Non-fullerene Acceptors Research Articles

(Invited) Precise Determination of Exciton Binding Energy of Organic Semiconductors

Strongly bound excitons in organic semiconductors are crucial for the operation of organic optoelectronic devices. However, accurate experimental data on the exciton binding energy of organic semiconductors are lacking. In this study, we determine the exciton binding energy as the difference between the optical and transport bandgaps with a precision of 0.1 eV. From the systematic study of the exciton binding energy of 42 organic semiconductors (15 non-fullerene acceptors, 4 fullerene acceptors, 13 low-bandgap polymers, 7 organic light-emitting diode materials, and 3 crystalline materials), we found that the exciton binding energy is a quarter of the transport gap regardless of the material. We interpret this unexpected relationship in terms of a hydrogen atom model.

The correlation of alkyl chain modulation and the efficiency of Y-series non-fullerene acceptor: A DFT approach

Y6, a highly efficient non-fullerene acceptor, could present distinctly different power conversion efficiency in organic solar cells when its alkyl side chains are replaced by others. However, the underlying mechanism is still unclear. To dissect this point, ten kinds of Y-series acceptors with variant side chains are studied. The density functional theory is used to calculate their ground geometry, frontier molecular orbital, ultraviolet–visible absorption spectra, the free energy of solvation, intramolecular electron transfer, and electronic structures on the excited state in chloroform. Results reveal that the presence of the branched R1 could reduce the dihedral angle and balance the planarity of molecular skeletons. The free energy of solvation is less than −1.5 eV and R2 has a greater impact on it than R1. Acceptors with longer R2 have better light absorption between 400 nm and 600 nm. Molecular dynamics simulation is also applied to the blending films with these acceptors and PM6. Molecules that possess branched R1 and longer R2 have a better morphology and are found to be promising candidates for highly efficient acceptors.

Development of cross-linked donor polymers for polymer solar cells with improved efficiency and thermal stability

Due to low glass-transition temperature and high diffusion coefficient of small molecule acceptors (SMAs), the bi-continuous interpenetrating morphology of SMA-based polymer solar cells (PSCs) are metastable and therefore most SMA-based PSCs exhibit poor operational and thermal stability. Cross-linked donor polymers have been proposed because the cross-linked behavior can “freeze” the initial morphology and avoid its further evolution. In this work, two new cross-linked donor polymers P1 and P2 were designed for application in PSCs with a non-fullerene acceptor Y6, in which the photovoltaic performance and thermal stability of PSCs were significantly improved. The P1-based PSCs achieved a superior power conversion efficiency (PCE) of 17.29 % with a Jsc of 25.99 mA cm−2, a Voc of 0.860 V and a FF of 77.35 %, which surpassed 9 % for PM6-based PSCs (15.86 %). Furthermore, the P1-based PSCs exhibited better thermal stability with a 17 % efficiency loss under continuous 60 °C for 210 h due to the fixation of active layer morphology, while PM6-based PSCs suffered from a serve PCE reduction of over 26 %. These results demonstrate that cross-linked donor polymer has a great application prospect to improve the PCE and thermal stability of PSCs.

Photon energy loss in ternary polymer solar cells based on nonfullerene acceptor as a third component

Photon energy loss in ternary polymer solar cells based on nonfullerene acceptor as a third component

Advancements in non‐fullerene acceptors for organic solar cells: Brief review of research trends

AbstractIn the last few decades, fullerene‐based acceptors have been extensively utilized in organic solar cells (OSCs). However, OSCs based on fullerenes suffer from low photocurrent and photovoltage due to their poor light absorption capacity and limited tunability of energy levels, which impedes efficiency improvements. Recently, non‐fullerene acceptors (NFAs) have enabled rapid progress in OSCs, owing to their favorable properties such as strong light absorption, facile energy level tuning, and improved charge transport characteristics. These features have led to rapid efficiency enhancements. This review discusses the latest research trends related to NFAs, covering several factors that can be applied in the development of NFAs. Finally, we suggest prospects and challenges for high‐performance NFA‐based OSCs on the path to commercialization.

