Zero‐Dimensional Perovskite Nanocrystals for Efficient Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are able to efficiently harvest solar energy through large‐area photovoltaic windows, where fluorophores are delicately embedded. Among various types of fluorophores, all‐inorganic perovskite nanocrystals (NCs) are emerging candidates as absorbers/emitters in LSCs due to their size/composition/dimensionality tunable optical properties and high photoluminescence quantum yield (PL QY). However, due to the large overlap between absorption and emission spectra, it is still challenging to fabricate high‐efficiency LSCs. Intriguingly, zero‐dimensional (0D) perovskites provide a number of features that meet the requirements for a potential LSC absorber, including i) small absorption/emission spectral overlap (Stokes shift up to 1.5 eV); ii) high PL QY (>95% for bulk crystal); iii) robust stability as a result of its large exciton binding energy; and iv) ease of synthesis. In this work, as a proof‐of‐concept experiment, Cs₄PbBr₆ perovskite NCs are used to fabricate semi‐transparent large‐area LSCs. Cs₄PbBr₆ perovskite film exhibits green emission with a high PL QY of ≈58% and a small absorption/emission spectral overlap. The optimized LSCs exhibit an external optical efficiency of as high as 2.4% and a power conversion efficiency of 1.8% (100 cm²). These results indicate that 0D perovskite NCs are excellent candidates for high‐efficiency LSCs compared to 3D perovskite NCs.     

Highly Emissive and Color‐Tunable CuInS2‐Based Colloidal Semiconductor Nanocrystals: Off‐Stoichiometry Effects and Improved Electroluminescence Performance

The off‐stoichiometry effects and gram‐scale production of luminescent CuInS₂‐based semiconductor nanocrystals, as well as their application in electroluminescence devices are reported. The crystal structures and optical properties of CuInS₂ nanocrystals can be significantly influenced by controlling their [Cu]/[In] molar ratio. A simple model adapted from the bulk materials is proposed to explain their off‐stoichiometry effects. Highly emissive and color‐tunable CuInS₂‐based NCs are prepared by a combination of [Cu]/[In] molar ratio optimization, ZnS shell coating, and CuInS₂–ZnS alloying. The method is simple, hassle‐free, and easily scalable to fabricate tens of grams of nanocrystal powders with photoluminescence quantum yields up to around 65%. Furthermore, the performance of high‐quality CuInS₂‐based NCs in electroluminescence devices is examined. These devices have lower turn‐on voltages of around 5 V, brighter luminance up to approximately 2100 cd m^?2 and improved injection efficiency of around 0.3 lm W^?1 (at 100 cd m^?2) in comparison to recent reports.     

Ligand‐Free Synthesis of Aluminum‐Doped Zinc Oxide Nanocrystals and their Use as Optical Spacers in Color‐Tuned Highly Efficient Organic Solar Cells

The color of polymer solar cells using an opaque electrode is given by the reflected light, which depends on the composition and thickness of each layer of the device. Metal‐oxide‐based optical spacers are intensively studied in polymer solar cells aiming to optimize the light absorption. However, the low conductivity of materials such as ZnO and TiO₂ limits the thickness of such optical spacers to tenths of nanometers. A novel synthesis route of cluster‐free Al‐doped ZnO (AZO) nanocrystals (NCs) is presented for solution processing of highly conductive layers without the need of temperature annealing, including thick optical spacers on top of polymer blends. The processing of 80 nm thick optical spacers based on AZO nanocrystal solutions on top of 200 nm thick polymer blend layer is demonstrated leading to improved photocurrent density of 17% compared to solar cells using standard active layers of 90 nm in combination with thin ZnO‐based optical spacers. These AZO NCs also open new opportunities for the processing of high‐efficiency color tuned solar cells. For the first time, it is shown that applying solution‐processed thick optical spacer with polymer blends of different thicknesses can process solar cells of similar efficiency over 7% but of different colors.     

