Fused-ring Electron Acceptors Research Articles

Designing A-D-A Type Fused-Ring Electron Acceptors with a Bulky 3D Substituent at the Central Donor Core to Minimize Non-Radiative Losses and Enhance Organic Solar Cell Efficiency.

Designing and synthesizing narrow bandgap acceptors that exhibit high photoluminescence quantum yield (PLQY) and strong crystallinity is a highly effective, yet challenging, approach to reducing non-radiative energy losses (∆Enr) and boosting the performance of organic solar cells (OSCs). We have successfully designed and synthesized an A-D-A type fused-ring electron acceptor, named DM-F, which features a planar molecular backbone adorned with bulky three-dimensional camphane side groups at its central core. These bulky substituents effectively hinder the formation of H-aggregates of the acceptors, promoting the formation of more J-aggregates and notably elevating the PLQY of the acceptor in the film. As anticipated, DM-F showcases pronounced near-infrared absorption coupled with impressive crystallinity. Organic solar cells (OSCs) leveraging DM-F exhibit a high EQEEL value and remarkably low ∆Enr of 0.137 eV-currently the most minimal reported value for OSCs. Moreover, the power conversion efficiency (PCE) of binary and ternary OSCs utilizing DM-F has reached 16.16% and 20.09%, respectively, marking a new apex in reported efficiency within the OSCs field. In conclusion, our study reveals that designing narrow bandgap acceptors with high PLQY is an effective way to reduce ∆Enr and improve the PCE of OSCs.

Design of non-fused ring electron acceptors with “embracing-type” side chains for organic solar cells

Backbone planarity is an inherent structural feature for fused ring electron acceptors and one of important prerequisites for their high photovoltaic performance. However, for non-fused ring electron acceptors (NFREAs), it is a challenging task to achieve it. In this work, two NFREAs (Ph2TIC and BT2TIC) containing thiophene-benzene-thiophene or thiophene-benzothiadiazole-thiophene three non-fused aromatic rings in central scaffold were designed with an attempt to use a pair of 2,4,6-triisopropylphenyl substituents (referred as “embracing”-type side chains) at 3,3′-positions of the two thiophene units to lock the backbone in a planar conformation. X-ray crystallography revealed that Ph2TIC has a quasi-planar backbone, whereas BT2TIC shows a twisted one, but both tend to form two-dimensional layered arrays through intermolecular π-π stacking involving terminal acceptor moieties. UV–vis light-absorption spectroscopy showed that both compounds have similar optoelectronic properties either in dilute solutions or in solid film state, thus confirming the conformation-lock function of the “embracing”-type side chains. While for photophysical properties, BT2TIC displays a red-shifted broader light absorption spectrum as compared to Ph2TIC due to the introduction of an additional acceptor moiety in the structure. Ph2TIC and BT2TIC were evaluated as the acceptor materials in organic solar cells in combination with PM6, a typical donor polymer, and delivered the optimized power conversion efficiencies of 4.31 % and 3.78 %, respectively. Although the photovoltaic performances are modest, the work provides a new thought for maintaining the backbone planarity in NFREAs and would be useful in their future design.

Chalcogen effect on the photovoltaic performance of nonfused-ring small molecular electron acceptors for efficient organic solar cells

Fused-ring electron acceptors (FREAs) are the current working horse for the top performing organic solar cells (OSCs). Nevertheless, these FREAs surfer from high synthetic complexity, production costs and poor scalability, hindering their industrialization. Developing nonfused-ring electron acceptors (NFREAs) is a more feasible alternative solution towards future photovoltaic applications. In this work, a series of NFREAs have been designed and synthesized by introducing chalcogen atoms on the side chains for OSCs. The introduced chalcogen atoms (O, S, and Se) not only modulated the energy levels but also finely tuned the intermolecular interactions. Especially, the formed S‧‧‧S and Se‧‧‧Se intermolecular interactions in TTS-4F and TTSe-4F resulted in higher molecular crystallinity than TTO-4F. Unexpectedly, the strong Se‧‧‧Se interactions also led the aggregation of TTSe-4F and formation of large domains in the PM6:TTSe-4F blend. The moderate S‧‧‧S interactions in TTS-4F enabled an optimal phase separation with more ideal nano fibrils distributed in the PM6:TTS-4F blend, facilitating the charge separation and transport. As a result, TTS-4F based devices achieved a champion power conversion efficiency (PCE) of 14.74%, higher than the TTO-4F (8.76%) and TTSe-4F (11.56%) based devices.

