Entropy promotes charge separation in bulk heterojunction organic photovoltaics

Kawashima, Eisuke; Fujii, Minoru; Yamashita, Koichi

doi:10.1016/j.jphotochem.2019.111875

Cited by 8 publications

(6 citation statements)

References 40 publications

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“…[82,83] Finally, several recent publications have put forward the role of entropy and disorder as driving forces to charge-pair dissociation and to the creation of a potential barrier limiting non-geminate carrier recombination. [84][85][86][87][88][89] Considering the small number of molecules investigated in HOSCs of different architectures and the various mechanisms already evoked, it seems evident that the understanding of the photogeneration of charges in HOSCs requires an intensification of research in the near future. In this context, a discussion of structure-properties relationships aiming at the definition of possible synthetic principles is of course premature.…”

Section: Discussionmentioning

confidence: 99%

Single‐Material Organic Solar Cells Based on Small Molecule Homojunctions: An Outdated Concept or a New Challenge for the Chemistry and Physics of Organic Photovoltaics?

Roncali

2021

Advanced Energy Materials

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Single‐material organic solar cells (SMOSCs) are on the forefront of research on organic photovoltaics (OPV). The generic term of SMOSCs encompasses a large variety of chemical structures implying very different basic concepts. Polydisperse «double cable» polymers and oligomers with acceptor groups linked to the conjugated backbone by a flexible spacer and donor–acceptor block copolymers are at present, the most investigated and efficient systems with spectacular progress in conversion efficiency achieved in the past 2–3 years. However, besides this mainstream SMOSCs research, a few recent publications describe OPV cells constituted of homojunctions based on small π‐conjugated molecules. While the process of charge generation in such systems is still a matter of debate due in particular to the possible direct photogeneration of charge–carriers, devices with significant performance have been recently reported. After a brief overview of the most recent advances on the various types of SMOSCs, recent remarkable results on homojunction OPV cells based on small π‐conjugated molecules are discussed in order to highlight the potential fundamental and technological interest of this emerging field of research.

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Section: Discussionmentioning

confidence: 99%

Single‐Material Organic Solar Cells Based on Small Molecule Homojunctions: An Outdated Concept or a New Challenge for the Chemistry and Physics of Organic Photovoltaics?

Roncali

2021

Advanced Energy Materials

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“…The increase of disorder 𝜎 induces a decrease in the energy barrier, but its negative effect on charge recombination should also be considered. [125] The CT exciton separation at the D/A interface depends critically on the molecule packing, as the anisotropic nature of organic crystal. However, it is difficult to investigate this in BHJ samples which are used in most OSCs for enhancing the signal originated from interfaces.…”

Section: Morphology Effect For Entropy Effectmentioning

confidence: 99%

The Role of Entropy Gains in the Exciton Separation in Organic Solar Cells

Yan

Zhang

Memon

et al. 2022

Macromol. Rapid Commun.

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In organic solar cells (OSCs), the lower dielectric constant of organic semiconductor material induces a strong Coulomb attraction between electron–hole pairs, which leads to a low exciton separation efficiency, especially the charge transfer (CT) state. The CT state formed at the electron‐donor (D) and electron‐acceptor (A) interface is regarded as an unfavorable property of organic photovoltaic devices. Since the OSC works in a nonzero temperature condition, the entropy effect would be one of the main reasons to overcome the Coulomb energy barrier and must be taken into account. In this review, the present understanding of the entropy‐driven charge separation is reviewed and how factors such as the dimensionality of the organic semiconductor, energy disorder effect, the morphology of the active layer, are described, as well as how the nonequilibrium effect affects the entropy contribution in compensating the Coulomb dissociation barrier for CT exciton separation and charge generation process. The investigation of the entropy effect on exciton dissociation mechanism from both theoretical and experimental aspects is focused on, which provides pathways for understanding the underlying mechanisms of exciton separation and further enhancing the efficiency of OSCs.

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“…Recent literature has extensively discussed the potential role of configurational entropy ( S = -k B ln[ Ω (r) ] ), 8,9,[35][36][37] disorder, 2,6,9 and charge-transfer state delocalization 2,38-40 (related to P(r) ) in understanding charge separation in OPV materials. These studies guide our choice of r c , the form of Ω (r) , and that of P(r) , drawing particular inspiration from the work of Ratner, 11 Gregg, 8 Kassal.…”

Section: W + /mentioning

confidence: 99%

Short and Long-Range Electron Transfer Compete to Determine Free-Charge Yield in Organic Semiconductors

Carr

Allen

Larson

et al. 2021

Preprint

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Understanding how Frenkel excitons efficiently split to form free-charges in low-dielectric constant organic semiconductors has proven challenging, with many different models proposed in recent years to explain this phenomenon. Here, we present evidence that a simple model invoking a modest amount of charge delocalization, a sum over the available microstates, and the Marcus rate constant for electron transfer can explain many seemingly contradictory phenomena reported in the literature. We use an electron-accepting fullerene host matrix dilutely sensitized with a series of electron donor molecules to test this hypothesis. The donor series enables us to tune the driving force for photoinduced electron transfer over a range of 0.7 eV, mapping out normal, optimal, and inverted regimes for free-charge generation efficiency, as measured by time-resolved microwave conductivity. However, the photoluminescence of the donor is rapidly quenched as the driving force increases, with no evidence for inverted behavior, nor the linear relationship between photoluminescence quenching and charge-generation efficiency one would expect in the absence of additional competing loss pathways. This behavior is self-consistently explained by competitive formation of bound charge-transfer states and long-range or delocalized free-charge states, where both rate constants are described by the Marcus rate equation. Moreover, the model predicts a suppression of the inverted regime for high-concentration blends and efficient ultrafast free-charge generation, providing a mechanistic explanation for why Marcus-inverted-behavior is rarely observed in device studies.

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Entropy promotes charge separation in bulk heterojunction organic photovoltaics

Cited by 8 publications

References 40 publications

Single‐Material Organic Solar Cells Based on Small Molecule Homojunctions: An Outdated Concept or a New Challenge for the Chemistry and Physics of Organic Photovoltaics?

Single‐Material Organic Solar Cells Based on Small Molecule Homojunctions: An Outdated Concept or a New Challenge for the Chemistry and Physics of Organic Photovoltaics?

The Role of Entropy Gains in the Exciton Separation in Organic Solar Cells

Short and Long-Range Electron Transfer Compete to Determine Free-Charge Yield in Organic Semiconductors

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