#TeddersRecommends

Solution processing of inorganic thin films has become an important thrust in material research community because it offers low-cost and high-throughput deposition of various functional coatings and devices. Especially inorganic thin film solar cells – macroelectronic devices that rely on consecutive deposition of layers on large-area rigid and flexible substrates – could benefit from solution approaches in order to realize their low-cost nature. This article critically reviews existing deposition approaches of functional layers for chalcogenide solar cells with an extension to other thin film technologies. Only true solutions of readily available metal salts in appropriate solvents are considered without the need of pre-fabricated nanoparticles. By combining three promising approaches, an air-stable Cu(In,Ga)Se₂ thin film solar cell with efficiency of 13.8% is demonstrated where all constituent layers (except the metal back contact) are processed from solutions. Notably, water is employed as the solvent in all steps, highlighting the potential for safe manufacturing with high utilization rates.

A Cu(In,Ga)Se₂ thin film solar cell with efficiency of 13.8% is demonstrated where all constituent layers (except the metal back contact) are processed from aqueous solutions of metal salts using industrially scalable processes.

A high mobility of 109.0 cm² V⁻¹ s⁻¹ is obtained by thin-film transistors (TFTs) comprising a composite made by aligning SnO₂ nanowires (NWs) in amorphous InGaZnO (a-IGZO) thin films. This composite TFT reaches an on-current density of 61.4 μA μm⁻¹ with a 10 μm channel length. Its performance surpasses that of single-crystalline InGaZnO and is comparable with that of polycrystalline silicon.

We have independently investigated the source of tramadol, a synthetic analgesic largely used for treating moderate to severe pain in humans, recently found in the roots of the Cameroonian medicinal plant, Nauclea latifolia. We found tramadol and its three major mammalian metabolites (O-desmethyltramadol, N-desmethyltramadol, and 4-hydroxycyclohexyltramadol) in the roots of N. latifolia and five other plant species, and also in soil and local water bodies only in the Far North region of Cameroon. The off-label administration of tramadol to cattle in this region leads to cross-contamination of the soil and water through feces and urine containing parent tramadol as well as tramadol metabolites produced in the animals. These compounds can then be absorbed by the plant roots and also leached into the local water supplies. The presence of tramadol in roots is, thus, due to an anthropogenic contamination with the synthetic compound.

The root of the problem: Tramadol, a synthetic analgesic, was recently detected in the roots of the Cameroonian medicinal plant Nauclea latifolia. However, tramadol is not a natural product. Along with its major mammalian metabolites, tramadol is present in the roots of N. latifolia and other plant species, as well as in soil and local water sources in the Far North region of Cameroon as a result of anthropogenic contamination.

Graphitic carbon nitride can be imprinted with a twisted hexagonal rod-like morphology by a nanocasting technique using chiral silicon dioxides as templates. The helical nanoarchitectures promote charge separation and mass transfer of carbon nitride semiconductors, enabling it to act as a more efficient photocatalyst for water splitting and CO₂ reduction than the pristine carbon nitride polymer. This is to our knowledge a unique example of chiral graphitic carbon nitride that features both left- and right-handed helical nanostructures and exhibits unique optical activity to circularly polarized light at the semiconductor absorption edge as well as photoredox activity for solar-to-chemical conversion. Such helical nanostructured polymeric semiconductors are envisaged to hold great promise for a range of applications that rely on such semiconductor properties as well as chirality for photocatalysis, asymmetric catalysis, chiral recognition, nanotechnology, and chemical sensing.

Helical nanorods: Graphitic carbon nitride with helical nanorod-like morphology similar to spiral vines is constructed for artificial photosynthesis. This helically conjugated polymer can also be fabricated with opposite chirality and shows photocatalytic activity for water splitting and CO₂ conversion.