Low-Cost Nonfused-Ring Electron Acceptors Enabled by Noncovalent Conformational Locks.

ConspectusSince the first bilayer-structured organic solar cells (OSCs) in 1986, fullerenes and their derivatives have dominated the landscape for two decades due to their unique properties. In recent years, the breakthrough in nonfullerene acceptors (NFAs) was mainly attributed to the development of fused-ring electron acceptors (FREAs), whose photovoltaic performance surpassed that of fullerene derivatives. Through the unremitting efforts of the whole community, the power conversion efficiencies (PCEs) have surpassed 19% in FREA-based OSCs. However, FREAs generally suffered from complex synthetic approaches and high product costs, which hindered large-scale production. Therefore, many researchers are seeking a new type of NFA to achieve cost-effective, highly efficient OSCs.In collaboration with Marks and Facchetti in 2012, Huang et al. (Huang, H. J. Am. Chem. Soc. 2012, 134, 10966-10973, 10.1021/ja303401s) proposed the concept of "noncovalent conformational locks" (NoCLs). In the following years, our group has been focusing on the theoretical and experimental exploration of NoCLs, revealing their fundamental nature, formulating a simple descriptor for quantifying their strength, and employing this approach to achieve high-performance organic/polymeric semiconductors for optoelectronics, such as OSCs, thin-film transistors, room-temperature phosphorescence, and photodetectors. The NoCLs strategy has been proven to be a simple and effective approach for enhancing molecular rigidity and planarity, thus improving the charge transport mobilities of organic/polymeric semiconductors, attributed to reduced reorganization energy and suppressed nonradiative decay.In 2018, Chen et al. (Li, S. Adv. Mater. 2018, 30, 1705208, 10.1002/adma.201705208) reported the first example of nonfused-ring electron acceptors (NFREAs) with intramolecular noncovalent F···H interactions. The NoCLs strategy is essential in NFREAs, as it simplifies the conjugated structures while maintaining high coplanarity comparable to that of FREAs. Due to their simple structures and concise synthesis routes, NFREAs show great potential for achieving cost-effective and highly efficient OSCs. In this Account, we provide an overview of our efforts in developing NFREAs with the NoCLs strategy. We begin with a discussion on the distinct features of NFREAs compared with FREAs, and the structural simplification from FREAs to NFREAs to completely NFREAs. Next, we examine several selected typical examples of NFREAs with remarkable photovoltaic performance, aiming to provide an in-depth exploration of the molecular design principle and structure-property-performance relationships. Then, we discuss how to achieve a balance among efficiency, stability, and cost through a two-in-one strategy of polymerized NFREAs (PNFREAs). Finally, we offer our views on the current challenges and future prospects of NFREAs. We hope this Account will trigger intensive research interest in this field, thus propelling OSCs into a new stage.

