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Ultrafast processes in molecules

This presentation discusses the ultrafast processes in Organic Photovoltaics (OPVs), highlighting their advantages such as low cost, flexibility, and ease of processing, alongside challenges like low efficiency and short lifetime. It covers key architectural types like Planar Heterojunction and Bulk Heterojunction, presenting recent computational methods and results related to charge transfer and separation dynamics. The insights gained from molecular computations aid in understanding the electronic structure and optimizing the performance of D-A complexes for enhanced energy conversion in OPVs.

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Ultrafast processes in molecules

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  1. Ultrafast processes in molecules VII – Organic photovoltaics Mario Barbatti barbatti@kofo.mpg.de

  2. Organic Photovoltaics (OPV) • OPV advantages: • Potential low cost • Flexible, light, and thin • Easy processing • Recent reviews: • Mishra and Bäuerle, AngewChemInt Ed 51, 2020 (2012) • Shang, Li, Meng, Wang, Shuai, TheorChemAcc129, 291 (2011) • Carsten, Vladimir, Rep ProgPhys73, 096401 (2010) • Brédas, Norton, Cornil, Coropceanu, AccChem Res 42, 1691 (2009)

  3. Efficiency h • Green, Emery, Hishikawa, Warta, Dunlop, Prog Photovolt 19, 565 (2011)

  4. Organic Photovoltaics (OPV) • OPV advantages: • Potential low cost • Flexible, light, and thin • Easy processing • OPV drawbacks • Low efficiency (recombination, low charge mobility) • Short lifetime (oxidation, photochemical degradation)

  5. OPV architecture Planar heterojunction (PHJ) Bulk heterojunction (BHJ) • a Mayer, Lloyd, Herman, Kasen, Malliaras, Appl Phys Lett 85, 6272 (2004) • b Sakai, Taima, Yamanari, Yoshida, Fujii, Ozaki, Jpn J Appl Phys 49, 032301 (2010) • c Sakai, Taima, Saito, Org Electron 9, 582 (2008)

  6. OPV operation J V Vmp VOC Jmp JSC • OPVs are characterized in terms of macro-quantities like h, VOC, and JSC • We do not expect to compute such quantities • But molecular computations may provide an indication of the adequacy of a D-A

  7. 1. Photoexcitation A A’ D D’ LE D’ 2. Exciton diffusion LE D 5. Recombination OPV photophysics 3. Charge transfer + - CT D→A 4. Charge separation + - CT D’→A’

  8. Ideal electronic structure D A 1. D shouldbephotoexcited Large oscillator strength between 500 nm and 700 nm 2. CT D→A should be quickly populated with hot polarons LE D should be above and near CT D→A LE Dbright 3. CT D→A should have a long lifetime Large energy gap between CT D→A and states below CT D→A GS • To check these features, we need to classify the electronic states of the D-A complex

  9. State classification Using a Mulliken partition: A B Degree of delocalization over A and B j Amount of CT between A and B i Considering a multielectronic wavefunction: • Crespo-Otero and Barbatti, TheorChem Acc 131, 1237 (2012)

  10. State classification

  11. Benchmarking… • Systematic investigation of the effects of: • D-A distance • D-A orientation • Oligomer size • Chemical environment • Chemical functionalization • Theoretical level

  12. Computationaldetails • wB97X-D • 6-31G(d) • S0D-A optimization • TDDFT: 40-70 states • G09 • State classification • Example: P3HT-PCBM • 240 atoms • 2 days (Xeon 3.3 GHz 10 cores) 3

  13. Dependence on functional • Extremely dependent on functional! • CTs are wrong without range-separation • C60 bands are blue shifted with range separation.

  14. Double excitations… • Constant blue shift between TDDFT and DFT/MRCI • Exceptions: states with large multiple excitation character

  15. Photophysics of D-A junctions LE Dbright CT D→A • How much does the electronic structure of the D-A complexes resemble the ”Ideal Electronic Structure”? GS

  16. Dependence on D-A distance • LOC does not show strong dependence on D-A distance • CT is stabilized at short distances (Coulomb r-1 is expected)

  17. Dependence on D-A orientation • LOCs are not affected • CTs are strongly affected • CTs are stabilized by stacking LOC(A) LOC(D) DELOC CT D→A CT A→D

  18. Dependence on D size • LOC(D)s are affected, but not LOC(A)s • CTs are affected

  19. Functionalization

  20. Environment

  21. Next lecture Quantum dynamicsmethods Contact barbatti@kofo.mpg.de

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