The Double-Cross of Benzotriazole-Based Polymers as Donors and Acceptors in Non-Fullerene Organic Solar Cells.

Organic solar cells (OSCs) are considered a very promising technology to convert solar energy to electricity and a feasible option for the energy market because of the advantages of light weight, flexibility, and roll-to-roll manufacturing. They are mainly characterized by a bulk heterojunction structure where a polymer donor is blended with an electron acceptor. Their performance is highly affected by the design of donor-acceptor conjugated polymers and the choice of suitable acceptor. In particular, benzotriazole, a typical electron-deficient penta-heterocycle, has been combined with various donors to provide wide bandgap donor polymers, which have received a great deal of attention with the development of non-fullerene acceptors (NFAs) because of their suitable matching to provide devices with relevant power conversion efficiency (PCE). Moreover, different benzotriazole-based polymers are gaining more and more interest because they are considered promising acceptors in OSCs. Since the development of a suitable method to choose generally a donor/acceptor material is a challenging issue, this review is meant to be useful especially for organic chemical scientists to understand all the progress achieved with benzotriazole-based polymers used as donors with NFAs and as acceptors with different donors in OSCs, in particular referring to the PCE.

Highly thermo- and photo-stable acceptor-acceptor′-acceptor type perylenediimide dimeric acceptors: Role of monofluorinating the central benzo[c][1,2,5]thiadiazole linking core

Acceptor-Acceptor′-Acceptor (A-A′-A) type perylene diimide (PDI)-based dimeric non-fullerene electron acceptors (NFEAs) have drawn the special attention but have not been sufficiently exploited. With that in mind, two A-A′-A type NFEAs, DTBT-(PDI-HD)2 and DTFBT-(PDI-HD)2, in which 4,7-di(thien-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) and its monofluorinated 4,7-di(thien-2-yl)-5-fluorobenzo[c][1,2,5]thiadiazole (DTFBT) as central linkers, both N,N′-(2-hexyldecyl)-PDI (PDI-HD) units as flanked biwings, were successfully synthesized to probe into the influence of different electron-deficient linkers on thermo- and photo-stability, absorption, energy level, molecular configuration, exciton dissociation, charge mobility and solar cell performance. Both A-A′-A type acceptors with excellent thermal- and photo-stability possessed the complementary absorption ranging from 300 nm to 780 nm and matched ELUMO of −3.84 eV to −3.91 eV when pairing with PTB7-Th. Moreover, increased molecular dipole moment, slightly blue-shifted the maximum absorption peak, decreased the absorption coefficient and deepened the ELUMO were found after monofluorinating the benzothiadiazole. Conversely, the augmented aggregation both in solution and film effectively tuned the miscibility and morphology of active layer, contributing to the improvements of the exciton dissociation, charge mobility and photocurrent. Benefiting from this, monofluorinating the electron-deficient benzothiadiazole linker made the JSC increase from 3.72 to 7.19 mA cm−2, the FF enhance from 34.99 % to 38.18 % in DTFBT-(PDI-HD)2-based device when blending with PTB7-Th, contributing to 1.09 times increased PCE from 1.03 % to 2.15 % even in the adverse condition of 0.01 V dropped VOC. These results suggest that monofluorinating the electron-deficient benzothiadiazole linker is an effective approach for boosting the device efficiency via tuning the molecular aggregation and affecting the related morphology and charge mobility during the A-A′-A type PDI-based NFEAs.

