Synthesis and Transport Properties of ZnSnP2-yAsy Chalcopyrite Solid Solutions

This work focuses on the synthesis and properties of quaternary ZnSnP2-yAsy chalcopyrite solid solutions. Full miscibility of the solid solution is achieved using ball milling followed by hot press sintering. The measured electrical conductivity increases substantially with As substitution from 0.03 S cm-1 for ZnSnP2 to 10.3 S cm-1 for ZnSnAs2 at 715 K. Band gaps calculated from the activation energies show a steady decrease with increasing As concentration from 1.4 eV for ZnSnP2 to 0.7 eV for ZnSnAs2. The Seebeck coefficient decreases significantly with As substitution from nearly 1000 μV K-1 for ZnSnP2 to -100 μV K-1 for ZnSnAs2 at 650 K. Thermal conductivity is decreased for the solid solutions due to alloy phonon scattering, compared to the end members with y = 0 and y = 2, with the y = 0.5 and y = 1.0 samples exhibiting the lowest values of 1.4 W m-1 K-1 at 825 K. Figure of merit values are increased for the undoped solid solutions at lower temperatures when compared to the end members due to the decreased thermal conductivity, with the y = 0.5 sample reaching zT = 1.6 × 10-3 and y = 1 reaching 2.1 × 10-3 at 700 K. The largest values of the figure of merit zT for the undoped series was found for y = 2 with zT = 2.8 × 10-3 at 700 K due to the increasing n-type Seebeck coefficient. Boltztrap calculations reveal that p-doping could yield zT values above unity at 800 K in case of ZnSnAs2, comparable with ZnSnP2.

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

Optical study of back-contacted CIGS solar cells

A novel back-contacted solar cell based on a submicron copper indium gallium (di)selenide (CIGS) absorber is proposed and optically investigated. First, charge carrier collection feasibility is studied by band diagram analysis. Then, two back-contacted configurations are suggested and optimized for maximum current production. The results are compared with a reference front/back-contacted CIGS solar cell with a 750-nm-thick absorber. Current density production of 38.84 mA/cm² is predicted according to our simulations for a realistic front-side texturing. This shows more than 38% improvement in optical performance compared to the reference cell and only 7.7% deviation from the theoretical Green absorption benchmark.

Solution based synthesis of Cu(In,Ga)Se2 microcrystals and thin films

Herein, for the first time, we report the synthesis of quaternary Cu(In,Ga)Se₂ microcrystals (CIGSe MCs) using a facile and economical one-pot heating-up method. The most important parameters such as reaction temperature and time were varied to study their influences on the structural, morphological, compositional and optical properties of the MCs. Based on the results, the formation of CIGSe was initiated from binary β-CuSe and then converted into pure phase CIGSe by gradual incorporation of In³⁺ and Ga³⁺ ions into the β-CuSe crystal lattice. As the reaction time increases, the band gap energy was increased from 1.10 to 1.28 eV, whereas the size of the crystals increased from 0.9 to 3.1 μm. Besides, large-scale synthesis of CIGSe MCs exhibited a high reaction yield of 90%. Furthermore, the CIGSe MCs dispersed in the ethanol was coated as thin films by a drop casting method, which showed the optimum carrier concentration, high mobility and low resistivity. Moreover, the photoconductivity of the CIGSe MC thin film was enhanced by three order magnitude in comparison with CIGSe NC thin films. The solar cells fabricated with CIGSe MCs showed the PCE of 0.59% which is 14.75 times higher than CIGSe NCs. These preliminary results confirmed the potential of CIGSe MCs as an active absorber layer in low-cost thin film solar cells.

Herein, for the first time, we report the synthesis of quaternary Cu(In,Ga)Se₂ microcrystals (CIGSe MCs) using a facile and economical one-pot heating-up method.

"Green" Aqueous Synthesis and Advanced Spectral Characterization of Size-Selected Cu2ZnSnS4 Nanocrystal Inks

Structure, composition, and optical properties of colloidal mercaptoacetate-stabilized Cu2ZnSnS4 (CZTS) nanocrystal inks produced by a "green" method directly in aqueous solutions were characterized. A size-selective precipitation procedure using 2-propanol as a non-solvent allows separating a series of fractions of CZTS nanocrystals with an average size (bandgap) varying from 3 nm (1.72 eV) to 2 nm (2.04 eV). The size-selected CZTS nanocrystals revealed also phonon confinement, with the main phonon mode frequency varying by about 4 cm-1 between 2 nm and 3 nm NCs.

