Non-fullerene Electron Acceptors Research Articles

Nanostructured films of PM6 and Y6 and their assembly using Langmuir–Schaefer technique

This study investigates the molecular-level assembly and electronic properties of Langmuir and Langmuir–Schaefer (LS) films formed from Y6 (a non-fullerene electron acceptor) and PM6 (a wide bandgap electron donor polymer). We employ a variety of techniques including surface pressure-area isotherms, Brewster angle microscopy, ultraviolet-visible spectroscopy, Raman spectroscopy, and electrical characterizations to explore the behavior of these films. Our findings reveal that the films exhibit a tendency to aggregate and form non-homogeneous layers, which impacts their optical and electrical properties. The LS films show a significant sensitivity to solar radiation, which is evident from the changes in conductivity under illumination. This study highlights the potential of these materials in the development of photovoltaic applications, emphasizing the importance of molecular packing and film morphology in optimizing device performance.

Machine learning assisted designing of non-fullerene electron acceptors: A quest for lower exciton binding energy

The designing of acceptors materials for the organic solar cells is a hot topic. The normal experimental methods are tedious and expensive for large screening. Machine learning guided exploration is more suitable solution. Bagging regression, random forest regression, gradient boosting regression, and linear regression are trained to predict exciton binding energy. Breaking Retrosynthetically Interesting Chemical Substructures (BRICS) methodology has utilized for designing of new non-fullerene acceptors (NFAs). The predicted values were used to select the designed NFAs. On the selected NFAs, clustering and chemical similarity analyses are also performed. Chemical fingerprints are used for this purpose, and the synthetic accessibility score of the new NFAs is also investigated.30 NFAs have selected with low exciton binding energy values. This approach will allow for the rapid screening of NFAs for organic solar cells. Our proposed framework stands out as a valuable tool for strategically selecting the most effective NFAs for organic solar cells and offers a streamlined approach for material discovery.

19.7% efficiency binary organic solar cells achieved by selective core fluorination of nonfullerene electron acceptors

19.7% efficiency binary organic solar cells achieved by selective core fluorination of nonfullerene electron acceptors

Excited state properties of an A-D-A non-fullerene electron acceptor: a LC-TD-DFTB study.

Understanding charge transfer processes is essential to estimate the performance of organic photovoltaic technologies. Although experimental production is on the rise, predictability strongly relies on theoretical modeling, which is limited to the size of semiconductors. As a computationally favorable approach, we benchmarked the long-range corrected (LC) time-dependent (TD) formulation of the semi-empirical density functional-based tight-binding method (DFTB) for three polycyclic aromatic hydrocarbons (PAHs) and studied the DTP-IC-4Ph molecule, a PAH-based non-fullerene electron acceptor (NFA) with an A-D-A backbone structure. After a thorough investigation into the long-range parameter (ω) tuning for naphthalene, anthracene and pyrene, the excitation energies, oscillator strengths and Natural Transition Orbitals (NTOs) were compared with the standard ωB97X-D/6-31G(d,p) level of theory and the ADC2/6-31G(d,p) multiconfigurational method. We estimated mobility-related properties of the NFA and considered 1000 thermally accessible configurations to qualitatively reproduce the experimental absorption profile and investigate the energetic disorder. Finally, we conducted a fragment-based analysis using the one-electron transition density matrix (1TDM) to determine the character of the excited states and investigate the effect of side chains on exciton formation. Our results are sensitive to the level of theory and highly dependent on the long-range parameter but suggest that the presence of alkyl chains promotes a higher average charge delocalization and allows for additional hopping mechanisms, favoring the charge transfer dynamics.

Unveiling the impact of exchange-correlation functionals on the description of key electronic properties of non-fullerene acceptors in organic photovoltaics.