Luminescent solar concentrators employing new Eu(TTA)3phen‐containing parylene films

New robust luminescent solar concentrators were produced by growing Eu(TTA)₃phen‐containing parylene thin films on poly(methyl methacrylate) slabs through a novel co‐deposition process. As it is the first time that Eu(TTA)₃phen molecules were inserted in parylene matrix, the deposition process was investigated, and the features of the samples were extensively analyzed. Fourier transform infrared spectroscopy analysis demonstrated the integrity of the vacuum‐sublimated Eu(TTA)₃phen molecules. Atomic force microscope analysis showed the very flat surface of the films (Rq = 3.7 nm), which is strategic in luminescent solar concentrators (LSCs) for minimizing the light scattering at the air/film interface. The optical measurements demonstrated that Eu(TTA)₃phen‐containing parylene exhibits higher absorption than the conventional Eu(TTA)₃phen films and the luminescence peaks characteristic of Eu(TTA)₃phen compound. This indicates that the Eu complex is not affected by the matrix interaction thus maintaining its extremely large Stokes shift. Moreover, the parylene matrix improves the luminescence intensity of the films: in fact, under the same absorption, these films show a luminescence intensity more than two times higher than standard Eu(TTA)₃phen ones. The current–voltage (I–V) measurements show that, under the same quantity of Eu(TTA)₃phen, Eu(TTA)₃phen‐containing parylene LSCs produce a current density more than twice as high as LSC without parylene and that their efficiency decreases more than ten times slower than organic‐based LSCs at increasing illuminating area, thus highlighting the feasibility of developing large size LSCs. Copyright © 2014 John Wiley & Sons, Ltd.     

Scale up the collection area of luminescent solar concentrators towards metre‐length flexible waveguiding photovoltaics

Luminescent solar concentrators (LSCs) are cost‐effective components easily integrated in photovoltaics (PV) that can enhance solar cells' performance and promote the integration of PV architectural elements into buildings, with unprecedented possibilities for energy harvesting in façade design, urban furnishings and wearable fabrics. The devices' performance is dominated by the concentration factor (F), which is higher in cylindrical LSCs compared with planar ones (with equivalent collection area and volume). The feasibility of fabricating long‐length LSCs has been essentially limited up to ten of centimetres with F < 1. We use a drawing optical fibre facility to easily scale up large‐area LSCs (length up to 2.5 m) based on bulk and hollow‐core plastic optical fibres (POFs). The active layers used to coat the bulk fibres or fill the hollow‐core ones are Rhodamine 6G‐ or Eu³⁺‐doped organic–inorganic hybrids. For bulk‐coated LSCs, light propagation occurs essentially at the POFs, whereas for hollow‐core device light is also guided within the hybrid. The lower POFs' attenuation (~0.1 m⁻¹) enables light propagation in the total fibre length (2.5 m) for bulk‐coated LSCs with maximum optical conversion efficiency (η_opt) and F of 0.6% and 6.5, respectively. For hollow‐core LSCs, light propagation is confined to shorter distances (6–9 × 10⁻² m) because of the hybrids' attenuation (1–15 m⁻¹). The hollow‐core optimised device displays η_opt = 72.4% and F = 12.3. The F values are larger than the best ones reported in the literature for large‐area LSCs (F = 4.4), illustrating the potential of this approach for the development of lightweight flexible high‐performance waveguiding PV. Copyright © 2016 John Wiley & Sons, Ltd.     

Solar‐Energy Conversion in TiO2/CuInS2 Nanocomposites

The search for low‐cost thin‐film solar cells, to replace silicon multi‐crystalline cells in due course, calls for new combinations of materials and new cell configurations. Here we report on a new approach, based on semiconductor nanocomposites, towards what we refer to as the three‐dimensional (3D) solar‐cell concept. Atomic layer chemical vapor deposition is employed for infiltration of CuInS₂ inside the pores of nanostructured TiO₂. In this way it is possible to obtain a nanometer‐scale interpenetrating network between n‐type TiO₂ and p‐type CuInS₂. X‐ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, scanning electron microscopy, transmission electron microscopy, and current–voltage measurements are used to characterize the nanostructured devices. The 3D solar cells obtained show photovoltaic activity with a maximum monochromatic incident photon‐to‐current conversion efficiency of 80 % and have an energy‐conversion efficiency of 4 %.     

Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

Recent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (E_g = 3·6–3·8 eV) than the conventional buffer material CdS (E_g = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS₂ (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm² in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm² (total area: 22·5 mA/cm²). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm² substrates have been prepared and already reach an efficiency (active area: 17·2 cm²) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.     