Tailoring the Electron‐Deficient Central Core on Fused‐Ring Nonfullerene Acceptors: Deciphering the Relationships Between Structure, Property, and Photovoltaic Performance

In the field of organic solar cells, organic semiconductors with Y6‐based chemical structure and their corresponding quinoxaline‐based assemblies are settled down as promising materials in the field of OSCs. However, the chemical structure of Y6 derivatives does not allow an expansion in the electron‐withdrawing central core, in contrast to their quinoxaline‐based analogues. For this reason, herein, two different quinoxaline‐based A–D–A′–D–A derivatives with 2D π‐extended cores endowed with electroactive rylenimide moieties, named as Y6‐1Napht and Y6‐1Pery, are designed and synthesized. These enlarged fused‐ring electron acceptors (FREAs) allow to study the influence of the length of the ryleneimide moiety on its structural, optical, electrochemical, and thermal properties as well as the tailoring of the frontier molecular orbitals. These results are also supported by quantum chemical calculations and photoluminescence measurements that provide additional information about their thermal stability and the microstructure of the films. Finally, as a proof of concept and to study the influence of these chemical modifications on the central core, solar cells based on these π‐conjugated nonfullerene acceptors are fabricated.

A Fused-Ring Electron Acceptor with Phthalimide-Based Ending Groups for Efficient Ternary Organic Solar Cells.

The ternary strategy has been widely applied and recognized to be a valid strategy to enhance the organic photovoltaics' (OPVs) performance. Here, a new fused-ring electron acceptor, BTP-PIO, is designed and synthesized, whose ending groups were replaced by a phthalimide-based group (2-butylcyclopenta[f]isoindole-1,3,5,7(2H,6H)-tetraone) from traditional 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile. The phthalimide-based ending groups endow BTP-PIO with the highest lowest unoccupied molecular orbital (LUMO) level and wider band gap than those of Y6. The ternary device based on PM6:Y6 with BTP-PIO as a guest electron acceptor achieved an elevated open-circuit voltage (VOC) of 0.848 V, a short-circuit current density (JSC) of 27.31 mA cm-2, and a fill factor (FF) of 73.9%, generating a remarkable power conversion efficiency (PCE) of 17.10%, which is superior to the PM6:Y6 binary device of 16.08%. The ternary device exhibited improved charge transfer, suppressed carrier recombination, and lower energy loss. BTP-PIO exhibited a good miscibility with Y6, and an alloy phase between BTP-PIO and Y6 was formed in the ternary bulk heterojunction, leading to better phase separation and molecular packing. This research reveals that ending group modification of Y6 derivatives is a feasible way to produce highly efficient ternary devices.

Impact of length of branched alkyl side chains on thiazolothiazole-based small molecular acceptors in non-fullerene polymer solar cells.

It has been reported that the length of branched alkyl side chains on fused-ring electron acceptors confers different impacts on properties versus solubility of BJH blends. However, because this impact on a non-fused acceptor backbone has rarely been studied, we examined the impact of molecular optimization from alkyl chain tuning based on non-fused thiazolothiazole small-molecule acceptors. The length of the side chain on the thiophene bridge was modified from 2-butyloctyl to 2-ethylhexyl, which corresponds to small molecules TTz3(C4C6) and TTz3(C2C4), respectively. Compared with the reported TTz3(C6C8) with long alkyl side chains, TTz3(C4C6) and TTz3(C2C4) exhibited stronger molecular aggregation, higher absorption coefficients, and greater redshifted UV absorption. Unexpectedly, after the alkyl chain was slightly shortened in this type of acceptor system, devices were successfully fabricated, but it was necessary to reduce the blending concentration at low rotation speeds due to the sharp decrease in the solubility of corresponding acceptor materials. Thus, the obtained unfavorable thickness and morphology of the active layer caused a decrease in Jsc and FF. As a consequence, TTz3(C4C6)- and TTz3(C2C4)-based devices showed an unsatisfactory power conversion efficiency of 6.02% and 2.71%, respectively, when donors were paired with the wide bandgap donor J71, which is inferior to that of TTz3(C6C8)-based devices (8.76%). These results indicate that it is challenging to determine the limit of the adjustable range of side chains to modify non-fused thiazolothiazole small-molecule acceptors for high-performance non-fullerene solar cells.

