Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Structural Isomeric Effect on Spin Transport in Molecular Semiconductors

Molecular semiconductor (MSC) is a promising candidate for spintronic applications benefiting from its long spin lifetime caused by light elemental-composition essence and thus weak spin-orbit coupling (SOC). According to current knowledge, the SOC effect, normally dominated by the elemental composition, is the main spin-relaxation causation in MSCs, and thus the molecular structure-induced SOC change is one of the most concerned issues. In theoretical study, molecular isomerism, a most prototype phenomenon, has long been considered to possess little difference on spin transport previously, since elemental compositions of isomers are totally the same. However, here in this study, quite different spin-transport performances are demonstrated in ITIC and its structural isomers BDTIC experimentally, for the first time, though the charge transport and molecular stacking of the two films are very similar. By further experiments of electron-paramagnetic resonance and density-functional-theory calculations, it is revealed that noncovalent-conformational locks (NCLs) formed in BDTIC can lead to enhancement of SOC and thus decrease the spin lifetime. Hence, this study suggests the influences from the structural-isomeric effect must be considered for developing highly efficient spin-transport MSCs, which also provides a reliable theoretical basis for solving the great challenge of quantificational measurement of NCLs in films in the future.

Silicon- and Germanium-Functionalized Perylene Diimides: Synthesis, Optoelectronic Properties, and Their Application as Non-fullerene Acceptors in Organic Solar Cells

Organic solar cells have been continuously studied and developed through the last decades. A major step in their development was the introduction of fused-ring non-fullerene electron acceptors. Yet, beside their high efficiency, they suffer from complex synthesis and stability issues. Perylene-based non-fullerene acceptors, in contrast, can be prepared in only a few steps and display good photochemical and thermal stability. Herein, we introduce four monomeric perylene diimide acceptors obtained in a three-step synthesis. In these molecules, the semimetals silicon and germanium were added in the bay position, on one or both sides of the molecules, resulting in asymmetric and symmetric compounds with a red-shifted absorption compared to unsubstituted perylene diimide. Introducing two germanium atoms improved the crystallinity and charge carrier mobility in the blend with the conjugated polymer PM6. In addition, charge carrier separation is significantly influenced by the high crystallinity of this blend, as shown by transient absorption spectroscopy. As a result, the solar cells reached a power conversion efficiency of 5.38 %, which is one of the highest efficiencies of monomeric perylene diimide-based solar cells recorded to date.

Symmetry-Induced Ordered Assembly of a Naphthobisthiadiazole-Based Nonfused-Ring Electron Acceptor Enables Efficient Organic Solar Cells

Nonfused-ring electron acceptors (NFREAs) have received increasing attention for use in organic solar cells (OSCs) because of their synthetic simplicity and tunable optical spectra. However, their fundamental molecular interactions and the mechanism by which they govern the property-function relations of OSCs remain elusive. Here, to investigate the effects of the structural symmetry of NFREAs, two acceptor-donor-acceptor'-donor-acceptor (A-D-A'-D-A)-type NFREAs, 2,2'-(((naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole-5,10-diylbis(4,4-bis(2-butyloctyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-6,2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NTz-4F) and 2,2'-(((benzo[c][1,2,5]thiadiazole-4,7-diylbis(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-6,2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (BT-4F), are designed and synthesized. They have different A' cores: NTz-4F has a modified centrosymmetric NTz core, whereas BT-4F has a modified axisymmetric BT core. In pristine films, the NTz-4F, which has a centrosymmetric core, shows substantially enhanced intermolecular interaction and microstructural crystalline ordering compared with BT-4F, which has an axisymmetric core. Even in blends with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8,-dione))] (PBDB-T), NTz-4F retains its highly crystalline structure, whereas BT-4F loses crystalline packing. These changes in NTz-4F result in increased electron transport and suppressed nonradiative voltage loss, resulting in a power conversion efficiency of 9.14% for PBDB-T:NTz-4F vs 7.18% for PBDB-T:BT-4F. This work demonstrates that centrosymmetric-structured cores are promising building blocks for high-performance NFREA-based OSCs.

