Dear Sabah,

Your expression of monochromatic light may be misleading. The solar cells are thought to convert the incident solar radiation into electricity.This radiation is not a monochromatic  light  bu a wide spectrum light similar to a blackbody radiation at 6000K. Therefore. for single junction solar cells, there is an optimum bandgap for converting such radiation with the maximum efficiency. The dependence of the conversion efficiency on the energy on the energy gap of the material has been intensively studied in the literature. It was found the optimum bandgap is about 1.35 eV which is approximate equals that GaAs.

In order to convert more incident solar radiation one uses the multi-junction solar cells, where the wide solar spectrum is divided into subbands made to be absorbed by a predetermined junction of the stack of the junctions.

For selecting among the different silicon solar cell technologies there is different criterion for this selection;

- The conversion efficiency

As the conversion efficiency increases, the area of the PV array decreases and the installation cost decreases.. Also , the foot print decreases which means less land is requires.. 

- The approval and maturation of the technology which means granted longer lifetime and high reliability.

- The overall cost of the PV station producing the same pea power.

- May be a figure of merit is the expected cost of one KWH  on the whole life of the array taking into consideration the maintenance  and operation cost.

The figure of merit may favor the single crystal silicon solar cells?

wish you success

You need a technology that has the band gap energy close to the photon energy of the monochromatic radiation. Too high and you lose current (too few photons will result in electrons) and too low a bandgap and you will not generate as much voltage as you could. Incidentally the efficiency of a well designed cell converting monochromatic radiation is going to be way above the efficiency obtained when it is illuminated by a black-body spectrum.

Tony Maine gave a correct response.

Note that this is a very hypothetical question, as most source generate black body radiation, and that is not monochromatic.

Solar cells are used for more than just converting sunlight. This application is no doubt the widest use, but they are also used in eg thermoelectic generators (for spacecraft eg), where a body is heated by radioactive decay of some substance (eg plutonium), and the black body radiation is transferred to electricity by solar cells, with a lower bandgap than conventional solar cells. In this case, a selective emitter is used, so that the spectral width of the radiation is reduced, and the efficiency is improved.

GaAs has an optimal band gap for the sunlight in space, i.e. AM0 sunlight. This does not at all apply for a hypothetical monochromatic light. For a monochromatic light, the optimal semiconductor will be one having a band gap slightly below the photon energy. 

You asked which technology will have better conversion, i.e. higher efficiency? I am not sure what you mean by technology! Technology will determine factors like cost, etc.

If you have a choice of silicon solar cells - monocrystalline, polycrystalline, thin film - whatever, there may be zero conversion if the monochromatic light energy is larger than the silicon band gap!

So the question goes back to you, Sabah Gaznaghi: What is the wavelength (or the photon energy) of the monochromatic light that you have in mind (to be converted to electrical energy)?

Well, let's say the hypothetical monochromatic light has a wavelength of 0.86 micron (Infrared).

Please note that solar light is not subject in this question, but a monochromatic light like a laser beam incidents a converting cell. GaAs cells have a direct and sharp bandgap, which could make a better coupling with a monochromatic light with almost equal energies. Some other factors like high purity films made by epitaxy deposition make GaAs a good candidate within this wavelength but it is expensive and needs relatively complicated manufacturing processes. Silicon cell can be used in the wavelength range of 0.8 to 1 micron too, but with lower conversion efficiency.

Sabah,

Your question now for me is clear. You have a monochromatic light of wavelength lambda= .85 um and you want to sense it with a solar cell. Then the parameter of inerest is spectral response of the solar cell at this wavelength  in A/W. This also is called the sensitivity of the detector. Mono crystalline silicon can work well in this wavelength. For more information please see the link:http://www.bentham.co.uk/pdf/PV_Technical_Note.pdf

wish you success

Coming back to your explicit question ("But if you have to choose between mono-crystalline silicon solar cells, poly crystalline silicon solar cells and thin film silicon solar cells, which one will you choose?"), I think the answer is more or less clear: Your first choice is mono-crystalline silicon, since it has the highest coversion efficiency of the three silicon solar cell types mentioned. If you are fine with a few percent (absolute) less conversion efficiency, then multi-crystalline silicon cells (especially those based on "high-performance multi" wafers developed in recent years) will satisfy you, too -- and they are way cheaper!

However, if for some reason you need a thin-film device, then you should have a look at others than silicon-based ones, since the latter don't perform so well. Unfortunately, you cannot use CdTe (cadmium telluride), because its band gap of 1.56 eV is too high for your purpose, since it makes CdTe transparent for wavelengths longer than 795 nm. So there are CIGS and co waiting to please you; however, I'm not sure about the actual band-gap energy of the absorber layer used in those modules (usually, they don't come in single cells)...

But why, on the other hand, do you speak of solar cells at all -- aren't there any photo diodes around that you could use? Or is the idea to establish a directed transfer of a significant amount of energy over a certain range (in the order between 0.1 m and 10 m, say) by using a high-intensity infrared laser?

I appreciate your replies. Indeed I am thinking about feasibility of wireless power transport via laser power beaming.

Dear Sabah,

I believe you find answer to your question in a recent paper of us (Höhn, Walker, Bett, Helmers, Applied Physics Letters 108(24), DOI: 10.1063/1.4954014). Here we investigate the ideal wavelength for GaAs PV laser power converters. The model can also be applied for different materials, i.e. laser wavelengths (e.g. 0.6 or 1.3-1.5 µm).

Best regards, Henning

I would advise a degree of caution if you want to transmit any real level of power using a laser. It might be quite dangerous for power levels over the 10-3 watt level. There are strict limits on the level of laser light intensity allowable in free space systems.

Sure. But the system which I am interested in uses fiber optics as a safe medium. Replacing copper wires with fiber optics would be very useful for many applications.

OK, Sabah, but that isn't a major issue. If you disconnect a fibre, and look at the end of it, and the radiation is not visible, you could still damage your eyes. Solar energy of 1 kW/m2 is about 1 nW per sq micron, so over the area of a single mode fibre core (about 50 sq microns) you get 50 nW. Using a 100 mW laser and putting it all into a fibre gives you an energy level equal to 20,000 suns at the exit point, and this can be focussed on the retina - not nice! So there are also safety limits to what power level you can put into a fibre, as well as practical ones.

Dear Dr.Maine,

Thank you for your clear and helpful explanation. Certainly it is hazardous, if someone's eye is exposed to a bright light of laser. But the question is, why should this happen!!?? I absolutely believe that, the optical power transmission is safer than wiring. (if you consider the risks of wiring).

Dear Sabah, very sadly it might happen because accidents happen. You would think that with modern understanding of electricity and its dangers that nobody would get electrocuted. But it does happen. If you have invisible beams of IR transmitting real power at very high intensities then sooner or later one will get into someone's eye. To mitigate this problem you could design connectors that blank off the fibre if they are disconnected (this might reflect the energy back into the laser and destroy it, so that's not a sure fire solution either). Some electrical connectors do something like this. And it will be an improvement - of course it will - until the safety mechanism fails, or more likely, gets overridden by a careless or over zealous technician.