**1. Introduction**

110 Solar Cells – Thin-Film Technologies

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It is a common viewpoint that the adscription of the PV research and industry in future has to be the lower cost and higher efficiency. However, those monocrystal as well as multicrystalline silicon wafer require very expensive processing techniques to produce low defect concentrations, and they are made by complicated wet chemical treatment, hightemperature furnace steps, and time-cost metallization. Thus, a high PV module cost exists for the first-generation technology. Recently, a strong motivation in R&D roadmap of PV cells has been put forward in thin film materials and heterojunction device fields. A large variety of possible and viable methods to manufacture low-cost solar cells are being investigated. Among these strategies, transparent conductive oxides (TCOs) and polycrystalline silicon thin films are promising for application of PV and challenging to develop cheap TCOs and TCO/c-Si heterojunction cells.

Converting solar energy into electricity provides a much-needed solution to the energy crisis in the world is facing today. Solar cells (SC) fabricated on the basis of semiconductor– insulator– semiconductor (SIS) structures are very promising because it is not necessary to obtain a p–n junction and the separation of the charge carriers generated by the solar radiation is realized by the electrical field at the insulator–semiconductor interface. Such SIS structures are obtained by the deposition of thin films of TCO on the oxidized semiconductor surface. One of the main advantages of SIS based SC is the elimination of high temperature diffusion process from the technological chain, the maximum temperature at the SIS structure fabrication by PVD/CVD being not higher than 450 ◦C. Besides that, the superficial layer of silicon wafer, where the electrical field is localized, is not affected by the impurity diffusion. The TCO films with the band gap in the order of 2.5–4.5 eV are transparent in the whole region of solar spectrum, especially in the blue and ultraviolet regions, which increase the photo response in comparison with the traditional SC. The TCO layer assists the collection of charge carriers and at the same time is an antireflection coating. The most utilized TCO layers are SnO2, In2O3 and their mixture ITO, as well as zinc oxide (ZnO). The efficiency of these kinds of devices can reach the value of more than 10% (Koida et al., 2009).

Transparent conducting oxides (TCOs), such as ZnO, Al-doped ZnO or ITO (SnO2:In2O3), are an increasingly significant component in photovoltaic (PV) devices, where they act as electrodes, structural templates, and diffusion barriers, and their work function are

TCO-Si Based Heterojunction Photovoltaic Devices 113

10-5 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, SnO2, In2O3 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 In2O3, (ITO) and antimony or fluorine doped SnO2 (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

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

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 104 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 (RS) 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−<sup>4</sup> cm, and its source materials are inexpensive and non-toxic. However, the development of large area, high rate deposition techniques is

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, CuAlO2, Cu2SrO2, and CuGaO2 thin films. These materials have not yet found a place in actual applications owing to the

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

maximizing the conductivity and the transparency can be faulty.

diodes and photovoltaic devices.

needed.

stability.

films.

dominant to the open-circuit voltage. The desirable characteristics of TCO materials that are common to all PV technologies are similar to the requirements for TCOs for flat-panel display applications and include high optical transmission across a wide spectrum and low resistivity. Additionally, TCOs for terrestrial PV applications must be used as low-cost materials, and some may be required in the device-technology specific properties. The fundamentals of TCOs and the matrix of TCO properties and processing as they apply to current and future PV technologies were discussed.

As an example, the In2O3:SnO2(ITO) transparent conducting oxides thin film was successfully used for the novel ultraviolet response enhanced PV cell with silicon-based SINP configuration. The realization of ultraviolet response enhancement in PV cells through the structure of ITO/SiO2/np-Silicon frame (named as SINP), which was fabricated by the state of the art processing, have been elucidated in the chapter. The fabrication process consists of thermal diffusion of phosphorus element into p-type texturized crystal Si wafer, thermal deposition of an ultra-thin silicon dioxide layer (15-20Å) at low temperature, and subsequent deposition of thick In2O3:SnO2 (ITO) layer by RF sputtering. The structure, morphology, optical and electric properties of the ITO film were characterized by XRD, SEM, UV-VIS spectrophotometer and Hall effects measurement, respectively.

The results showed that ITO film possesses high quality in terms of antireflection and electrode functions. The device parameters derived from current-voltage (I-V) relationship under different conditions, spectral response and responsivity of the ultraviolet photoelectric cell with SINP configuration were analyzed in detail. We found that the main feature of our PV cell is the enhanced ultraviolet response and optoelectronic conversion. The improved short-circuit current, open-circuit voltage, and filled factor indicate that the device is promising to be developed into an ultraviolet and blue enhanced photovoltaic device in the future.

On the other hand, the novel ITO/AZO/SiO2/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 1.075×10-5A, respectively. In addition, the values of IF/IR (IF and IR 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.
