Phase control in the colloidal synthesis of well-defined nickel sulfide nanocrystals.

Morphologically well-defined colloidal nanocrystals of Ni3S4, NiS, Ni9S8, and Ni3S2 were independently prepared through a solution-phase synthesis using N,N'-disubstituted thioureas as the sulfur precursor. Synthetic control over phase and composition of the resulting colloidal nickel sulfide nanocrystals was achieved by primarily adjusting the reactivity of substituted thioureas as well as tuning the key reaction parameters of temperature and precursor ratio. In general, the more reactive N,N'-diphenyl thiourea yields more sulfur-rich phases (Ni3S4 and NiS) while less reactive N,N'-dibutyl thiourea yields sulfur-poor phases (Ni9S8 and Ni3S2). This phase control can be further tuned through the use of 1-dodecanethiol as an important secondary reactivity-directing agent. In the presence of 1-dodecanethiol, nanocrystals of more sulfur-deficient phases are prepared, while in the absence of 1-dodecanethiol, more sulfur-rich phases are prepared. Under the most sulfur-rich synthetic conditions (i.e., with N,N'-diphenyl thiourea and no thiol) a phase progression from Ni3S4 to the α-NiS and ß-NiS phases was observed upon an increase in reaction temperature and sulfur-to-nickel precursor ratio. This study establishes, for the first time, a systematic evaluation of factors that simultaneously control the phase and yield well-defined nickel sulfide nanocrystals.

Ultrathin high band gap solar cells with improved efficiencies from the world's oldest photovoltaic material.

Selenium was used in the first solid state solar cell in 1883 and gave early insights into the photoelectric effect that inspired Einstein's Nobel Prize work; however, the latest efficiency milestone of 5.0% was more than 30 years ago. The recent surge of interest towards high-band gap absorbers for tandem applications led us to reconsider this attractive 1.95 eV material. Here, we show completely redesigned selenium devices with improved back and front interfaces optimized through combinatorial studies and demonstrate record open-circuit voltage (V OC) of 970 mV and efficiency of 6.5% under 1 Sun. In addition, Se devices are air-stable, non-toxic, and extremely simple to fabricate. The absorber layer is only 100 nm thick, and can be processed at 200 ˚C, allowing temperature compatibility with most bottom substrates or sub-cells. We analyze device limitations and find significant potential for further improvement making selenium an attractive high-band-gap absorber for multi-junction device applications.Wide band gap semiconductors are important for the development of tandem photovoltaics. By introducing buffer layers at the front and rear side of solar cells based on selenium; Todorov et al., reduce interface recombination losses to achieve photoconversion efficiencies of 6.5%.

Nanoscale Characterization of Back Surfaces and Interfaces in Thin-Film Kesterite Solar Cells.

Combinations of sub 1 µm absorber films with high-work-function back surface contact layers are expected to induce large enough internal fields to overcome adverse effects of bulk defects on thin-film photovoltaic performance, particularly in earth-abundant kesterites. However, there are numerous experimental challenges involving back surface engineering, which includes exfoliation, thinning, and contact layer optimization. In the present study, a unique combination of nanocharacterization tools, including nano-Auger, Kelvin probe force microscopy (KPFM), and cryogenic focused ion beam measurements, are employed to gauge the possibility of surface potential modification in the absorber back surface via direct deposition of high-work-function metal oxides on exfoliated surfaces. Nano-Auger measurements showed large compositional nonuniformities on the exfoliated surfaces, which can be minimized by a brief bromine-methanol etching step. Cross-sectional nano-Auger and KPFM measurements on Au/MoO3/Cu2ZnSn(S,Se)4 (CZTSSe) showed an upward band bending as large as 400 meV within the CZTSSe layer, consistent with the high work function of MoO3, despite Au incorporation into the oxide layer. Density functional theory simulations of the atomic structure for bulk amorphous MoO3 demonstrated the presence of large voids within MoO3 enabling Au in-diffusion. With a less diffusive metal electrode such as Pt or Pd, upward band bending beyond this level is expected to be achieved.

