Question

2. (a) Explain about heterojunction and draw different types of heterojunction band-diagram?

(c) Using semiconductors in (b), explain the structure of thin-film heterojunction solar cell which is capable of separating the photoexcited electrons and holes efficiently, and show the direction of incoming solar light in order to achieve the optimized energy conversion efficiency.

Answer 1

A.

Heterojunction

A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in many solid state device applications including semiconductor lasers, solar cells and transistors to name a few. The combination of multiple heterojunctions together in a device is called a heterostructure although the two terms are commonly used interchangeably. The requirement that each material be a semiconductor with unequal band gaps is somewhat loose especially on small length scales where electronic properties depend on spatial properties. A more modern definition of heterojunction is the interface between any two solid-state materials, including crystalline and amorphous structures of metallic, insulating, fast ion conductor and semiconducting materials. In 2000, the Nobel Prize in physics was awarded jointly to Herbert Kroemer and Zhores I. Alferov for "developing semiconductor heterostructures used in high-speed- and opto-electronics".

Energy band alignment

The three types of semiconductor heterojunctions organized by band alignment.

Band diagram for straddling gap, n-n semiconductor heterojunction at equilibrium.

The behaviour of a semiconductor junction depends crucially on the alignment of the energy bands at the interface. Semiconductor interfaces can be organized into three types of heterojunctions: straddling gap (type I), staggered gap (type II) or broken gap (type III) as seen in the figure.Away from the junction, the band bending can be computed based on the usual procedure of solving Poisson's equation.

Various models exist to predict the band alignment.

B.

HIT (heterojunction with intrinsic thin-layer) cell, shown schematically in figure below, combines crystalline and amorphous silicon .n-type crystalline silicon wafers have layers of intrinsic and doped amorphous silicon applied to the front and rear. The junction has intrinsic amorphous layers between the n-type crystalline and p-type amorphous silicon regions to reduce recombination. Contacts are made by metal electrodes and layers of transparent conducting oxide (TCO). Unwelcome absorption in the TCO and electrically inactive doped amorphous layers is compensated by the excellent passivation of the crystalline silicon by the amorphous. Their negative temperature coefficient(−0.33%/°C) is smaller than that of conventional silicon cells. Large-area laboratory cell efficiency of 21.3% has been reported, and nominal cell production efficiency is up to 19.5%. HIT200 module has nominal cell and module efficiencies of 19.5% and 17%, respectively. Sanyo produced about 16 MWpduring 2001 and reportedly intends to increase production from to 120 MWp per annum by 2005.

Thin-film solar cell, type of device that is designed to convert lightenergy into electrical energy (through the photovoltaic effect) and is composed of micron-thick photon-absorbing material layers deposited over a flexible substrate. Thin-film solar cells were originally introduced in the 1970s by researchers at the Institute of Energy Conversion at the University of Delaware in the United States. The technology continuously improved so that in the early 21st century the global thin-film photovoltaic market was growing at an unprecedented rate and was forecast to continue to grow. Several types of thin-film solar cells are widely used because of their relatively low cost and their efficiency in producing electricity

C.

Recent achievements in the development of polycrystalline thin film solar cells are reviewed. Properties of compound semiconductors are analyzed with respect to solar cell applications and compared with the other commonly used photovoltaic materials. Cu(In,Ga)Se2 and CdTe, presently the main candidates, are discussed in detail. Basic problems such as the influence of defects and impurities on electronic properties of the semiconductors and limitations of performance due to the polycrystalline structure of the film are investigated. Advantageous properties of Chalcogenide semiconductors, e.g. low grain boundary and surface recombination, and the favorable interaction of oxygen with the surface of these compounds are pointed out. Wide bandgap semiconductors for the use in heterojunctions are selected. Key problems of heterojunction solar cells such as energy band alignments, lattice match, chemical stability of the interface, and recombination of minority carriers in the bulk and at the interface are considered. The different technological approaches for thin film deposition of compound semiconductors for solar cells are reviewed in respect to scalability and thin film quality and hence expected solar cell performance.

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