Quantum modelling of multi-directional fused-ring electron acceptors for organic photovoltaics

Multi-directional fused-ring electron acceptors (FREAs) feature a fused-ring backbone that extends in multiple directions through bridging and acceptor units. This unique structural design imparts several advantageous properties, like increased light absorption, precise energy level adjustments, improved electron mobility, reduced energy loss, and enhanced efficiency in organic photovoltaic (OPV) devices. In this study, we designed seven novel multi-directional FREAs and conducted a comprehensive computational analysis using Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT). The designed molecules feature a benzotrithienopyrrole central core connected to a π-bridge acceptor, either benzothiadiazole or benzotriazole, and three varying terminal acceptors, including malononitrile, 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile, and 2-(3-methyl-4-oxothiazolidin-2-ylidene)malononitrile. Our investigation encompassed the evaluation of geometry, frontier molecular orbitals (FMOs), optical properties, density of states (DOS), charge mobilities, dipole moment, global reactivity descriptors, molecular electrostatic potential (MEP), transition density matrix (TDM), and OPV device-related parameters. The results revealed significant potential of the designed molecules as FREAs for OPVs, thus highlighting their suitability for further experimental exploration.

Low bandgap fused-ring electron acceptors based on π-expended asymmetric benzotrithiophene core: Regulating aggregation and photovoltaic performance by side chain isomerization

Asymmetric molecular design strategy has received increasing attention in the development of fused-ring electron acceptors (FREAs). Benzo [2,1-b: 3,4-b’: 5,6-b”] trithiophene (asym-BTT) with intrinsically asymmetric and electron-rich π-plane is an ideal unit to construct asymmetric FREAs, whereas asym-BTT-based FREAs are still very limited. Herein, two asymmetric FREAs based on the π-expended asym-BTT fused-core with para-hexylphenyl and meta-hexylphenyl side chains (p-FABTT and m-FABTT) have been developed. Owing to strong electron-donating ability and planar configuration of π-expended asym-BTT fused-core, two acceptors exhibit effective intramolecular charge transfer and intermolecular interaction, leading to near infrared absorption above 900 nm with narrow bandgaps of around 1.34 eV. Although the same π-conjugated backbones of two acceptors afford the nearly identical energy levels and solution absorptions, side chain isomerization leads to significant differences in film absorption, molecular stacking orientation and crystallinity. In neat film, the m-FABTT with meta-hexylphenyl side chains exhibits slightly redshifted absorption with stronger 0-0 absorption peak from J-aggregation compared to p-FABTT with para-hexylphenyl side chains. More orderly molecular stacking and stronger π-π interactions with shorter π-π stacking distance in both the neat film and active layer can be observed for m-FABTT. When blended with donor polymer D18, m-FABTT exhibits broadened spectral region and proper phase separation morphology for effective exciton dissociation and charge transport. Therefore, the OSCs device based on m-FABTT achieves an optimal PCE of 7.14 % with a higher JSC of 15.39 mA cm−2 and an improved FF of 57.96 % as comparing with p-FABTT-based device (a PCE of 4.81 % with a JSC of 11.67 mA cm−2 and a FF of 48.51 %).

A convenient strategy to develop sp2 hybridized fused Y123 monomers for photovoltaic and optical ON-OFF switches with economic energy offsets

This paper presents a design strategy, their efficient visible light converting optimizations offsets and dye anchorage utility as N-doped fused ring electron acceptors of organic dye Y123 in photovoltaic and optical devices. Five novel dyes (Y1–Y5) are investigated, identifying major electronic contributions. Results shows favourable energy levels and oscillator strengths (f) for efficient charge transfer upon photoexcitation. The dye Y1 had an ionization potential (IP) of 3.00eV, electron affinity (EA) of 1.01eV, and dipole moment of 2.01D. The dye Y3 exhibited a high global hardness (η) of 0.85 and low softness (σ) of 0.12eV, indicating stability and strong electron-accepting ability. Evaluation of photovoltaic performance revealed impressive characteristics, such as Y2 with a light harvesting efficiency (LHE) of 98 %, open-circuit voltage of 1.74eV, fill factor of 0.9235, and short-circuit current (Jsc) density of 50.92 mA/cm2, resulting in a maximum power output (Pmax) of 81.82W. This study also included a detailed analysis of transition density matrix (TDM) and electron-hole overlap, providing insights into exciton dissociation and charge separation mechanisms in Y123 dyes. These analyses enhance the design of efficient N-doped fused ring electron acceptors for organic photovoltaic and optical devices. The study demonstrates the potential of N-doped fused ring electron acceptors as promising materials for efficient energy conversion in next-generation organic photovoltaic and optical devices.

