Cadmium telluride , thin-film

Ubale, A. U. Dhokne, R. I Chikhlikar, P. S. Sangawar, V. S. Kulkanari, K. 2006. Characterization of nanocrystalline cadmium telluride thin films grown by successive ionic layer adsorption and reaction (SILAR) method. Bull. Mater. Sci. 29 165-168. 

Camarero et al. [41] have prepared graded cadmium-mercury-telluride thin films (CdxFlgi cTe) applying cathodic electrodeposition at variable deposition voltage. Atomic proportions of mercury in the range 0.05-0.15 were considered. 

Neumann-Spallart M., Tamizhami G. and Levy-C16ment C. (1990), Photoelectro-chemical properties of semiconducting cadmium mercury telluride thin films with bandgap between 1.47 and 1.08 eV , J. Electrochem. Soc. 137, 3434-3437. 

Cadmium Telluride. Cadmium teUuride [1306-25-8] CdTe, is another promising thin film. CdTe is a well-known semiconductor often used in high performance infrared sensors. CdTe absorbs visible light very strongly, and very thin films (1—2 llm) are sufficient to absorb most sunlight. 

A prime contender for leading thin film technology as applied to solar cells is cadmium telluride (CdTe). Its bandgap is almost ideal for use as a solar cell for energy conversion from the Sun s spectrum. Here, CdTe and cadmium sulfide (CdS) are used to produce a low cost thin film solar cell... 

Still another method used to produce PV cells is provided by thin-film technologies. Thin films are made by depositing semiconductor materials on a solid substrate such as glass or metal sheet. Among the wide variety of thin-film materials under development are amorphous silicon, polycrystalline silicon, copper indium diselenide, and cadmium telluride. Additionally, development of multijunction thin-film PV cells is being explored. These cells use multiple layers of thin-film silicon alloys or other semiconductors tailored to respond to specific portions of the light spectrum. 

Although conventional solar cells based on silicon are produced from abundant raw materials, the high-temperature fabrication routes to single-crystal and polycrystalline silicon are energy intensive and expensive. The search for alternative solar cells has therefore focused on thin films composed of amorphous silicon and on other semiconductor heterojunction cells (e.g., cadmium telluride and copper indium... 

Another exciting thin-film company is FirstSolar (www.firstsolar.com). First Solar claimed to achieve a manufacturing cost of only 1.08 per watt during the third quarter of 2008. The company uses cadmium telluride as a semiconducting material, which is a byproduct of the mining and production of base metals such as zinc and copper. First Solar has entered into many excellent long-term sales agreements with major electric utility companies. 

An imager having an element packing density of 90% is disclosed in US-A-4104674. Infrared photovoltaic detectors of mercury cadmium telluride are mounted on a silicon substrate. Electrical contacts are made by thin-film metallizations. 

The imager disclosed in WO-A-9202959 comprises thin film transistors deposited on a substrate, a planarization layer deposited on the transistors followed by a mercury cadmium telluride layer and a top electrode layer. 

A plurality of thin film field effect transistors 11 are deposited onto a substrate 12. Each of the transistors has a source electrode 13, a drain electrode 14 and a gate electrode 15. Source lines 17 link the source electrodes in each row of the transistors and drain lines 18 link the drain electrodes in each column of the transistors. The source lines and drain lines are electrically isolated by a planarization layer 19. A mercury cadmium telluride layer 20 is deposited onto the planarization layer followed by a top electrode layer 23. The gate electrodes are connected with the mercury cadmium telluride layer by connectors 21. A cross-section of the imager is shown below. 

Here, 41 indicates the thin film transistors, 51 the substrate, 43 a dielectric layer, 49 polysilicon gates, 50 gate electrodes, 55 contact plugs, 56 bottom electrodes, 53 the planarization layer, 54 the mercury cadmium telluride layer and 57 the top electrode layer. The planarization layer is formed from silicon oxide, silicon nitride, silicon oxide nitride or from a polyimide. The planarization layer may be formed as a double or triple layer. 

A thin p-type body 2 of Hgo.79Cdo.21Te is bonded to a silicon integrated circuit 3 by an intermediate film of epoxy adhesive. The upper and lower surfaces of the body 2 are passivated by a surface layer of cadmium telluride or ZnS. Gaps 40 are etched in the body 2 to separate the islands 28 from the remainder of the body. Then, a thin p-type body 1 of Hgo.7Cdo.3Te is bonded to the upper surface of the body 2 by an intermediate film or epoxy adhesive. 

The basic structure of polycrystalline cadmium telluride (CdTe) thin-film cells has a glass superstrate and a layer of TCO as front contact, a near-transparent n-type cadmium sulfide (CdS) window layer, p-type CdTe, and a metallic rear contact. The CdTe is usually deposited by three families of techniques. In the first group (vapor transport deposition, close space sublimation, physical vapor deposition, and sputtering) elemental vapors of Cd and Te condense and react on the substrate. In the second (electrodeposition), Cd + and HTe02" ions in acidic electrolyte are galvanically reduced at the surface ... 

The most prospective thin film technology is BP s proprietary cadmium telluride process. It offers a better combination of performance, robustness and costs than competing materials. The process technology is straightforward - a cheap glass is coated with a transparent conductive layer of tin oxide, which is sequentially coated with thin layers of CdS and then CdTe to make the basics of the cell. The CdTe layer is black and an extremely efficient absorber of light. 

Theoretical conversion efficiencies of photovoltaic systems depend on the semiconductor materials used in the cells and on the ambient tanperatuie. The materials currently used to make photovoltaic cells can be grouped into three broad categories 1) expensive, efficient monocrystalline silicon, 2) less efficient but much lower cost polycrystalline silicon, and 3) the lowest cost and poorest performer, amorphous silicon material. Conversion efficiencies of commercial polycrystaUine silicon cells are 10 to 15 percent. Now the primary development areas are in how to use monocrystalline silicon with solar concentrators and making thin-film cells by depositing a 5- to 20-micron film of silicon onto an inexpensive substrate, because the estimated efficiency of these cells is above 20 percent. Work is ongoing with other materials, including amorphous silicon (a-Si), copper indium diselenide (CuInSe2 or CIS) and related materials, and cadmium telluride (CdTe). 

Compared to homojunction stmcture amorphous silicon thin-film cells use a P-I-N hetrojunction stmcture, whereas cadmium telluride (CdTe) cells utilize a N-I-P arrangement. The overall picture embraces a three-layer sandwich with a middle intrinsic (/-type or undoped) layer between an N-t) e layer and a P-type layer (Fig. 3b). Multiple junction cells have... 

Lepiller C, Cowache P, Guillemoles JF, Gibson N, Ozsan E, Lincot D (2000) Fast electrodeposition route for cadmium telluride solar cells. Thin Solid Films 361-362 118... 

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