Suppressing Exciton-Vibration Coupling to Prolong Exciton Lifetime of Nonfullerene Acceptors Enables High-Efficiency Organic Solar Cells

The limited exciton lifetime (τ, generally <1 ns) leads to short exciton diffusion length (LD ) of organic semiconductors, which is the bottleneck issue impeding the further improvement of power conversion efficiencies (PCEs) for organic solar cells (OSCs). However, efficient strategies to prolong intrinsic τ are rare and vague. Herein, we propose a facile method to efficiently reduce vibrational frequency of molecular skeleton and suppress exciton-vibration coupling to decrease non-radiative decay rate and thus prolong τ via deuterating nonfullerene acceptors. The τ remarkably increases from 0.90 ns (non-deuterated L8-BO) to 1.35 ns (deuterated L8-BO-D), which is the record for organic photovoltaic materials. Besides, the inhibited molecular vibration improves molecular planarity of L8-BO-D for enhanced exciton diffusion coefficient. Consequently, the LD increases from 7.9 nm (L8-BO) to 10.7 nm (L8-BO-D). The prolonged LD of L8-BO-D enables PM6 : L8-BO-D-based bulk heterojunction OSCs to acquire higher PCEs of 18.5 % with more efficient exciton dissociation and weaker charge carrier recombination than PM6 : L8-BO-based counterparts. Moreover, benefiting from the prolonged LD , D18/L8-BO-D-based pseudo-planar heterojunction OSCs achieve an impressive PCE of 19.3 %, which is among the highest values. This work provides an efficient strategy to increase the τ and thus LD of organic semiconductors, boosting PCEs of OSCs.

Binding Energy of Triplet Excitons in Nonfullerene Acceptors: The Effects of Fluorination and Chlorination

One strategy to improve the photovoltaic properties of nonfullerene acceptors (NFAs), employed in state-of-art organic solar cells, is the rational fluorination or chlorination of these molecules. Although this modification improves important acceptor properties, little is known about the effects on the triplet states. Here, we combine the polarizable continuum model with an optimally tuned range-separated hybrid functional to investigate this issue. We find that fluorination or chlorination of NFAs decreases the degree of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) overlap along these molecules. Consequently, the energy gap between T₁ and S₁ states, ΔE_ST = E_S1 - E_T1, also decreases. This effect reduces the binding energy of triplet excitons, which favors their dissociation into free charges. Furthermore, the reduction of ΔE_ST can contribute to mitigating the losses produced by the nonradiative deactivation of the T₁ excitons. Interestingly, although Cl has a lower electronegativity than F, chlorination is more effective to reduce ΔE_ST. Since the chlorination of NFAs is easier than fluorination, Cl substitution can be a useful approach to enhance solar energy harvesting using triplet excitons.

A Functional Hexaphenylbenzene Library Comprising of One, Three, and Six Peripheral Rylene-Diimide Substituents

Synthesis and characterization of a series of rylene-diimide substituted hexaphenylbenzenes (HPBs) is presented. The direct connection of the rylene-diimide units to the HPBs via the imide-N-position without any linkers as well as the use of naphthalene-diimides (NDIs) next to perylene-diimides (PDIs) is unprecedented. While mono-substituted products were obtained by imidization reactions with amino-HPB and the respective rylene-monoimides, key step for the formation of tri- and hexa-substituted HPBs is the Co-catalysed cyclotrimerization. Particular emphasis for physic-chemical characterization was on to the number of NDIs/PDIs per HPB and the overall substitution patterns. Lastly, Scholl oxidation conditions were applied to all HPB systems to generate the corresponding hexa-peri-hexabenzocoronenes (HBCs). Importantly, the efficiency of the transformation strongly depends on the number of NDIs/PDIs. While three rylene-diimide units already hinder the Scholl reaction, the successful synthesis of mono-substituted HBCs is possible.

