ICMCTF2016 Session C4-2: Thin Films for Energy Related Applications

Zinc sulfide (ZnS) thin films have attracted great attention in recent years due to their unique semiconductor properties, making them suitable for various micro- and optoelectronic applications. ZnS is a II-VI group semiconductor having a large band gap of 3.6 eV and a high refractive index of 2.27 at near infrared and in the visible range. Thin films of ZnS are suitable for heterojunction solar cells and applicable for near ultra-violet and visible light detectors.

Thin films of ZnS may be deposited by different methods, however thermal evaporation is one of simplest and reproducible ways to reach them. Due to different applications, these films should be doped for a required concentration. Our work was devoted to deposition and investigation of ZnS semiconductor thin films of n-type with various concentration of the major charge carriers. In our experiments, we used a plasma enhanced method to introduction the ionized nitrogen atoms and molecules in the growing ZnS film. For this goal, we developed the specific deposition setup, which combines the vacuum thermal evaporation method with the plasma discharge in the nitrogen. Nitrogen ions were directed into the growing thin film through evaporation process. The ionized particles flux was monitored using an in-situ emission spectroscopic method.

Electrical and optical properties of obtained thin films were investigated. As shown, the semiconductor ZnS thin films with various charge carriers concentration may be obtained by our method.

Using in-situ ultra-high vacuum variable-temperature scanning tunneling microscopy (STM), we investigate the chemical vapor deposition kinetics of hexagonal boron nitride (hBN) monolayer growth on Pd(111) substrates as a function of substrate temperature, borazine flux, and deposition time. All of the experiments were carried out on sputter-deposited Pd(111)/Al₂O₃(0001) thin films. In each experiment, STM images were acquired while resistively heating the Pd(111) samples on the STM stage at temperatures between 300 K and 773 K and in the presence of borazine (10-⁷ ~ 10-⁶ Torr) for times between 60 s and 1800 s. We observe the nucleation and growth of chemisorbed (and presumably partially dissociated) borazine islands on the Pd surfaces. From the STM images, we measured the island density and size as a function of time, temperature, and borazine flux. We find that both the island density and size increase with increasing borazine flux and deposition time. We also find that borazine islands form on Pd(111) ‘up-steps.’ After achieving monolayer coverage of borazine, the samples are annealed in ultra-high vacuum at 1020 K for 60 seconds to form hBN monolayers. We then determined of the number density of rotational domains in the hBN layers, based upon which we identify the deposition parameters critical to the growth of single-domain hBN layers on Pd(111).

The commercial exploitation of Fusion Energy is one of the longstanding dreams of Humankind. During the decades of intensive research in Magnetic Fusion, it has become evident that issues arising from the interactions of the hot plasma with the surrounding elements are paramount. Although tungsten has been initially chosen for the design of the elements exposed directly to the plasma in the Divertor of ITER, it is now clear that its use in a future Reactor can be extremely challenging, and a strong need of alternative solutions is acknowledged. In the last decades, the development of concepts based in the use of liquid metals as plasma facing components has been steadily growing. Due to the fact that liquid metals are electrically conductive, induced currents will interact with the required strong magnetic fields leading to splashing issues. Moreover, minimization of the total volume of liquid metal in a Reactor is required for safety and tritium inventory issues and hence, mostly concepts based on thin liquid films are considered at present for the design of these alternative solutions. Furthermore, the use of capillary structures to hold liquid metals against the MHD forces (the so called CPS concept) has been preferred and widely tested worldwide. In this presentation, an overview of the present status of the use of liquid metal films for Fusion applications will be given and the open issues will be addressed.

