates that the solvothermal synthesis of nanocrystalline CuInS₂ thin films using the amino acid l-cysteine as sulfur source is facile and robust against variation of reaction time and temperature. Synthesis was carried out in a reaction time range of 3–48 h (at 150 °C) and a reaction temperature range of 100–190 °C (for 18 h). It was found that at least a time of 6 h and a temperature of 140 °C is needed to produce pure nanocrystalline CuInS₂ thin films as proven by X-ray and electron diffraction, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Using UV-vis spectroscopy, a good absorption behavior as well as direct band gaps between 1.46 and 1.55 eV have been determined for all grown films. Only for a reaction time of 3 h and temperatures below 140 °C CuInS₂ is not formed. This is attributed to the formation of metal ion complexes with l-cysteine and the overall slow assembly of CuInS₂. This study reveals that the reaction parameters can be chosen relatively free; the reaction is completely nontoxic and precursors and solvents are rather cheap, which makes this synthesis route interesting for industrial up scaling.

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.

Hydroxyl-Terminated CuInS2-Based Quantum Dots: Potential Cathode Interfacial Modifiers for Efficient Inverted Polymer Solar Cells

The use of interfacial modifiers on cathode or anode layers can effectively reduce the recombination loss and thus have potential to enhance the device performance of polymer solar cells. In this work, we demonstrated that hydroxyl-terminated CuInS₂-based quantum dots could be potential cathode interfacial modifiers on ZnO layer for inverted polymer solar cells. By casting of a thin film of CuInS₂-based quantum dots onto ZnO layer, the controlled devices show obvious enhancements of open-circuit voltage, short-circuit current, and fill factor. With an optimized interfacial layer with ∼7 nm thickness, an improvement of power conversion efficiency up to 16% is obtained and the optimized power conversion efficiency of PTB7-based (PTB7: poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b] thiophenediyl]]) polymer solar cells approaches 8.51%. Detailed analysis shows that the performance enhancement can be explained to the improved light absorption, modified work function, reduced surface roughness, and the increased electron transfer of ZnO cathode interlayer.

Improvement of Catalytic Activity by Nanofibrous CuInS2 for Electrochemical CO2 Reduction

The current study reports the application of chalcopyrite semiconductor CuInS₂ (CIS) nanofibers for the reduction of CO₂ to CO with a remarkable Faradaic efficiency of 77 ± 4%. Initially the synthesis of CuInS₂ nanofibers was carried out by adaptable electrospinning technique. To reduce the imperfection in the crystalline fiber, polyacrylonitrile (PAN) was selected as template polymer. Afterward, the desired chemical structure of nanofibers was achieved through sulfurization process. Making continuous CuInS₂ nanofibers on the cathode surface by the electrospinning method brings the advantages of being economical, environmentally safe, and versatile. The obtained nanofibers of well investigated size and diameter according to the SEM (scanning electron microscope) were used in electrochemical studies. An improvement of Faradaic efficiency was achieved with the catalytic active CuInS₂ in nanofibrous structure as compared to the solution processed CuInS₂. This underlines the important effect of the electrode fabrication on the catalytic performance. Being less contaminated as compared to solution processing, and having a well-defined composition and increased catalytically active area, the CuInS₂ nanofiber electrodes prepared by the electrospinning technique show a 4 times higher Faradaic efficiency. Furthermore, in this study, attention was paid to the stability of the CuInS₂ nanofiber electrodes. The electrochemical reduction of CO₂ to CO by using CIS nanofibers coated onto FTO electrodes was carried out for 10 h in total. The observed current density of 0.22 mA cm^-2 and the stability of CIS nanofiber electrodes are found to be competitive with other heterogeneous electrocatalysts. Hence, we believe that the fabrication and application of nanofibrous materials through the electrospinning technique might be of interest for electrocatalytic studies in CO₂ reduction.