Promoting the Efficiency and Stability of Nonfullerene Organic Photovoltaics by Incorporating Open-Cage [60]Fullerenes in the Nonfullerene Nanocrystallites

The device efficiency of PM6:Y6-based nonfullerene organic solar cells is fast advanced recently. To maintain organic solar cells (OSCs) with high power conversion efficiency over 16% in long-term operation, however, remains a challenge. Here, a novel non-volatile additive, an open-cage [60]fullerene (8OC₆₀Me), is incorporated into PM6:Y6-based OSCs for high-performance with high durability. With optimized addition of 1.0 wt % 8OC₆₀Me, the PCE value of PM6:Y6/8OC₆₀Me OSCs can be promoted to 16.5% from 15.0%. Most strikingly, such a high PCE performance can maintain nearly 100% for over 500 h at room temperature; at an elevated operation temperature of 80 °C, the PCE can be stabilized above 15.0% after 45 h of operation. Grazing incidence small- and wide- angle X-ray scattering studies reveal improved orientation and crystallinity of Y6 in a fractal-like network structure of PM6 in PM6:Y6/8OC₆₀Me films under in situ annealing, parallel to the enhanced electron mobility. Analysis of charge distributions lines up possible van der Waals interaction between the thienyl/carbonyl moiety of 8OC₆₀Me and difluorophenyl-based FIC-end groups of Y6. This result is of great contrast to those devices with the best-selling PC₆₁BM as the additives─8OC₆₀Me might be of interest to be incorporated into future Y6-based OSCs for concomitantly improved PCE and excellent stability.

Design, Regioselective Synthesis, and Photophysical Properties of Perfluoronaphthalene-Based Donor-Acceptor-Donor Fluorescent Dyes

A one-step route to a series of perfluoronaphthalene-based donor (D)-acceptor (A)-D fluorescent dyes with various electron-donating groups was developed. The perfluoronaphthalene moiety in the D-A-D dyes served as a good electron-accepting aromatic ring with excellent intramolecular charge transfer properties, as determined by density functional theory calculations and measurements of the fluorescence properties in solution, in poly(methyl methacrylate) (PMMA) films, and in crystal form. Notably, replacing the naphthalene ring with perfluoronaphthalene in the D-A-D dyes carrying the phenothiazine moiety not only stabilized the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels but also reduced the energy band gap to change the emission color from blue to yellow. Among the four synthesized perfluoronaphthalene D-A-D dyes, those bearing diphenylamino groups afforded the best fluorescence quantum yields in Et₂O solution (0.60) and in PMMA film (0.65) because the propeller structure of the diphenylamino group that acts as a donor substituent effectively suppresses radiation-free deactivation. In contrast, in the crystalline state, the carbazoyl-bearing D-A-D dye provided the best fluorescence quantum yield (0.35) because the radiation-free inactivation was suppressed by π-π_F stacking at the donor site, which was confirmed by single-crystal X-ray analysis.

Glycol bearing perylene monoimide based non-fullerene acceptors with increased dielectric permittivity

Perylene monoimide based electron acceptors have great properties for use in organic solar cells, like thermal stability, strong absorption, and simple synthesis. However, they typically exhibit low values for the dielectric permittivity. This hinders efficient exciton dissociation, limiting the achievable power conversion efficiencies. In this work, we present the synthesis and utilization of two new acceptor-donor-acceptor (A-D-A) molecules, comprising perylene monoimide as electron withdrawing A unit. Oligo ethylene glycol side chain modified carbazole (PMI-[C-OEG]) and fluorene (PMI-[F-OEG]) linkers were used as electron rich D units, respectively. The polar side chains are expected to increase the polarizability of the molecules and, thus, their permittivity according to the Clausius-Mossotti relationship. We found that the incorporation of glycol chains improved the dielectric properties of both materials in comparison to the reference compounds with alkyl chains. The permittivity increased by 18% from 3.17 to 3.75 for the carbazole-based non-fullerene acceptor PMI-[C-OEG] and by 12% from 3.10 to 3.47 for the fluorene-based acceptor PMI-[F-OEG]. The fabricated solar cells revealed power conversion efficiencies of 3.71 ± 0.20% (record 3.92%) with PMI-[C-OEG], and 1.21 ± 0.06% (record 1.51%) with PMI-[F-OEG].

