Efficient Nonfullerene Acceptors Research Articles

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.

Enhancing the Chemical Stability and Photovoltaic Properties of Highly Efficient Nonfullerene Acceptors by Chalcogen Substitution: Insights from Quantum Chemical Calculations

Enhancing the Chemical Stability and Photovoltaic Properties of Highly Efficient Nonfullerene Acceptors by Chalcogen Substitution: Insights from Quantum Chemical Calculations

Stabilizing Non-Fullerene Organic Photodiodes through Interface Engineering Enabled by a Tin Ion-Chelated Polymer.

The recent emergence of non-fullerene acceptors (NFAs) has energized the field of organic photodiodes (OPDs) and made major breakthroughs in their critical photoelectric characteristics. Yet, stabilizing inverted NF-OPDs remains challenging because of the intrinsic degradation induced by improper interfaces. Herein, a tin ion-chelated polyethyleneimine ethoxylated (denoted as PEIE-Sn) is proposed as a generic cathode interfacial layer (CIL) of NF-OPDs. The chelation between tin ions and nitrogen/oxygen atoms in PEIE-Sn contributes to the interface compatibility with efficient NFAs. The PEIE-Sn can effectively endow the devices with optimized cascade alignment and reduced interface defects. Consequently, the PEIE-Sn-OPD exhibits properties of anti-environmental interference, suppressed dark current, and accelerated interfacial electron extraction and transmission. As a result, the unencapsulated PEIE-Sn-OPD delivers high specific detection and fast response speed and shows only slight attenuation in photoelectric performance after exposure to air, light, and heat. Its superior performance outperforms the incumbent typical counterparts (ZnO, SnO2 , and PEIE as the CILs) from metrics of both stability and photoelectric characteristics. This finding suggests a promising strategy for stabilizing NF-OPDs by designing appropriate interface layers.

Eco-Friendly Solvent-Processed Dithienosilicon-Bridged Carbazole-Based Small-Molecule Acceptors Achieved over 25.7% PCE in Ternary Devices under Indoor Conditions.

Terminal acceptor atoms and side-chain functionalization play a vital role in the construction of efficient nonfullerene small-molecule acceptors (NF-SMAs) for AM1.5G/indoor organic photovoltaic (OPV) applications. In this work, we report three dithienosilicon-bridged carbazole-based (DTSiC) ladder-type (A-DD'D-A) NF-SMAs for AM1.5G/indoor OPVs. First, we synthesize DTSiC-4F and DTSiC-2M, which are composed of a fused DTSiC-based central core with difluorinated 1,1-dicyanomethylene-3-indanone (2F-IC) and methylated IC (M-IC) end groups, respectively. Then, alkoxy chains are introduced in the fused carbazole backbone of DTSiC-4F to form DTSiCODe-4F. From solution to film absorption, DTSiC-4F exhibits a bathochromic shift with strong π-π interactions, which improves the short-circuit current density (Jsc) and the fill factor (FF). On the other hand, DTSiC-2M and DTSiCODe-4F display up-shifting lowest unoccupied molecular orbital (LUMO) energy levels, which enhances the open-circuit voltage (Voc). As a result, under both AM1.5G/indoor conditions, the devices based on PM7:DTSiC-4F, PM7:DTSiC-2M, and PM7:DTSiCOCe-4F show power conversion efficiencies (PCEs) of 13.13/21.80%, 8.62/20.02, and 9.41/20.56%, respectively. Furthermore, the addition of a third component to the active layer of binary devices is also a simple and efficient strategy to achieve higher photovoltaic efficiencies. Therefore, the conjugated polymer donor PTO2 is introduced into the PM7:DTSiC-4F active layer because of the hypsochromically shifted complementary absorption, deep highest occupied molecular orbital (HOMO) energy level, good miscibility with PM7 and DTSiC-4F, and optimal film morphology. The resulting ternary OSC device based on PTO2:PM7:DTSiC-4F can improve exciton generation, phase separation, charge transport, and charge extraction. As a consequence, the PTO2:PM7:DTSiC-4F-based ternary device achieves an outstanding PCE of 13.33/25.70% under AM1.5G/indoor conditions. As far as we know, the obtained PCE results under indoor conditions are one of the best binary/ternary-based systems processed from eco-friendly solvents.

