WW-Colloquium: Prof. Dr. Frédéric Laquai, ‘How the Energetic Landscape in Non-Fullerene Acceptor Organic Solar Cells Controls the Device Performance’.

Prof. Dr. Frédéric Laquai
Fakultät für Chemie und Pharmazie, Department Chemie, Physikalische
Chemie und Spektroskopie von Energiematerialien, LMU München

How the Energetic Landscape in Non-Fullerene Acceptor Organic Solar Cells Controls the Device Performance

Organic solar cells (OSCs) using blends of electron donor materials and non-fullerene acceptors (NFAs) have now achieved power conversion efficiencies exceeding 20% in single-layer bulk heterojunction devices. Their device photophysics are distinctly different from other semiconductors such as hybrid perovskites due to the large exciton binding energy in organic materials, demanding energy offsets at the donor-acceptor interface sufficient to create a driving force for exciton dissociation and charge separation. This is typically associated with additional energy and quantum yield penalties limiting device efficiency, which must be mitigated.
In this talk, I will present our insights into how the energetic landscape in state-of-the-art non-fullerene acceptor (NFA) bulk heterojunction organic solar cells determines their efficiency obtained from various time-resolved optical and electro-optical spectroscopies on organic semiconductor thin films and devices. First, I will introduce how spectroscopically-determined parameters such as rate constants and reaction yields can be linked to the device figures-of-merit. Second, I will show that the A-D-A type structural motif of most NFAs leads causes quadrupole moments, which not only impact the exciton-to-charge transfer state conversion, but also the charge separation, its field dependence, as well as the charge recombination processes. In fact, exceptionally long exciton diffusion coupled with efficient energy transfer from the donor to the acceptor occur in NFA-based blends and precedes hole transfer, leading to a distinct dependence of the devices’ charge generation efficiency on the blend’s ionization energy (IE) offset. While in most donor-NFA combinations, offsets of about 0.4 eV are required to ensure quantitative charge separation, recent results on blends using Y-derivatives indicate that they maintain high charge separation yields at even lower offsets.
Understanding the impact of the molecular structure on the energetic landscape of the heterojunction and the device photophysics is crucial for a guided design of new organic electronic materials by, for instance, computational material chemistry approaches, predicting efficient target structures.

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