Ultrasmall CuCo2S4 Nanocrystals: All‐in‐One Theragnosis Nanoplatform with Magnetic Resonance/Near‐Infrared Imaging for Efficiently Photothermal Therapy of Tumors

Copper‐based ternary bimetal chalcogenides have very promising potential as multifunctional theragnosis nanoplatform for photothermal treatment of tumors. However, the design and synthesis of such an effective platform remains challenging. In this study, hydrophilic CuCo₂S₄ nanocrystals (NCs) with a desirable size of ≈10 nm are synthesized by a simple one‐pot hydrothermal route. The as‐prepared ultrasmall CuCo₂S₄ NCs show: 1) intense near‐infrared absorption, which is attributed to 3d electronic transitions from the valence band to an intermediate band, as identified by density functional theory calculations; 2) high photothermal performance with a photothermal conversion efficiency up to 73.4%; and 3) capability for magnetic resonance (MR) imaging, as a result of the unpaired 3d electrons of cobalt. Finally, it is demonstrated that the CuCo₂S₄ NCs are a promising “all‐in‐one” photothermal theragnosis nanoplatform for photothermal cancer therapy under the irradiation of a 915 nm laser at a safe power density of 0.5 W cm^?2, guided by MR and infrared thermal imaging. This work further promotes the potential applications of ternary bimetal chalcogenides for photothermal theragnosis therapy.     

Synthesis and characterization of CuInS2 microsphere under controlled reaction conditions and its application in low-cost solar cells

CuInS₂ microspheres were synthesized by Ultrasonic method in propylene glycol as solvent and copper oxalate, indium chloride and thioacetamde (TAA) as precursors. Optimum conditions such as reaction time, solvent type, sulfur source, and ultrasonic power were determined. Then, a thin film of CuInS₂ was prepared and its application in solar cells was investigated. Photovoltaic characteristics such as V_oc, J_sc and FF were measured. X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy were performed to characterize the CuInS₂ microsphere. The optical band gap of the CuInS₂ microsphere was estimated to be 2.28 eV.     

Effect of zinc doping and temperature on the properties of sprayed CuInS2 thin films

Copper indium disulfide (CuInS₂) is an efficient absorber material for photovoltaic and solar cell applications. The structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified by suitably doping CuInS₂ thin films with dopants such as Zn, Sn, Bi, Cd, Na, N, O, P and As. In this work Zn (0.01 M) doped CuInS₂ thin films are (Cu/In=1.25) deposited on to glass substrates in the temperature range 300–400 °C. It is observed that the film growth temperature, ion ratio (Cu/In=1.25) and Zn-doping affect structural, optical, photoluminescence and electrical properties of sprayed CuInS₂ thin films. As the XRD patterns depict, Zn-doping facilitates the growth of CuInS₂ thin films along (112) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Zn in the films. The optical studies show, about 90% of light transmission occurs in the IR regions; hence Zn-doped CuInS₂ can be used as an IR transmitter. The absorption coefficient (α) in the UV–visible region is found to be in the order of 10⁴–10⁵ cm⁻¹ which is the optimum value for an efficient absorber. The optical band gap energies increase with increase of temperatures (1.66–1.78 eV). SEM photographs reveal crystalline and amorphous nature of the films at various temperature ranges. Photoluminescence study shows that well defined broad Blue and Green band emissions are exhibited by Zn-doped CuInS₂ thin films. All the films present low resistivity (ρ) values and exhibit semiconducting nature. An evolution of p-type to n-type conductivity is obtained in the temperature range 325–350 °C. Hence, Zn species can be used as a donor and acceptor impurity in CuInS₂ thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters.     

Industrially feasible,dopant‐free,carrier‐selective contacts for high‐efficiency silicon solar cells

Dopant‐free, carrier‐selective contacts (CSCs) on high efficiency silicon solar cells combine ease of deposition with potential optical benefits. Electron‐selective titanium dioxide (TiO₂) contacts, one of the most promising dopant‐free CSC technologies, have been successfully implemented into silicon solar cells with an efficiency over 21%. Here, we report further progress of TiO₂ contacts for silicon solar cells and present an assessment of their industrial feasibility. With improved TiO₂ contact quality and cell processing, a remarkable efficiency of 22.1% has been achieved using an n‐type silicon solar cell featuring a full‐area TiO₂ contact. Next, we demonstrate the compatibility of TiO₂ contacts with an industrial contact‐firing process, its low performance sensitivity to the wafer resistivity, its applicability to ultrathin substrates as well as its long‐term stability. Our findings underscore the great appeal of TiO₂ contacts for industrial implementation with their combination of high efficiency with robust fabrication at low cost. Copyright © 2017 John Wiley & Sons, Ltd.     

