Papers

ICMCTF2011 Session C2/F4-1: Thin Films for Photovoltaics and Active Devices: Synthesis and Characterization

Monday, May 2, 2011 1:30 PM in Room Sunset

Monday Afternoon

Start	Invited?	Item
1:30 PM		C2/F4-1-1 Plasma Processing for Photovoltaics: Fundamentals and Applications Richard van de Sanden (Eindhoven University of Technology, Netherlands) In this presentation I will address the different roles atomic hydrogen and ions play in the synthesis of thin amorphous and crystalline materials. In particular I will address the role of atomic H during the growth of amorphous and microcrystalline silicon as determined from in situ studies using optical emission spectroscopy (to monitor the hydrogen flux) and evanscent wave cavity ring down spectroscopy to measure the subsurface defect density. Controlled monoenergetic ion energy distribution functions, by means of pulsed biasing scheme, will be used to illustrate the different roles of ion flux and ion energy in the densification of hydrogenated amorphous silicon. Moreover during ion bombardment studies on thin film amorphous silicon films an ion induced Staebler-Wronski effect was observed. It will be demonstrated that defects in this case are created by UV photon generation as a result of electron-ion recombination on the surface and is not due to ion collision cascade effects as normally assumed. In addition the role of H in ZnO:Al electronic transport properties will be addressed.
2:10 PM		C2/F4-1-3 Glancing Angle Deposited Sculptured Titania Films for Light Scattering Enhancement in Solar Cells Kai-Hsiang Hung (Industrial Technology Research Institute, Taiwan); Guan-Di Chiou, Ming-Show Wong (National Dong Hwa University, Taiwan); Yu-Chih Wang, Wei-Ting Kuo, I-Shan Chung, Chun-Ming Yeh (Industrial Technology Research Institute, Taiwan) Light trapping is a crucial approach to eliminate optical losses and to capture solar radiation effectively in solar cells. By texturing surface of front transparent conductive electrodes in which the incident light is scattered, its energy conversion efficiency can be substantially enhanced. For example, textured fluorine-doped tin oxides (FTO, Asahi U-type) exhibit rough surface morphology with excellent light scattering ability, and they are widely used as conductive electrodes in silicon-based solar cells. However, FTO can’t endure in high hydrogen plasma and generally needs protective layers such as titania (TiO₂) or zinc oxide films during silicon thin film growth. In specific, TiO₂ films have not only excellent resistance against hydrogen environments but also act as anti-reflective coatings at TCO/silicon interface. Nonetheless, previous studies do not investigate the light scattering ability of TiO₂ films and their application on conductive electrodes. In this study, a promising technique to enhance the hydrogen-radical durability of FTO and its light scattering ability are demonstrated. A series of TiO₂ films were evaporated by glancing angle deposition (GLAD) method. The sculptured TiO₂ films were deposited on FTO glass at a fixed oblique angle of 86° under various substrate rotation rates from 0.16 to 1 rpm. As a result, the optical transmittance of FTO films retained at 80% after coating porous TiO₂ films. More significantly, the optical haze, a measurable parameter of light scattering ability, was markedly enhanced from 33 to 52% (at wavelength=400 nm). This haze values increase up to 57% improvement. In addition, the sheet resistance of TiO₂/FTO films were slightly reduced from 8.4 to 5.7 Ω/□. Transmission electron microscopy depicts that the TiO₂/FTO films exhibited pyramid-like texture accompanied with sculptured nanoporous TiO₂ films. By covering of sculptured TiO₂ films, the electrical and optical scattering properties as well as hydrogen-radical durability of FTO films are enhanced simultaneously, and they still exhibit transparent conducting behavior. We suggest that the glancing angle deposited sculptured TiO₂ films are promising for light scattering enhancement, and the sculptured TiO₂/FTO films may utilize as transparent conductive electrodes.