Non-fullerene electron acceptors have emerged as promising alternatives to traditional electron-acceptors in the active layers of organic photovoltaics. This is due to their tunable energy levels, optical response in the visible light spectrum, high electron mobility, and photochemical stability. In this study, the electronic properties of two representative non-fullerene acceptors, ITIC and Y5, have been calculated within the framework of density functional theory using a range of hybrid and non-hybrid density functionals. Screened range-separated hybrid (SRSH) approaches were also tested. The results are analyzed in light of the previously reported experimental outcomes. Specifically, we have calculated the oxidation and reduction potentials, fundamental and optical gaps, the highest occupied molecular orbital and lowest unoccupied molecular orbital energies, and exciton binding energies. Additionally, we have investigated the effects of the medium dielectric constant on these properties employing a universal implicit solvent model. It was found that hybrid functionals generally perform poorly in predicting oxidation potentials, while non-hybrid functionals tend to overestimate reduction potentials. The inclusion of a large Hartree-Fock contribution to the global or long range was identified as the source of inaccuracy for many hybrid functionals in predicting both redox potentials and the fundamental and optical gaps. Corroborating with the available literature, ∼50% of all tested functionals predicted very small exciton binding energies, within the range of ±0.1eV, that become even smaller by increasing the dielectric constant of the material. Finally, the OHSE2PBE and tHCTHhyb functionals and the optimal tuning SRSH approach emerged as the best-performing methods, with good accuracy in the description of the electronic properties of interest.

B─N Covalent Bond-Based Nonfullerene Electron Acceptors for Efficient Organic Solar Cells.

The optoelectronic properties and photovoltaic performance of nonfullerene electron acceptors (NFEAs) in organic solar cells (OSCs) are greatly influenced by the rational structure regulation of the central core unit. This study introduces a novel type of six-membered fused electron-donating core containing B─N covalent bonds to construct acceptor-donor-acceptor (A-D-A)-type NFEAs. By modulating the branching alkyl chains on the nitrogen atom, two NFEAs, BN910 and BN1014, are synthesized and characterized. Both molecules exhibit strong near-infrared absorption, narrow bandgaps (≈1.45eV), appropriate energy levels, and tunable molecular packing behaviors, positioning them as promising candidates for efficient NFEAs in OSCs. The investigation reveals that BN1014, with longer and C2-branched alkyl chains, demonstrates superior intermolecular packing and morphology within active layers, leading to enhanced exciton dissociation, improved charge transfer, and reduced charge recombination in OSCs. As a result, a power conversion efficiency (PCE) of 10.02% is achieved for D18:BN1014-based binary OSCs. Notably, BN1014 can be utilized as the third component in the D18:DT-Y6 binary system to fabricate the ternary OSCs, and a PCE of 17.65% is achieved, outperforming 17.05% of D18:DT-Y6-based binary OSCs. These findings highlight the potential of heteroarenes featuring B─N covalent bonds for constructing high-efficiency NFEAs in OSCs.

Low‐Cost Pentacyclic Nonfullerene Acceptors to Exceed 13% Efficiency by Thermal Annealing‐Free Treatment

Nonfullerene electron acceptors (NFAs) improve the photoelectrical conversion efficiency of organic solar cells (OSCs), but they suffer from high synthetic cost and time‐consuming thermal annealing treatment. Nevertheless, low‐cost NFAs and simple device fabrication are still crucial issues to boosting the commercial application of OSCs. Herein, one kind of low‐cost pentacyclic NFAs based on benzothiadiazole and thiophene derivatives named L1‐EH and L1‐BO with different substitutions is successfully synthesized and characterized. The optimized OSCs based on PM6:L1‐BO (2‐butyloctyl side chain) display an outstanding power conversion efficiency of 13.15%. Moreover, OSC devices based on these acceptors operate in thermal annealing‐free conditions. Further studies manifest that the proper side chains in NFAs can induce more favorable face‐on molecule orientation, enhance carrier mobility, balance charge transport, and reduce recombination losses. This research demonstrates that these types of low‐cost NFAs are promising active layer candidates of high‐efficiency organic solar cells featuring simple device manufacture.

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.

Observation of an Exciton-Plasma Transition in a Molecular Semiconductor.

The nonfullerene electron acceptors (NFAs) for organic solar cells are attracting intense research efforts due to their impressive performance. Understanding the temporal evolution of the excited states in NFAs is essential to gain insights into the working mechanism of these state-of-the-art devices. Here we characterized the photoconductivities of a neat Y6 film and a Y6:PM6 blend film using time-resolved terahertz spectroscopy. Three different types of excited states were identified based on their distinct terahertz responses, i.e., plasma-like carriers, weakly bound excitons, and spatially separated carriers. Under high-intensity excitation, the many-body interaction of excitons in the Y6 film leads to the plasma-like state, giving rise to a terahertz response characteristic for a dispersive charge transport. This transient state decays quickly into exciton gas due to fast Auger annihilation. Under low-intensity excitation, only isolated excitons are created and the plasma state is absent.

