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.