Semiconductor solar cells, metal-insulator

We note in this context that in Si based MIS (metal-insulator-semiconductor) solar cells one of the roles of the 20-60A thick Si02 layer may well be reduction of the recombination velocity at the Si surface. Chambouleyron and Soucedo noted a decrease in the recombination velocity at the conductive Sn02/Si interface8 relative to that at the Si surface and Michel and Lasnier find a recombination velocity of less than 2xl04 cm/sec at the conductive indium tin oxide/Si interface.9 In both cases heating the metal oxides present on the elemental Si produces an intermediate Si02 layer. 

Figure 6. Energy band diagram for metal/insulator/n-type semiconductor solar cell.

Because of the high functional values that polyimides can provide, a small-scale custom synthesis by users or toll producers is often economically viable despite high cost, especially for aerospace and microelectronic applications. For the majority of industrial applications, the yellow color generally associated with polyimides is quite acceptable. However, transparency or low absorbance is an essential requirement in some applications such as multilayer thermal insulation blankets for satellites and protective coatings for solar cells and other space components (93). For interlayer dielectric applications in semiconductor devices, polyimides having low and controlled thermal expansion coefficients are required to match those of substrate materials such as metals, ceramics, and semiconductors used in those devices (94). 

Four different types of junctions can be used to separate the charge earners in solar cells (1) a homojunction joins semiconductor materials of the same substance, e.g., the homojunction of a p — n silicon solar cell separates two oppositely doped layers of silicon (2) a heterojunction is formed between two dissimilar semiconductor substances, e.g., copper sulfide, Cu S, and cadmium sulfide, CdS, in CuxS—CdS solar cells (3) a Schottky junction is formed when a metal and semiconductor material are joined and (4) in a metal-insulator-semiconductor junction (MIS), a thin insulator layer, generally less than 0.003-pim thick, is sandwiched between a metal and semiconductor material. 

The metal-insulator—semiconductor (MIS) structure is employed in the heterophase blocking where the thickness of the insulator used is the key factor in satisfying the blocking requirement. Differently from the solar cell with MIS structure, in which an insulating film tens of angstroms thick is used to avoid the back-diffusion of photoinduced carriers (Wronski et al., 1981), the photoreceptor of electrophotography has necessarily a rather thick insulating film to block carrier transport. 

The above biaxial Ge and CdTe semiconductor films were grown on oblique angle deposited CaF buffer layer on amorphous substrates. The biaxial CaF is an insulator and may not be suitable for devices that need electrical conduction. One may ask if it is possible to use a biaxial metal film as a buffer layer to grow epitaxial semiconductors for solar cell and display applications. Recently we developed a flipping rotation method in the dynamic oblique angle deposition technique. This flipping rotation method produces biaxial metallic films such as Mo and W on amorphous substrates This opens up another route to grow heteroepitaxial semiconductor films directly on metallic surfaces. 

As an alternative configuration in which to study recombination effects, consider a metal-semiconductor junction solar cell. For generality it will be supposed that an insulating layer is inserted as shown in Fig.11.1 and 11.2, making this an M.I.S. cell. Such cells can be of class n or of class p type, as shown. This corresponds to the use of an n-type or p-type semiconductor respectively. Light can fall onto the cell from the left, and the structure is best approached from the right, where the electric field E (x) is... 

Band-bending and hence open-circuit voltage in Schottky barrier solar cells are favoured by an insulating layer with acceptor/ donor surface states which are in better communication with the n-type/p-type semiconductor than they are with the metal. 

In practice it has been found that intimate contacts formed between metal films and crystalline semiconductors exhibit poor photovoltaic response. This is caused by the fact that the thermionic emission dark current at the Schottky barrier leads to significantly higher dark currents than is normally encountered in a homojunction or heterojunction structure. This problem, however, can be got round while still preserving the potential advantages of the Schottky barrier by allowing a very thin oxide or insulating layer to be formed between the semiconductor and the metal contact. The introduction of this layer leads to the ccxnmon form of Metal-Insulator-Semiconductor Schottky Barrier Solar Cell (MIS SBSC) with hich we are primarily concerned here. 

ZnO, and heterostructure devices. Among the devices, light emitters, microcavities, optically pumped lasers, photodiodes, metal-insulator-semiconductor diodes, field-effect transistors, transparent conducting oxides, and transparent thin-fihn transistors based on ZnO, piezoelectric devices in the form of surface acoustic wave devices, and gas and biosensor followed by solar cells cap the discussion. 

---