ICMCTF2015 Session C4-2: Thin Films for Energy Related Applications

Zinc oxide (ZnO) is a wide-band gap semiconductor at the heart of new gallium-indium-zinc-oxide (GIZO) transparent optoelectronic materials that are now deployed as transparent TFT’s in many state-of-the-art displays. However, despite an existing pathway for zinc-oxide based devices there remain some significant stumbling blocks in forming a complete understanding of the basic ZnO optical properties.

The low temperature luminescence from ZnO remains completely dominated by emission from defect-related sources. Our low-temperature photoluminescence (PL) experiments reveal more than 10 different excitonic features involving electron hole-pairs bound to different impurities and second order exciton effects. A challenge when comparing samples of ZnO is that the separation between such excitonic features is comparable to experimental accuracy. Thus careful calibration is required before definitive statements can be made about the defects contained within a particular sample.

Variations in PL emission from different crystallographic faces show intriguing effects that hint at the strong influence of the ionic character of ZnO surfaces. Interestingly surface polarity dependent PL can be modified by both annealing and metal coverage indicating that PL may also play a diagnostic role in device processing. A clue to understanding such ZnO properties is to look at the role of hydrogen. Hydroxl groups on the surfaces appear to act as donors creating unusual 2-D surface electron gases that are possible sources of the polarity differences. Additionally, hydrogen implanted into the bulk material is observed to create new emission lines from hydrogen-other-defect complexes.

This work will present current results from our luminescence studies of ZnO surfaces following treatment processes including annealing and ion implantation.

Pyroelectricity is the property of certain materials that generates voltage by temperature variations. The position of the atoms within the crystal structure of these materials slightly changes by simply heating/cooling the material. The displacement of the atoms will alter the polarization state which will result a temporary voltage across the crystal. The zinc oxide semiconductor is one of the materials that possesses great pyroelectric feature. Crystallinity and orientation of zinc oxide thin film play a crucial role in this property. In most studies, ZnO nanostructures are deposited on sapphire substrates in order to obtain a c-axis oriented thin film which is the most thermodynamically favorable and preferred orientation for pyroelectric applications [1,2,3].

Although sapphire has the same hexagonal type lattice structure with ZnO, it is expensive and hard to find in large size compared to silicon. However there is a large lattice missmatch between ZnO and silicon. Porous silicon can be the solution that reduce the degree of missmatch and a suitable substrate for c-axis oriented growth [4]. Porous silicon is not only integrated with silicon, but it also is easy to fabricate and low cost.

In this study the pyroelectrical characteristic of ZnO thin film has been investigated as a sensing element for a pyroelectric sensor application. In order to observe the effect of porous-Si layer, the thin films were deposited on both Si and porous-Si/Si substrates.

The results taken via XRD, AFM and SEM indicated that, the ZnO/porous-Si layers exhibited a dominant peak of (002) plane which means that the film was grown with c-axis orientation. Moreover results showed that ZnO film has larger grains on porous-Si/Si substrate than the thin film on Si substrate and a rough surface.

The pyroelectric analysis shows that the device with porous-Si/Si substrate has considerably high pyroelectric coefficient compared to Si substrated device. In addition the pyroelectric voltage obtained by porous-Si device is also higher than Si device.

The optimization of design and detailed investigation are being carried out to increase pyroelectric voltage and the measurement for thermal isolation effect of Porous-Si is underway.

[1] Wei C.S., Lin Y.Y., Hu Y.C., Wu C.W., Shih C.K., Huang C.T., Chang S.H. Sens. Actuators A. 128,18–24, 2006.

[2] Hsiao C.C., Huang K Y and Hu Y C. Sensors. 8, 185–92, 2008.

[3] Heiland G. and Ibach H. Solid State Communications. 4,353-354, 1966.

[4] Stolyarova S., Baskin E., Nemirovsky Y. Journal of Crystal Growth, 360,131–133, 2012.

It has previously been shown that alloying of Zinc Telluride (ZnTe), with ZnO modifies the band structure of ZnTe by the anticrossing interaction between extended conduction band (CB) states with localized O states at ~0.2 eV below CB edge of ZnTe [1]. On the other hand, incorporating Te into ZnO is expected to modify the valence band (VB) of the ZnO matrix through the valence-band anticrossing interaction between localized Te states at ~1 eV above the VB edge of ZnO and extended VB states of ZnO. The interaction leads to the formation of a new Te-derived VB edge above that of ZnO. The large upward shift of the VB edge could lead to stable p-type doping, and results in a strong reduction of the ZnO band-gap toward the visible range, making this material suitable for solar energy conversion devices[2].

