Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss

Donor–acceptor organic solar cells often show low open-circuit voltages (VOC) relative to their optical energy gap (Eg) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between Eg and that of intermolecular charge transfer (CT) states at the donor–acceptor interface. Here we study charge generation occurring in PIPCP:PC61BM, a system with a very low driving energy for initial charge separation (Eg−ECT ~ 50 meV) and a high internal quantum efficiency (ηIQE ~ 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:PC61BM, 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.

Citation

Menke SM, Cheminal A, Conaghan P, Ran NA, Greehnam NC, et al. (2018) Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss. Nature Communications 9. Available: http://dx.doi.org/10.1038/s41467-017-02457-5.

Sponsors

We would like to acknowledge Professor René Janssen and Professor Weiwei Li for supply of the PDPP polymers and fruitful discussion. S.M.M. and R.H.F. acknowledge support from the KAUST Competitive Research Grant Program. N.A.R., T.-Q.N. and G.C.B. acknowledge support from the Department of the Navy, Office of Naval Research (award nos. N00014-14-1-0580 and N00014-16-1-25200). A.C., P.C., N.C.G. and A.R. acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability.