As‐Cast Ternary Organic Solar Cells Based on an Asymmetric Side‐Chains Featured Acceptor with Reduced Voltage Loss and 14.0% Efficiency

A new small‐molecule nonfullerene acceptor based on the benzo[1,2‐b:4,5‐b′]dithiophene (BDT) fused central core with asymmetrical alkoxy and thienyl side chains, namely TOBDT , is designed and synthesized. The alkoxy unit helps narrow the bandgap, and thienyl side chain helps enhance the intermolecular interaction. As a result, TOBDT is suitable to match the deep‐lying highest occupied molecular orbital (HOMO) of polymer donor PM6 . Then, a strong crystalline acceptor IDIC is introduced as the third component to fabricate as‐cast nonfullerene ternary devices to achieve absorption and morphology control. Addition of IDIC not only mixes well with TOBDT but modulates the morphology of the blend film, which helps to balance the charge transport properties and reduce the photovoltage loss of ternary devices. All these contribute to synergetic improvement of J_sc, V_oc, and fill factor parameters, leading to a power conversion efficiency of 14.0% for the as‐cast fullerene‐free ternary device.     

Balancing Intermolecular Interactions between Acceptors and Donor/Acceptor for Efficient Organic Photovoltaics

Promoted by uninterrupted materials and device innovation, organic solar cells have achieved impressive development. However, the complicated intermolecular interactions inside active layers are less understood. Herein, the intermolecular interactions are studied from the dual perspectives of acceptor/acceptor (A/A) and donor/acceptor (D/A), and how these interactions synergistically control the final efficiencies. Three small molecular acceptors (SMAs) are designed with different end-caps, which manipulate the crystallinity and electrostatic potential (ESP) distributions of acceptors, and accordingly, the A/A and D/A intermolecular interactions. The results show that SMA LA17 with low A/A interactions exhibits inferior performance around 12%, owing to its strong D/A interaction with donor PM6, which shapes too miscible morphology and increases charge recombination. Instead, LA16 with strong A/A interactions and moderate D/A interactions delivers improved bulk-heterojunction (BHJ) networks, and therefore, enhances charge transport and diminishes geminate or trap-assisted charge recombination. Consequently, PM6:LA16 records the competitive efficiency of up to 13.74% among the three systems. Therefore, this study deepens the synergistic or balancing effect of the D/A and A/A interactions on BHJ blends for efficient organic solar cells.     

Atomic Optimization on Pyran-Fused Nonfullerene Acceptor Enables Organic Solar Cells With an Efficiency Approaching 16% and Reduced Energy Loss

Atomic replacement on platforms of nonfullerene acceptor (NFA) with already excellent performance is expected to further optimize the energy levels, absorptions, and even charge transfer dynamics of NFAs effectively without greatly destroying their superior molecular conformations. On the basis of high-performance F-series NFAs, the structural optimization at atomic level is performed by replacing sulfur atoms in FO-2Cl with selenium atoms, thus affording a new NFA labeled as FOSe-2Cl. FOSe-2Cl not only inherits the good planar configuration of FO-2Cl, but also exhibits more suitable energy levels, redshifted absorption, enhanced molecular packing, and accelerative charge transfer/transport dynamics compared with those of FO-2Cl. With a widely used polymer PM6 as the donor, organic solar cell (OSC) based on FOSe-2Cl affords a significantly improved power conversion efficiency (PCE) of 15.94% with a reduced energy loss (E_loss) of 0.670 eV, with respect to that of FO-2Cl-based OSC with a PCE of 14.94% and E_loss of 0.706 eV. The result represents the best performance reported to date for pyran-fused NFAs and F-series NFAs-based binary OSCs, providing another promising platform to achieve the state-of-the-art OSCs in addition to the well-known Y-series NFAs.     

The relationship between intermolecular interactions and charge transport properties of trifluoromethylated polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) with the electron-withdrawing groups such as halogen atom, cyanide, perfluoroalkyl (PFA), or perfluoroary, etc. exhibit good air stability and better solid-state charge carrier mobility. To obtain a better understanding of structure property relationships of this kind of compounds, a series PAH(CF₃)_n derivatives a1, a2, b1, b2, c1, and c2, which contain different numbers of trifluoromethyls and benzene rings, were chosen and studied by both band-like model and hopping model. Their crystals contain different intermolecular interactions. It turns out that intermolecular hydrogen bonding interactions are mainly responsible for electron transport, while π-stacking interactions dominate hole transport. When the π-stacking and intermolecular hydrogen bonding interactions coexist in the same direction, a competitive relationship occurs between hole and electron transport, which tend to cause enhancement of electron transport, and restrain hole transport.     

