Solar Cells / Discussion

This is the discussion related to the wiki page Solar Cells.

Dharma Maddala 28 Apr 2009 16:11

nice work kuoting,
what are suitable materials for solar cells, i mean, what is the band gap where there is high efficiency of conversion. (helps us in answering question 2 in homework 3)

Kuoting 28 Apr 2009 16:33

The absorption coefficient of a material indicates how far light having a specific wavelength (or energy) can penetrate the material before being absorbed. A small absorption coefficient means that light is not readily absorbed by the material. Again, the absorption coefficient of a solar cell depends on two factors: the material making up the cell, and the wavelength or energy of the light being absorbed. Solar cell material has an abrupt edge in its absorption coefficient. The reason is that light whose energy is below the material's bandgap cannot free an electron. And so, it isn't absorbed.

Multijunction Devices

This structure, also called a cascade or tandem cell, can achieve a higher total conversion efficiency by capturing a larger portion of the solar spectrum. In the typical multijunction cell, individual cells with different bandgaps are stacked on top of one another. The individual cells are stacked in such a way that sunlight falls first on the material having the largest bandgap. Photons not absorbed in the first cell are transmitted to the second cell, which then absorbs the higher-energy portion of the remaining solar radiation while remaining transparent to the lower-energy photons. These selective absorption processes continue through to the final cell, which has the smallest bandgap.These cells have efficiencies of more than 35% under concentrated sunlight—which is high for PV devices.

Hello Dharma, I hope these information can help you with the question in HW:)

Re:

yuan xue 05 May 2009 03:37

Hi, Kuoting,
Great information about the multijunction cell. The problem 2 in this homework introduced 2 materials, ZnS and GaP. ZnS is not satisfied for solar cell, but it can be used in multilayer soler cell, is this can be explained by this information? Thanks!

Re:

Jialan Zhang 05 May 2009 04:26

Hello~Yuan,
ZnS can be used as buffer layer in the multijunction cell. Buffer layer can make the light source expitaxial, and reduce the mismatch
between the layer.
Also, doped ZnS can reduce its energy gap,which can make it suitable for solar cell.

I hope I answered your question:)

Re:

yuan xue 05 May 2009 14:34

Thanks, Jialan, your answer helped me a lot!

Suitable material

satyesh 30 Apr 2009 18:10

There are various approach other than multi-junction approach as using single junction but modifying solar spectrum by up-conversion and down-conversion of photons.

Increase effeciency

satyesh 30 Apr 2009 18:13

In order to increase efficiency of solar cell not only band gap have to be right but exciton separation should be fast and efficient.

Re: Increase effeciency

Kuoting 02 May 2009 15:15

That's correct, thank you for sharing this information. :)

mariael84 01 May 2009 15:42

Hey Kuo-ting, very nice work. In these multijunction devices that you describe in your answer, are all the band gaps of the materials in the range of the energy of visible light, or the first cell for example could have a higher band gap?

Re:

Kuoting 02 May 2009 15:07

Hello Maria, For my understanding, The top and bottom layers in a multijunction devices have different roles. The top layer, or "window" layer, is a material with a high bandgap selected for its transparency to light. The window allows almost all incident light to reach the bottom layer.
For example, If use ZnMgO(wide band gap material), it can cover from UV to blue light.

I hope I answered you question:)

Fangxiao 02 May 2009 18:03

Hi, Guoting,

Very interesting topic! As I am working mostly on the experimental side, I am very curious to know the commercial application of solar cell. Is the production of solar cell commercially available in industry? Is there some challenge to make it a commercial product? Thank you!

Re:

Kuoting 05 May 2009 06:29

Hello Fangxiao,
As I know, people are trying to make solar cell with high efficiency and low cost materials and method.
These are two important factor of solar cell.

A-Rivera 04 May 2009 22:55

Hi Guoting,

Do you know how much is the higher efficiency ever in a solar cell, any possible configuration???

Re:

Kuoting 05 May 2009 06:13

Hello A-rivera,
As I seach on the website, the higher efficiency of solar cell will be a triple-junction GaInP/GaInAs/Ge
cell fabricated by Spectrolab and measured at NREL, which can exceed 40% energy conversion
efficiency. An efficiency of 407% is reported at 240 suns concentration under the low aerosol optical depth direct-beam spectrum.

circuit diagram

Gokhan Bakan 05 May 2009 13:25

Hi,

Nice page,

I have a question: in the second paragraph of the "connection to an external load" part, it is stated that "From the equivalent circuit it is evident that the current produced by the solar cell is equal to that produced by the current source, minus that which flows through the diode, minus that which flows through the shunt resistor…"

My question is: How does its equivalent circuit look like and why is there a shunt resistor?

Thanks.

Liang 05 May 2009 15:47

Hi, Kuoting, good job! I am interested in the electrodes attached to the n-side. Which material do they use to allow illumination enter?