Anatomy of the energetic driving force for charge generation in organic solar cells

Nakano, Kyohei; Chen, Yujiao; Xiao, Bo; Han, Weining; Huang, Jianming; Yoshida, Hiroyuki; Zhou, Erjun; Tajima, Kazuo

doi:10.1038/s41467-019-10434-3

Cited by 118 publications

(128 citation statements)

References 61 publications

(59 reference statements)

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“…For homogeneous media, theory predicts a CT binding energy of around 400 meV, in clear contrast to the high external quantum efficiency (EQE) for photocurrent generation of many NFA‐containing devices. Several studies showed high EQE to be related to a larger driving force . This situation reminds of the “hot” dissociation model, where exciton dissociation creates a more loosely bound electronically/vibronically excited CT state .…”

mentioning

confidence: 72%

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

et al. 2020

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Organic solar cells are currently experiencing a second golden age thanks to the development of novel non‐fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high‐performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near‐unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.

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mentioning

confidence: 72%

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

et al. 2020

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“…[4][5][6] As the field of OSCs now moves towards efficient non-fullerene based D-A systems with low voltage losses, surprisingly high external quantum efficiencies (EQEs) are observed even in the absence of a considerable driving force. [7][8][9][10][11] For a number of such D-A systems, it has been shown that the field induced by the presence of electrodes assists the charge generation mechanism. [12,13] This calls for a re-evaluation of charge generation mechanisms in new materials and systems, including the effects of the external field, delocalization, entropy, and driving force.…”

Section: Introductionmentioning

confidence: 99%

Field Effect versus Driving Force: Charge Generation in Small‐Molecule Organic Solar Cells

Nikolis

Dong

Kublitski

et al. 2020

Advanced Energy Materials

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Efficient charge generation in organic semiconductors usually requires an interface with an energetic gradient between an electron donor and an electron acceptor in order to dissociate the photogenerated excitons. However, single‐component organic solar cells based on chloroboron subnaphthalocyanine (SubNc) have been reported to provide considerable photocurrents despite the absence of an energy gradient at the interface with an acceptor. In this work, it is shown that this is not due to direct free carrier generation upon illumination of SubNc, but due to a field‐assisted exciton dissociation mechanism specific to the device configuration. Subsequently, the implications of this effect in bilayer organic solar cells with SubNc as the donor are demonstrated, showing that the external and internal quantum efficiencies in such cells are independent of the donor‐acceptor interface energetics. This previously unexplored mechanism results in efficient photocurrent generation even though the driving force is minimized and the open‐circuit voltage is maximized.

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“…Compound BThIND‐Cl exhibits somewhat lower energy levels when compared to the ones of BThIND, that may be ascribed to the electron‐withdrawing effect of Cl atom that is increased relative to the H atom, as reported in the literature, [ 59 ] and also be attributed to the dipole moment differences between the carbon–chlorine bond and carbon‐hydrogen bond, with the former being greater. [ 60 ] The HOMO and LUMO energy levels of PM6 are ‐5.47 and −3.61 eV. As shown in the energy level diagram (Figure 3) that the HOMO/LUMO offsets between the BThIND‐Cl/BThIND and PM6 are 0.17/0.35 and 0.13/0.33 eV, respectively.…”

Section: Resultsmentioning

confidence: 97%

Enhancement of photovoltaic efficiency through fine adjustment of indacene‐based non‐fullerene acceptor by minimal chlorination for polymer solar cells

et al. 2020

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Two non‐fullerene small molecule acceptors (NFSMAs, BThIND‐Cl and BThIND) with an A–D–A configuration containing indacenedithiophene donor core unit and terminal unit with IC (BThIND) and chlorinated IND were synthesized for their application as an acceptor in polymer solar cells (PSCs). Compared to BThIND, the chlorinated molecule BThIND‐Cl exhibits lower energy levels and red‐shifted absorption spectrum but also exhibits improved crystallinity and molecular packing. These two NFSMAs are used as acceptors along with a well‐known conjugated polymer PM6, that exhibits complementary absorption to both NFSMAs, and has suitable frontier energy levels (highest occupied molecular orbital at –5.47 eV and lowest unoccupied molecular orbital at 3.61 eV) as a donor, in fabricating PSCs. Having optimized the active layers, the PSC based on PM6: BThIND‐Cl showed higher power conversion efficiency (PCE) of 12.36% and a fill factor (FF) of 0.71 than that of PM6: BThIND counterpart (PCE = 8.04% with a FF of 0.66). Moreover, for the PM6: BThIND‐Cl based PSC a quite high open‐circuit voltage of 0.96 V was achieved in addition to a low energy loss of 0.53. Notably higher short circuit current and FF value were found for the BThIND‐Cl based PSCs that are attributed to originate from the measured high values of charge carrier mobilities (and more balanced), to an advantageous nanoscale morphology, and finally to increased efficiency of exciton dissociation as compared to BThIND counterparts. These results demonstrate that substitution of the single chlorine atom in the IC terminal unit of A–D–A NFSMA is sufficient to enhance the PCE of PSCs

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Anatomy of the energetic driving force for charge generation in organic solar cells

Cited by 118 publications

References 61 publications

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

Field Effect versus Driving Force: Charge Generation in Small‐Molecule Organic Solar Cells

Enhancement of photovoltaic efficiency through fine adjustment of indacene‐based non‐fullerene acceptor by minimal chlorination for polymer solar cells

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