Diketopyrrolopyrrole based highly crystalline conjugated molecules for application in small molecule donor-polymer acceptor nonfullerene organic solar cells

In order to specifically investigate the low efficiency of small molecule donor-polymer acceptor (M-P) nonfullerene organic solar cells, we have successfully modify the synthesis of a series of D-π-A-π-D conjugated molecules containing diketopyrrolopyrrole (DPP) and different end groups. By incorporation of end group with different size of π-conjugation (benzene, naphthalene and pyrene), we further improved the fill factor (FF) and short current density (J_sc) of the donors molecule. Our experimental results and theoretical calculations have proven that the size of the end groups can influence the molecule crystallinity, mobility and intermolecular packing by altering the molecular coplanarity. As the result of improved crystallinity, morphology and fine-tuned mobilities, we demonstrated an increased FF, a high J_sc of ∼4.5 mA/cm² and a power conversion efficiency of 2.05%, which is among the highest efficiency reported for M-P nonfullerene solar cells. Our results provide opportunities and possibilities of achieving higher performance M-P nonfullerene solar cells in the future.     

Theoretical studies on the hole transport property of tetrathienoarene derivatives: The influence of the position of sulfur atom,substituent and π-conjugated core

Chemical modifications such as changing the position of heteroatom, introducing different substituents and π-conjugated cores are powerful molecular design tools to modulate their optical and electrochemical performance. In this context, in-depth density functional theory investigations on the geometries, frontier molecular orbitals, reorganization energies, transfer integrals, anisotropic mobilities and band structures of tetrathienoarene derivatives were carried out to provide insights into the effects of these chemical modifications on their hole transport properties. The electrostatic potential, Hirshfeld surface analysis, energy decomposition analysis (EDA) and anisotropic mobility were also employed to shed light on the intricate interplays among molecular packings, intermolecular interactions and transport properties. It is found that compared with 2, 1a with sulfur atom inside has lower frontier orbital energy level, smaller reorganization energy and larger transfer integral and hole mobility. However, more S⋯S interactions in 2 could provide more effective transport channels for charge carrier transport. The introduction of hexylthienyl groups (1c) results in an enhancement of π–π interaction and leads to an increase in the highest occupied molecular orbital (HOMO) and transfer integrals. Meanwhile, the strongest intermolecular interaction energy of pathway A in 1c renders its transport behavior with typical one-dimensional (1D) transport. Moreover, anthracene as the π-conjugated core seemed to possess better transport properties in comparison with dibenzothiophene or chrysene acting as core. In addition, the dispersion energy of all investigated compounds plays a leading role in determining the energetically accessible stacking motifs. We hope that our speculation would facilitate the future design and preparation of high-performance charge-transport materials.     

Improving Photovoltaic Performance of Non-Fullerene Polymer Solar Cells Enables by Fine-Tuning Blend Microstructure via Binary Solvent Mixtures

Studies of the relationship between blend microstructure and photovoltaic performance are becoming more common, which is a prerequisite for rationally improving device performance. Non-fullerene acceptors usually have planar backbone conformation and strong intermolecular packing, normally resulting in excessive phase separation. Herein, an effective co-solvent blending strategy to turn the molecular organization of a chlorinated small molecule acceptor Y6-2Cl and phase separation of the corresponding active layer with PM6 as donor is demonstrated. The in situ photoluminescence measurements and relevant morphological characterizations illustrate that the film-forming process is fine-turned when using the mixtures of chloroform (CF) and chlorobenzene (CB) solvents, and the blend showed high domain purity with suitable phase-separated networks. Thus, better exciton dissociation and charge generation, more balanced charge transport, and less recombination loss are obtained in the co-solvent blade-coated devices. As a result, a maximum power conversion efficiency (PCE) of 16.17% is achieved, which is much higher than those of CF- and CB-bladed devices (14.08% and 11.44%, respectively). Of note is that the use of this co-solvent approach in the other two high-performance photovoltaic systems is also confirmed, demonstrating its good generality of employing in the printing organic solar cells.     