Size-dependent polymer/CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots

This paper reports the size-dependent performance in polymer/CuInS₂ solar cells with tunable synthesis of chalcopyrite CuInS₂ quantum dots (QDs) by the solvothermal method. The CuInS₂ QDs of 3.2–5.4 nm in size are fine tuned by the reaction time in the solvothermal process with the slow supply of In³⁺ ions during the crystallization, and the band gaps increased with QDs sizes decreasing according to the results from the characterization of sizes, morphologies, component elements, valence states and band gaps of CuInS₂ QDs. We fabricated MEH-PPV/CuInS₂ solar cells, and the photoactive layer of device displayed size-dependent light-harvesting, charge separation and transport ability. Moreover, the solar cells exhibit size-dependent short circuit current (J_sc) and open circuit voltage (V_oc), with higher performance in both J_sc and V_oc for smaller CuInS₂ QDs, resulting in the maximum power conversion efficiency of ca. 0.12% under the monochromic illumination at 470 nm; CuInS₂ QDs actually serve as an effective electron acceptor material for the MEH-PPV/CuInS₂ solar cells with the wide spectral response extending from 300 to 900 nm.     

Localization of Au Nanoclusters on Layered Double Hydroxides Nanosheets: Confinement‐Induced Emission Enhancement and Temperature‐Responsive Luminescence

Gold nanoclusters (Au NCs) stand for a new type of fluorescent nanomaterials with outstanding optical properties due to their discrete electronic energy and direct electron transition. However, relative low quantum yield (QY) of Au NCs in aqueous or solid state has limited their photofunctional applications. To improve the fluorescent performances of Au NCs and find an effective approach for the fabrication of Au NCs‐based films, in this work, Au NCs are localized onto 2D layered double hydroxides (LDHs) nanosheets via a layer‐by‐layer assembly process; the as‐fabricated (Au NCs/LDH)_n ultrathin films (UTFs) show an ordered and dense immobilization of Au NCs. The localization and confinement effects imposed by LDH nanosheets induce significantly increased emissive Au(I) units as confirmed by X‐ray photoelectron spectroscopy and periodic density functional theoretical simulation, which further results in promoted QY (from 2.69% to 14.11%) and prolonged fluorescence lifetime (from 1.84 µs to 14.67 µs). Moreover, the ordered (Au NCs/LDH)_n UTFs exhibit well‐defined temperature‐dependent photoluminescence (PL) and electrochemiluminescence (ECL) responses. Therefore, this work supplies a facile strategy to achieve the immobilization of Au NCs and obtain Au NCs‐based thin films with high luminescent properties, which have potential applications in PL and ECL temperature sensors.     

Hybrid Solar Cells Based on Nanoparticles of CuInS2 in Organic Matrices

We report on solution‐processed hybrid solar cells consisting of a nanocrystalline inorganic semiconductor, CuInS₂, and organic materials. Synthesis of quantized CuInS₂ nanoparticles was performed using a colloidal route, where the particle surface was shielded by an organic surfactant. First attempts were made to use nanocrystalline CuInS₂ with fullerene derivatives to form flat‐interface donor–acceptor heterojunction solar cells. We investigated also bulk heterojunctions by replacing the CuInS₂ single layer by a blend of CuInS₂ and p‐type polymer (PEDOT:PSS; poly(3,4‐ethylenedioxythiophene:poly(styrene sulfonic acid) in the same cell configuration. Bulk heterojunction solar cells show better photovoltaic response with external quantum efficiencies up to 20 %.     