The tuning of charge carrier concentrations in semiconductor is necessary in order to approach high performance of the electronic and optoelectronic devices. It is demonstrated that the charge-carrier density of single-layer (SL), bilayer (BL), and few-layer (FL) MoS₂ nanosheets can be finely and reversibly tuned with N₂ and O₂ gas in the presence of deep-ultraviolet (DUV) light. After exposure to N₂ gas in the presence of DUV light, the threshold voltages of SL, BL, and FL MoS₂ field-effect transistors (FETs) shift towards negative gate voltages. The exposure to N₂ gas in the presence of DUV light notably improves the drain-to-source current, carrier density, and charge-carrier mobility for SL, BL, and FL MoS₂ FETs. Subsequently, the same devices are exposed to O₂ gas in the presence of DUV light for different periods and the electrical characteristics are completely recovered after a certain time. The doping by using the combination of N₂ and O₂ gas with DUV light provides a stable, effective, and facile approach for improving the performance of MoS₂ electronic devices.

The charge-carrier density of single-, bi-, and few-layer MoS₂ nanosheets can be finely and reversibly tuned with N₂ and O₂ gas in the presence of deep-ultraviolet (DUV) light. The exposure to N₂ gas in the presence of DUV light significantly improves the drain-to-source current, carrier density, and charge-carrier mobility for MoS₂ nanosheets.

Transition-metal chalcogenide (TMC) nanoflakes of composition MX₂ (where M=Ti, Zr and Hf; X=S and Se) crystallize preferentially in equilateral hexagons and exhibit a pronounced lateral quantum confinement. The hexagonal shape of octahedral (1T) TMC nanoflakes is the result of charge localization at the edges/vertices and the resulting Coulomb repulsion. Independent of their size, all nanoflakes have the M_nX_2n−2 stoichiometry and thus an unoxidized metal center which results in dopant states. These states become relevant for small nanoflakes and lead to metallic character, but for larger nanoflakes (>6 nm) the 2D monolayer properties dominate. Finally, coordination of Lewis bases at the nanoflake edges has no significant effect on the electronic structure of these species confirming the viability of colloidal synthetic approaches.

Six-sided flakes: Transition-metal dichalcogenide nanoflakes of composition MX₂ (where M=Ti, Zr and Hf; X=S and Se) grow preferentially in equilateral hexagons and exhibit a pronounced lateral quantum confinement. The hexagonal shape arises from the charge location at the edges and vertices and the resulting Coulombic repulsion.

Traditional lithium-ion batteries that are based on layered Li intercalation electrode materials are limited by the intrinsically low theoretical capacities of both electrodes and cannot meet the increasing demand for energy. A facile route for the synthesis of a new type of composite nanofibers, namely carbon nanofibers decorated with molybdenum disulfide sheets (CNFs@MoS₂), is now reported. A synergistic effect was observed for the two-component anode, triggering new electrochemical processes for lithium storage, with a persistent oxidation from Mo (or MoS₂) to MoS₃ in the repeated charge processes, leading to an ascending capacity upon cycling. The composite exhibits unprecedented electrochemical behavior with high specific capacity, good cycling stability, and superior high-rate capability, suggesting its potential application in high-energy lithium-ion batteries.

Carbon nanofibers (CNFs) decorated with molybdenum disulfide sheets are fabricated by a facile hydrothermal process with low-cost, biomass-derived carbonaceous nanofibers as the supports. On reacting with lithium, the nanofibers undergo novel electrochemical processes that are triggered by a synergistic lithium storage effect, leading to enhanced cycling and rate performance of the lithium-ion battery.

Lead free perovskite solar cells based on a CsSnI₃ light absorber with a spectral response from 950 nm is demonstrated. The high photocurrents noted in the system are a consequence of SnF₂ addition which reduces defect concentrations and hence the background charge carrier density.

Organolead halide perovskites have attracted extensive attentions as light harvesting materials for solar cells recently, because of its high charge-carrier mobilities, high photoconversion efficiencies, low energy cost, ease of deposition, and so on. Herein, with CH₃NH₃PbI₃ film deposited on flexible ITO coated substrate, the first organolead halide perovskite based broadband photodetector is demonstrated. The organolead halide perovskite photodetector is sensitive to a broadband wavelength from the ultraviolet light to entire visible light, showing a photo-responsivity of 3.49 A W⁻¹, 0.0367 A W⁻¹, an external quantum efficiency of 1.19×10³%, 5.84% at 365 nm and 780 nm with a voltage bias of 3 V, respectively. Additionally, the as-fabricated photodetector exhibit excellent flexibility and robustness with no obvious variation of photocurrent after bending for several times. The organolead halide perovskite photodetector with high sensitivity, high speed and broad spectrum photoresponse is promising for further practical applications. And this platform creates new opportunities for the development of low-cost, solution-processed and high-efficiency photodetectors.