Non-Halogenated-Solvent Processed and Additive-Free Tandem Organic Solar Cell with Efficiency Reaching 16.67%

Organic solar cells (OSCs) have recently reached a remarkably high efficiency and become a promising technology for commercial application. However, OSCs with top efficiency are mostly processed by halogenated solvents and with additives that are not environmentally friendly, which hinders large-scale manufacture. In this study, high-performance tandem OSCs, based on polymer donors and two small-molecule acceptors with different bandgaps, are fabricated by solution processing with non-halogenated solvents without additive. Importantly, the two active layers developed from non-halogenated solvents show better phase segregation and charge transport properties, leading to superior performance than halogenated ones. As a result, a tandem OSC with high efficiency of up to 16.67% is obtained, showing unique advantages in future massive production.     

Effects of electron-withdrawing group and electron-donating core combinations on physical properties and photovoltaic performance in D-π-A star-shaped small molecules

The first representatives of star-shaped molecules having 3-alkylrhodanine (alkyl-Rh) electron-withdrawing groups, linked through bithiophene π-spacer with electron-donating either triphenylamine (TPA) or tris(2-methoxyphenyl)amine (m-TPA) core were synthesized. The physical properties and photovoltaic performance of these novel molecules with 3-ethylrhodanine groups were comprehensively studied and compared to their full analogs having dicyanovinyl (DCV) units as the other type of well-known and frequently used acceptor groups. On one hand, the former demonstrate several advantages such as higher solubility and better photovoltaic performance in bulk-heterojunction (BHJ) organics solar cells (OSCs) as compared to the latter. Nevertheless, the former have slightly lower optical/electrochemical bandgaps and higher thermooxidation stability. On the other hand, molecules of both series based on m-TPA core along with higher solubility and higher position of HOMO energy levels have more pronounced tendency to crystalize as compared to the TPA-based molecules. Detailed investigation of the structure-property relationships for these series of molecules revealed that donor and acceptor unit combinations influence both charge generation and charge transport/recombination properties, as demonstrated by the ultrafast photoinduced absorption spectroscopy, space charge limited current measurements and transient photovoltage technique. These results give more insight how to fine-tune and predict physical properties and photovoltaic performance of small molecules having either alkyl-Rh or DCV units in their chemical structures and thus providing a molecular design guideline for the next generation of high-performance photovoltaic materials.     

End-chain effects of non-fullerene acceptors on polymer solar cells

Four acceptor1-acceptor2-donor-acceptor2-acceptor1 (A1-A2-D-A2-A1) structural electron acceptors with different end-chains were designed and synthesized which all possessed indacenodithiophene (IDT) core, benzothiadiazole (BT) bridge as acceptor2, and rhodanine (R) end groups as acceptor1. The non-fullerene acceptor attached with ethyl group is called IDT-BT-R2 and used as control compound. And the other three of them are attached with methoxymethyl, trifluoroethyl and 1-piperidino groups generating IDT-BT-RO, IDT-BT-RF3 and IDT-BT-RN, respectively. The influence of end-chains on their optoelectronic properties were compared between four non-fullerene acceptors. Compared with IDT-BT-R2, the molecule IDT-BT-RF3 show red-shifted light absorption and lower LUMO level because of the electron withdrawing property of fluorine atoms. OSCs based on IDT-BT-RF3 display more efficient charge separation and lower degree of monomolecular recombination, allowing OSCs to show higher short-circuit current (J_sc) than the system of IDT-BT-R2. OSCs based on IDT-BT-RO also show more efficient charge separation and less monomolecular recombination. Due to the elevated LUMO level of the acceptor IDT-BT-RN, organic solar cells (OSCs) utilizing this material as acceptor display high open-circuit voltage (V_oc) of 1.10 eV and low energy loss of 0.49 eV when maintaining a relatively high power conversion efficiency (PCE) of 7.09%. We demonstrated that the end-chain engineering could finely tune the light absorption properties and energy levels of novel non-fullerene acceptors and eventually improved OSCs performance can be harvested.     

The investigations of two conjugated polymers that show distinctly different photovoltaic properties in polymer solar cells

In fullerene-free polymer solar cells (PSC), the non-fullerene small molecular acceptors (NF-SM) demonstrates quite different film morphology in comparison with the conventional fullerene acceptors when blended with the same conjugated polymer donor. In this work, a NF-SM acceptor, ITIC, was introduced into two efficient binary PSCs based on fullerene to fabricate ternary solar cells. It is found that the addition of ITIC led fundamentally different results. After carefully investigated the difference on film morphology and charge carriers' mobility, the results showed that the good miscibility between ITIC and the polymer may deteriorate the favorable film morphology with appropriate phase separation and suppress the formation of continues charge transport channel. This work offered a useful consideration to produce high-performance ternary PSCs by rational selecting the donors and acceptors.     