Latest Updates of Single-Junction Organic Solar Cells up to 20% Efficiency

Single-junction organic solar cells have reached a power conversion efficiency of 20% with narrow bandgap non-fullerene electron acceptor materials such as Y6, as well as with large band gap electron donor materials and their derivatives. The power conversion efficiency improvement of single-junction organic solar cells is a result of highly efficient light harvesting in the near-infrared light range and reduced energy losses with the most promising active layer layout currently available, Bulk-Heterojunction. Ternary blending is known to be the most advanced strategy to construct Bulk-Heterojunction structures in organic solar cells at present. In this review, we examine different devices based on Bulk-Heterojunction structures with efficient electron donors and acceptors. Then, we review the performance of binary and ternary organic solar cells with high power conversion efficiency, in conjunction with different anode and cathode interfaces used in recent studies of high-power conversion efficiency. Finally, we present perspectives on the future development of single-junction organic solar cells.

Improved Crystallization of High-Solubility Non-Fullerene Electron Acceptors for Enhanced Photoelectric Conversion Efficiency: Effect of the Terminal Group

Organic small-molecule acceptors with acceptor–donor–acceptor–donor–acceptor (A-DA′D-A)-type configurations typically exhibit photoinduced intra- and intermolecular charge transfer (CT) dynamics. Herein, using steady-state and transient absorption spectroscopy, we studied the impact of the three-dimensional (3D) terminal group on the intra- and intermolecular CT in the A-DA′D-A-type small-molecular acceptor and the effect of the photoelectric conversion process in the corresponding blend film. The results show that 3D terminal groups have little impact on intramolecular charge transfer but significantly improve solubility and regulate intermolecular interactions. 3D terminal groups can improve exciton generation and enhance exciton diffusion distance by preventing the film from over-aggregating, thus benefiting the charge transfer and photoelectric conversion in the blend film. This work demonstrated the advantages of the 3D terminal group and revealed the fundamental importance of regulating the intra- and intermolecular CT in enhancing photoelectric conversion.

Efficient ternary organic solar cells enabled by asymmetric nonfullerene electron acceptor with suppressed nonradiative recombination

The open-circuit voltage (Voc) of organic solar cells (OSCs) is still far from the Shockley-Queisser limit due to the large non-radiative voltage loss (ΔVocnonrad). Reducing energy loss (Eloss) to obtain higher Voc without sacrificing Jsc and FF is the key to achieve further improvement in power conversion efficiency (PCE) of OSCs. In this work, we designed and synthesized a new asymmetric nonfullerene acceptor via a dual asymmetric strategy, named as SN-O. Under the synergistic effect of alkoxy chain and nitrogen atom substitution, the LUMO and HOMO energy levels of SN-O are significantly elevated compared to Y6, allowing SN-O to display a high open-circuit voltage with a reduced nonradiative energy loss (ΔEnonrad = 0.177 eV). The alkoxy side chain of SN-O brings an intramolecular conformational locking effect and the large dipole moment of asymmetric molecules facilitate the aggregation properties in thin films. The good miscibility allows Y6 and SN-O to form alloy acceptor attributing to the similar chemical structures. The addition of SN-O as the third component into the D18:Y6 system showed a significant reduction in the nonradiative energy loss and a favorable blend film morphology, thus improves exciton dissociation, charge transport and collection. Consequently, a high PCE of 18.3% was achieved for D18:Y6:SN-O ternary system with simultaneously enhancement of Voc, Jsc and FF, which is much higher than that of the binary devices.

Novel Rigidochromic and Anti-Kasha Dual Emission Fluorophores Based on D-π-A Dyads as the Promising Materials for Potential Applications Ranging from Optoelectronics and Optical Sensing to Biophotonics and Medicine.