Solar light harvesting with multinary metal chalcogenide nanocrystals

The paper reviews the state of the art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into the electric current or the accumulation of solar energy in the form of products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to produce the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

Transparent and flexible fingerprint sensor array with multiplexed detection of tactile pressure and skin temperature

We developed a transparent and flexible, capacitive fingerprint sensor array with multiplexed, simultaneous detection of tactile pressure and finger skin temperature for mobile smart devices. In our approach, networks of hybrid nanostructures using ultra-long metal nanofibers and finer nanowires were formed as transparent, flexible electrodes of a multifunctional sensor array. These sensors exhibited excellent optoelectronic properties and outstanding reliability against mechanical bending. This fingerprint sensor array has a high resolution with good transparency. This sensor offers a capacitance variation ~17 times better than the variation for the same sensor pattern using conventional ITO electrodes. This sensor with the hybrid electrode also operates at high frequencies with negligible degradation in its performance against various noise signals from mobile devices. Furthermore, this fingerprint sensor array can be integrated with all transparent forms of tactile pressure sensors and skin temperature sensors, to enable the detection of a finger pressing on the display.

Next-generation mobile security devices require fingerprint sensors that can be incorporated directly into the display. Here, Park et al. demonstrate a highly transparent, multifunctional capacitive fingerprint sensor array that simultaneously detects tactile pressure and finger skin temperature.

Luminescence and photoelectrochemical properties of size-selected aqueous copper-doped Ag-In-S quantum dots

Ternary luminescent copper and silver indium sulfide quantum dots (QDs) can be an attractive alternative to cadmium and lead chalcogenide QDs. The optical properties of Cu-In-S and Ag-In-S (AIS) QDs vary over a broad range depending on the QD composition and size. The implementation of ternary QDs as emitters in bio-sensing applications can be boosted by the development of mild and reproducible syntheses directly in aqueous solutions as well as the methods of shifting the photoluminescence (PL) bands of such QDs as far as possible into the near IR spectral range. In the present work, the copper-doping of aqueous non-stoichiometric AIS QDs was found to result in a red shift of the PL band maximum from around 630 nm to ∼780 nm and PL quenching. The deposition of a ZnS shell results in PL intensity recovery with the highest quantum yield of 15%, with almost not change in the PL band position, opposite to the undoped AIS QDs. Size-selective precipitation using 2-propanol as a non-solvent allows discrimination of up to 9 fractions of Cu-doped AIS/ZnS QDs with the average sizes in the fractions varying from around 3 to 2 nm and smaller and with reasonably the same composition irrespective of the QD size. The decrease of the average QD size results in a blue PL shift yielding a series of bright luminophors with the emission color varies from deep-red to bluish-green and the PL efficiency increases from 11% for the first fraction to up to 58% for the smallest Cu-doped AIS/ZnS QDs. The rate constant of the radiative recombination of the size-selected Cu-doped AIS/ZnS QDs revealed a steady growth with the QD size decrease as a result of the size-dependent enhancement of the spatial exciton confinement. The copper doping was found to result in an enhancement of the photoelectrochemical activity of CAIS/ZnS QDs introduced as spectral sensitizers of mesoporous titania photoanodes of liquid-junction solar cells.

Structural and electronic properties of defects at grain boundaries in CuInSe2

We report on a first-principles study of the structural and electronic properties of a Σ3 (112) grain boundary model in CuInSe₂. The study focuses on a coherent, stoichiometry preserving, cation-Se terminated grain boundary, addressing the properties of the grain boundary as such, as well as the effect of well known defects in CuInSe₂. We show that in spite of its apparent simplicity, such a grain boundary exhibits a very rich phenomenology, providing an explanation for several of the experimentally observed properties of grain boundaries in CuInSe₂ thin films. In particular, we show that the combined effect of Cu vacancies and cation antisites can result in the observed Cu depletion with no In enrichment at the grain boundaries. Furthermore, Cu vacancies are unlikely to produce a hole barrier at the grain boundaries, but Na may indeed have such an effect. We find that Na-on-Cu defects will tend to form abundantly at the grain boundaries, and can provide a mechanism for the carrier depletion and/or type inversion experimentally reported.

Room temperature synthesis of CuInS2 nanocrystals

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