2:30 PM		C2/F4-1-4 Optical and Mechanical Characterisation of Nanostructured Antireflectance Coatings for Solar Cells Jonathan Moghal (University of Oxford, UK); James Best, Martin Gardener (Oxford Advanced Surfaces Group plc, UK); Andrew Watt (Univeristy of Oxford, UK); Gareth Wakefield (Oxford Advanced Surfaces Group plc, UK) Anti-Reflection (AR) coatings can be applied to a wide range of optical elements to significantly reduce reflections caused by step changes in the refractive index at the window surface. For example, on typical solar cells glass windows and plastic lenses reflect approximately 5% of incident light per surface. Current anti-reflection technology relies primarily upon expensive vacuum deposition techniques of multilayer stacks. There is a growing need to develop low cost processes which can be scaled to a wide range of applications and substrates. We have previously demonstrated an AR coating that can achieve a tuneable reflection minimum between 400nm and 1900nm and also shown that the short circuit current (I_sc) and the power conversion efficiency (PCE) of a solar cell is increased by between4.5% and 5.0%. High optical performance is achieved by using a binder system in conjunction with the mesoporous silica nanoparticles. The ratio of nanoparticles to binder and the process conditions are used to optimise the optical and mechanical properties of the film. This paper discusses the mechanical properties of the anti reflection coatings on polymer substrates. We describe the elastic behaviour of the coatings using nano-indentation, nano-scratch tests and impact testing. Even though the AR coating consists of a high (>50%) loading of inorganic nanoparticles, they have been shown to flex with the underlying polymer substrate. The coating can flex and recover from deformations of up to 40 times the original thickness under mechanical load.
2:50 PM		C2/F4-1-7 Polymeric Materials and Self-Assembled Interlayers for Printed Photovoltaic Cells Antonio Facchetti (Polyera Corporation and Northwestern University) Polymeric Materials and Self-Assembled Interlayers for Pritned Photovoltaic Cells Antonio Facchetti Polyera Corporation and Northwestern University In this presentation we will report the design, synthesis, and characterization of new molecular precursors for the fabrication of self-assembled (inter)layers on SiO_x, metal oxide, metals, and organic films. These building blocks are designed to strongly adhere to the surfaces of these conductors altering charge injection, charge trapping, and light absorption. Several photovoltaic cell device architectures based on these interlayers are fabricated and shows that these materials enable improved performance or new functions. Furthermore, I will report on the synthesis of new donor polymers for bulk-heterojuction solar cells achieving efficiencies > 7%.
3:30 PM		C2/F4-1-9 Fabrication of Large Area TiO₂ NT Dye-Sensitized Solar Cell on Stainless-Steel by Thermal Spraying and Anodizing Methods Chien-Chon Chen (National United University, Taiwan); Chung-Kwei Lin, Chi-Jung Chang (Feng Chia University, Taiwan); Cheng-Hsun Hsu (Tatung University, Taiwan); Wern-Dare Jheng (National Chin-Yi University of Technology, Taiwan) This article is to provide large-scale dye-sensitized solar cells (DSSC) and methods for their manufacture by thermal spraying and anodizing. An DSSC device that containing a photosensitive dye (N3) adsorbing on a large area surfaces of anode, a transparent conductive cathode disposed to be opposite to the anode and wherein a platinum(Pt) nano-catalytic particles are adhered to its surfaces, and an electrolytic solution sealed between the anode and the transparent conductive cathode. A titania nanotube (TiO₂ NT) film was fabricated by thermo-sprayed titanium film which was on 304 stainless-steel substrate. The film of Ti was then through anodization, TiCl₄ treatment, and heat treatment to form anatase TiO₂ NT film which was as an anode of DSSC. An indium doped tin oxide (ITO) glass coated with platinum (Pt) particles by sputtering was used as a counter electrode. Electrolyte containing 0.5 M lithium iodide (LiI) and 0.05M iodine (I₂) in acetonitrile (CH₃CN, 99.9%) was introduced into the electrodes. The photo-current conversion efficiency was tested under an AM 1.5 Solar Simulator. The DSSC device has short current density (Jsc) of 9.32 mA cm^-2, open voltage (Voc) of 0.58V, fill factor (FF) of 0.59, and conversion efficiency (η) of 3.2%. The internal impedance of DSSC were detected and simulated using an electrical impedance spectra (EIS) technique with inductance, resistors, and capacitors characteristics. The impedance of the bulk materials was simulated by using L₀+R₀+R_b; the impedance of the working electrode was simulated by (C₁//R₁)//(R_a+(C₂//R₂); the electrolyte was simulated by C₃//R₃; and the counter electrode was simulated by C₄//R₄. The equivalent circuit of DSSC was illustrated express as: (L₀+R₀+R_b)+{[(C₁//R₁)//(R_a+(C₂//R₂))}+(C₃//R₃)+(C₄//R₄).