Tuning of Optoelectronic Characteristics of Quinoxalineimide-based Novel Non-Fullerene Acceptors for Organic Photovoltaic Applications

Organic solar cells (OSCs) have made remarkable progress due to developing novel materials and device architectures. Herein, we designed six new non-fullerene acceptors (QM1–QM6) having a quinoxalineimide (QI) as the central-building-block. We aimed to explore the potential of QI-containing structures for developing high-performance acceptors for OSCs. These designed molecules possess a fusion of the QI unit with a thienylthiophene backbone and are equipped with various efficient end-capping groups. Extensive theoretical calculations were carried out to evaluate the structural and charge distribution characteristics, including the optical, optoelectronics, density of states, transition density matrix, molecular electrostatic potential (ESP), reorganization energies, and photovoltaic properties. The results revealed that the designed series exhibited improved electron–hole pair coherences, efficient charge transportation, and favorable ESP patterns, suggesting their suitability for enhanced optical and electronic characteristics. Furthermore, the hole and electron overlap analyses demonstrated significant overlap in QM1–QM6, indicating efficient charge transfer and higher charge mobilities. Additionally, the analysis of reorganization energies showed low reorganization energy values, indicating favorable charge mobility rates and potential for high-efficiency solar cell devices. This research establishes a strong foundation for utilizing QI-containing fused units as central building blocks for designing high-performance acceptors, opening up new avenues for advancements in OSCs technology.

All‐Small‐Molecule Ternary Organic Solar Cell with 16.35% Efficiency Enabled by Chlorinated Terminal Units

In the last few years, there have been notable developments in organic solar cells using both small molecule donor and acceptor. It has been noted that adding halogens to the end groups of small molecules could enhance the film structure and, consequently, the performance of the devices. In this study, three novel small molecule donors are created. The molecules include a vinyl‐CPDT oligomer with three units, with end‐caps made up of indanedione groups and containing four H, four Cl, and four F substituents. The purpose of the study is to investigate how the halogen substituent affects the photovoltaic characteristics of binary devices made with the non‐fullerene acceptor (NFA) TOCR2 as the acceptor. Having the halogen in the device enhances its effectiveness, and FG5, which has 4‐Cl substituents in the end groups, shows the highest efficiency among all devices with a PCE of 14.39%. Incredibly, the ternary device that is created in normal atmospheric conditions with chloro‐substituted FG5 as the donor, TOCR2 as the acceptor, and the wide band gap NFA DICTF as the third element shows significantly improved efficiency, achieving PCE values of up to 16.35%.

Nanometer-Thick ITIC Bulk Heterojunction Films as Non-Fullerene Acceptors in Organic Solar Cells

Nanometer-Thick ITIC Bulk Heterojunction Films as Non-Fullerene Acceptors in Organic Solar Cells

Improving Miscibility of Polymer Donor and Polymer Acceptor by Reducing Chain Entanglement for Realizing 18.64 % Efficiency All Polymer Solar Cells.

All-polymer solar cells have experienced rapid development in recent years by the emergence of polymerized small molecular acceptors (PSMAs). However, the strong chain entanglements of polymer donors (PDs) and polymer acceptors (PAs) decrease the miscibility of the resulting polymer mixtures, making it challenging to optimize the blend morphology. Herein, we designed three PAs, namely PBTPICm-BDD, PBTPICγ-BDD and PBTPICF-BDD, by smartly using a BDD unit as the polymerized unit to copolymerize with different Y-typed non-fullerene small molecular acceptors (NF-SMAs), thus achieving a certain degree of distortion and giving the polymer system enough internal space to reduce the entanglements of the polymer chains. Such effects increase the chances of the PD being interspersed into the acceptor material, which improve the solubility between the PD and PA. The PBTPICγ-BDD and PBTPICF-BDD displayed better miscibility with PBQx-TCl, leading to a well optimized morphology. As a result, high power conversion efficiencies (PCEs) of 17.50 % and 17.17 % were achieved for PBQx-TCl : PBTPICγ-BDD and PBQx-TCl : PBTPICF-BDD devices, respectively. With the addition of PYFT-o as the third component into PBQx-TCl : PBTPICγ-BDD blend to further extend the absorption spectral coverage and finely tune microstructures of the blend morphology, a remarkable PCE of 18.64 % was realized finally.