Magnetic/Fluorescent Barcodes Based on Cadmium‐Free Near‐Infrared‐Emitting Quantum Dots for Multiplexed Detection

Magnetic/fluorescent barcodes, which combine quantum dots (QDs) and superparamagnetic nanoparticles in micrometer‐sized host microspheres, are promising for automatic high‐throughput multiplexed biodetection applications and “point of care” biodetection. However, the fluorescence intensity of QDs sharply decreases after addition of magnetic nanoparticles (MNPs) due to absorption by MNPs, and thus, the encoding capacity of QDs becomes more limited. Furthermore, the intrinsic toxicity of cadmium‐based QDs, the most commonly used QD in barcodes, has significant risks to human health and the environment. In this work, to alleviate fluorescence quenching and intrinsic toxicity, cadmium‐free NIR‐emitting CuInS₂/ZnS QDs and Fe₃O₄ MNPs are successfully incorporated into poly(styrene‐co‐maleic anhydride) microspheres by using the Shirasu porous glass membrane emulsification technique. A “single‐wavelength” encoding model is successfully constructed to guide the encoding of NIR QDs with wide emission spectra. Then, a “single‐wavelength” encoding combined with size encoding is used to produce different optical codes by simply changing the wavelength and the intensity of the QDs as well as the size of the barcode microspheres. 48 barcodes are easily created due to the greatly reduced energy transfer between the NIR‐emitting QDs and MNPs. The resulting bifunctional barcodes are also combined with a flow cytometer using one laser for multiplexed detection of five tumor markers in one test. Assays based on these barcodes are significantly more sensitive than non‐magnetic and traditional ELISA assays. Moreover, validating experiments also show good performance of the bifunctional barcodes‐based suspension array when dealing with patient serum samples. Thus, magnetic/fluorescent barcodes based on NIR‐emitting CuInS₂/ZnS QDs are promising for multiplexed bioassay applications.     

Size‐Controlled Synthesis of Cu2‐xE (E = S,Se) Nanocrystals with Strong Tunable Near‐Infrared Localized Surface Plasmon Resonance and High Conductivity in Thin Films

A facile method for preparing highly self‐doped Cu_2‐xE (E = S, Se) nanocrystals (NCs) with controlled size in the range of 2.8–13.5 nm and 7.2–16.5 nm, for Cu_2‐xS and Cu_2‐xSe, respectively, is demonstrated. Strong near‐infrared localized surface plasmon resonance absorption is observed in the NCs, indicating that the as‐prepared particles are heavily p‐doped. The NIR plasmonic absorption is tuned by varying the amount of oleic acid used in synthesis. This effect is attributed to a reduction in the number of free carriers through surface interaction of the deprotonated carboxyl functional group of oleic acid with the NCs. This approach provides a new pathway to control both the size and the cationic deficiency of Cu_2‐xSe and Cu_2‐xS NCs. The high electrical conductivity exhibited by these NPs in metal‐semiconductor‐metal thin film devices shows promise for applications in printable field‐effect transistors and microelectronic devices.     

Investigation of the morphology and electrical characteristics of hybrid blends based on poly(3-hexylthiophene) and colloidal CuInS2 nanocrystals of different shapes

Colloidally synthesized CuInS₂ nanocrystals are a promising candidate for hybrid solar cell applications due to suitable optical and transport properties of copper indium disulfide and being it an eco-friendly material. However, as opposite to solar cells where CuInS₂ is synthesized in situ in a conductive polymer matrix, advances in the field of hybrid solar cells containing colloidal CuInS₂ nanocrystals that are blended after synthesis with a polymer are still negligible. Here, we report about the influence of pyridine, alkylamine, and hexanethiol stabilizing ligands on the morphology of the active layer and the electrical characteristics of solar cells based on elongated and pyramidal CuInS₂ nanocrystals blended with poly(3-hexylthiophene) (P3HT). All CuInS₂ nanocrystals used within this study had a wurtzite crystal structure as revealed by X-ray diffraction. With pyridine as ligand, the morphology was found to depend strongly on the shape of the nanocrystals. Strong agglomeration was observed in the case of elongated nanocrystals and explains the low performance of corresponding solar cells. Employment of hexanethiol as ligand resulted in an improvement of the morphology of the CuInS₂/P3HT layers and enhancement of the rectification ratio of the laboratory solar cells. Nevertheless, it was found that morphology of the active layer is not the main limiting factor in the CuInS₂/P3HT system. According to cyclic voltammetry measurements, unsuitable alignment of the energy levels for CuInS₂ nanocrystals and P3HT was observed. Taking this fact into account, appropriate donor materials for CuInS₂ based bulk heterojunctions are discussed.     