Photoelectrochemical Properties of Vertically Aligned CuInS2 Nanorod Arrays Prepared via Template-Assisted Growth and Transfer

Although copper-based chalcopyrite materials such as CuInS2 have been considered promising photocathodes for solar water splitting, the fabrication route for a nanostructure with vertical orientation has not yet been developed. Here, a fabrication route for vertically aligned CuInS2 nanorod arrays from an aqueous solution using anodic aluminum oxide template-assisted growth and transfer is presented. The nanorods exhibit a phase-pure CuInS2 chalcopyrite structure and cathodic photocurrent response without co-catalyst loading. Small particles of CdS and ZnS were conformally decorated onto CuInS2 nanorods using a successive ion layer adsorption and reaction method. With surface modification of CdS/ZnS, the photoelectrochemical properties of CuInS2 nanorod arrays are enhanced via flat-band potential shift, as determined by analyses of onset potential and Mott-Schottky plots.

Colloidal CuInS2 Quantum Dots as Inorganic Hole-Transporting Material in Perovskite Solar Cells

To develop novel hole-transporting materials (HTMs) is an important issue of perovskite solar cells (PSCs), especially favoring the stability improvement and the cost reduction. Herein, we use ternary quantum dots (QDs) as HTM in mesoporous TiO2/CH3NH3PbI3/HTM/Au solar cell, and modify the surface of CuInS2 QDs by cation exchange to improve the carrier transport. The device efficiency using CuInS2 QDs with a ZnS shell layer as HTM is 8.38% under AM 1.5, 100 mW cm(-2). The electrochemical impedance spectroscopy suggested that the significantly enhanced performance is mainly attributed to the reduced charge recombination between TiO2 and HTM. It paves a new pathway for the future development of cheap inorganic HTMs for the high efficiency PSCs.

Tuning carrier mobilities and polarity of charge transport in films of CuInSe(x)S(2-x) quantum dots

CuInSe(x)S(2-x) quantum dot field-effect transistors show p-type, n-type, and ambipolar behaviors with carrier mobilities up to 0.03 cm(2) V(-1) s(-1). Although some design rules from studies of cadmium and lead containing quantum dots can be applied, remarkable differences are observed including a strong gating effect in as-synthesized nanocyrstals with long ligands.

The phase transformation of CuInS2 from chalcopyrite to wurtzite

In the present work, CuInS2 nanoparticles have been successfully synthesized by water-bath method with deionized water as solvent and thioglycolic acid as complexing agent at 80°C. The phase transition of CuInS2 from chalcopyrite to wurtzite was realized by adjusting the pH value of reaction solution. The emergence of Cu2S in the condition of higher pH value of reaction solution led to the formation of wurtzite CuInS2. This facile method that controls the phase structure by adjusting the solution pH value could open a new way to synthesize other I-III-VI2 ternary semiconductor compounds.

Solution transformation of Cu₂O into CuInS₂ for solar water splitting

Though Cu2O has demonstrated high performance as a photocathode for solar water splitting, its band gap is too large for efficient use as the bottom cell in tandem configurations. Accordingly, copper chalcopyrites have recently attracted much attention for solar water splitting due to their smaller and tunable band gaps. However, their fabrication is mainly based on vacuum evaporation, which is an expensive and energy consuming process. Here, we have developed a novel and low-cost solution fabrication method, and CuInS2 was chosen as a model material due to its smaller band gap compared to Cu2O and relatively simple composition. The nanostructured CuInS2 electrodes were synthesized at low temperature in crystalline form by solvothermal treatment of electrochemically deposited Cu2O films. Following the coating of overlayers and decoration with Pt catalyst, the as-fabricated CuInS2 electrode demonstrated water splitting photocurrents of 3.5 mA cm(-2) under simulated solar illumination. To the best of our knowledge, this is the highest performance yet reported for a solution-processed copper chalcopyrite electrode for solar water splitting. Furthermore, the electrode showed good stability and had a broad incident photon-to-current efficiency (IPCE) response to wavelengths beyond 800 nm, consistent with the smaller bandgap of this material.