Fused-Ring Electron-Acceptor Single Crystals with Chiral 2D Supramolecular Organization for Anisotropic Chiral Optoelectronic Devices.

Supramolecular chiral organization gives π-conjugated molecules access to fascinating specific interactions with circularly polarized light (CPL). Such a feature enables the fabrication of high-performance chiral organic electronic devices that detect or emit CPL directly. Herein, it is shown that chiral fused-ring electron-acceptor BTP-4F single-crystal-based phototransistors demonstrate distinguished CPL discrimination capability with current dissymmetry factor exceeding 1.4, one of the highest values among state-of-the-art direct CPL detectors. Theoretical calculations prove that the chirality at the supramolecular level in these enantiomeric single crystals originates from chiral exciton coupling of a unique quasi-2Dsupramolecular organization consisting of interlaced molecules with opposite helical conformation. Impressively, such supramolecular organization produces a higher dissymmetry factor along the preferred growth direction of the chiral single crystals in comparison to that of the short axis direction. Furthermore, the amplified, inverted, and also anisotropic current dissymmetry compared to optical dissymmetry is studied by finite element simulations. Therefore, a unique chiral supramolecular organization that is responsible for the excellent chiroptical response and anisotropic electronic properties is developed, which not only enables the construction of high-performance CPL detection devices but also allows a better understanding of the structure-property relationships in chiral organic optoelectronics.

Oligomerized Fused-Ring Electron Acceptors for Efficient and Stable Organic Solar Cells.

Organic solar cells (OSCs) have attracted wide research attention in the past decades. Very recently, oligomerized fused-ring electron acceptors (OFREAs) have emerged as a promising alternative to small-molecular/polymeric acceptor-based OSCs due to their unique advantages such as well-defined structures, batch reproducibility, good film formation, low diffusion coefficient, and excellent stability. So far, rapid advances have been made in the development of OFREAs consisting of directly/rigidly/flexibly linked oligomers and fused ones. In this minireview, we systematically summarized the recent research progress of OFREAs, including structural diversity, synthesis approach,molecular conformation and packing, and long-term stability. Finally, we conclude with future perspectives on the challenges to be addressed and potential research directions. We believe that this review will encourage the development of novel OFREAs for OSC applications.

Configuration-dependent photovoltaic properties of diketopyrrolopyrrole (DPP) based Ir complexes for organic solar cells

Organometallic complexes with diverse spatial configurations have been rarely investigated in organic solar cells (OSCs). In this work, two octahedral cyclometalated Ir(III) complexes of bis-heteroleptic DPPTBzIr and tris-homoleptic DPPTBz3Ir are designed from a highly planar diketopyrrolopyrrole (DPP)-containing ligand DPPTBz. Upon coordination, gradually increased optical absorption was observed with the maximum molar extinction coefficient (ε) of 3.80×104, 5.19×104 and 1.39×105 M-1 cm-1 for DPPTBz, DPPTBzIr and DPPTBz3Ir, respectively. Unexpectedly, owing to the large π-conjugation and strong intraligand donor-acceptor interactions from the cyclometalated DPPTBz ligand, no obvious triplet character is determined for the DPP-containing Ir complexes from both experimental and theoretical methods. When employed as electron donor materials for OSCs by blending with fused-ring electron acceptor N3, substantially improved power conversion efficiency (PCE) with average values from 0.74%, 2.2% to 3.41% are achieved for DPPTBz, DPPTBzIr and DPPTBz3Ir, respectively. The best photovoltaic performance for DPPTBz3Ir is originated from strengthened light harvest, suitable energy levels, manipulated blend morphologies and favorable carrier dynamics in the corresponding devices.