An Asymmetric Perylene Diimide Dimer Acceptor for High Performance Organic Solar Cells.

The vinylene-bridged helical perylene diimide (PDI) dimer (PDI2) with a build-in twisted configuration is an alternative building block to the parent PDI for the construction of efficient non-fullerene acceptor (NFAs). Moreover, it has been proved asymmetric strategy plays a vital role in the development of NFAs. Herein, we designed and synthesized a pair of acceptor-donor-acceptor (A-D-A) type PDI2 derivatives, namely IDTIC-PDI and IDT-diPDI2, which contain asymmetric and symmetric end-cap units, respectively. To determine the structure-performance relationships of asymmetric strategy, the organic solar cells (OSCs) based on these two molecules were fabricated and measured. The asymmetric IDTIC-PDI based device exhibits a much higher PCE of 8.23 % than that of symmetric IDT-diPDI2 (5.21 %). These results reveal that symmetry breaking provides an effective way to optimize the photovoltaic performances of PDI2 based OSCs.

Open-Shell Donors and Closed-Shell Acceptors in Organic Solar Cells

The active materials of organic solar cells are recognized to show a singlet ground state, and their photo-excited states have been widely investigated in previous work. Herein, we disclose the inherent open-shell singlet ground state of all the donors with obvious electron spin resonance signals. In contrast, the well-known highly efficient non-fullerene acceptors are almost electron spin resonance-silent. These results are carefully studied and confirmed via the combination of variable-temperature nuclear magnetic resonance, electron spin resonance, superconducting quantum interference device, and theoretical calculation. More interestingly, the ground state can be readily tunable between open-shell and closed-shell by changing the quinoidal character of their conjugated backbone. This work provides a new perspective to understand the intrinsic molecular structure of active materials in organic solar cells.

Efficient perylene-diimides-based nonfullerene acceptors with triazine cores synthesized via a simple nucleophilic substitution reaction

Noble-metal catalyst is widely-used to synthesize high-efficiency photovoltaic materials. However, expensive catalysts are difficult to remove and the residual catalysts have deleterious effects on photovoltaic performance. In this study, novel efficient non-fullerene acceptors (NFAs) are synthesized via simple nucleophilic substitution without any noble-metal catalyst. An impressive power conversion efficiency (PCE) of 8.91% is obtained using a single triazine-cored NFA, called NAQ-3. To the best of our knowledge, this is the highest PCE for NFAs synthesized via simple nucleophilic substitution or noble-metal-free reactions. Moreover, a delicate four-armed NFA with a bitriazine core, called NAQ-4, is synthesized as an extension example of this synthesis method. Furthermore, the absorption spectra and energy levels of NAQ-3 and NAQ-4 are compared, and the photovoltaic performance, dissociation process and underlying recombination mechanisms of NAQ-3- and NAQ-4-based organic solar cells are analyzed to determine the structure-performance relationship. The results indicate a successful example of high-performance NFAs synthesized via nucleophilic substitution reactions.

In Silico Designing of Thieno[2,3-b]thiophene Core-Based Highly Conjugated, Fused-Ring, Near-Infrared Sensitive Non-fullerene Acceptors for Organic Solar Cells.

The performance of organic solar cells (OSCs) has been improving steadily over the last few years, owing to the optimization of device fabrication, fine-tuning of morphology, and thin-film processing. Thiophene core containing fused ring-type non-fullerene acceptors (NFAs) achieved significant proficiency for highly efficient OSCs. Quantum chemical computations are utilized herein with the motive of suggesting new NIR sensitive, highly efficient low-band gap materials for OSCs. A series of extended conjugated A-π-D-π-A architectured novel fused-ring NFAs (FUIC-1-FUIC-6) containing thieno[2,3-b]thiophene-based donor core are proposed by substituting the end-capped units of synthesized molecule F10IC. Different properties including frontier molecular orbital analysis, density of states analysis, transition density matrix analysis, excitation energy, reorganizational energies of both holes (λh) and electrons (λe), and open-circuit voltage (V oc) were performed employing the density functional theory approach. Charge transfer analysis of the best-designed molecule with the donor complex was analyzed to comprehend the efficiency of novel constructed molecules (FUIC-1-FUIC-6) and compared with the reference. End-caped acceptor alteration induces the reduction of the energy gap between HOMO-LUMO (1.88 eV), tunes the energy levels, longer absorption in the visible and near-infrared regions, larger V oc, smaller reorganizational energies, and binding energy values in designed structures (FUIC-1-FUIC-6) in comparison to reference (FUIC). The designed molecules show the best agreement with the PTBT-T donor polymer blend and cause the highest charge from the HOMO to the LUMO orbital. Our findings predicted that thieno[2,3-b] thiophene-based newly designed molecules would be efficient NFAs with outstanding photovoltaic characteristics and can be used in future applications of OSCs.

