# Diodes, silicon, structures

Discussion in 'Electronics Homework Help' started by Marian000, Aug 15, 2016.

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1. ### Marian000

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Aug 15, 2016
Can you help me with answering to these questions?

1. Amorphous silicon has quite different properties to those of crystalline silicon. Why does amorphous silicon have a narrower spectral bandwidth (nm)?

2. How does a typical solar cell differ from a Shockley diode?

3. Why do short wavelength photons generate a lower photocurrent than longer wavelengths?

4. Amorphous silicon is fundamentally more efficient at absorbing light, compared to crystalline silicon. Why should this be?

5. Name the limiting factors that affect the absorption of sunlight by a solar cell.

6. Explain briefly how each of these limitations affect the potential photocurrent?

2. ### davennModerator

13,669
1,891
Sep 5, 2009
hi there
welcome

have you actually bothered to google any of these questions before asking us to do that for you ?

we don't give direct answers for homework questions, rather we prefer to aim people to learn how to
do their own research as it helps then learn how to study

so with all that in mind, tell us what you have discovered about each of your questions so far

Dave

3. ### Marian000

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Aug 15, 2016
I was trying to google out these things. That is what I found to specific questions.

1.
For crystalline silicon the energy gap Eg = 1.12 eV.
For amorphous silicon the energy gap Eg = 1.75 eV

The situation is very lossy for crystalline silicon as it is an indirect band gap material. Amorphous silicon is however a direct band gap material.
For wavelengths longer than the critical value c, the photon energy is no longer sufficient to excite electrons to the conduction band and the material becomes transparent to those wavelengths.
Amorphous silicon has a higher band gap (1.75 eV) than crystalline silicon (c-Si) (1.12 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. a-Si:H In early studies of amorphous silicon, a-Si, it was determined that plasma-deposited amorphous silicon contained a significant percentage of hydrogen atoms within the amorphous silicon structure. These atoms were discovered to be essential to the improvement of the electronic properties of the material.

but Im not sure does it cover the problem

2.

The I–V characteristic of an ideal diode in either forward, reverse bias or zero bias are expressed by [exp( ) 1]  0  k T qV I I diode Where: Idiode is the diode current Io is the reverse bias saturation current V is the voltage across the diode k is Boltzmann’s constant 1.380 ×10−23 JK −1 q is the charge on an electron 1.6 ×10−19 C In the absence of light, the I-V characteristics of a solar cell are also expressed by the Schockley Equation: [exp( ) 1]  0  k T qV I I diode Io I V When light is absorbed and photo current generated, Iphoto, the output current is represented by I – this is simply expressed as the difference between the photocurrent, Iphoto and the diode current, Idiode I = Iphoto - Idiode [exp( ) 1]   0  k T qV I I I photo

I couldn't found the better specification

3.

The current generated is proportional to the incident light or radiation power. The light is absorbed exponentially with distance and is proportional to the absorption coefficient. The absorption coefficient is very high for shorter wavelengths in the UV region and is small for longer wavelengths Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while silicon becomes transparent to light wavelengths longer than 1200 nm. Moreover, photons with energies smaller than the band gap are not absorbed at all.

4.

Amorphous silicon has a higher band gap (1.75 eV) than crystalline silicon (c-Si) (1.12 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. a-Si:H In early studies of amorphous silicon, a-Si, it was determined that plasma-deposited amorphous silicon contained a significant percentage of hydrogen atoms within the amorphous silicon structure. These atoms were discovered to be essential to the improvement of the electronic properties of the material.

5.

• the number of photons absorbed by the cell
• the electron charge
• the quantum efficiency
• the photon wavelength
• the surface area

not sure whether these are relevant factors

6.

Trying to describe the meaning of factors from previous question.

I need the verification of my answers. If any ambiguities or mistakes please give me the hints or send me the links to websites where could I find out more. I'll be grateful for your assistance

4. ### Ratch

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Mar 10, 2013
I don't think any of us in this forum know enough about silicon applications at the quantum level to be of much help to you. Your questions appear to be at the post graduate level in material science. Sorry, but we are mostly skilled in electronic and electrical applications.

Ratch

5. ### Marian000

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Aug 15, 2016
http://www.speedyshare.com/8sfrT/Renewable-energy-technology.pdf
The only textbook I have obtained from my lecturer was this attached pdf. Unfortunately that appears to me to be not sufficient to answer to all of these questions. I need the verification whether I answeredright and maybe some advise where could I get the rest of the essential knowledge from.

6. ### Ratch

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331
Mar 10, 2013
I could not download the contents of the above link without going through some silly verification process, which I refuse to do.

Ratch

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Aug 15, 2016
8. ### Ratch

1,088
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Mar 10, 2013
OK, got it. It is going to take some time to assimilate this material. I will get back when I finish.

Edit: I looked at the link andI would advise you to get another reference to study. The linked material does not systematically build the basic knowledge needed, and refers to terms and concepts in a haphazard manner. It also does not fully define many of the terms it throws at you. Interspersed within are many historical items that are interesting, but history does not teach technical knowledge. I think this link disperses knowledge in a discombobulated manner.

Ratch

Last edited: Aug 16, 2016