Exciton Transport in the Nonfullerene Acceptor O-IDTBR from Nonadiabatic Molecular Dynamics.

Theory, computation, and experiment have given strong evidence that charge carriers in organic molecular crystals form partially delocalized quantum objects that diffuse very efficiently via a mechanism termed transient delocalization. It is currently unclear how prevalent this mechanism is for exciton transport. Here we carry out simulation of singlet Frenkel excitons (FE) in a molecular organic semiconductor that belongs to the class of nonfullerene acceptors, O-IDTBR, using the recently introduced FE surface hopping nonadiabatic molecular dynamics method. We find that FE are, on average, localized on a single molecule in the crystal due to sizable reorganization energy and moderate excitonic couplings. Yet, our simulations suggest that the diffusion mechanism is more complex than simple local hopping; in addition to hopping, we observe frequent transient delocalization events where the exciton wave function expands over 10 or more molecules for a short period of time in response to thermal excitations within the excitonic band, followed by de-excitation and contraction onto a single molecule. The transient delocalization events lead to an increase in the diffusion constant by a factor of 3-4, depending on the crystallographic direction as compared to the situation where only local hopping events are considered. Intriguingly, O-IDTBR appears to be a moderately anisotropic 3D "conductor" for excitons but a highly anisotropic 2D conductor for electrons. Taken together with previous simulation results, two trends seem to emerge for molecular organic crystals: excitons tend to be more localized and slower than charge carriers due to higher internal reorganization energy, while exciton transport tends to be more isotropic than charge transport due to the weaker distance dependence of excitonic versus electronic coupling.

Asymmetrical Elongation of Branched Alkyl Chains in Non‐Fullerene Acceptors for Large‐Area Organic Solar Modules

AbstractWith the drive toward the development of large‐area organic solar cells (OSCs), there is a critical need for advanced fabrication techniques that ensure both their efficiency and scalability. In particular, a shift from toxic halogenated solvents to safer non‐halogenated alternatives such as o‐xylene, which have lower environmental and health impacts, is required. However, transitioning to non‐halogenated solvents can lead to serious problems, including aggregation within the active layer, which compromises film morphology and the resulting efficiency of OSCs. To address this aggregation, in the present study, the 2‐ethylhexyl (EH) groups in L8‐BO(EH‐EH) are replaced with longer chains (2‐heptylundecyl [HU], 2‐decyltetradecyl [DT], and 2‐dodecylhexadecyl [DH] groups) to synthesize the non‐fullerene acceptors (NFAs) of L8‐BO(HU‐HU), L8‐BO(HU‐DT), and L8‐BO(HU‐DH). The NFAs with the longer alkyl chains are highly soluble in o‐xylene and produce highly uniform films, making them more suitable for use in large‐area OSCs. Using the NFAs, slot‐die‐coated organic solar modules with an active area of 200 cm2 are fabricated; the L8‐BO(HU‐DT)‐based module exhibits an impressive power conversion efficiency of 11.44%. This work thus underscores the asymmetrical elongation of alkyl chains in the NFAs to mitigate severe NFA phase separation and improve film printability in the practical production of organic solar modules.

Progress in Non-Fullerene Acceptors: Evolution from Small to Giant Molecules.

With the rapid development of non-fullerene acceptors (NFAs), the power conversion efficiency (PCE) of organic solar cells (OSCs) is increasing. According to their different chemical structures, NFAs can initially be divided into two categories: small molecule acceptors (SMAs) and polymerized small molecule acceptors (PSMAs). Due to the strong absorption capacity and controllable energy levels, the PCE of devices based on SMAs has approached 20%. Compared with SMAs, PSMAs have advantages in stability and flexibility, and the PCE of PSMA-based devices has exceeded 18%. However, the higher synthesis cost and lower batch repeatability hinder its further development. Recently, the concept of giant molecule acceptors (GMAs) has been proposed. These materials have a clear molecular structure and are considered novel acceptor materials that combine the advantages of SMAs and PSMAs. Currently, the PCE of devices based on GMAs has exceeded 19%. In this review, we will introduce the latest developments in SMAs, PSMAs, and GMAs. Then, the advantages of GMAs and the relationship between their structure and performance will be analyzed. In the end, perspectives on the opportunities and challenges of these materials are provided, which could inspire further development of NFAs for advanced OSCs.