We use six batches of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)-carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) to systematically investigate the effects of molecular weight (Mw) and polydispersity indices (PDI) on the optical properties of PTB7:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) blend films (PTB7:PCBM). The optical properties of the blends were determined using multi-sample (5 film thicknesses), multi-angle of incidence spectroscopic ellipsometry and transmission measurements. Corresponding solar cell devices were prepared. The optimised power conversion efficiency (PCE), as function of thickness, varied from 4.8% to 7.8% depending on the PTB7 properties. We found that variations in Mw and PDI of PTB7 affect the optical properties; however an increase in extinction coefficient does not necessarily imply increased power conversion efficiency. Low performing photovoltaic devices were generally prepared from PTB7 with larger PDI values and lower Mw that presented distinctive morphology with coarse phase separation, large domain size, and higher surface roughness. In addition they also show lower hole mobilities and lower extinction coefficient for the lowest energy transition for perpendicular polarization. However, we found that good PCE (6.9− 7.8%) could be achieved in a wide range of PTB7 properties: with higher MW and higher PDI, or with lower MW and lower PDI. These studies illustrate the importance of Mw and PDI to optimize the bulk heterojunction morphology for high performance solar cells.

Acknowledgements: This work was supported by the Strategic Research Theme, University Development Fund, and Small Project Grant (administrated by The University of Hong Kong) are also acknowledged. J.A.Z. acknowledges funding from GRF Project 122812 from the Research Grants Council of Hong Kong. Partial support of the work is provided by the RGC Theme based Research Scheme (Grant Number HKU T23-713/11). C.S. would like to acknowledge funding support from the Clarea Au Endowment Professorship. The authors would like to thank Organtec Materials Inc. for GPC measurements. [1] Chap Hang To, et al. "Effect of PTB7 Properties on the Performance of PTB7:PC71BM Solar Cells" ACS Appl. Mater. Interfaces 2015, 7, 13198−13207.

WAlN/WAlON/Al₂O₃ coating was successfully deposited on stainless steel (SS) substrates using reactive DC and RF magnetron sputtering. Excellent spectrally selective property with a high absorptance of 0.958 in the solar wavelength range and low emittance of 0.08 in the infrared region was achieved by optimising the composition and thickness of the coating. WAlN and WAlON layers act as main absorber part of the coating, whereas the topmost Al₂O₃ layer acts as an anti-reflecting layer. A thick tungsten layer, as an infrared reflector and diffusion barrier was deposited on SS substrate to reduce the emissivity of the coatings, without affecting the absorptance. Various properties of the absorber coatings like composition, structure, surface topography, optical properties were studied using X-Ray diffraction, solar spectrum reflectometer and emissometer, UV-Vis-NIR reflectance spectroscopy, Fourier transform Infrared spectroscopy, field emission scanning electron microscopy and atomic force microscopy. While assessing the thermal stability of the absorbers, the coatings were subjected to heat treatment in air at different temperatures (350°-550°C) for 2 hrs. The absorber deposited on SS substrates showed high solar selectivity (α/ε) of 0.920/0.11, even after heat treatment in air up to 500°C for 2 h. At 550°C, the solar selectivity (0.820/0.50) decreased significantly on SS substrates. Taken together, the present study demonstrated that the WAlN/WAlON/Al₂O₃-based selective absorbing coating with excellent thermal stability could be a promising material for photo-thermal conversion at temperatures upto 500°C.

Silt erosion is a predominant wear phenomenon in hydro turbine components affecting their life and efficiency. This undesirable type of erosion increases the unscheduled shutdown maintenance and repairing cost. Various hard coatings through different processes such as tungsten carbide coating by HVOF process, plasma nitriding, nitro-carburising by diffusion process and ceramic coatings by D-gun/plasma spray process have been studied to combat silt erosion in hydro turbine components. These types of coatings are reasonably successful against erosion at the low angle of impingement but show fragment type of failure at high angle of impingement. Recently, the efforts have been made to develop hard and tough ternary transition metal nitride coatings using magnetron sputtering process for improving erosion resistance of base material. It has been reported that nano-composite Zr-W-N/Zr-W-B-N hard metal nitride coating exhibited a combination of high thermal stability, wear resistance and fracture toughness. In the present study, Zirconium Tungsten Nitride (Zr_1-x-W_x-N) nanostructure coatings has been deposited on 13Cr-4Ni stainless steel substrate by pulsed DC magnetron sputtering and the effect of Zr/W ratio on silt erosion resistance of Zr_1-x-W_x-N coatings has been discussed in detail. It is observed that the silt erosion resistance of Zr-W-N thin films can be optimized by controlling Zr-W ratio of the composition.