Today we see an increasing demand for new fluorescent materials exhibiting various sensory abilities due to their broad applicability ranging from the construction of flexible devices to bioimaging. In this paper, we report on the new fluorescent pigments AntTCNE, PyrTCNE, and PerTCNE which consist of 3-5 fused aromatic rings substituted with tricyanoethylene fragments forming D-π-A diad. Our studies reveal that all three compounds exhibit pronounced rigidochromic properties, i.e., strong sensitivity of their fluorescence to the viscosity of the local environment. We also demonstrate that our new pigments belong to a very rare type of organic fluorophores which do not obey the well-known empirical Kasha'rule stating that photoluminescence transition always occurs from the lowest excited state of an emitting molecule. This rare spectral feature of our pigments is accompanied by an even rarer capability of spectrally and temporally well-resolved anti-Kasha dual emission (DE) from both higher and lowest electronic states in non-polar solvents. We show that among three new pigments, PerTCNE has significant potential as the medium-bandgap non-fullerene electron acceptor. Such materials are now highly demanded for indoor low-power electronics and portable devices for the Internet-of-Things. Additionally, we demonstrate that PyrTCNE has been successfully used as a structural unit in template assembling of the new cyanoarylporphyrazine framework with 4 D-π-A dyads framing this macrocycle (Pyr4CN4Pz). Similarly to its structural unit, Pyr4CN4Pz is also the anti-Kasha fluorophore, exhibiting intensive DE in viscous non-polar medium and polymer films, which strongly depends on the polarity of the local environment. Moreover, our studies showed high photodynamic activity of this new tetrapyrrole macrocycle which is combined with its unique sensory capacities (strong sensitivity of its fluorescent properties to the local environmental stimuli such as viscosity and polarity. Thus, Pyr4CN4Pz can be considered the first unique photosensitizer that potentially enables the real-time combination of photodynamic therapy and double-sensory approaches which is very important for modern biomedicine.

Non-Fullerene Electron Acceptors Based on Hybridisation of Corannulene and Thiophene-S,S-Dioxide Motifs.

Herein we show that hybridisation of buckybowl corannulene and thiophene-S,S-dioxide motifs is a general approach for the preparation of high electron affinity molecular materials. The devised synthesis is modular and relies on thienannulation of corannnulene-based phenylacetylene scaffolds. The final compounds are highly soluble in common organic solvents. These compounds also exhibit interesting optical properties such as absorption and emission in the blue/green regions of the electromagnetic spectrum. Importantly, a bis-S,S-dioxide derivative exhibits three reversible reductions similar in their strength to the prevalent fullerene-based electron acceptor phenyl-C61 -butyric acid methyl ester (PC61 BM).

Spatiotemporal Mapping Uncouples Exciton Diffusion from Singlet–Singlet Annihilation in the Electron Acceptor Y6

Understanding the spatial dynamics of nanoscale exciton transport beyond the temporal decay is essential for further improvements of nanostructured optoelectronic devices, such as solar cells. The diffusion coefficient (D) of the nonfullerene electron acceptor Y6 has so far only been determined indirectly, from singlet-singlet annihilation (SSA) experiments. Here, we present the full picture of the exciton dynamics, adding the spatial domain to the temporal one, by spatiotemporally resolved photoluminescence microscopy. In this way, we directly track diffusion and we are able to decouple the real spatial broadening from its overestimation given by SSA. We measured the diffusion coefficient, D = 0.017 ± 0.003 cm2/s, which gives a Y6 film diffusion length of nm. Thus, we provide an essential tool that enables a direct and free-of-artifacts determination of diffusion coefficients, which we expect to be pivotal for further studies on exciton dynamics in energy materials.

Crystal structures in state-of-the-art non-fullerene electron acceptors

The relationship between molecular design strategies, the packing information of small molecule acceptors is demonstrated. This review highlights the recent advances in the study of single crystal structures of non-fullerene acceptors (NFAs).

Versatile π-bridges in nonfullerene electron acceptors of organic solar cells

The development of nonfullerene electron acceptors (NFEAs) is a landmark advance in the field of organic solar cells (OSCs).

A narrow band gap non-fullerene electron acceptor based on a dithieno-3,2-b:2',3'-dlpyrrole unit for high performance organic solar cells with minimal highest occupied molecular orbital offset.

