MIT scientists and students are looking for cheaper and more efficient solar cells, either by incorporating materials that are so abundant that they could support a major boom in the industry or by cutting production costs for conventional solar cells.

The research is a systematic exploration of the kinds of materials that could be used for future solar cells. More than 500 compounds made from Earth elements have the necessary semiconductor properties. Of these, about 10 elements are selected that appeared most promising for more detailed study. The first of these compounds is cuprous oxide (Cu₂O). "It is promising, the optical properties are just right," Buonassisi said. "But the electrical properties are not up to snuff," so they are using defect engineering methods to try to improve that.

Another important direction of research - is trying to cut the cost of the highly purified silicon used to make conventional solar panels. Normally, silicon-rich ore such as quartz is heated in an arc furnace so that the impurities can be separated out -- a very energy-intensive process. The impurities are not uniformly distributed in the quartz, but are concentrated in small pockets or clustered. The researchers develop a process for removing these impurities before melting that could eliminate steps in purifying.

Buonassisi and other MIT colleagues are also working on improving the efficiency of solar cells made using multicrystalline silicon. But in the multicrystalline material the defects within the grains called dislocations exist that tend to soak up a lot of the energy produced. By controlling the temperature just right, the energy-sapping dislocations can be reduced by more than a hundred fol. This part of the research is already well advanced. The MIT team is beginning to work with manufacturers to bring it to market. Pilot runs are expected within a year, and full-scale production soon thereafter.

One of effective directions of semiconductor silicon structures efficiency increasing is expounded in the report of experimental laboratory of electronics of Massachusetts Technological Institute (Boosting power solar sells. December 3, 2008. David Chandler, MIT News Office MIT's Research Laboratory of Electronics). He consists in depositing of diffraction grating on the backside of silicon structure with the purpose of increase the active phase of the sun radiation interaction with a photosensitive medium. The computer design of sunlight absorption process which was executed by Lionel Kimerling, Peter Bermel and other researchers showed possibility of 50-percent transformation of the sun radiation energy into the electric signal. For three years the researchers hope to get the industrial prototypes of similar high effective silicon structures.  

Part of scientific searches is directed to reduction of dependence of the sun modules producers from the producers of silicon structures due to the use of concentrative elements coincided with the photo-electric elements of less size. In such combined systems along with small solar cells the optical concentrators are used: refractive lenses, mirror elements and diffraction elements (zone Fresnel plates). They provide focusing of optical radiation on the small surface of photosensitive element and thus diminish the relative value of silicon elements in the total worth of the system «concentrator – photo-electric element».

One of similar revolutionary technologies is offered on the court of scientific public in the article «MIT opens new 'window' on solar energy. Cost effective devices expected on market soon». Elizabeth A. Thomson, MIT News Office. July 10, 2008.

Marc A. Baldo, Shalom Goffri, Michael Currie, Jon Mapel and Timothy Heidel - all from Massachusetts Technological Institute - suggested using as concentrator the parallel-sided plate which was produced from optically transparent polymeric material. During the plate making process the polymeric material was doped by the dye mixture composed from one or more ingredients. The dye matrix which was created this way takes in sunlight and then radiates him mainly in a plane of the parallel-sided plate. Silicon cells that are set on the ends of plate get thus concentrated light and convert him into electric current. Developers expect that the channeling of sun radiation made in such way will help to attain efficiency of light-signal transformation close to 50%. They created an industrial company to commercialize a project by bringing of investments of venture funds in.

Yet at the beginning of 90th Nizhyn laboratories of scanning devices ltd executed development of electric relay display on the basis of the use of waveguide properties of light transparent polyethylene (RMMA) plate. As light active material a photoluminescence dye re-irradiative in the range of wave-length approximately 0,6 μm (Red) was used. Light waveguides of different form – parallel-sided plates, cylinder type - were made by extrusion from fusion and by pressing. The edges of waveguide were polishing to receive the necessary cleanness. While waveguides were frontally illuminated by solar light the effect of the certain strengthening of light was speculated on the edges of the doped plates. We did not conduct the quantitative measure, because we delivered the effect of strengthening on subjective perception. It is very interesting to acquaint with these previous results of measure of channeled optical power quantity relatively to the entrance optical power quantity.

Now Nizhyn laboratories of scanning devices ltd that has an experience of holographic scanner development work on creation of the new type of the optic holographic concentrators for the sun modules. The main peculiarity of this optical holographic concentrator is the possibility of uses him as follow:  

·     optic filter for admission (reflection) of certain wave-length radiation;

·     optic concentrator for focusing of optical radiation on a small photosensitive element;

·     tracking element, which allows correcting the trajectory of light beam scanning.

These different useful qualities are set (programming) to the holographic concentrative element during wave front record making process. 

The engineers of NLSD created a row of charts of solar modules with scattered bar electrodes system and combined with holographic concentrators, working on transmission and reflectance of light.

A solar holographic concentrator, working on transmission, is composed of many holographic components, situated one near other and created by means of writing of interference patterns that appear in a moment of interaction of two coherent beams, one having a flat wave, other - spherical. Every next hologram differs from former by phase shifts, calculated before making of hologram. This composed hologram works in that way. Incident solar radiation, which consists of beams 1, 2, 3,…, N, falls on the elementary holographic aspherical lenses. Diffracted beams 1¹, 2¹, 3¹, …N¹ are cutting in a focal point, where a sensitive part of a solar cell is mounted.

Solar module consists of light sensitive semiconductor, placed on a base plate, and of row of scattered bar electrodes.  A system of scattered electrodes is made just as electrode system of charge coupled device. Under each electrode the potential wells appear that work as accumulators of charges, created under bombardment of light photons. Such a system permits to lower the speed of recombination of charge carriers and to raise the effectiveness of solar sell.

The principle of construction and work of the solar module with scattered bar electrodes system and combined with holographic concentrators, working on reflectance of light, differ insufficiently.

The selection of concrete chart of a solar cell is defined by conditions of use and by the possibilities of device-maker.  

The got results showed that flat holographic concentrating elements of Nizhyn laboratories of scanning devices ltd give a possibility of their high effective use both in the volume and flat solar cells systems.

The holographic elements are made with the use of well known technologies of wave front record. Their low cost is provided at the mass making. The efficiency of light-signal transformation that can be attained by means of holographic concentrating elements is high enough (30…70% depending on the chosen chart of the use). The serial production of the high efficiency sun modules with new holographic optical concentrators can be started during 10-15 months.

Technologies and equipments, which we develop, are universal enough for the receipt of electric current and providing electric power to different users. From other side, technologies and equipment have high degree of scientific features (in a world there is the limited amount of remote analogues). For the serial making of equipment in future both our enterprise and certain other highly technological enterprises can be involved.

 Vasil Sidorov E-mail: sidorovvasil@gmail.com

on May 05, 2009, QueltaNews Office