TCO-Si Based Heterojunction Photovoltaic Devices (2011)

On the other hand, the novel ITO/AZO/SiO 2 /p-Si SIS heterojunction has been fabricated by low temperature thermally grown an ultrathin silicon dioxide and RF sputtering deposition ITO/AZO double films on p-Si texturized substrate.The crystalline structural, optical and electrical properties of the ITO/AZO antireflection films were characterized by XRD, UV-VIS spectrophotometer, four point probes, respectively.The results show that ITO/AZO films have good quality.The electrical junction properties were investigated by I-V measurement, which reveals that the heterojunction shows strong rectifying behavior under a dark condition.The ideality factor and the saturation current of this diode is 2.3 and 1.075×10 -5 A, respectively.In addition, the values of I F /I R (I F and I R stand for forward and reverse current, respectively) at 2V is found to be as high as 16.55.It shows fairly good rectifying behavior indicating formation of a diode between AZO and p-Si.High photocurrent is obtained under a reverse bias when the crystalline quality of ITO/AZO double films is good enough to transmit the light into p-Si.In device physics, the tunneling effect of SIS solar cell has been investigated in our current work, depending on the thickness of the ultra-thin insulator layer, which is potential for the understanding of quantum mechanics in the photovoltaic devices. Review of TCO thin films2.1 Development of TCOs 2.1.1Feature of TCO Most optically transparent and electrically conducting oxides (TCOs) are binary or ternary compounds, containing one or two metallic elements.Their resistivity could be as low as www.intechopen.comTCO-Si Based Heterojunction Photovoltaic Devices 11310 -5  cm, and their extinction coefficient k in the visible range (VIS) could be lower than 0.0001, owing to their wide optical band gap (Eg) that could be greater than 3 eV.This remarkable combination of conductivity and transparency is usually impossible in intrinsic stoichiometric oxides; however, it is achieved by producing them with a non-stoichiometric composition or by introducing appropriate dopants.Badeker (1907) discovered that thin CdO films possess such characteristics.Later, it was recognized that thin films of ZnO, SnO 2 , In 2 O 3 and their alloys were also TCOs.Doping these oxides resulted in improved electrical conductivity without degrading their optical transmission.Al doped ZnO (AZO), tin doped In 2 O 3 , (ITO) and antimony or fluorine doped SnO 2 (ATO and FTO), are among the most utilized TCO thin films in modern technology.In particular, ITO is used extensively in acoustic wave device, electro-optic modulators, flat panel displays, organic light emitting diodes and photovoltaic devices.The actual and potential applications of TCO thin films include: (1) transparent electrodes for flat panel displays (2) transparent electrodes for photovoltaic cells, (3) low emissivity windows, (4) window defrosters, (5) transparent thin films transistors, (6) light emitting diodes, and (7) semiconductor lasers.As the usefulness of TCO thin films depends on both their optical and electrical properties, both parameters should be considered together with environmental stability, abrasion resistance, electron work function, and compatibility with substrate and other components of a given device, as appropriate for the application.The availability of the raw materials and the economics of the deposition method are also significant factors in choosing the most appropriate TCO material.The selection decision is generally made by maximizing the functioning of the TCO thin film by considering all relevant parameters, and minimizing the expenses.TCO material selection only based on maximizing the conductivity and the transparency can be faulty.Recently, the scarcity and high price of Indium needed for ITO materials, the most popular TCO, as spurred R&D aimed at finding a substitute.Its electrical resistivity (ρ) should be ~10 -4  cm or less, with an absorption coefficient ( ) smaller than 10 4 cm -1 in the near-UV and VIS range, and with an optical band gap >3eV.A 100 nm thick film TCO film with these values for and will have optical transmission (T) 90% and a sheet resistance (R S ) of < 10 /.At present, AZO and ZnO:Ga (GZO) semiconductors are promising alternatives to ITO for thin-film transparent electrode applications.The best candidates is AZO, which can have a low resistivity, e.g. on the order of 10 -4  cm, and its source materials are inexpensive and non-toxic.However, the development of large area, high rate deposition techniques is needed.Another objective of the recent effort to develop novel TCO materials is to deposit p-type TCO films.Most of the TCO materials are n-type semiconductors, but p-type TCO materials are required for the development of solid lasers, as well as TFT or PV cells.Such p-type TCOs include: ZnO:Mg, ZnO:N, ZnO:In, NiO, NiO:Li, CuAlO 2 , Cu 2 SrO 2 , and CuGaO 2 thin films.These materials have not yet found a place in actual applications owing to the stability.Published reviews on TCOs reported exhaustively on the deposition and diagnostic techniques, on film characteristics, and expected applications.The present paper has three objectives: (1) to review the theoretical and experimental efforts to explore novel TCO materials intended to improve the TCO performance, (2) to explain the intrinsic physical limitations that affect the development of an alternative TCO with properties equivalent to those of ITO, and (3) to review the practical and industrial applications of existing TCO thin films. 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