Investigating the Role of Novel Benzotrithiophene-Based Bat-Shaped Non-Fullerene Acceptors for High Performance Organic Solar Cells

The development of an effective building blocks considered the boon in constructing highly efficient nonfullerene electron acceptors (NFEAs). The first theoretical design and investigation of environmental-friendly groups for changing potential end-capped electron acceptor molecules for high-performance environmental-friendly OSCs is proposed in this study. We developed BTT-based [Formula: see text]–[Formula: see text]–[Formula: see text]–[Formula: see text]–[Formula: see text] (acceptor–bridge–core–bridge–acceptor) type, neoteric Bat-shaped environmental-friendly acceptor molecules (K1–K6) for the first time by replacing the electronegative –F group of BTT with four electron-withdrawing (–CN, –COOCH3, –NO2, –Cl) groups. K1–K6 has its exciton-binding energy ([Formula: see text]), open-circuit voltage ([Formula: see text]), frontier molecular orbital analysis (FMO), transition density matrix (TDM) analysis, electron and hole reorganization energy ([Formula: see text]e, [Formula: see text]h), density of state (DOS) graphs, and transition energy Ex values computed and compared to the recently proposed high-performance BTT molecule. According to the research, the photovoltaic, photo-physical and electrical applications of proposed molecules K1–K6 are comparable to those of [Formula: see text]. When compared to reference [Formula: see text] and proposed molecules, K6 tested to be the proverbial optoelectronic compound for OSCs due to its low [Formula: see text] (2.05[Formula: see text]eV), lowest [Formula: see text] (1.57[Formula: see text]eV), highest [Formula: see text] max values of 790.61[Formula: see text]nm in CHCl3, 0.95[Formula: see text]V value for [Formula: see text] and comparable [Formula: see text] (0.482[Formula: see text]eV). The superposition of orbitals and lucrative charge shift from the highest occupied molecular orbital (HOMO) (PTB7-Th) to the lowest unoccupied molecular orbital (LUMO) were verified by charge transfer study among the K6: PTB7-Th combine. As a result, the proposed molecules (K1–K6) with exceptional optoelectronic capabilities are suggested as the best harmless alternative materials for creating proficient and environmental-friendly OSCs.

A novel chlorinated small molecule donor for efficient binary and ternary all-small-molecule organic solar cells

A new acceptor-donor-acceptor (A-D-A) type small molecule donor named BTEHR-CT has been developed, taking the chlorothienyl-substituted benzodithiophene (BDT) as the central unit and 3-(2-ethylhexyl)rhodanine as the terminal group. BTEHR-CT shows a low-lying highest occupied molecular orbital (HOMO) level of −5.56 eV and the suitable molecular aggregation benefiting from the introduction of chlorine atoms, which contributes to achieving excellent photovoltaic performances. Based on an efficient non-fullerene acceptor F–2Cl, the optimized binary device exhibits a high power conversion efficiency (PCE) of 11.13% with an open-circuit voltage ( V oc ) of 0.946 V, short-circuit current densities ( J sc ) of 17.05 mA cm −2 and FF of 0.690. With the incorporation of PC 71 BM, the ternary blend film based on BTEHR-CT:F–2Cl:PC 71 BM (weight ratio of 1:0.8:0.2) shows a fibrous morphology with suitable domain size and favorable phase separation after solvent vapor annealing (SVA) treatment, leading to a superior J sc of 18.53 mA cm −2 and fill factor (FF) of 0.692, thus a higher PCE of 12.05%. To our best knowledge, the PCEs of 11.13% and 12.05% are among the highest values reported to date for non-Y series acceptors based binary and ternary all-small-molecule organic solar cells (ASM-OSCs), respectively. The results indicate that the combination of careful molecular design and suitable morphology control is an effective strategy to achieve high performances for ASM-OSCs. • A new chlorinated small molecule donor named BTEHR-CT was designed and synthesized. • BTEHR-CT exhibits excellent optical and electrochemical properties. • The binary ASM-OSCs based on BTEHR-CT:F–2Cl gave a high PCE of 11.13%. • BTEHR-CT based ternary ASM-OSCs showed a higher PCE of 12.05% with the incorporation of PC 71 BM. • The PCEs are among the highest values reported to date for non-Y series acceptors based ASM-OSCs.