Development of nonstoichiometric CuInS₂ as a light-harvesting photoanode and catalytic photocathode in a sensitized solar cell

A simple one-pot approach was developed to obtain nonstoichiometric CuInS2 nanocrystals. Using this approach, both In-rich and Cu-rich CuInS2 nanocrystals could be reliably synthesized by tuning stoichiometric combinations of [Cu]/[In] precursor constituents. By designing Cu-rich CuInS2 heteronanostructures to serve as counter electrodes, quantum-dot-sensitized solar cells (QDSSCs) equipped with In-rich CuInS2 and CdS cosensitizers delivered a power conversion efficiency of 2.37%, which is significantly more efficient than conventional Pt counter electrodes. To the best of our knowledge, this study represents the first report utilizing nonstoichiometric CuInS2 nanocrystals as a photon-harvesting sensitizer comprised of a photoanode and photocathode in QDSSCs; also unique to this report, these nonstoichiometric CuInS2 nanocrystals were formed by simply changing the cationic molar ratios without complicated precursor preparation. Impedance spectroscopy and Tafel polarization indicated that these Cu-rich CuInS2 heteronanostructures had electrocatalytic activities (used for reducing S(2-)/Sn(2-)) that were superior to a Pt catalyst. Moreover, we demonstrated that Cu-rich CuInS2 heteronanostructures were also useful counter electrodes in dye-sensitized solar cells, and this finding revealed a promising conversion efficiency of 6.11%, which was ∼96% of the efficiency in a cell with a Pt-based counter electrode (6.32%).

Magneto-Optical Properties of CuInS2 Nanocrystals

We compare the absorption, photoluminescence, and magneto-optical properties of colloidal CuInS2 (CIS) nanocrystals with two closely related and well-understood binary analogs: Cu-doped ZnSe nanocrystals and CdSe nanocrystals. In contrast with conventional CdSe, both CIS and Cu-doped ZnSe nanocrystals exhibit a substantial energy separation between emission and absorption peaks (Stokes shift) and a marked asymmetry in the polarization-resolved low-temperature magneto-photoluminescence, both of which point to the role of localized dopant/defect states in the forbidden gap. Surprisingly, we find evidence in CIS nanocrystals of spin-exchange coupling between paramagnetic moments in the nanocrystal and the conduction/valence bands of the host lattice, a behavior also observed in Cu-doped ZnSe nanocrystals, where the copper atoms incorporate as paramagnetic Cu(2+) ions.

Crystal-bound vs surface-bound thiols on nanocrystals

The use of thiol ligands as a sulfur source for nanocrystal synthesis has recently come en vogue, as the products are often high quality. A comparative study was performed of dodecanethiol-capped Cu2S prepared with elemental sulfur and thiol sulfur reagents. XPS and TGA-MS provide evidence for differing binding modes of the capping thiols. Under conditions where the thiol acts only as a ligand, the capping thiols are "surface-bound" and bond to surface cations in low coordination number sites. In contrast, when thiols are used as a sulfur source, "crystal-bound" thiols result that sit in high coordination sites and are the terminal S layer of the crystal. A (1)H NMR study shows suppressed surface reactivity and ligand exchange with crystal-bound thiols, which could limit further application of the particles. To address the challenge and opportunity of nonlabile ligands, dodecyl-3-mercaptopropanoate, a molecule possessing both a thiol and an ester, was used as the sulfur source for the synthesis of Cu2S and CuInS2. A postsynthetic base hydrolysis cleaves the ester, leaving a carboxylate corona around the nanocrystals and rendering the particles water-soluble.

Emission-tunable CuInS2/ZnS quantum dots: structure, optical properties, and application in white light-emitting diodes with high color rendering index

Synthesis and application of CuInS2/ZnS core/shell quantum dots (QDs) with varying [Cu]/[In] ratios were conducted using a stepwise solvothermal route. CuInS2 (CIS) core QDs with varying [Cu]/[In] ratios exhibited deep-red emissions result from donor-acceptor pair recombination. The absorption and emission band gap of the CuInS2 QDs increased with the decrease in Cu content. The emission bands of the CuInS2/ZnS were tuned from 550 to 616 nm by controlling the [Cu]/[In] ratio after coating ZnS layer. The CIS QDs model was developed to elucidate the synthesized crystal structure and photoluminescence of the QDs with various [Cu]/[In] ratios. Temperature-dependent photoluminescence spectra of the CIS/ZnS QDs were also investigated. The temperature dependency of the photoluminescence energy and intensity for various CIS/ZnS QDs were studied from 25 to 200 °C. Efficient white light-emitting diodes with high color rendering index values (Ra = 90) were fabricated using CIS/ZnS QDs as color converters in combination with green light-emitting Ba2SiO4:Eu(2+) phosphors and blue light-emitting diodes.

