Diodes, silicon, structures

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

  1. Marian000

    Thread Starter New Member

    Aug 15, 2016
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    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. WBahn

    Moderator

    Mar 31, 2012
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    Where are these questions coming from? They sound like homework questions. Are they?
     
  3. Martin976

    New Member

    Aug 14, 2016
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    Yes, exactly. I don't expect the full answers from you, mainly the tips where could I get these information from to cover the questions.
     
  4. Martin976

    New Member

    Aug 14, 2016
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    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 :)
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    I don't mean to be critical, but I find the internet to be a poor source for detailed technical material. The writing is generally unsuited for newcomers to a subject and often omits necessary background material relevant to the exposition at hand. If the articles you read contain references, I would check those out. One or more of them might be just the ticket. As a last resort you can ask your instructor to recommend some appropriate material. I'd hate to rely on material that is wrong or incomplete when I walk into an exam.

    It has numerous similarities to using Cliff's Notes to write a term paper on a novel or play you just don't have time to read. It is the fastest way to get a D or an F that I know of.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    What material do you have for this course in terms of a textbook or something similar? That is what I would rely on first and foremost (since that is the material that the instructor is probably relying on for the homework and exams so, even if it isn't the best and most accurate material, it will be the best match for what you will be being graded against).
     
  7. Martin976

    New Member

    Aug 14, 2016
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    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 answered right and maybe some advise where could I get the rest of the essential knowledge from.
     
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