Electron Trapping in Conjugated Polymers

Abbaszadeh, Davood; Kunz, Alexander; Kotadiya, Naresh B.; Mondal, Anirban; Andrienko, Denis; Michels, Jasper J.; Wetzelaer, Gert‐Jan A. H.; Blom, Paul W. M.

doi:10.1021/acs.chemmater.9b01211

Cited by 87 publications

(77 citation statements)

References 46 publications

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“…Measured and calculated DOS widths exclude effects of nonequilibrium relaxation of charges which could assist charge splitting. Note that the electron affinity and ionization energy are both within the “trap‐free” window, hinting at a trap‐free ambipolar transport in pristine Y6 films . In addition, the calculated excited state reorganization energy of Y6 has a very small value of 0.24 eV, which promotes large exciton diffusion rates and lengths and makes it less sensitive to the morphological variation in a bulk heterojunction.…”

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confidence: 94%

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: 94%

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

et al. 2020

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“…Electron transport is particularly affected by reactions with air, and it is essential to prevent the electron polaron from reducing ambient species. To accomplish this, the LUMO energy must be low enough to prevent excited electrons to reduce hydrated O2 complexes to O2 − (one of the most favourable electrochemical reactions) 5 or H2O to OH − . These unwanted electrochemical processes can lower charge transport and also lead to further irreversible reactions within a semiconductor.…”

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confidence: 99%

The role of chemical design in the performance of organic semiconductors

et al. 2020

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Organic semiconductors are solution-processable, lightweight and flexible, such that they are increasingly being used as the active layer in a wide range of new technologies. The versatility of synthetic organic chemistry enables the materials to be tuned such that they can be incorporated into biological sensors, wearable electronics, semi-transparent photovoltaics and flexible displays. These devices can be improved not only by developing their synthetic chemistry but also by improving the analytical and computational techniques that enable us to understand the factors that govern material properties. Judicious molecular design provides control of the semiconductor frontier molecular orbital energy distribution and guides the hierarchical assembly of organic semiconductors into functional films where we can control the properties and motion of charges and excited states. This Review describes how molecular design plays an integral role in developing organic semiconductors for electronic devices in present and emerging technologies. [H1] Molecular orbital design considerations The energies of frontier molecular orbitals and the distribution of the orbitals in a π-conjugated molecule play critical roles in intra-and intermolecular charge transport, light absorption/emission, charge injection/extraction/trapping and electrochemistry. This is true for organic small molecules and conjugated polymers alike. In each case, the energy of the highest occupied molecular orbital (HOMO) largely depends on the electron density and delocalization of the π electrons throughout a π-conjugated backbone. Substituents that donate electron density mesomerically (for example, lone pair donation from N, O or S heteroatoms) or inductively (for example, alkyl chains) can contribute to raising the HOMO energy EHOMO, decreasing the solid-state ionization potential (IP, Box 1). Conversely electronwithdrawing groups, such as-F,-C(O)R or-C≡N groups, can act to lower both the HOMO and lowest unoccupied molecular orbital (LUMO) energy, leading to an increase in the solid-state electron affinity (EA). BOX 1 | Energy levels in an isolated organic molecule and a molecular crystal or polymer.

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“…Furthermore, in an unaged operating PLED, the density of free electrons is much smaller than the density of free holes due to the present of electron traps. [48] Therefore, the interactions between triplet excitons (triplet-triplet annihilation) and between triplet excitons and free holes (triplet-polaron interaction) are expected to be dominant processes in an operating PLED.…”

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Role of Singlet and Triplet Excitons on the Electrical Stability of Polymer Light‐Emitting Diodes

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et al. 2020

Adv Elect Materials

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In contrast, small-molecule organic LEDs exploiting triplet excitons using phosphorescent molecules (ph-OLEDs) or thermally activated delayed fluorescence (TADF) clearly outperform fluorescent PLEDs in terms of efficiency. The low efficiency is also detrimental for the operational stability of PLEDs, since higher currents are required to reach a required light-output, which accelerates degradation. However, to reach high efficiencies, both ph-OLEDs and TADF based OLEDs generally have complex device architectures comprising different injection, transport, emission, and blocking layers. Despite dissimilar architectures, the multilayer TADF and phosphorescent OLEDs as well as single-layer PLEDs all show typical degradation features of a voltage increase and a luminance decrease under continuous electrical operation. [4] For multilayer OLEDs the physical processes behind degradation are hard to elucidate due to the presence of many materials and interfaces. [5,6] The standard PLED device structure, being an emitting polymer layer sandwiched between two different work function electrodes, is more basic, making degradation processes more straightforward to analyze. In earlier work on degradation of poly(phenylene vinylene) (PPV) based LEDs the effects of molecular weight, molecular structure, and defects that arise during synthesis on lifetime have been discussed. [7] In particular halogen related defects in PPVs are pointed out to have a negative influence on the lifetime. With regard to physical degradation mechanisms, in 2001, Silvestre et al. [4] were the first to propose that "voltage increase" and "luminance decrease" during degradation shared a common origin, namely the formation of trap states. Furthermore, Pekkola et al. [8] studied the influence of triplet excitons on the electrical stability of conjugated polymers. A triplet sensitizer was introduced into a PPV-based LED and a negative influence of triplet excitons on the lifetime was reported. As a possible mechanism the energy transfer from a PPV triplet to an oxygen triplet state, creating the reactive singlet oxygen molecule, was suggested. [9,10] Singlet oxygen has the ability to attack the vinyl bonds of PPVs, providing a chemical pathway for degrading the material. [11] Degradation due to extrinsic effects, such as oxygen and water, is typically minimized by proper encapsulation of organic LEDs and by working in a controlled environment. [5−7,12] Also in ph-OLEDs the harmful influence of Stability under electrical stress is an important aspect for the function of organic light-emitting diodes (OLEDs). Degradation is currently one of the key topics in this field, concerning all types of OLEDs, including fluorescent-, phosphorescent-, and thermally activated delayed fluorescence-based OLEDs. For single-layer polymer light-emitting diodes (PLEDs) it has recently been found that degradation is the result of hole trap formation due to excitonpolaron interactions. However, whether singlet or triplet excitons are responsible for degradation is an open questi...

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Electron Trapping in Conjugated Polymers

Cited by 87 publications

References 46 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 role of chemical design in the performance of organic semiconductors

Role of Singlet and Triplet Excitons on the Electrical Stability of Polymer Light‐Emitting Diodes

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