Photovoltaic response of hybrid solar cells with alloyed ZnS–CuInS2 nanorods

CuInS₂ and ZnS are miscible so that quaternary ZnS–CuInS₂ alloys can be obtained. This opens the possibility to tune optical properties of the material in a wide range via control of the elemental composition. In the present work, ZnS–CuInS₂ nanorods were synthesized by means of colloidal chemistry. Their absorption properties were studied in detail, and different types of optical transitions identified. In view of optoelectronic applications, the nanoparticles were examined for their suitability as absorber material in hybrid polymer/nanoparticle solar cells. Therefore, the nanorods were combined with a common low band gap polymer. Cyclic voltammetry and electron spin resonance were used to study the alignment of the energy levels at the heterojunction as well as the possibility of charge transfer. The material combination forms a type II heterojunction, but with the nanoparticles acting as electron donor material. The blends were implemented in hybrid solar cells. Although the photocurrent density and efficiency achieved were relatively low, the system showed a high open-circuit voltage exceeding the value 1 V.     

Plasmonic Au/TiO2‐Dumbbell‐On‐Film Nanocavities for High‐Efficiency Hot‐Carrier Generation and Extraction

Plasmon‐induced hot carriers have vast potential for light‐triggered high‐efficiency carrier generation and extraction, which can overcome the optical band gap limit of conventional semiconductor‐based optoelectronic devices. Here, it is demonstrated that Au/TiO₂ dumbbell nanostructures assembled on a thin Au film serve as an efficient optical absorber and a hot‐carrier generator in the visible region. Upon excitation of localized surface plasmons in such coupled particle‐on‐film nanocavities, the energetic conduction electrons in Au can be injected over the Au/TiO₂ Schottky barrier and migrated to TiO₂, participating in the chemical reaction occurring at the TiO₂ surface. Compared with the same dumbbell nanostructures on an indium tin oxide (ITO) film, such nanocavities exhibit remarkable enhancement in both photocurrent amplitude and reaction rate that arise from increased light absorption and near‐field amplification in the presence of the Au film. The incident‐wavelength‐dependent photocurrent and reaction rate measurements jointly reveal that Au‐film‐mediated near‐field localization facilitates more efficient electron–hole separation and transport in the dumbbells and also promotes strong d‐band optical transitions in the Au film for generation of extra hot electrons. Such nanocavities provide a new plasmonic platform for effective photoexcitation and extraction of hot carriers and also better understanding of their fundamental science and technological implications in solar energy harvesting.     

Virtually Bare Nanocrystal Surfaces: Significantly Enhanced Electrical Transport in CuInSe2 and CuIn1−xGaxSe2 Thin Films upon Ligand Exchange with Thermally Degradable 1‐Ethyl‐5‐Thiotetrazole

A facile and safe ligand exchange method for readily synthesized CuInSe₂ (CIS) and CuIn_1‐xGa_xSe₂ (CIGS) nanocrystals (NCs) from oleylamine to 1‐ethyl‐5‐thiotetrazole, preserving the colloidal stability of the chalcopyrite structure, is presented. 1‐Ethyl‐5‐thiotetrazole as thermally degradable ligand is adapted for the first time for trigonal pyramidal CIS (18 nm), elongated CIS (9 nm) and CIGS NCs (6 nm). Exchanged NC solutions are processed onto gold electrodes yielding ordered thin films. These films are thermally annealed at 260 °C to completely remove 1‐ethyl‐5‐thiotetrazol leaving individual closely assembled NCs with virtually bare surfaces. The current–voltage characteristics of the NC solids are measured prior to ligand thermolysis in the dark and under illumination and after ligand thermolysis in the same manner. The conductivity of trigonal pyramidal CIS increases by four orders of magnitude (1.4 × 10^?9 S cm^?1 (dark) to 1.4 × 10^?5 S cm^?1 (illuminated)) for ligand‐free NC films. Elongated CIS NC films show a three orders of magnitude conductivity increase and CIGS NC films exhibit improved conductivity by two orders of magnitude. Conductivity enhancement thereby depends on the NC size accentuating the role of trap‐states and internal grain boundaries in ligand‐free NC solids for electrical transport. This approach for the first time offers the possibility to address chalcopyrite materials’ electrical properties in a virtually ligand‐free state.