Charge Separation from an Intra-Moiety Intermediate State in the High-Performance PM6:Y6 Organic Photovoltaic Blend

Abstract

Bulk-heterojunction organic photovoltaic devices with nonfullerene acceptors (NFAs) exhibit efficient hole transfer with small interfacial energy offset, which results in power conversion efficiencies above 17% in single junction devices using the high-performance NFA of Y6. However, the underlying mechanism responsible for the hole transfer channel in the polymer/Y6 blends remains poorly understood. Herein, we report that the hole transfer channel of photocharge generation is mediated by an intra-moiety excited state in a blend of donor polymer PM6 and NFA Y6 using broadband transient absorption (TA) spectroscopy. By comparing the TA data recorded from the solution and film Y6 samples, we identify the ultrafast formation of an intra-moiety excimer state together with the conversion from the primary local excitation on a time scale of ∼0.2 ps in the Y6 film. The intra-moiety excimer state acts as the intermediate for the hole transfer channel, which dissociates into free polarons on a time scale of ∼15 ps in the PM6/Y6 blend at room temperature. The intra-moiety intermediate state, arising from the intermolecular coupling in Y6 domains, is markedly different from the interfacial charge transfer state, which is commonly accepted as the intermediate state for the electron transfer channel. These findings suggest that manipulating the interplay between intra-moiety and interfacial excited species can provide a promising route for further improving device performance.

Over 14% efficiency all-polymer solar cells enabled by a low bandgap polymer acceptor with low energy loss and efficient charge separation

“…To further confirm the efficient hole transfer from PF5-Y5 to PBDB-T in the all-PSCs despite their small HOMO offset of 0.14 eV, herein, the hole transfer dynamics of photo-induced carriers in the all-polymer blend film were probed using femtosecond-resolved transient absorption (TA) spectroscopy measurements under a nitrogen atmosphere. 48b, 53 According to the absorption of PBDB-T and PF5-Y5, the pump wavelength was selected as 800 nm. Moreover, according to the results of excitation intensity-dependent measurements of acceptor material pure films under a pump wavelength of 800 nm (Fig.…”

Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells

“…3d–f ). We highlight that by tracking the growth of the transient EA signal we monitor the separation dynamics of excitons/CTEs into free charges, while other reports to date focused on measuring the charge transfer time across the D/A heterojunctions (i.e., dissociation of excitons into CTEs) in non-fullerene OSCs 29 , 40 – 42 .

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An Insight into the Excitation States of Small Molecular Semiconductor Y6

“…In semiconductors, many species, such as triplet states and charge states, could induce long-lived signal for the bleaching signal. Y6 is a small molecule, the primary excitation is LE states [ 33 ]; accordingly, the long-lived component could not be attributed to the decay of charge states. Considering the formation of triplet states via intersystem crossing is a common process in organic semiconductors, and the decay of triplet states is relatively long, we deduced that the long-lived component observed in the bleaching kinetics of Y6 solution was most likely due to the formation of the triplet state.…”

“…This increased intrinsic exciton lifetime in the film could be correlated with the formation of intermolecular excitons, because of the aggregation of the molecular backbone in the film. Very recently, Zhang et al found that the intramoiety excited states, which were similar to pseudo charge transfer state or intermolecular exciton, could be formed from local excitation on a time scale of ~0.2 ps in the Y6 film [ 33 ]. By comparing TA kinetics of Y6 in solution and film under the same excitation fluence.…”