Enhancing the Performance of Organic Solar Cells by Prolonging the Lifetime of Photogenerated Excitons

Exciton lifetime (τ) is crucial for the migration of excitons to donor/acceptor interfaces for subsequent charge separation in organic solar cells (OSCs); however, obvious prolongation of τ has rarely been achieved. Here, by introducing a solid additive 9-fluorenone-1-carboxylic acid (FCA) into the active layer, which comprises a nonfullerene acceptor, 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (IT-M), τ is substantially prolonged from 491 to 928 ps, together with obvious increases in fluorescence intensity and quantum yield. Time-resolved transient infrared spectra indicate the presence of an intermolecular vibrational coupling between the electronic excited state of IT-M and the electronic ground state of FCA, which is first observed here and which can suppress the internal conversion process. IT-M-based OSCs display an improved short-circuit current and fill factor after the addition of FCA. Thus, the power conversion efficiency is increased, particularly for devices with a large donor/acceptor ratio of 1:4, whose efficiency is increased by 56%. This study describes a novel method, which is also applicable to other nonfullerene acceptors, for further improving the performance of OSCs without affecting their morphology and light absorption properties.

Selenium and Tellurium Derivatives of Corannulene: Serendipitous Discovery of a One-Dimensional Stereoregular Coordination Polymer Crystal Based on Te-O Backbone and Side-Chain Aromatic Array

Monobromo-, tetrabromo-, and pentachloro-corannulene are subjected to nucleophilic substitution reactions with tolyl selenide and phenyl telluride-based nucleophiles generated in situ from the corresponding dichalcogenides. In the case of selenium nucleophile, the reaction provides moderate yields (52-77 %) of the targeted corannulene selenoethers. A subsequent oxidation of the selenium atoms proceeds smoothly to furnish corannulene selenones in 81-93 % yield. In the case of tellurides, only monosubstitution of the corannulene scaffold could be achieved albeit with concomitant oxidation of the tellerium atom. Unexpectedly, this monotelluroxide derivative of corannulene (RR'Te=O, R=Ph, R'=corannulene) is observed to form a linear coordination polymer chain in the crystalline state. In this chain, Te-O constitutes the polymer backbone around which the aromatic groups (R and R') arrange as polymer side-chains. The polymer crystal is stabilized through intramolecular π-π stacking interactions of the side-chains and intermolecular hydrogen and halogen bonding interactions with the solvent (chloroform) molecules. Interestingly, each diad of the polymer chain is racemic. Therefore, in terms of stereoregularity, the polymer chain can be described as syndiotactic.

Perylene Diimide-Based Conjugated Polymers for All-Polymer Solar Cells

In recent decades, non-fullerene acceptors (NFAs) are undergoing rapid development and emerging as a hot area in the field of organic solar cells. Among the high-performance non-fullerene acceptors, aromatic diimide-based electron acceptors remain to be highly promising systems. This review discusses the important progress of perylene diimide (PDI)-based polymers as non-fullerene acceptors in all-polymer solar cells (all-PSCs) since 2014. The relationship between structure and property, matching aspects between donors and acceptors, and device fabrications are unveiled from a synthetic chemist perspective.

Triplet Acceptors with a D-A Structure and Twisted Conformation for Efficient Organic Solar Cells

Triplet acceptors have been developed to construct high-performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T1 ) are close to those of charge-transfer states (3 CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π-conjugated core and D-A structure, were confirmed to be triplet materials, leading to high-performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D-A structures result in large spin-orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T1 is close to 3 CT, facilitating the split of triplet exciton to free charges.

The Synthesis and Optoelectronic Applications for Tellurophene-Based Small Molecules and Polymers

Tellurophene-based small molecules and polymers have received great attentions owing to their applications in thin-film transistors, solar cells, and sensors. This article reviews the current progress of the synthesis and applications of tellurophene-based small molecules and polymers. The physicochemical properties and optoelectronic applications of tellurophene-based materials are summarized and discussed. In the end, the challenges and outlook of tellurophene-based materials are presented.

Naphthobisthiadiazole-Based Selenophene-Incorporated Quarterchalcogenophene Copolymers for Field-Effect Transistors and Polymer Solar Cells