Rational tuning of intermolecular and intramolecular interactions enabling high-efficiency indoor organic photovoltaics

Indoor organic photovoltaics (IOPVs) are one of promising candidates for transferring artificial illumination to power the Internet of Things (IoT). However, their power conversion efficiencies (PCE) are limited by the fact that only a few efficient non-fullerene acceptors are available for IOPVs. Herein, new strategy is adopted to tune the intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT) to achieve high PCEs under indoor environment in the combination of changing skeleton from symmetry to asymmetry and end group from chlorination to non-chlorination. The study reveals that, in comparison with symmetric molecule BTP-2ThCl, asymmetric structure TB-SCl possesses a smaller intra-CT resulting in higher VOC and strengthened inter-CT for neighboring acceptors resulting in more efficient carrier collection and thus higher FF. Moreover, the non-chlorine molecule TB-S exhibits smaller intra and inter-CT leading to further promoting VOC but smaller FF compared with TB-SCl under 1-sun illumination. When the illumination is switched to artificial light, the mitigated trap state, minimized leakage current, and suppressed non-radiative energy loss (∆Enonrad) enable a novel acceptor TB-S yielding high-efficiency rigid PCE of 23.3 % which surpasses TB-SCl and flexible PCE of 20.02% which is one of the best PCE for flexible IOPVs. Our observation provides a new insight into molecule structure modification to promote the indoor photovoltaic performance.

Ternary organic solar cells with enhanced charge transfer and stability combining the advantages of polymer acceptors and fullerene acceptors

Organic solar cells (OSCs) based on highly efficient non-fullerene acceptors (NFAs) have recently developed rapidly, and achieved effective charge separation and high open-circuit voltage ( V OC ) under low driving force. Nonetheless, the low driving force inescapably reduces the short-circuit current ( J SC ) and potentially restricts the improvement of external quantum efficiency (EQE). Herein, we introduce the fullerene acceptor PC 71 BM into the efficient NFA system. And the PM6:PY-IT system is selected as a typical NFA system for its high V OC above 0.90 V. Benefiting from the deeper energy level, excellent electron transport characteristic, and unique spherical structure of PC 71 BM, the driving force for charge transfer is effectively improved. Accordingly, the ternary OSC based on PM6:PY-IT:PC 71 BM demonstrates a high efficiency of 16.36%, a much-enhanced J SC of 25.06 mA cm −2 , and the maximum EQE value is successfully increased to 87%, which is superior to binary OSC (efficiency of 14.84%, J SC of 23.10 mA cm −2 and maximum EQE value of 80%). Meanwhile, the introduction of PC 71 BM improves the molecular stacking structure, resulting in excellent stability in non-encapsulated ternary OSCs, which retain 80% initial efficiency after 6 h of continuous illumination treatment and 85% initial efficiency after 33 days of storage (the efficiencies decrease to 68% and 76% respectively in binary OSCs). In addition, similar conclusions can be drawn in another classical NFA system PM6:Y6, with an optimal efficiency of 17.49%. This work proves that PC 71 BM has unique advantages and great research potential in improving the driving force for charge transfer. • The highlights of this work can be summarized as follows: • The PC 71 BM was incorporated into the PM6:PY-IT system to enhance the driving force and construct efficient OSCs. • Benefiting from the enhanced driving force, the maximum EQE and J SC are significantly increased, and the V OC is 0.942 V. • The ternary OSC obtained an optimal efficiency of 16.36%, which is superior to binary OSC (efficiency of 14.84%). • The stability of the non-encapsulated optimized ternary device was improved by the incorporation of PC 71 BM.