3:50 PM		C2/F4-1-10 Effect of (poly)Phosphate Anion Structure on Characteristics of Pulsed DC PEO Coatings on Ti, for Dye Sensitised Solar Sell Applications Po-Jen Chu, Aleksey Yerokhin (University of Sheffield, UK); Ju-Liang He (Feng Chai University, Taiwan); Allan Matthews (University of Sheffield, UK) Plasma electrolytic oxidation (PEO) of Ti can be used to prepare TiO₂-based coatings with useful functional properties, e.g. photocatalytic and biological activity. Of particular interest is development of PEO coatings for dye sensitised solar cell (DSSC) applications. Appropriate coatings are required to be porous and enriched with metastable anatase phase. Anatase stabilisation requires approaches that are substantially different from those dominated in development of protective PEO coatings. One possibility consists in incorporation into the coating structure of anatase stabilizers, such as Si and/or P. In this study, the detailed investigation of microstructure, surface morphology and porosity of porous titania coatings was carried out. Aqueous solutions of tri-sodium orthophosphate (Na₃PO₄), sodium pyrophosphate (Na₄P₂O₇) and sodium hexametaphosphate (Na₂PO₄-(NaPO₃)_x-Na₂PO₃, x = 10…12) of 0.01 to 0.03 mol l^-1 concentration were utilised. A pulsed DC PEO mode was employed with voltages, pulse frequencies and duty cycles varied between 200 V to 600 V, 100 Hz to 4000 Hz and 0.11 to 4, respectively. The oxidation was carried out for 10 minutes. The coating microstructure, surface morphology and porosity was analyzed by X-ray diffractometry, scanning electron microscopy and electrochemical impedance microscopy. Inverse-type ITO glass/Pt/electrolyte (I₂+LiI)/dye/TiO₂/Ti devices were assembled using the above PEO-treated specimens. The DSSC photovoltaic efficiency was evaluated by exposing it to a fixed output-power solar simulation. Results show that the PEO titania coating growth rate varies from 0.08 to 1.5μm min^-1 and 6 to 8 μm thick coatings can be produced in orthophosphate electrolytes, containing maximum amount of anatase phase. The uniformly rough surface morphology is featured by small pores (200 to 300 nm in diameter) and deep craters (1 to 2 μm in diameter). Such a porous morphology is deemed to be beneficial for dye penetration and associated increment in photovoltaic efficiency for DSSC application.
4:10 PM		C2/F4-1-6 Effects of Additive in Cu Solution for Electrodeposition of CuInSe₂ Film Tsung-Wei Chang (National Cheng Kung University, Taiwan); Wen-Hsi Lee (National Cheng-Kung University, Taiwan); Yin-Hsien Su, Fang-I Chih (National Cheng Kung University, Taiwan) In this work, we discuss the development of inexpensive, high efficiency, large-area solar cells of the thin-film CuInSe₂(CIS) type. We show that it is possible to fabricate p-type CIS entirely by electrodeposition. CIS is considered to be one of the best absorber materials for use in polycrystalline thin-film photovoltaic solar cells. It is an inexpensive, non-vacuum deposition technology which is inherently scalable to large-area deposition. A smooth under layer will lead to better qualities of CuInSe₂ films. Large grain size, great crystallinity, and fine morphology were characterized in the CuInSe₂ films. We conclude that better qualities of CuInSe₂ films will be achieved on smoother Cu under layers, and the addition of CH₄N₂S leveler in the Cu solution will help the formation of smoother Cu layers. We electrodeposit CIS to control ratio, and to fabricate the superior structural properties of these selenized films were also clearly reflected by X-ray diffraction (XRD). Field-emission-scanning electron microscopy indicates that the ordered copper indium diselenide thin-films are entirely filled, and the structure of the molybdenum thin-film. X-ray diffraction result shows that the copper indium diselenide thin-films are crystalline and have the highly preferential orientation. Energy dispersive spectrometer analysis observation shows the composition atomic ratio of copper indium diselenide.