Cyclization of Inner Linear Alkyl Chains in Fused‐Ring Electron Acceptors Toward Efficient Organic Solar Cells

By changing the structures of the side chain on the backbone of the Y6 acceptor, many Y‐series acceptors have been synthesized to investigate the relationships between structures and properties of the fused‐ring electron acceptors (FREAs). However, not much attention has been paid to the effects of cycloalkane substituents on optoelectronic, morphological, and photovoltaic properties of the FREAs. Here, a brand‐new FREA D12 is developed by incorporating cyclohexylmethyl functional groups into the pyrrole rings of the backbone of the Y6 acceptor, and a similar compound E12 with the linear n‐heptane side chains is also synthesized for comparison. After the cyclization of the alkyl side‐chain, the molecular orientation of D12 exhibits favorable face‐on dominant, which endows D12 with higher charge mobility. Consequently, the organic solar cells (OSCs) with PM6 as donor and D12 as acceptor deliver the maximum power conversion efficiency of 17.3%, along with an excellent fill factor of 0.794, which is much higher than those of the PM6:E12‐based OSCs. The results reveal that introducing an inner cyclized alkyl chain on the pyrrole motif is a simple and feasible strategy to regulate molecular orientation and improve photovoltaic properties, which might provide guidelines for developing high‐performance electron acceptors.

Enhancing performance of tin-based perovskite solar cells via fused-ring electron acceptor

The performance of tin-based perovskite solar cells has been substantially hampered by voltage loss caused by energy level mismatch, charge recombination, energetic disorder, and other issues. Here, a fused-ring electron acceptor based on indacenodithiophene (IDIC) was for the first time introduced as a transition layer between a tin-based perovskite layer and a C60 electron transport layer, leading to better matched energy levels in the device. In addition, coordination interactions between IDIC and perovskite improved the latter's crystallinity. The introduction of IDIC raised the power conversion efficiency from 8.98% to 11.5% and improved the device's stability. The decomposition mechanism of tin-based perovskite was also revealed by detecting the optical properties of perovskite microdomains through innovative integration of confocal laser scanning microscopy and photoluminescence spectroscopy.

Non-fused Polymerized Small-Molecule Acceptors with a Benzothiadiazole Core for All-Polymer Solar Cells.

Polymerized small-molecule acceptors (PSMAs) have made significant progress as the application in all-polymer solar cells (all-PSCs). Most PSMAs are constructed by near-infrared fused-ring electron acceptors via Stille polymerization, such as Y-series acceptors. However, very limited non-fused electron acceptors with simplified synthetic complexity have been used in PSMAs. In this work, two non-fused PSMAs with a benzothiadiazole (BT) core are developed for application in all-PSCs. The S-O non-covalent interaction and the regioregularity have been introduced to improve the crystallinity and charge transport properties. As a result, a high-power conversion efficiency (PCE) of 11.42% with an enhanced photocurrent has been obtained in these regioregular PBTO-γ-based solar cells, representing the highest PCEs based on non-fused PSMAs. The encouraging results will intrigue more design of non-fused PSMAs toward high-performance all-PSCs.

The Multiplicity of π-π Interactions of Fused-Ring Electron Acceptor Polymorphs on the Exciton Migration and Charge Transport.

Efficient long-range exciton migration and charge transport are the key parameters for organic photovoltaic materials, which strongly depend on the molecular stacking modes. Herein, we extracted the stacked structures of the archetype fused-ring electron acceptor molecule, ITIC, based on the information on four polymorphic crystals and investigated the relationship between molecular stacking modes and exciton migration/charge transport properties through the intermolecular Coulomb coupling and charge transfer integral calculation. Experimentally, the thin film texture is crystallized through a post-annealing treatment through grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, which lead to the enhanced exciton migration through exciton-exciton annihilation in the femtosecond transient absorption (fs-TA) measurements. This work demonstrates the relationship between the molecular arrangement and the exciton migration and electron transport and highlights the significance of optimizing molecular stacking for the development of high-performance electron acceptor materials.