Inner/Outer Side Chain Engineering of Non-Fullerene Acceptors for Efficient Large-Area Organic Solar Modules Based on Non-Halogenated Solution Processing in Air.

Achieving efficient and large-area organic solar modules via non-halogenated solution processing is vital for the commercialization yet challenging. The primary hurdle is the conservation of the ideal film-formation kinetics and bulk-heterojunction (BHJ) morphology of large-area organic solar cells (OSCs). A cutting-edge non-fullerene acceptor (NFA), Y6, shows efficient power conversion efficiencies (PCEs) when processed with toxic halogenated solvents, but exhibits poor solubility in non-halogenated solvents, resulting in suboptimal morphology. Therefore, in this study, the impact of modifying the inner and outer side-chains of Y6 on OSC performance is investigated. The study reveals that blending a polymer donor, PM6, with one of the modified NFAs, namely N-HD, achieved an impressive PCE of 18.3% on a small-area OSC. This modified NFA displays improved solubility in o-xylene at room temperature, which facilitated the formation of a favorable BHJ morphology. A large-area (55 cm2) sub-module delivered an impressive PCE of 12.2% based on N-HD using o-xylene under ambient conditions. These findings underscore the significant impact of the modified Y6 derivatives on structural arrangements and film processing over a large-area module at room temperature. Consequently, these results are poised to deepen the comprehension of the scaling challenges encountered in OSCs and may contribute to their commercialization.

Air processed, high open‐circuit voltage indoor organic photovoltaic cells based on side chain modified N‐annulated perylene diimides

AbstractTo achieve high‐performance indoor organic photovoltaics (OPVs), it is important to match the photoactive layer optical absorption with the light‐source emission. This can be accomplished by developing organic photoactive materials that can efficiently absorb visible light and thus minimize energy losses. While indoor OPVs have achieved efficiencies above 33% under low light intensities, the power output is limited by low open circuit voltages (VOC), often well below 1 V. In this study, we present a series of visible‐light absorbing (energy gap >1.90 eV) non‐fullerene acceptors (NFAs) based on perylene diimide dimers, which have been systematically modified with side chains of varying polarity and steric bulk (trimethyl benzyl, ethyl adamantane, trialkoxyl phenyl, and oligo ethylene glycol). Our results show that the incorporation of sterically bulky side chains such as ethyl adamantane and trimethyl benzyl, blended with the common widegap polymer PTQ10, provides photoactive layers with absorption greater than 2.0 eV, and consequently, VOCs higher than 1.2 V are achieved under AM 1.5 G illumination. Importantly, we found that the NFA with ethyl adamantane based side chains (tPDI2N‐ethyl adamantane, compound 4) exhibited the best performance, with minimized energy loss. As a result, devices using PTQ10:tPDI2N‐ethyl adamantane photoactive layers demonstrated excellent indoor efficiencies of over 16% and 18 μW cm−2 power output under a 2700 K LED lamp at 300 lux, and showed better repeatability compared to other systems. The PTQ10:tPDI2N‐ethyl adamantane based devices maintained a high VOC (>1.0 V) across a wide range of indoor lighting conditions, including 2700 K and 6500 K LED lamps. Overall, this work provides a sidechain engineering method to create NFAs for efficient indoor OPV devices.

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.