Fused porphyrin-single-walled carbon nanotube hybrids: efficient formation and photophysical characterization.

A systematic study of the interaction between π-extended porphyrins and single-walled carbon nanotubes (SWNTs) is reported here. Zinc porphyrins with 1-pyrenyl groups in the 5,15-meso positions, 1, as well as compounds where one or both of the pyrene groups have been fused at the meso and ß positions of the porphyrin core, 2 and 3, respectively, have been examined. The strongest binding to SWNTs is observed for porphyrin 3, leading to debundling of the nanotubes and formation of stable suspensions of 3-SWNT hybrids in a range of common organic solvents. Absorption spectra of 3-SWNT suspensions are broad and continuous (λ=400-1400 nm), and the Q-band of 3 displays a significant bathochromic shift of 33 nm. The surface coverage of the SWNTs in the nanohybrids was estimated by spectroscopic and analytical methods and found to reach 64% for (7,6) nanotubes. The size and shape of π-conjugated porphyrins were found to be important factors in determining the strength of the π-π interactions, as the linear anti-3 isomer displays more than 90% binding selectivity compared to the bent syn-3 isomer. Steady-state photoluminescence measurements show quenching of porphyrin emission from the nanohybrids. Femtosecond transient absorption spectroscopy reveals that this quenching results from ultrafast electron transfer from the photoexcited porphyrin to the SWNT (1/kCT=260 fs) followed by rapid charge recombination on a picosecond time scale. Overall, our data demonstrate that direct π-π interaction between fused porphyrins and SWNTs leads to electronically coupled stable nanohybrids.

Effect of an oxygen scavenger on the stability of preservative-free flour tortillas.

UNLABELLED: Along with purge and moisture control, oxygen scavenging is a prominent active packaging technology employed by many food processors. The objective of this study was to determine the effect of an oxygen scavenger system (OSS) on the shelf life of preservative-free tortillas stored at varying storage conditions. The shelf life of the tortillas was evaluated at accelerated storage (AS; 37 °C, 75% relative humidity [rh]), room temperature (RT; 22 °C, 57% rh), and refrigeration (R; 4 °C, 42% rh) conditions. The OSS consisted of a multilayer, coextruded bag paired with an oxygen scavenger sachet. A resealable bag made of low-density polyethylene/linear low-density polyethylene was used as a control. The diameter, thickness, CIELab color, water activity, pH, texture, and microbial growth within the sample tortillas were measured before and after exposure to the storage conditions. The results showed that the OSS had superiority when compared to the control. The weight and thickness under RT remained unchanged, while lightness was superior to the control under R conditions. Under AS, gradient remained constant, and force followed the same pattern under RT and R conditions. At the same time, microbial growth as measured by aerobic plate count and yeast and molds showed no changes under both AS and RT conditions. Future studies will investigate the effect of a faster acting oxygen scavenger on shelf life of this type of tortillas. PRACTICAL APPLICATION: The results of this study show promise for the use of oxygen scavenging technology in the packaging of natural and preservative-free tortillas.

Tin and germanium monochalcogenide IV-VI semiconductor nanocrystals for use in solar cells.

The incorporation of colloidal semiconductor nanocrystals into the photoabsorbant material of photovoltaic devices may reduce the production costs of solar cells since nanocrystals can be readily synthesized on a large scale and are solution processable. While the lead chalcogenide IV-VI nanocrystals have been widely studied in a variety of photovoltaic devices, concerns over the toxicity of lead have motivated the exploration of less toxic materials. This has led to the exploration of tin and germanium monochalcogenide IV-VI semiconductors, both of which are made up of earth abundant elements and possess properties similar to the lead chalcogenides. This feature article highlights recent efforts made towards achieving synthetic control over nanocrystal size and morphology of the non-lead containing IV-VI monochalcogenides (i.e., SnS, SnSe, SnTe, GeS and GeSe) and their application toward photovoltaic devices.