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00706-022-02956-2.

Phenylene-Bridged Perylene Monoimides as Acceptors for Organic Solar Cells: A Study on the Structure-Property Relationship

A series of non-fullerene acceptors based on perylene monoimides coupled in the peri position through phenylene linkers were synthesized via Suzuki-coupling reactions. Various substitution patterns were investigated using density functional theory (DFT) calculations in combination with experimental data to elucidate the geometry and their optical and electrochemical properties. Further investigations of the bulk properties with grazing incidence wide angle X-ray scattering (GIWAXS) gave insight into the stacking behavior of the acceptor thin films. Electrochemical and morphological properties correlate with the photovoltaic performance of devices with the polymeric donor PBDB-T and a maximum efficiency of 3.17 % was reached. The study gives detailed information about structure-property relationships of perylene-linker-perylene compounds.

Novel Pyrrolo [3,4-b] dithieno [3, 2-f:2",3"-h] quinoxaline-8,10 (9H)-dione Based Wide Bandgap Conjugated Copolymers for Bulk Heterojunction Polymer Solar Cells

Two D-A copolymers consisted of fused ring pyrrolo-dithieno-quinoxaline acceptor are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2-ethylhexyloxy (P117) side chains. These copolymers are used as donor and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron-deficient fused ring pyrrolo-bithieno-quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells based on P134:Y6 attained the power conversion efficiency of 15.42%, which is higher than P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 could be associated with more well-adjusted charge transport, weak charge recombination, proficient exciton generation and dissociation into free charge carriers and their subsequent charge collection owing to the dense π-π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor-weak donor tactic for developing efficient D-A copolymers consists of quinoxaline acceptor for polymer solar cells. This article is protected by copyright. All rights reserved.

Wide-bandgap organic solar cells with a novel perylene-based non-fullerene acceptor enabling open-circuit voltages beyond 1.4 V

A perylene-based acceptor (PMI-FF-PMI), consisting of two perylene monoimide (PMI) units bridged with a dihydroindeno[1,2-b]fluorene molecule was developed as a potential non-fullerene acceptor (NFA) for organic solar cells (OSCs). The synthesized NFA was combined with the high-performance donor polymer D18 to fabricate efficient OSCs. With an effective bandgap of 2.02 eV, the D18:PMI-FF-PMI blend can be categorized as a wide-bandgap OSC and is an attractive candidate for application as a wide-bandgap sub-cell in all-organic triple-junction solar cell devices. Owing to their large effective bandgap, D18:PMI-FF-PMI solar cells are characterized by an extremely high open-circuit voltage (V _OC) of 1.41 V, which to the best of our knowledge is the highest reported value for solution-processed OSCs so far. Despite the exceptionally high V _OC of this blend, a comparatively large non-radiative voltage loss (ΔV ^non-rad _OC) of 0.25 V was derived from a detailed voltage loss analysis. Measurements of the electroluminescence quantum yield (ELQY) of the solar cell reveal high ELQY values of ∼0.1%, which contradicts the ELQY values derived from the non-radiative voltage loss (ΔV ^non-rad _OC = 0.25 V, ELQY = 0.0063%). This work should help to raise awareness that (especially for BHJ blends with small Δ_HOMO or Δ_LUMO offsets) the measured ELQY cannot be straightforwardly used to calculate the ΔV ^non-rad _OC. To avoid any misinterpretation of the non-radiative voltage losses, the presented ELQY discrepancies for the D18:PMI-FF-PMI system should encourage OPV researchers to primarily rely on the ΔV ^non-rad _OC values derived from the presented voltage loss analysis based on EQE_PV and J-V measurements.