Geminate and Nongeminate Pathways for Triplet Exciton Formation in Organic Solar Cells

AbstractOrganic solar cells (OSCs) have recently shown a rapid improvement in their performance, bringing power conversion efficiencies to above 18%. However, the open‐circuit voltage of OSCs remains low relative to their optical gap and this currently limits efficiency. Recombination to spin‐triplet excitons is a key contributing factor, and is widely, but not universally, observed in donor–acceptor blends using both fullerene and nonfullerenes as electron acceptors. Here, an experimental framework that combines time‐resolved optical and magnetic resonance spectroscopies to detect triplet excitons and identify their formation mechanisms, is reported. The methodology is applied to two well‐studied polymer:fullerene systems, PM6:PC60BM and PTB7‐Th:PC60BM. In contrast to the more efficient nonfullerene acceptor systems that show only triplet states formed via nongeminate recombination, the fullerene systems also show significant triplet formation via geminate processes. This requires that geminate electron–hole pairs be trapped long enough to allow intersystem crossing. It is proposed that this is a general feature of fullerene acceptor systems, where isolated fullerenes are known to intercalate within the alkyl sidechains of the donor polymers. Thus, the study demonstrates that engineering good donor and acceptor domain purity is key for suppressing losses via triplet excitons in OSCs.

Charge transfer processes via tandem modification of efficient non-fullerene acceptors for organic solar cells

High-efficiency non-fullerene acceptors (NFAs) have extensive applications in organic solar cells (OSCs) instead of fullerene-based acceptors. Herein, the dithienothiophen[3.2-b]-pyrrolo-benzothiadiazole (BDP), chlorinated BDP-2Cl, BDP-4Cl and designed BDPN-4Cl have been investigated. One performed the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to study their optimized structures, frontier molecular orbitals (FMOs) and other photoelectric parameters, which showed that designed BDPN-4Cl displayed narrower band gaps, smaller reorganization energy, larger electrophilic index and wider absorption spectrum than prototypical NFAs, and it had a greater ratio of charge separation rate/charge recombination rate (KCS/KCR) for heterojunction. It was also found that the designed BDPN-4Cl exhibited the largest electron transport ability, which was conducive to the interfacial process. Structure modification at the end positions of NFAs, such as the π-extension at the end-capping, showed a synergistic effect on performance. Furthermore, one also estimated the ideal performance (VOC, VLOSS, JSC) of all the compounds to reveal the influence of halogen atomic substitution and π-extension at the end-capping on performance. The present investigation suggests viable strategies for further designing NFAs in OSCs.

Designing benzothiadiazole based highly efficient non-fullerene acceptor molecules for organic solar cells

This research is carried out to investigate photovoltaic characteristics of the six modified molecules derived from benzothiadiazole core-based reference molecule JY5, using four different non-fullerene acceptors at ωB97XD/6-31G** via TD-DFT approach. To check the efficiency of these designed molecules dipole moment, band gap, binding energy, chemical-potential, soft and hard character, electrophilicity, light harvesting efficiency, open circuit voltage and fill factor are studied along with transition density matrix, molecular electro-potential surface, density of states and photoinduced electron transfer (PET). Amidst JY5-W1 to JY5-W6 structures, JY5-W4 molecule shows a broad absorption range with a λmax of 458 nm. JY5-W3 and JY5-W4 molecules shows the highest electron mobilities. The outstanding light harvesting efficiencies (>0.90) make the designed molecules a good candidate for organic solar cells. HOMO-LUMO gaps parameterize Voc as a key factor in evaluating photovoltaic outcome of designed molecules (electrophilic in nature) so, PTB7-Th is considered as a preferred donor as its lower HOMO (−5.01 eV) and higher LUMO level (−2.60 eV) results in increased Voc predicting an efficient organic solar cell.

Noncovalent Conformational Locks Enabling Efficient Nonfullerene Acceptors

A simple synthesis of highly planar extended π‐electron molecules is of particular interest for the development of efficient nonfullerene acceptors in organic solar cells with high light absorption and high mobility. Herein, two small‐molecule acceptors (MPU7 and MPU8) with a diketopyrrolopyrrole core connected to CPTCN end‐capping groups (A2) via thienylethynylselenophene (MPU7) or thienylethynylthiophene (MPU8) linkers are conceived and synthesized. Planarity of the conjugated skeleton is achieved in both molecules, thanks to the existence of four (Se⋯O or S⋯O) noncovalent, through‐space intramolecular interactions. Both nonfullerene small‐molecule acceptors show broad absorption in the visible and near‐infrared region (up to 930 nm). As a result, the binary solar cells constructed together with a polymer donor (P) display power conversion efficiencies as high as 14.12%. Devices built with MPU7 (containing the selenophene) show better film morphology, electron mobility, and higher efficiency than those containing thiophene (MPU8).

