Ultrafast processes in molecules

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