Progress in development of polymer organic solar cells
Scientists from the IMC have successfully explained the peculiarities observed during the time evolution of excitation energy in thin films of conjugated metallo-supramolecular polymers after absorption of a laser pulse. In the future, this mechanism could play a role in polymer organic solar cells made of these materials. The results of this study were published in the journal Frontiers in Chemistry.
The polymer under study represents a system of one-dimensional chains of organic molecules (ditopic thiophene-bridged terpyridines), linked together by metal cations (Zn2+, see Figure). After photoexcitation two excited species are formed: singlet and triplet excitons which differ by the value of their spins. “Our detailed analysis of time-resolved optical absorption spectra showed that photoexcited singlets move on polymer chains and collide, yielding the bound triplet pairs that subsequently dissociate into two independent triplet excitons. Such a mechanism could play a role in polymer organic solar cells made of these materials,” describes Dr. Miroslav Menšík from the Department of Polymers for Electronics and Photonics.
This recently published work follows the researchers’ earlier publication completed in 2017, where they experimentally confirmed the existence of both singlet and triplet states and their mutual transformation during relaxation of the polymer after excitation, and where they found an unusual decay of their concentration in time. Meanwhile, scientists from the IMC developed mathematical techniques to better describe and understand such processes.
“Our research would not be possible without having suitable materials for our studies. The metallo-supramolecular polymers used in our research were synthesized in the group of Prof. Vohlídal at the Faculty of Sciences at Charles University in Prague. Our theoretical work brought the long-term cooperation with his team,” says Dr. Miroslav Menšík.
Upper part: Structure of the metallo-supramolecular polymer, where the repeating units of ditopic thiophene-bridged terpyridines are linked by Zn2+ cations.
Bottom part: The scheme of the kinetics of photoexcited states: after a collision of two excited singlet states (S1), one of the molecules is deexcited, while the second one is excited to a higher excited state (Sn). This state Sn undergoes an energy transfer process, resulting in the formation of a triplet pair state T1T1, which dissociates into two independent triplet states T1.
