TCO-Si Based Heterojunction Photovoltaic Devices

Abstract

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