Abstract
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.
Highlights
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%
As this signal plateaus on the 1–5 ps timescale, we infer that the timescale for charge transfer (CT) state separation is between 1 and 5 ps for PIPCP: phenyl-C61-butyric acid methyl ester (PCBM)
We have calibrated the intensity of the EA response using quasi-steady-state EA measurements using an externally applied electric field
Summary
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%. In fullerene-based organic solar cells, for example, this offset of ~300 meV allows electrons to be injected from the donor into a set of delocalised states within a fullerene aggregate, resulting in ballistic electron-hole separation in under 100 fs[5,6] This fast timescale ensures that free charge carrier generation is nearly 100% efficient. We probe the dynamics of the initial electron-hole separation for an organic heterojunction based on the donor polymer PIPCP and the acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (Fig. 1a, b)[18,21] This system is of particular interest because there is only a ~50 meV difference between the optical energy gap (Eg) and the energy of the interfacial CT state (ECT) as determined by complementary, sensitive measurements of absorption and electroluminescence[18]. We attribute the ability to generate charges slowly yet efficiently to the remarkably low disorder in the system and present a conceptual model invoking the Urbach tail in the density of states that can inform us of the upper bound for the electronic disorder tolerable to achieve efficient carrier separation in systems with small offset energies
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