Tackling voltage losses

Voltage losses limit the performance of organic solar cells, yet their origins are not fully understood. Now, a theoretical model encompassing electronic state hybridization and thermal population of vibrational states explains the reduced non-radiative voltage losses in efficient non-fullerene acceptor systems.

High-Performance Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors Enabled by Asymmetric Cores with Enhanced Noncovalent Intramolecular Interactions.

Nonfullerene acceptors (MQ3, MQ5, MQ6) are synthesized using asymmetric and symmetric ladder-type heteroheptacene cores with selenophene heterocycles. Although MQ3 and MQ5 are constructed with the same number of selenophene heterocycles, the heteroheptacene core of MQ5 is end-capped with selenophene rings while that of MQ3 is flanked with thiophene rings. With the enhanced noncovalent interaction of O⋅⋅⋅Se compared to that of O⋅⋅⋅S, MQ5 shows a bathochromically shifted absorption band and greatly improved carrier transport, leading to a higher power conversion efficiency (PCE) of 15.64 % compared to MQ3, which shows a PCE of 13.51 %. Based on the asymmetric heteroheptacene core, MQ6 shows an improved carrier transport induced by the reduced π-π stacking distance, related with the increased dipole moment in comparison with the nonfullerene acceptors based on symmetric cores. MQ6 exhibits a PCE of 16.39 % with a VOC of 0.88 V, a FF of 75.66 %, and a JSC of 24.62 mA cm-2 .

Over 13% Efficient Organic Solar Cells Based on Low‐Cost Pentacyclic A‐DA′D‐A‐Type Nonfullerene Acceptor

Recent studies have almost focused on finding active layer materials with extended π‐conjugation structures for high‐performance organic solar cells (OSCs). However, with the extension of conjugate length, the synthesis difficulty and cost of materials will increase. Achieving high efficiency while reducing material costs is a prerequisite for the commercialization of OSCs. Herein, two low‐cost A‐DA′D‐A‐type (where A and D represent an electron‐withdrawing unit and an electron‐donating unit, respectively) nonfullerene acceptors (Y25,Y26) are synthesized with pentacyclic fused backbone as the DA′D electron‐deficient core and 5,6‐difluoro‐3‐(dicyandiamethyl) indigo as the end groups. Compared with classical Y series acceptors with heptacyclic backbone, although Y25 and Y26 own the reduced conjugated length, they still show moderate performance (11.65% and 13.34%), and the cost of synthesis is significantly reduced. Therefore, we provide a new molecular design idea for commercially efficient nonfullerene OSCs acceptors. We also find that adding alkyl chains to the β site of thiophenes is beneficial to obtaining the reduced energetic disorder, dominant molecular stacking, and desirable morphology, which can facilitate charge carrier transport and prompt higher short‐circuit current density (Jsc) as well as fill factor.

Non-fullerene acceptors with branched side chains and improved molecular packing to exceed 18% efficiency in organic solar cells

Molecular design of non-fullerene acceptors is of vital importance for high-efficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach, as it may introduce an undesirable steric hindrance that reduces charge transport in non-fullerene acceptors. Here we show the design and synthesis of a highly efficient non-fullerene acceptor family by substituting the beta position of the thiophene unit on a Y6-based dithienothiophen[3,2-b]-pyrrolobenzothiadiazole core with branched alkyl chains. It was found that such a modification to a different alkyl chain length could completely change the molecular packing behaviour of non-fullerene acceptors, leading to improved structural order and charge transport in thin films. An unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of the branched alkyl chain topology in tuning the molecular packing and blend morphology, which leads to improved organic photovoltaic performance. Molecular design of acceptor and donor molecules has enabled major progress in organic photovoltaics. Li et al. show that branched alkyl chains in non-fullerene acceptors allow favourable morphology in the active layer, enabling a certified device efficiency of 17.9%.