Does Electron Delocalization Influence Charge Separation at Donor–Acceptor Interfaces in Organic Photovoltaic Cells?

Kahle, Frank‐Julian; Saller, Christina; Olthof, Selina; Li, Cheng; Lebert, Jenny; Weiß, Sebastian; Herzig, Eva M.; Hüttner, Sven; Meerholz, Klaus; Strohriegl, Peter; Köhler, Anna

doi:10.1021/acs.jpcc.8b06429

Cited by 36 publications

(33 citation statements)

References 85 publications

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“…We propose that this improvement in e–h separation efficiency with fullerene aggregation results from the population of more spatially delocalized CT states at the interfaces with fullerene domains and the greater number of such states that are accessible at room temperature ( Figure 5 ). 30 , 58 , 59 This improved separation efficiency should also be facilitated by enhanced electron mobility in the larger fullerene domains. 60 In the case of the lower offset system, polymer exciton dissociation is also ultrafast and efficient, but nanosecond geminate recombination occurs even in the presence of fullerene domains, because the majority of low-lying excited states have excitonic or partial charge-transfer character and so are unable to dissociate readily into separate charges.…”

Section: Discussionmentioning

confidence: 99%

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends

Dimitrov

Azzouzi

et al. 2019

J. Am. Chem. Soc.

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Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron–hole separation at organic donor–acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron–hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer–fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer–fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron–hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron–hole recombination is observed revealing the importance of spatially localized electron–hole pairs (bound CT states) in the electron–hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron–hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron–hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.

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Section: Discussionmentioning

confidence: 99%

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends

Dimitrov

Azzouzi

et al. 2019

J. Am. Chem. Soc.

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“…In contrary, other investigations reported charge carrier generation efficiency basically independent of the excess energy suggesting that the relaxed CT excitons are the major precursors of free charge carriers [75][76][77]. Several factors have been discussed that may reduce the CT state binding energy enabling dissociation of the relaxed CT state: a) a high dielectric permittivity of PCBM [78], b) the delocalization of charge carriers forming CT states [79,80], c) the formation of interfacial dipoles creating repulsive forces repelling carriers away from the interface [81], d) the increase in entropy during the charge separation, reducing the free energy of separated charges [82,83].…”

Section: Carrier Photogeneration In Organic Solar Cellsmentioning

confidence: 97%

Charge carrier mobility dynamics in organic semiconductors and solar cells

Gulbinas¹

2020

physics

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Charge carrier mobility in organic semiconductors is not a constant value unambigously characterizing some particular material, but depends on the electric field, temperature and even on time after it was generated or injected. The time dependence is particularly important for the thin-film devices where charge carriers pass the organic layer before mobility reaching its stationary value. Here we give a review of experimental techniques with ultrafast timeresolution enabling one to address the mobility kinetics and analyse properties of the time-dependent mobility in conjugated polymers and organic solar cells. We analyse kinetics during the charge carrier generation and extraction of free charge carriers. The mobility typically decreases by several orders of magnitude on a picosecond-nanosecond time scale; however, its kinetics also depends on the investigation technique. The mobility kinetics in blends for bulk heterojunction solar cells strongly depends on the stoichiometric ratio of donor and acceptor materials.

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“…A complete picture on charge generation, of course, has to involve both the effects of morphology, and the presence and influence of the charge-separated state, and is beyond the scope of this work. [67][68][69][70][71][72][73][74][75][76][77][78]…”

Section: Effect Of Hybridization On Device Performancementioning

confidence: 99%

Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells

et al. 2019

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A number of recent studies have shown that the non-radiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor:acceptor blends to determine the effect that energetic offset has on both radiative and non-radiative recombination of the charge transfer (CT) state. We find that for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the non-radiative voltage loss to values as low as 0.23V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low non-radiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low non-radiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states, and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low non-radiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

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Does Electron Delocalization Influence Charge Separation at Donor–Acceptor Interfaces in Organic Photovoltaic Cells?

Cited by 36 publications

References 85 publications

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends

Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends

Charge carrier mobility dynamics in organic semiconductors and solar cells

Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells

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