Need help understanding organic copolymers in OPV devices

[Peffe]

Hi, I've been reading PF threads for a long time but never really had a question to ask.. until recently.

I am doing some of my own studies on OPV devices (i'm still a high school student, so I have no prior research experience of in-depth knowledge on processes that take place at the atomic level during the operation of the device). i have been studying organic copolymers for use in OPV devices now, and just need to clarify a few questions hanging about my mind..

1) i understand that the longer the conjugation length in a polymer, the lower the energy of the photon it is able to absorb (the higher the wavelength of light it is able to absorb). however, i have gotten some experimental data that seems to prove otherwise.. i have copolymer units with anthracene, naphthalene and phenylene (lets call them A, N and P respectively). my data shows that the maximum absorption decreases from P->N->A. i don't get it - isn't the trend supposed to be the other way round (ie, maximum absorption increases from P->N->A)?

2) i always read about the absorption of photon energy, and that as long as the photon has enough energy to overcome the energy barrier between the HOMO and LUMO energies, the substance will be able to absorb the light. what does this mean? does it mean that photons with energy ABOVE the barrier will also be absorbed, just that the excess energy will be "wasted" (returned back to surroundings thru other processes like releasing heat)? or does it mean that only photons with the EXACT energy as the energy barrier can be absorbed?

all help is appreciated :)

 

[Alkim]

Just an idea: The polymer chain with conjugated bonds may act as an optical antenna and therefore resonance wavelength is comparable to chain length. Absorption has to do with antenna efficiency (losses).

 

[Peffe]

Thank you so much! I went to check up a little on what you said and checked my stuff again.. It does indeed seem like there is some curvature happening in the molecule so that's highly likely the reason why the trend is not as expected.

Thanks!

I still don't know the second question tho hehe. It's not really smth I don't know. I have always thot of photon absorption as any photon w energy content above the energy barrier can be absorbed, but there are many things proving my way of thinking wrong, so I just want to check from others.

 

[Alkim]

Of course the polymer chains are not straight but I am pretty sure the antenna formalism still could be applied in your case. I have read quite a few articles about development of branched organic molecules for use as energy harvesting antennas. The idea is that a molecule with a large absorption cross section can be designed to collect light energy from a large area and from all directions and channel that energy to a point of interest along the chains containing the conjugated bonds as if they were metal cables. The energy could be re-emited as light (i.e fluorescence) or converted into electrical current. Absorption by itself is equivalent to loss, fluorescence (or electrical current) is gain.

 

[alphy]

Hello,

Thank you for reaching out with your questions about organic copolymers in OPV devices. I can understand how these concepts may be confusing, especially without prior research experience. I am happy to provide some clarification and guidance on these topics.

1) The trend you have observed in your experimental data is actually correct. The maximum absorption of a polymer does not always increase with longer conjugation length. It depends on the specific structure and composition of the polymer. In your case, the anthracene unit (A) has the longest conjugation length, but it may also have a higher energy barrier between the HOMO and LUMO energies compared to the naphthalene (N) and phenylene (P) units. This means that the photons with higher energy may not be able to overcome this barrier and be absorbed, resulting in a decrease in maximum absorption. This is a complex phenomenon and can also be affected by factors such as molecular packing and intermolecular interactions in the polymer.

2) The absorption of photon energy in a material depends on the energy levels of the HOMO and LUMO. When a photon with enough energy hits the material, it can excite an electron from the HOMO to the LUMO, creating an electron-hole pair. This is the absorption of light. The energy barrier between the HOMO and LUMO determines the minimum energy required for this process to occur. This means that photons with energy equal to or greater than this barrier can be absorbed, but photons with lower energy will not be able to excite the electrons. So, to answer your question, only photons with energy equal to or greater than the energy barrier can be absorbed. Any excess energy will be dissipated through other processes like heat generation or fluorescence.

I hope this helps to clarify your understanding of these concepts. Keep up the great work with your studies and research! Don't hesitate to reach out if you have any further questions.