Structural Fusion Yields Guest Acceptors that Enable Ternary Organic Solar Cells with 18.77 % Efficiency.

The design and selection of a suitable guest acceptor are particularly important for improving the photovoltaic performance of ternary organic solar cells (OSCs). Herein, we designed and successfully synthesized two asymmetric silicon-oxygen bridged guest acceptors, which featured distinct blue-shifted absorption, upshifted lowest unoccupied molecular orbital energy levels, and larger dipole moments than symmetric silicon-oxygen-bridged acceptor. Ternary devices with the incorporation of 14.2 wt % these two asymmetric guest acceptors exhibited excellent performance with power conversion efficiencies (PCEs) of 18.22 % and 18.77 %, respectively. Our success in precise control of material properties via structural fusion of five-membered carbon linkages and six-membered silicon-oxygen connection at the central electron-donating core unit of fused-ring electron acceptors can attract considerable attention and bring new vigor and vitality for developing new materials toward more efficient OSCs.

Monomer morphology selection rules for an accurate design of bulk heterojunction: An updated theoretical account

Theoretical simulation of film morphology with bulk heterojunction (BHJ) has become a technological breakthrough point for material and performance development in recent years. Stemming from the precision of calculation and the boom of materials, the unified process for predicting the relevant properties of BHJ through monomers needs to be further updated. This study analyzed the sensibilities of environmental changes (monomers, the acceptors of D/A and A/A in morphology) on structures and properties from fullerene to non-fullerene materials. The results showed that it is inaccurate to predict morphology only by optimizing the monomer through employing a unified process. Due to material specificity, excessive environmental sensitivity could lead to prediction errors. In different environments, the properties of C60 with rigid structure vary greatly. The stability of fused ring electron acceptors (FREAs) is determined by the type and number of bridge bonds (single and ethylenic bonds) in main backbones, where a small number and strong conjugation are favorable. As a result, flexible ITIC is insensitive to the environment, which could be performed using a unified process. This may be the main reason for their excellent properties. This work puts forward higher requirements for the accuracy of a theoretical calculation process, and also provides an updated theoretical account for an accurate design of bulk heterojunction.

Effect of Steric Hindrance at the Anthracene Core on the Photovoltaic Performance of Simple Nonfused Ring Electron Acceptors

Solving the contradiction between good solubility and dense packing is a challenge in designing high-performance nonfullerene acceptors. Herein, two simple nonfused ring electron acceptors (o-AT-2Cl and m-AT-2Cl) carrying ortho- or meta-substituted hexyloxy side chains can be facilely synthesized in only three steps. The two ortho-substituted phenyl side chains in o-AT-2Cl cannot freely rotate due to a big steric hindrance, which endows the acceptor with good solubility. Moreover, o-AT-2Cl displays a more ordered packing than m-AT-2Cl as revealed by the absorption measurement. When blended with polymer donor D18 for the fabrication of organic solar cells (OSCs), o-AT-2Cl-based devices exhibit a favorable morphology, more efficient exciton dissociation, and better charge transport. Consequently, the optimal OSCs based on D18:o-AT-2Cl exhibit a power conversion efficiency (PCE) of 12.8%, which is significantly higher than the moderate PCE (7.66%) for D18:m-AT-2Cl-based devices. Remarkably, o-AT-2Cl shows a higher figure-of-merit value compared with classic high-efficiency fused ring electron acceptors. As a result, our research succeeds in obtaining nonfused ring acceptors with cost-effective photovoltaic performance and provides a valuable experience for simultaneously improving solubility as well as ensuring ordered packing of acceptors through regulating the steric hindrance via changing the position of substituents.