In organic solar cells, we mostly discussed charge transport. I think it is true that in the blend material of the organic solar cells, the energy gap difference between donor and acceptor is favorable for the charge transport. I have two questions?
1) In the blend of the organic solar cells, is there any chance of energy transfer? If yes then how we can detect this energy transfer rate? More specifically if we want to measure it by using ultrafast transient absorption spectroscopy then how we can recognize the energy transfer rate from either the transient absorption peak spectra or lifetime constant?
2) If I take only the donor or acceptor of the blend material and use the ultrafast transient absorption spectroscopy, is there any chance of charge or energy transfer phenomena? if yes which process would more dominant?
Johar Zeb
welcome!
This question is very interesting.
However I could not exactly understand what do you mean by using ultrafast transient absorption spectroscopy.
I will try to explain the mechanism of acquiring of losing energy by valence electrons from the incident photons and the subsequent energy interactions during the disposal of the electrons and holes from the materials towards to the electrodes.
Let us assume only one photon with energy Eph greater than Eg the bandgap energy of the donor material as the donor is the photons absorber. Then the difference in energy Eph-Eg will be lost as a thermal energy causing heating of the donor material. This energy is called the thermalization energy of the generated excitons. This process is very rapid. After which the excitons diffuse to the interface of the donor acceptors where its energy will be dropped when its electron is transported to the the acceptor molecule. So an energy equal to Eld- Ele will be lost in form of thermal energy. So the energy lost at the donor acceptor interface = the lumo level of the donor Eld - the lumo level of the acceptor Ela.
This energy will be lost as a heat at the interface. At the interface of the of the acceptor layer and the ETL there will be an additional drop comprising the difference between the lumo level of the ETL Eletl and the lumo lumo level of the acceptor layer Ela. Likewise, the holes will be dropped from the donors to the HTL causing the holes to lose energy in equal amount of the difference between homolevel of the HTL Ehhtl and the homolevel of the donor Ehd. All these energy drops will be lost as thermal energy.
While the energy drops at the interface with the transport layer can be made zero by energy band matching, the drop at the donor acceptor interface is necessary of the dissociation of the excitons. It is also the greatest difference and so it could be observed at the donor acceptor interface as a local rise in temperature.
For the energy level diagrams and the positioning of the different energy levels in the organic solar cells please follow the paper:Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells
Best wishes
Dear Professor @Abdelhalim,
Thank you so much for such a great explanation. Highly appreciated it. I study and understand what you have discussed above. I very happy that i have learned something new. But I need some information regarding Ultrafast transient absorption spectroscopy that how we can differentiate between charge and energy transport on the basis of this spectroscopic technique? Is any proper sign of charge or energy on the basis of transient signal and lifetime constant? Will be looking forward for your response.
Thank you so much.
Best Regards
Johar Zeb
you can see the following links:
1. https://pubs.rsc.org/en/content/articlelanding/2014/EE/C3EE42484G
2. Article Organic Solar Cells: Impact of Acceptor Quadrupole Moment on...
3. Article Charge Transport in Pure and Mixed Phases in Organic Solar Cells
4. https://d-nb.info/1014891965/34
5. Article Modeling Charge Transport in Organic Photovoltaic Materials
- Badges
- Science topic