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


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Authors
Cheminal, Alexandre 
Conaghan, Patrick 
Ran, Niva A 
Greehnam, Neil C 
Abstract

Donor-acceptor organic solar cells often show low open-circuit voltages (V OC) relative to their optical energy gap (E g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E g 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 (E g-E CT ~ 50 meV) and a high internal quantum efficiency (η IQE ~ 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.

Description
Keywords
40 Engineering, 3403 Macromolecular and Materials Chemistry, 4016 Materials Engineering, 34 Chemical Sciences, 7 Affordable and Clean Energy
Journal Title
Nat Commun
Conference Name
Journal ISSN
2041-1723
2041-1723
Volume Title
9
Publisher
Springer Science and Business Media LLC
Sponsorship
Science and Technology Facilities Council (ST/M007073/1)
Engineering and Physical Sciences Research Council (EP/M005143/1)
Engineering and Physical Sciences Research Council (EP/M006360/1)
STFC (ST/M007073/1)