Researchers demonstrate 'avalanche effect' in solar cells

May 26, 2008
Researchers demonstrate 'avalanche effect' in solar cells
Visualisation of avalanche effect. Credit: TU Delft

Researchers at TU Delft (Netherlands) and the FOM Foundation for Fundamental Research on Matter have found irrefutable proof that the so-called avalanche effect by electrons occurs in specific, very small semiconducting crystals. This physical effect could pave the way for cheap, high-output solar cells. The findings are to be published in scientific journal Nano Letters this week.

Solar cells provide great opportunities for future large-scale electricity generation. However, there are currently significant limitations, such as the relatively low output of most solar cells (typically fifteen percent) and high manufacturing costs.

One possible improvement could derive from a new type of solar cell made of semiconducting nanocrystals (crystals with dimensions in the nanometre size range). In conventional solar cells, one photon (light particle) can release precisely one electron. The creation of these free electrons ensures that the solar cell works and can provide power. The more electrons released, the higher the output of the solar cell.

In some semiconducting nanocrystals, however, one photon can release two or three electrons, hence the term avalanche effect. This could theoretically lead to a maximum output of 44 percent in a solar cell comprising the correct semiconducting nanocrystals. Moreover, these solar cells can be manufactured relatively cheaply.

The avalanche effect was first measured by researchers at the Los Alamos National Laboratories in 2004. Since then, the scientific world has raised doubts about the value of these measurements. Does the avalanche effect really exist or not?

Within the Joint Solar Programme TU Delft’s Prof. Laurens Siebbeles has now demonstrated that the avalanche effect does indeed occur in lead selenide (PbSe) nanocrystals. It has been established, however, that the effect in this material is smaller than previously assumed. Siebbeles’ results are more reliable than those of other scientists thanks to more careful and more detailed measurement using ultra-fast laser methods.

Siebbeles believes that this research paves the way for further unravelling the secrets of the avalanche effect.

The paper can be found here (requires subscription).

Source: TU Delft

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User comments : 9

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2 / 5 (4) May 27, 2008
The scientific errors in this articles are pretty bad...

Avalanching alone doesn't do squat for efficiency.
3 / 5 (1) May 28, 2008
Quote "The scientific errors in this articles are pretty bad... " (sic)

Now, that's what I call Specific and Measurable Feedback! Does "this" refer to the particular article or by the vague "articles" you doom the whole literature altogether?
Could you be more specific albeit the aforism? Your constructive quantifiable scientific feedback will be much appreciated, certainly by an ignorant like me and might I say by the whole world (eventually). Cheerio!
3.8 / 5 (4) May 28, 2008
My fault on the unintended 's'.
Feel free to use the 'quote' button.

I was hoping someone more familiar with optoelectronics would prove me wrong, or someone else would agree/support that avalanching doesn't help your efficiency at all. Even if you have one photon in and many electrons out, you still have the same amount of energy coming in, and if it's one near equal energy electron out, or a few low energy electrons out, it really doesn't matter. You'd have to supply an external voltage to take advantage of (or possibly produce) these additional electrons, which seems to defeat the purpose. If the material is designed with a very wide/gradiented bandgap along with the avalanche effect then you could have some interesting effects, such as a higher sensitivity to a broader spectrum.

4.5 / 5 (2) May 28, 2008
In traditional solar cell, the excess energy of photon is rather transfered to heat than electricity. Carrier multiplication can use this excess energy to create one or more electrons (avalanche effect). Solar cell is characterized by voltage and current. The current is depend on number of electrons and voltage is depend on band gap of material. Avalanche effect increases number of electron and of course increases solar cell efficiency.
4 / 5 (1) Jun 02, 2008
Maybe in a thermovoltaic this is true... You need a mechanism to transfer energy from lattice vibrations(phonons) to electrons. The argument of avalanching increasing efficiency due to taking advantage of heating is completely invalidated because solar cells work BETTER at lower temperatures. This is because there is no mechanism of thermal to electrical energy in conventional solar cells.
4 / 5 (2) Jun 02, 2008
In this case, there is nothing to do with phonon or heat. In bulk semiconductor, the hot electrons (after absorbed photons) is relaxed to bottom of conduction band and the excess energy is transfered to heat in about few ps. But in quantum dot, this excess energy is used directly to create others electrons in about subps (faster than transfer to heat), this is so call impact ionization or multiple exciton generation (other name is carrier multiplication). The mechanism for this phenomenon is still not clear.
3 / 5 (1) Jun 02, 2008
>However, there are currently significant limitations, such as the relatively low output of most solar cells (typically fifteen percent)

Efficiency or output efficiency not just output. Output (power) should be given in wats.
not rated yet Jun 02, 2008
'One photon in and two electrons out' can lead to better efficiency then 'one photon in one electron out' if its the energy normally wasted as heat in the later case that is converted to this second electron excitation.
not rated yet Jun 10, 2008
This article is using "efficiency" in a somewhat ambiguous way.

When "efficiency" is defined as the number of electrons out per photon in, then avalanche improves the efficiency dramatically.

However, if "efficiency" is defined as power out / power in, then it is not clear that there is much gain from using the avalanche effect. (As other posters have noted, perhaps there is a recapture of the energy converted to heat, but I would be very surprised to find that contribution changes the efficiency from 15% to 45%.)

Obviously, for solar energy generation, it is the energy efficiency that matters most.

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