In this research, we developed six new selenophene-incorporated naphthobisthiadiazole-based donor-acceptor polymers PNT2Th2Se-OD, PNT2Se2Th-OD, PNT4Se-OD, PNT2Th2Se-DT, PNT2Se2Th-DT, and PNT4Se-DT. The structure-property relationships have been systematically established through the comparison of their structural variations: (1) isomeric biselenophene/bithiophene arrangement between PNT2Th2Se and PNT2Se2Th polymers, (2) biselenophene/bithiophene and quarterselenophene donor units between PNT2Th2Se/PNT2Se2Th and PNT4Se polymers, and (3) side-chain modification between the 2-octyldodecylthiophene (OD)- and 2-decyltetradecyl (DT)-series polymers. The incorporation of selenophene units in the copolymers induces stronger charge transfer to improve the light-harvesting capability while maintaining the strong intermolecular interactions to preserve the intrinsic crystallinity for high carrier mobility. The organic field-effect transistor device using PNT2Th2Se-OD achieved a high hole mobility of 0.36 cm² V^-1 s^-1 with an on/off ratio of 1.9 × 10⁵. The solar cells with PNT2Th2Se-OD:PC₇₁BM exhibited a power conversion efficiency of 9.47% with a V_oc of 0.68 V, an fill factor of 67%, and an impressive J_sc of 20.69 mA cm^-2.

Doping Sumanene with Both Chalcogens and Phosphorus(V): One-Step Synthesis, Coordination, and Selective Response Toward AgI

Heterasumanenes 4-6 containing chalcogen (S, Se, and Te) and phosphorus atoms have been synthesized in a one-pot reaction from trichalcogenasumanenes 1-3 by replacing one chalcogen atom with a P=S unit. The P=S unit makes 4-6 almost planar and shrinks the HOMO-LUMO gap as compared to 1-3. The bonding between Ag⁺ and S atom on P=S brings about a distinct change to the optical properties of 4-6; 4 in particular shows a selective fluorescence response toward Ag⁺ with LOD of 0.21 μm. Compounds 4-6 form complexes with AgNO₃ to be (4)₂ ⋅AgNO₃ , (5)₂ ⋅AgNO₃ , and (6)₂ ⋅(AgNO₃ )₃ . In complexes, the coordination between Ag⁺ and P=S is observed, which leads to shrinkage of C-P and C-X (X=S, Se, Te) bond lengths. As a result, 4, 5, and 6 are all bowl-shaped in complexes with bowl-depths reaching to 0.66 Å, 0.42 Å, and 0.40 Å, respectively. There are Ag-Te dative bonds between Ag⁺ and Te atom on telluorophene in (6)₂ ⋅(AgNO₃ )₃ .

Aerobic Solid State Red Phosphorescence from Benzobismole Monomers and Patternable Self-Assembled Block Copolymers

The synthesis of the first bismuth-containing macromolecules that exhibit phosphorescence in the solid state and in the presence of oxygen is reported. These red emissive high molecular weight polymers (>300 kDa) feature benzobismoles appended to a hydrocarbon scaffold, and were built via an efficient ring-opening metathesis (ROMP) protocol. Moreover, our general procedure readily allows for the formation of cross-linked networks and block copolymers. Attaining stable red phosphorescence with non-toxic elements remains a challenge and, thus, our new class of soluble (processable) polymeric phosphor is of great interest. Furthermore, the formation of bismuth-rich cores within organic-inorganic block copolymer spherical micelles is possible, leading to patterned arrays of bismuth in the film state.

Fluorination Induced Donor to Acceptor Transformation in A1-D-A2-D-A1-Type Photovoltaic Small Molecules

With the development of diversity of non-fullerene acceptors, it is found that there is no clear boundary between electron donors and electron acceptors. Modulation of the electron donating and withdrawing properties of organic semiconductors is necessary for organic photovoltaics research. In this work, we designed and synthesized three A1-D-A2-D-A1-type (A represents acceptor unit and D represents donor unit) small molecules, named as M-0F, M-1F, and M-2F, respectively containing zero, one, and two fluorine atoms in the terminal acceptor segments (A1), respectively. Fluorination substitution was found to be able to downshift the HOMO and LUMO energy level, red-shift the absorption, and enhance the electron mobility. The M-0F exhibited the highest efficiency of 5.99% as a donor in fullerene-containing system and the lowest efficiency of 0.58% as an acceptor in fullerene-free system. While the M-2F performed the lowest efficiency of 0.97% as the donor and the highest efficiency of 2.65% as the acceptor. The electron-donating and electron-withdrawing property of M-1F are in-between that of M-0F and M-2F. Among the three molecules, the electron mobility is increased while the hole mobility is decreased with increasing fluorination. This work provides a typical example of tuning of the electron donating and withdrawing property without changes to the backbone of the conjugated molecules, which is important for further designing high performance solution processable small molecules.