Chlorination of benzyl group on the terminal unit of A2-A1-D-A1-A2 type nonfullerene acceptor for high-voltage organic solar cells

Benzotriazole (BTA)-based A2-A1-D-A1-A2 type wide-bandgap (WBG) non-fullerene acceptors (NFAs) have shown promising potential in indoor photovoltaic, and in-depth investigation of their structure-property relationship is of great significance. Herein, we explored the chlorination effect of the side chain on the terminals. We introduced Cl atoms into the benzyl side chains in parent BTA5 to synthesize two NFAs, BTA5-Cl with mono-chlorinated benzyl groups and BTA5-2Cl containing bi-chlorinated benzyl groups. We chose D18-Cl with deep-energy levels and strong crystallinity to pair with these three acceptors, affording high photovoltage and photocurrent. With the stepwise chlorination, the open-circuit voltage (VOC) values decrease from 1.28, 1.22, to 1.20 V, while the corresponding power conversion efficiencies (PCEs) improve from 5.07 %, 9.15 %, to 10.96 %. Compared with BTA5-based OSCs, introducing Cl atoms downshifts the energy levels and slightly increases the non-radiative energy loss (0.14 < 0.17 < 0.19 eV), resulting in a sequential decrease in VOC. However, more chlorine atom replacements produce more effective exciton dissociation, higher charge transfer, and balanced carrier mobility in the blend films, ultimately achieving better PCEs. This work indicates that chlorination of the benzyl group on the terminals can improve the device's performance, implying good application potential in indoor photovoltaics.

Charge Generation Dynamics in Organic Photovoltaic Blends under One-Sun-Equivalent Illumination Detected by Highly Sensitive Terahertz Spectroscopy.

Organic photovoltaic (OPV) devices attain high performance with nonfullerene acceptors by utilizing the synergistic dual channels of charge generation that originate from excitations in both the donor and acceptor materials. However, the specific intermediate states that facilitate both channels are subject to debate. To address this issue, we employ time-resolved terahertz spectroscopy with improved sensitivity (ΔE/E < 10-6), enabling direct probing of charge generation dynamics in a prototypical PM6:Y6 bulk heterojunction system under one-sun-equivalent excitation density. Charge generation arising from donor excitations is characterized with a rise time of ∼9 ps, while that from acceptor excitations shows a rise time of ∼18 ps. Temperature-dependent measurements further reveal notably distinct activation energies for these two charge generation pathways. Additionally, the two channels of charge generation can be substantially manipulated by altering the ratio of bulk to interfaces. These findings strongly suggest the presence of two distinct intermediate states: interfacial and intramoiety excitations. These states are crucial in mediating the transfer of electrons and holes, driving charge generation within OPV devices.

Enhanced Fill Factor and Efficiency of Ternary Organic Solar Cells by a New Asymmetric Non-Fullerene Small Molecule Acceptor.

Asymmetric non-fullerene small molecules acceptor (as-NF-SMAs) exhibit greater vitality in photovoltaic materials compared to their symmetric counterparts due to their larger dipole moments and stronger intermolecular interactions, which facilitate exciton dissociation and charge transmission in organic solar cells (OSCs). Here, we introduced a new as-NF-SMAs, named IDT-TNIC, as the third component in ternary organic solar cells (TOSCs). The asymmetric IDT-TNIC used indacenodithiophene (IDT) as the central core, alkylthio-thiophene as a unilateral π-bridge and extended end groups as electron-withdrawing. Due to the non-covalent conformational lock (NCL) established between O⋅⋅⋅S and S⋅⋅⋅S, the IDT-TNIC molecule preserves its coplanar structure effectively. Furthermore, IDT-TNIC exhibits complementary absorption and excellent compatibility with donor and acceptor materials, as well as optimized ladder energy level arrangement, resulting in a higher and more balanced μh/μe value, more homogeneous and suitable phase separation morphology in TOSCs. Thus, the PCE of the TOSCs reached 17 % when the weight ratio of PM6 : Y6 : IDT-TNIC was 1 : 1.1 : 0.1, and it is noteworthy that when the device area was increased to 1 cm2, the PCE could still be maintained at over 14 %. Detailed studies and analysis indicate that IDT-TNIC has great potential as a third component in OSCs and for large-scale printing in the future.