Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 29348491.
- Also identified by DOI 10.1038/s41467-017-02457-5 and PMC identifier 5773693.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
Donor-acceptor organic solar cells often show low open-circuit voltages (V <sub>OC</sub>) relative to their optical energy gap (E <sub>g</sub>) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E <sub>g</sub> and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC<sub>61</sub>BM, a system with a very low driving energy for initial charge separation (E <sub>g</sub>-E <sub>CT</sub> ~ 50 meV) and a high internal quantum efficiency (η <sub>IQE</sub> ~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC<sub>61</sub>BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state.