Picture Story – How Do Organic Solar Cells Function?

After the introductory posts about organic solar cells – split in parts zero, one and two, – I would like to present a somewhat more intuitive picture today… well, picture indeed says it all;-)

Step 1: Light Absorption => Exciton Generation

osc bhj morphology scheme - 1.jpg absorption bands polymer vs cis.jpg
  • light is absorbed in the donor material, e.g., a conjugated polymer
  • excitons are thus created, strongly bound electron-hole pairs on the polymer chain
  • very high absorption coefficient, device thickness on ~100nm scale, as compared to the inorganic polycrystalline semiconductor CuInSe2 (~1 micron) and crystalline Silicon (~100 micron)
  • but: only narrow absorption bands, as shown for two conjugated polymers P3HT and PCPDTBT in comparison to CuInSe2. This drawback could be circumvented by synthesis of novel materials, or multijunction concepts (tandem solar cells).


Step 2: Exciton Diffusion => to Acceptor Interface

osc bhj morphology scheme - 2.jpg OPV bhj device scheme - pedot, light and morph.png
  • the photogenerated excitons are strongly Coulomb bound due to the low dielectric constant in organic materials, and the correspondingly low screening length: charges can ‘see’ each other very well
  • electrically neutral excitons can only move by diffusion
  • in order to disociate into an electron-hoe pair, it has to find an acceptor site (e.g., fullerene molecule)
  • short exciton diffusion length of only a few nanometres
  • therefore, no bilayer concept, instead bulk heterojunction solar cells of intermixed donor and acceptor materials (shown in the figure), such as conjugated polymers blended with fullerene derivatives

Step 3: Exciton Dissociation => Polaron Pair Generation

osc bhj morphology scheme - 3.jpg opv-ct-ppv-pcbm.jpg
  • excitons dissociate only at energetically favourable acceptor molecules such as the fullerenes, when the energy gain is larger than the exciton binding energy
  • then, an electron transfer (or charge transfer) takes place, dissociating the exciton into an electron on the fullerene acceptor, and a hole remaining on the polymer
  • this electron-hole pair is still Coulomb bound, and is called geminate pair or polaron pair

Step 4: Polaron Pair Dissociation => Free Electron–Hole Pairs!

osc bhj morphology scheme - 4.jpg opv-ct-diss.jpg
  • the polaron pairs are Coulomb bound
  • they also need to be dissociated, this time by an electric field ( = built-in voltage + applied voltage)
  • therefore, the photocurrent in organic solar cells depends strongly on the applied voltage
  • this is a major loss mechanism in organic solar cells

Step 5: Charge Transport => Photocurrent!

osc bhj morphology scheme - 5.jpg
  • the electrons and holes are transported to the respective electrodes, driven by the electric field, and moved by a hopping transport process
  • hopping: very slow charge transport, low carrier mobility, at least a factor of 1000 smaller than for crystalline Silicon… while the power conversion efficiency of organic solar cells is only factor 4 worse;-)
  • indeed, our current research indicates that a loss of free charge carriers by nongeminate recombination during the charge transport to the contacts is only marginal
  • and, higher mobility does not improve the power conversion efficiency significantly. Will be covered in a later post;-)

So much for now, see you later.

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14 Responses to Picture Story – How Do Organic Solar Cells Function?

  1. [...] in organic solar cells – Part 1 In at least two previous posts (Picture Story and How do organic solar cells function – Part 1), I highlighted the field dependence of the [...]

  2. armismelas says:

    Very nice! Crystal clear :)

    I have done my bachelor on C60/CuPc solar cells. I’ll keep reading this blog!

  3. yuliar says:

    Hi, thanks for all your posts. Its really helpful. I wonder if you could also discuss about the role of energy transfer in organic solar cells. Whether it could be favorable or otherwise for photocurrent.

    • deibel says:

      Thanks. Energy Transfer is an interesting topic. In principle it is a loss mechanism in competition with charge transfer for exciton dissociation, however, it can also lead to separate charges by subsequent charge transfer. Unfortunately, due to my time contraints I am not likely to write about it soon. Best, Carsten

  4. Matthias says:

    Hi,

    is there any source for the pictures you use here? I want to use them in my thesis and -of course – want to cite them. But I don’t like to cite an url…

    Thanks
    Matthias

  5. Salman says:

    Dear Professor,

    At first I would like to thank you for your great effort. It will really help us all who are working with organic solar cells. My question might sound silly but it will help me to understand the inner physics of an organic solar cell.

    1. You wrote, “light is absorbed in the donor material”. Can acceptor materials also absorb light? Does it not depend on the band-gap of the acceptor material? Is it for the ratio of acceptor and donor?

    2. Can we compare the acceptor as p-type and donor as n-type material? If it is true, then the separated electron should move along the n-type material (as it can be observed in P-N junction solar cells).

    3. Do the excitons dissociate with the help of electric-field between the acceptor and donor (my assumptions can be wrong)?

    It would be a great favour, if you answer my questions.

    • deibel says:

      1. The acceptor, e.g. a fullerene, can also absorb light, but usually less; it can also contribute to charge photogeneration (following a hole transfer from acceptor to the donor). The bandgap counts, but in contrast to inorganic materials such as GaAs or so, you need to consider which optical transitions are possible with which oscillator strength.

      2. Comparison to n- and p-type can be done in some respect; less in view of doping, but more in stating which is the dominant charge carrier type being transported after photogeneration in the given material. Thus, your 2nd statement does describe the situation approximately, but well.

      3. Only in neat polymers, but the yield is very low. Therefore, you use a two-component system in which the exciton mostly dissociates at the interface between the two semiconductors, leading to electron transfer from D=>A and/or hole transfer from A=>D. (The terms donor D and acceptor A usually refer to the electrons, but an electron acceptor can also be a hole donor.)

      Best, Carsten

      • Salman says:

        Thank you professor. Your explanations will really help me a lot. Again, I am very grateful to you.

        With best regards,
        S

  6. Hasnan Aziz says:

    hi Prof thank you for your explanation. I find it very useful for me to capture the concept of OSC.in future can i use any of your diagram for my presentation? I find it very clear compare than other journal that I have ever read before. thank you again for such a kind sharing of knowledge.

  7. Sarthak Ghosh says:

    Hi Dr.Deibel,I am Sarthak Ghosh, from India.I am an undergrad student.I have recently started studying about solar cells and organic solar cells.I have a fundamental doubt on how the effective band gap between donor and acceptor affects the open circuit voltage in an organic solar cell.Or in general , what are the factors which determine the open circuit voltage and how?It would be great if you can explain this, and als suggest some reading material fro me to get started in this field of study, especially related to the recombination processes in an organic solar cell.Thanks!

    • deibel says:

      Hi Sarthak, the open circuit voltage in organic solar cells is essentially given by effective gap minus charge carrier losses minus losses due to band bending (i.e. charge carrier concentration gradients). The effective band gap is often represented the the HOMO(donor)-LUMO(acceptor) gap, and more precisely related to the (charge transfer, CT) tail states in the device. That is the maximum energy the charge carrier pair can have in the solar cell. The actual open circuit voltage is clearly lower, due to the losses as mentioned above. For an overview, have a look at [Deibel 2010] Review, section 4, and the references therein, for CT states as effective gap in particular [Vandewal 2010]. Best, Carsten

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