Photocharging Artifacts in Measurements of Electron Transfer in Quantum-Dot-Sensitized Mesoporous Titania Films

Transient absorption and time-resolved photoluminescence measurements of high-performance mesoporous TiO2 photoanodes sensitized with CuInSexS2-x quantum dots reveal the importance of hole scavenging in the characterization of photoinduced electron transfer. The apparent characteristic time of this process strongly depends on the local environment of the quantum dot/TiO2 junction due to accumulation of long-lived positive charges in the quantum dots. The presence of long-lived photoexcited holes introduces artifacts due to fast positive-trion Auger decay (60 ps time constant), which can dominate electron dynamics and thus mask true electron transfer. We show that the presence of a redox electrolyte is critical to the accurate characterization of charge transfer, since it enables fast extraction of holes and helps maintain charge neutrality of the quantum dots. Although electron transfer is observed to be relatively slow (19 ns time constant), a high electron extraction efficiency (>95%) can be achieved because in well-passivated CuInSexS2-x quantum dots neutral excitons have significantly longer lifetimes of hundreds of nanoseconds.

Efficient perovskite solar cells based on low-temperature solution-processed (CH3NH3)PbI3 perovskite/CuInS2 planar heterojunctions

In this work, the solution-processed CH3NH3PbI3 perovskite/copper indium disulfide (CuInS2) planar heterojunction solar cells with Al2O3 as a scaffold were fabricated at a temperature as low as 250°C for the first time, in which the indium tin oxide (ITO)-coated glass instead of the fluorine-doped tin oxide (FTO)-coated glass was used as the light-incidence electrode and the solution-processed CuInS2 layer was prepared to replace the commonly used TiO2 layer in previously reported perovskite-based solar cells. The influence of the thickness of the as-prepared CuInS2 film on the performance of the ITO/CuInS2(n)/Al2O3/(CH3NH3)PbI3/Ag cells was investigated. The ITO/CuInS2(2)/Al2O3/(CH3NH3)PbI3/Ag cell showed the best performance and achieved power conversion efficiency up to 5.30%.

Electrophoretic Deposition, Excited-State Dynamics, and Photovoltaic Performance

Ternary metal chalcogenides such as CuInS2 offer new opportunities to design quantum dot solar cells (QDSC). Chemically synthesized CuInS2 quantum dots (particle diameter, 2.6 nm) have been successfully deposited within the mesoscopic TiO2 film using electrophoretic deposition (150 V cm(-1) dc field). The primary photoinduced process of electron injection from excited CuInS2 into TiO2 occurs with a rate constant of 5.75 × 10(11) s(-1). The TiO2/CuInS2 films are photoactive and produce anodic photocurrent with a power conversion efficiency of 1.14%. Capping the TiO2/CuInS2 film with a CdS layer decreases the interfacial charge recombination and thus offers further improvement in the power conversion efficiency (3.91%). The synergy of using CdS as a passivation layer in the composite film is also evident from the increased external quantum efficiency of the electrode in the red region where only CuInS2 absorbs the incident light.

Engineered CuInSexS2-x Quantum Dots for Sensitized Solar Cells

Colloidal CuInSexS2-x quantum dots (QDs) are an attractive less-toxic alternative to PbX and CdX (X = S, Se, and Te) QDs for solution-processed semiconductor devices. This relatively new class of QD materials is particularly suited to serving as an absorber in photovoltaics, owing to its high absorption coefficient and near-optimal and finely tunable band gap. Here, we engineer CuInSexS2-x QD sensitizers for enhanced performance of QD-sensitized TiO2 solar cells (QDSSCs). Our QD synthesis employs 1-dodecanethiol (DDT) as a low-cost solvent, which also serves as a ligand, and a sulfur precursor; addition of triakylphosphine selenide leads to incorporation of controlled amounts of selenium, reducing the band gap compared to that of pure CuInS2 QDs. This enables significantly higher photocurrent in the near-infrared (IR) region of the solar spectrum without sacrificing photovoltage. In order to passivate QD surface recombination centers, we perform a surface-cation exchange with Cd prior to sensitization, which enhances chemical stability and leads to a further increase in photocurrent. We use the synthesized QDs to demonstrate proof-of-concept QDSSCs with up to 3.5% power conversion efficiency.