Organometal halide perovskites have shown tremendous potential as incident light absorbers for optoelectronic applications. In this work, a broadband photo­detecotor is demonstrated based on the CH₃NH₃PbI₃ film, showing a photo-responsivity of 3.49 A W⁻¹, 0.0367 A W⁻¹, an external quantum efficiency of 1.19 × 10³ %, 5.84% at 365 and 780 nm, respectively. These results provide new opportunities for the development of high-efficiency photodetectors.

Photovoltaics based on organic−inorganic perovskites offer new promise to address the contemporary energy and environmental issues. These solar cells have so far largely relied on small-molecule hole transport materials such as spiro-OMeTAD, which commonly suffer from high cost and low mobility. In principle, polyfluorene copolymers can be an ideal alternative to spiro-OMeTAD, given their low price, high hole mobility and good processability, but this potential has not been explored. Herein, polyfluorene derived polymers-TFB and PFB, which contain fluorine and arylamine groups, are demonstrated and can indeed rival or even outperform spiro-OMeTAD as efficient hole-conducting materials for perovskite solar cells. In particular, under the one-step perovskite deposition condition, TFB achieves a 10.92% power conversion efficiency that is considerably higher than that with spiro-OMeTAD (9.78%), while using the two-step perovskite deposition method, about 13% efficient solar cells with TFB (12.80%) and spiro-OMeTAD (13.58%) are delivered. Photo­luminescence reveals the efficient hole extraction and diffusion at the interface between CH₃NH₃PbI₃ and the hole conducting polymer. Impedance spectroscopy uncovers the higher electrical conductivity and lower series resistance than spiro-OMeTAD, accounting for the significantly higher fill factor, photocurrent and open-circuit voltage of the TFB-derived cells than with spiro-MeOTAD.

Polyfluorene derivatives are applied for inorganic−organic hybrid perovskite solar cells, and indeed can rival or outperform spiro-OMeTAD as efficient hole-conducting materials for perovskite solar cells. In particular, with the one-step deposition method, TFB achieves a 10.92% power conversion efficiency, which is considerably higher than that with spiro-OMeTAD (9.78%).

Porous films of p-type CuInS₂, prepared by sulfurization of electrodeposited metals, are surface-modified with thin layers of CdS and TiO₂. This specific porous electrode evolved H₂ from photoelectrochemical water reduction under simulated sunlight. Modification with thin n-type CdS and TiO₂ layers significantly increased the cathodic photocurrent and onset potential through the formation of a p–n junction on the surface. The modified photocathodes showed a relatively high efficiency and stable H₂ production under the present reaction conditions.

Porous photocathodes: CuInS₂ porous films, prepared by sulfurization of electrodeposited metals and modified with thin layers of CdS and TiO₂, evolved H₂ from photoelectrochemical water reduction under simulated sunlight (see picture). The modified photocathodes showed a relatively high efficiency and stable H₂ production under the reaction conditions by surface modification.

Unintentionally formed nanocrystalline graphene (nc-G) can act as a useful seed for the large-area synthesis of a hexagonal boron nitride (h-BN) thin film with an atomically flat surface that is comparable to that of exfoliated single-crystal h-BN. A wafer-scale dielectric h-BN thin film was successfully synthesized on a bare sapphire substrate by assistance of nc-G, which prevented structural deformations in a chemical vapor deposition process. The growth mechanism of this nc-G-tailored h-BN thin film was systematically analyzed. This approach provides a novel method for preparing high-quality two-dimensional materials on a large surface.

A hexagonal boron nitride (h-BN) thin film with an atomically flat surface was obtained using unintentionally formed nanocrystalline graphene (nc-G). A wafer-scale dielectric h-BN thin film was synthesized on a bare sapphire substrate with the assistance of nc-G, which prevented structural deformations during chemical vapor deposition. The sp³-hybridized edges of nc-G play a key role during these processes.