Here, a new narrow band gap non-fullerene small molecular acceptor (NFSMA) based on a dithieno-3,2-b:2',3'-dlpyrrole(DTP) unit, namely SNIC-F, was designed and synthesized. Due to the strong electron-donating ability of the DTP-based fused-ring core, SNIC-F showed a strong intramolecular-charge transfer (ICT) effect and thus gave a narrow band gap of 1.32 eV. Benefiting from the low band gap and efficient charge separation, when pairing with a copolymer PBTIBDTT, the device optimized by 0.5% 1-CN gave a high short circuit current (Jsc) of 19.64 mA cm-2. In addition, a high open-circuit voltage (Voc) of 0.83 V was obtained due to the near 0 eV highest occupied molecular orbital (HOMO) offset between PBTIBDTT and SNIC-F. As a result, a high power conversion efficiency (PCE) of 11.25% was obtained, and the PCE was maintained above 9.2% as the active layer thickness increased from 100 nm to 250 nm. Our work indicated that designing a narrow band gap NFSMA-based DTP unit and blending it with a polymer donor with small HOMO offset is an efficient strategy for achieving high performance OSCs.

A computational study of thiophene containing small-molecule electron acceptors for non-fullerene organic photovoltaic cells

During the past few years, researchers have devoted extensive efforts to improve organic solar cell (OSC) performance to reach interesting power conversion efficiencies (PCE) exceeding 10 %. Among heterojunctions OSCs (BHJ) types, Fullerene based small molecule acceptors (SMAs) have proved to be a favorable option in virtue of their high-power conversion efficiency (PCE), good electronic conductivity and superior charge segregation. Yet, they represent some serious limitations, such as low light absorption over 600 nm, solubility in organic solvents, and inefficient processing. Accordingly, the so-called non-fullerene acceptors (NFA) organic group was developed and showed excellent characteristics over fullerene acceptors with their easily tunable band gap, strong absorption in the visible region, low voltage loss, good morphological stability and simple fabrication techniques. In the present paper, a series of non-fullerene electron acceptors (C1–C4) were designed by modifying the reference material R. we have obtained new conjugated organic structures by adding more functional capped units. The quantum chemical study (DFT/TD-DFT) approach was used to perform theoretical calculations in order to characterize the effect of end group redistribution via the frontier molecular orbital (FMO), optical absorption, reorganization energy in accordance with R. Using PTB7-Th as an electron donor, open circuit voltage (Voc), photovoltaic properties and intermolecular charge transfer have been also calculated for all the conceived compounds. The findings revealed that all engineered materials (C1–C4) possess narrow band gap and great optical characteristics. In addition the proposed structures have displayed comparatively lower electron and hole reorganization energies, we have found that C1 represents the lowest electron and hole reorganization energies, respectively 0.048 eV and 0.028 eV, consequently the highest electron and hole mobility [1]. These interesting outcomes could prove proposed electron acceptors to be excellent candidates in the improvement of optoelectronic properties of organic solar cell technology.

Versatile nonplanar perylene diimide-based acceptor for inverted organic solar cells and photodetectors with modified ZnO buffer layers

The versatile nonfullerene perylene diimide-based electron acceptor was applied for inverted organic solar cells and organic photodetectors with good device performance. The ZnO films were modified by ethylene glycol (EG), ethanedithiol (EDT) and propanediol (PDO) as electron transporting layers. The energy levels of ZnO buffer layers were tuned to facilitate charge injection between the electrodes and the active layers. The ideal miscibility and interpenetrated network of subsequently deposited active layer was achieved on EG modified ZnO interlayer. Compared to the conventional device, the efficiency of the inverted device using ZnO/EG as electron transporting layer was improved to 3.61%, with a higher Jsc of 10.06 mA/cm2. The dark current of the organic photodetector could be dramatically reduced by the interfacial modification. The ZnO/EG based OPD exhibited low dark current of 3.24 × 10−9A/cm2 at − 0.1 V and external quantum efficiency (EQE) of 48.6% at 700 nm, with a responsivity (R) of 0.274 A/W and a specific detectivity (D*) of 8.52 × 1012Jones.