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

Nikolis, Vasileios C.; Dong, Yifan; Kublitski, Jonas; Benduhn, Johannes; Zheng, Xijia; Huang, Chengye; Yüzer, Abdulcelil; Ince, Mine; Spoltore, Donato; Durrant, James R.; Bakulin, Artem A.; Vandewal, Koen

doi:10.1002/aenm.202002124

Cited by 22 publications

(28 citation statements)

References 47 publications

(55 reference statements)

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“…[46] However, this hypothesis has been questioned in a more recent work. [74] On the basis of the absence of charge photogeneration observed on pristine SubNc films, it was concluded that the process of charge generation is a field-assisted exciton dissociation specific of the device configuration and not an intrinsic property of the material. [74] However, based on the observed large magnitude of this field-effect, it was proposed that in certain neat organic semiconductors, this effect can lead to efficient photocurrent generation even though the driving force is minimized and the open-circuit voltage is maximized.…”

Section: Discussionmentioning

confidence: 99%

“…[74] On the basis of the absence of charge photogeneration observed on pristine SubNc films, it was concluded that the process of charge generation is a field-assisted exciton dissociation specific of the device configuration and not an intrinsic property of the material. [74] However, based on the observed large magnitude of this field-effect, it was proposed that in certain neat organic semiconductors, this effect can lead to efficient photocurrent generation even though the driving force is minimized and the open-circuit voltage is maximized. [74] In fact, the built-in electric field provided by the interlayers/contacts has been reported to contribute to exciton dissociation in BHJ cells based on a low bandgap polymer and PC 71 BM with low driving energy or in high efficiency BHJs based on NFA.…”

Section: Discussionmentioning

confidence: 99%

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Single‐Material Organic Solar Cells Based on Small Molecule Homojunctions: An Outdated Concept or a New Challenge for the Chemistry and Physics of Organic Photovoltaics?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

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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“…As another probe of free charge generation in Y6, we perform optical-pump-terahertz probe (OPTP) spectroscopy. 40,41 Figure 2e shows the conductivity spectrum at 3 ps probe delay of a neat film of Y6, excited with an 800 nm pump beam, at an excitation density of ~5×10 15 excitations/cm 3 . The non-zero real part of the terahertz (THz) spectrum (light blue line) is indicative of a fraction of free polarons, rather purely bound excitons.…”

Section: Exciton -Charge Dynamics and Equilibriummentioning

confidence: 99%

“…For the last thirty years, 4,12 this constraint has guided the development of organic photovoltaics (OPVs)which promise a step-change in flexible, lightweight, non-toxic solution-processed solar energy production, but are yet to be widely commercialised. Splitting bound excitons into free charges has required sharp molecular heterojunctions between donor and acceptor materials (though some have recently shown field-assisted exciton dissociation, or CT-state/polaron-pair formation [13][14][15][16] ). Rather than purely optimising charge harvesting, OPV devices have been optimised for exciton splitting at interfaces, and demand complex interpenetrating networks of donor and acceptor materials.…”

Section: Introductionmentioning

confidence: 99%

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Free Charge Photogeneration in a Single Component High Photovoltaic Efficiency Organic Semiconductor

Hodgkiss¹,

Price²,

Hume³

et al. 2021

Preprint

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Organic photovoltaic cells promise cheap, flexible and scalable solar energy. Whereas light directly generates free charges in silicon photovoltaic cells, bound electron and hole pairs known as excitons are understood to be the primary excitations in organic semiconductors due to their low dielectric constants. These excitons must then be split apart at molecular heterojunctions in order to extract current. Recent record efficiency organic photovoltaics utilise the small molecule, Y6, as a key component in the photon-absorbing blend layer. This molecule and its analogues – unlike previous organic semiconductors – have both low band-gaps and high dielectric constants. Here we show that, in a neat film of Y6, these factors lead to intrinsic free charge generation without the need for a molecular heterojunction to split the exciton. We use a suite of intensity-dependent optical spectroscopy measurements to show that a significant (20-90%) fraction of free charges exist in equilibrium with bound states at light intensity equivalent to 1 sun. Rapid bimolecular charge recombination constrains single component Y6 organic photovoltaic devices to low efficiencies, but this recombination is reduced by the introduction of small quantities of donor polymer. Quantum-chemical calculations reveal charge generation pathways through strong coupling between exciton and CT states, and an intermolecular polarisation pattern that drives exciton dissociation. Our results challenge the understanding of how current record efficiency organic photovoltaics operate, and point towards new future possibilities – offering a molecular picture of intrinsic charge generation as a platform to improve charge yields, and renewing the possibility of efficient single-component organic photovoltaic devices.

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Assessing the Photovoltaic Quality of Vacuum‐Thermal Evaporated Organic Semiconductor Blends

et al. 2021

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Vacuum‐thermal evaporation (VTE) is a highly relevant fabrication route for organic solar cells (OSCs), especially on an industrial scale as proven by the commercialization of organic light emitting diode‐based displays. While OSC performance is reported for a range of VTE‐deposited molecules, a comprehensive assessment of donor:acceptor blend properties with respect to their photovoltaic performance is scarce. Here, the organic thin films and solar cells of three select systems are fabricated and ellipsometry, external quantum efficiency with high dynamic range, as well as OTRACE are measured to quantify absorption, voltage losses, and charge carrier mobility. These parameters are key to explain OSC performance and will help to rationalize the performance of other material systems reported in literature as the authors’ methodology is applicable beyond VTE systems. Furthermore, it can help to judge the prospects of new molecules in general. The authors find large differences in the measured values and find that today's VTE OSCs can reach high extinction coefficients, but only moderate mobility and voltage loss compared to their solution‐processed counterparts. What needs to be improved for VTE OSCs is outlined to again catch up with their solution‐processed counterparts in terms of power conversion efficiency.

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

Cited by 22 publications

References 47 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?

Free Charge Photogeneration in a Single Component High Photovoltaic Efficiency Organic Semiconductor

Assessing the Photovoltaic Quality of Vacuum‐Thermal Evaporated Organic Semiconductor Blends

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