In this study we synthesized ZnO_1-xTe_x alloys with Te composition x < 0.23 by using pulsed laser deposition. We found that alloys with x < 0.06 are crystalline with a columnar growth structure while samples with higher Te content are polycrystalline with random grain orientation. It was found that Te atoms are randomly distributed with no observable clustering. Optical measurements show that incorporation of a small concentration of Te (x~0.01) red shifts the ZnO optical absorption edge by more than 1 eV. The minimum band gap obtained in this work is 1.8 eV for x = 0.23. The optical properties are consistent with the modification of the valence band of ZnO due to anticrossing interactions of the localized Te states with the ZnO valence band extended states. X-ray photoelectron spectroscopy (XPS) results confirm the upward shift of the valence band in ZnO_1-xTe_x alloys. The optical absorption results were explained using the band anticrossing model with the Te level located at 0.95 eV above the VB edge of ZnO and the anticrossing coupling constant of 1.35 eV. Combining these results with our previous work on band anticrossing in Te-rich alloys [1] allows the prediction of the compositional dependence of the band gap as well as the conduction and the valence band offsets in ZnO_1-xTe_x alloys over the full composition range. The large Te- induced upward shift of the VBE in O-rich ZnO_1-xTe_x could alleviate the notorious p-type doping problem in ZnO [3]. Effects of doping with different group V acceptors (N, As, Sb) on electrical properties of ZnO_1-xTe_x will be discussed.

[1] K. M. Yu, et al., Phys. Rev. Lett. 91, 246403 (2003).

[2] A. Janotti, et al., Rep. Prog. Phys.72, 126501 (2009).

[3] E. Fortunato, et al. (2007). MRS Bulletin, 32, pp 242-247.

Concentrating Solar Power (CSP) based on parabolic trough receivers is a well established and mature technology to obtain clean energy. Currently, there are around 3 GW of power plants installed world-wide. Nevertheless, there are issues that need to be addressed in order to improve the positioning of this technology in the energy mix. One key issue is to increase the performance of the plant to obtain higher productivity reducing the cost of energy. To this purpose it is necessary to increase the working temperature from current 400ºC to 600ºC. In this sense, it is necessary to have solar selective coatings with good spectral selectivity values which must be able to stand the new higher requirements. On the other hand, the lifetime required for the solar receivers operating in a power plant is 25 years. Currently, there is a lack of standard accelerated tests for solar collectors for high temperature operation. This is a problem to introduce in the market coatings to work at higher temperatures. Thus, it is important to understand how coatings the solar selective coatings behave in terms of degradation of their optical properties when working at high temperatures.

Solar selective coatings are obtained by means of a nanostructured multilayer stack. Each layer plays a different role in order to achieve spectral selectivity values suitable for CSP and optical design is necessary to obtain an optimal multilayer stack with the required properties. Thus the composition and thickness of the layers must be carefully designed and controlled during the deposition process.

In this work, multilayer selective nanocoatings are proposed and analyzed as candidates for 600ºC working temperature applications. Hence, alternatives such as Al and Mo as infrared reflector mirror layer to avoid thermal losses and Si3N4 and Al₂O₃ matrices for the absorber cermet are designed and deposited in combination with Al₂O₃ antidiffusion barrier layer and SiO₂ antireflective top layer. The proposed coatings are deposited by means of a preindustrial magnetron sputtering equipment able to coat 4 meter long tubes. The coatings obtained have been analyzed with regard to the degradation behavior of the solar selectivity values at high temperatures.

The bilayer solar cells based on Active layer and [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) by a solution process were fabricated and the effect of Active layer thickness on the cell efficiency was investigated.The results show that the power conversion efficiency (PCE) of 4.7% under simulated AM 1.5G irradiation (100 mW cm2). As the thickness of Active layer increases to 80 nm, a higher PCE of 7.0% could be obtained. This study indicates that the high exciton diffusion efficiency is enabled by the long diffusion length of Active layer relative to its thickness. Furthermore, the low exciton binding energy of Active layer implies that exciton splitting at the Active layer/ PC₆₁BM interface is very efficient.

LiFePO_4-xN_y thin films have been sputter deposited on micron carbon-fibers (MCFs) under N₂/Ar/H₂ mixture gas as electrode materials for pseudocapacitors. The MCFs were fabricated using thermal chemical vapor deposition on stainless steel substrates as current collectors. Various amounts of nitrogen were introduced and the gas contents were controlled by the flow ratios of N₂ to Ar/H₂. The LiFePO_4-xN_y thin films coated on the surface of MCFs can be observed by field emission scanning electron microscopy. The electrochemical properties of LiFePO_4-xN_y thin films were characterized using cyclic voltammogram and charge-discharge processes. The LiFePO_4-xN_y thin film electrode deposited under the optimal N₂ content exhibits a high specific capacitance of 722 F/g at 1 A/g. Even at a current as high as 20 A/g, it still delivers a capacitance of 298 F/g. The pseudocapacitors with LiFePO_4-xN_y thin film electrodes shows no significant capacity fading after 1000 cycles at 1 A/g. The results indicate that nitrogen-doped can improve the electrochemical performance, and LiFePO_4-xN_y can be a potential candidate as an active material for pseudocapacitors.