An Analysis of the Factors Determining the Efficiency of Photocurrent Generation in Polymer:Nonfullerene Acceptor Solar Cells

Cha, Hyojung; Tan, C. L.; Wu, Jiaying; Dong, Yifan; Zhang, Weimin; Hu, Chen; Rajaram, Sridhar; Narayan, K. S.; McCulloch, Iain; Durrant, James R.

doi:10.1002/aenm.201801537

Cited by 25 publications

(39 citation statements)

References 54 publications

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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 .…”

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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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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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“…Cha et al attributed this to the smaller driving force for charge separation and a greater degree of geminate recombination in PffBT4T-2OD:EH-IDTBR. 9,27 However, in-depth analysis of the transient absorption (TA) data has not been performed. The exciton and charge dynamics as well as the dynamics of CTS have not been clearly identified.…”

Section: Article Scitationorg/journal/jcpmentioning

confidence: 99%

The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation

Dong

Cha

Zhang

et al. 2019

The Journal of Chemical Physics

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Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the "reference" PffBT4T-2OD:PC 71 BM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.

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“…[48] A strong NFA π-π stacking has been reported to enhance charge transport, but not linked systematically with charge generation. [48] Therefore ultrafast spectroscopic studies have shown that charge carriers are also generated efficiently in NFA-based OSCs, [35][36][37][38][39][40] but a detailed photophysical understanding of NFA OSCs still lags behind the continuing, very promising, advances in their device efficiencies.…”

Section: Introductionmentioning

confidence: 99%

Exciton and Charge Carrier Dynamics in Highly Crystalline PTQ10:IDIC Organic Solar Cells

Cha

Zheng

Dong

et al. 2020

Advanced Energy Materials

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an electron donating conjugated polymer and an electron accepting small molecule, pure phases of donor and/or acceptor often coexist alongside a molecularly intermixed donor:acceptor phase. [5-9] The overall blend morphology, and the proportions and lengthscales of such pure and intermixed domains have been shown to be critical in determining device performance. [5-9] In recent years, the development of a range of nonfullerene acceptors (NFAs) has enabled, when blended with complementary absorbing donor polymers, impressive advances in OSC performance. [10,11] In the study herein, we focus on the recently reported PTQ10:IDIC blend system, which has been suggested to be particularly promising for low cost commercial application. [2] We find this blend system to be remarkably crystalline, and investigate the impact of this high crystallinity upon the exciton and charge carrier dynamics determining device performance. In BHJ blends, a large donor/acceptor interfacial area facilitates the exciton dissociation required for efficient photocurrent generation. [12,13] However, this donor/acceptor interface can also yield interfacial electron-hole pairs, often referred to as charge transfer (CT) states. [12,14-17] CT states formed in the highly intermixed regions of such blends have been reported to give rise Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. PTQ10:IDIC BHJs are shown to be particularly promising for low cost organic solar cells (OSCs). It is found that both PTQ10 and IDIC show remarkably high crystallinity in optimized BHJs, with GIWAXS data indicating pi-pi stacking coherence lengths of up to 8 nm. Exciton-exciton annihilation studies indicate long exciton diffusion lengths for both neat materials (19 nm for PTQ10 and 9.5 nm for IDIC), enabling efficient exciton separation with half lives of 1 and 3 ps, despite the high degree of phase segregation in this blend. Transient absorption data indicate exciton separation leads to the formation of two spectrally distinct species, assigned to interfacial charge transfer (CT) states and separated charges. CT state decay is correlated with the appearance of additional separate charges, indicating relatively efficient CT state dissociation, attributed to the high crystallinity of this blend. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in OSCs, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.

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An Analysis of the Factors Determining the Efficiency of Photocurrent Generation in Polymer:Nonfullerene Acceptor Solar Cells

Cited by 25 publications

References 54 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

The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation

Exciton and Charge Carrier Dynamics in Highly Crystalline PTQ10:IDIC Organic Solar Cells

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