Triisopropylsilyl-Substituted Benzo[1,2-b:4,5-c′]dithiophene-4,8-dione-Containing Copolymers with More Than 17% Efficiency in Organic Solar Cells

Considering the special functions of fused-ring aromatic building blocks and Si-atom in high-performance donor–acceptor-conjugated materials at the same time, herein the synthesis of a novel fused-ring tricyclic heterocycle, triisopropylsilyl-substituted benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (iBDD-Si), an isomer of well-known benzo[1,2-c:4,5-c′]dithiophene-4,8-dione is presented. The iBDD-Si-based copolymer series (PM6, PM6-5Si, PM6-10Si, and PM6-15Si) is synthesized via Stille polymerization, revealing fine-tuned optical and electrochemical properties, and molecular packing with varying iBDD-Si contents in the backbone. Organic solar cells are fabricated by pairing the copolymer donors with nonfullerene acceptor N3 and characterized. High power conversion efficiency of more than 17% is achieved using the PM6-5Si-based solar cell, which is attributed to the balanced charge transport, enhanced charge generation/dissociation kinetics, and minimized total energy and recombination losses. It is demonstrated that iBDD-Si is a promising backbone toolbox for various high-performance conjugated materials.     

Versatile Thermochromic Supramolecular Materials Based on Competing Charge Transfer Interactions

Stimuli‐responsive supramolecular materials are of paramount importance for a broad range of applications. It is essential to impart versatility, sustainability, and scalability into these materials. Herein the authors report the design and synthesis of a new class of thermochromic supramolecular materials, which can easily be processed from water via a reversible sol–gel transition. The supramolecular materials are composed of a bis‐bipyridinium acceptor, a π‐electron‐rich naphthalene derivative donor, and halogen counterions. Long helical nanofibers can be assembled in water, gelating at room temperature. Inked designs, thin films, and aerogels are solution‐processed to exhibit thermochromic behavior based on competing π → π* and n → π* charge transfer interactions. By using different π‐electron rich donors, and counterions, the authors demonstrate that both the color observed at room temperature and at high temperatures can be tailored. The results open up the door to develop novel amphiphile‐based thermochromes with water processability and a large tunable color palette.     

Conformation Locking on Fused‐Ring Electron Acceptor for High‐Performance Nonfullerene Organic Solar Cells

In this work, sidechain engineering on conjugated fused‐ring acceptors for conformation locking is demonstrated as an effective molecular design strategy for high‐performance nonfullerene organic solar cells (OSCs). A novel nonfullerene acceptor (ITC6‐IC) is designed and developed by introducing long alkyl chains into the terminal electron‐donating building blocks. ITC6‐IC has achieved definite conformation with a planar structure and better solubility in common organic solvents. The weak electron‐donating hexyl upshifts the lowest unoccupied molecular orbital level of ITC6‐IC, resulting in a higher V_OC in comparison to the widely used ITIC. The OSCs based on PBDB‐T:ITC6‐IC reveal a promising power conversion efficiency of 11.61% and an expected high V_OC of 0.97 V. The weaker π–π stacking induced by steric hindrance affords ITC6‐IC with enhanced compatibility with polymer donors. The blend film treated with suitable thermal annealing exhibits a fibril crystallization feature with a good bicontinuous network morphology. The results indicate that the molecular design approach of ITC6‐IC can be inspirational for future development of nonfullerene acceptors for high efficiency OSCs.     

Unravelling the Molecular Origin of Organic Semiconductors with High-Performance Thermoelectric Response

A decisive prerequisite toward systematic development of high-efficiency organic thermoelectric materials is not only thoroughly understanding the microscopic physical processes controlling the performance, but also precisely correlating such processes and the macroscopic properties to the basic chemical structures. Here, by using multiscale first-principles calculations, the interplay among thermoelectric properties, microscopic transport parameters, and molecular structures for the whole family of small-molecule organic thermoelectric materials is rationalized, and general molecular design principles are concurrently formulated. It is unveiled that thermoelectric power factor of a wide variety of molecular semiconductors is directly proportional to a unified quality factor, and high-performance thermoelectric response demands to boost the intermolecular electronic coupling, and to suppress the interaction of electron with lattice vibrations. Furthermore, it is uncovered that extending the π-conjugated backbones along the long axis, and maximizing the networks of intermolecular S···S or C H···π contacts meet the proposed material design rule.     

Small-molecule acceptors with long alkyl chains for high-performance as-cast nonfullerene organic solar cells

Three fused-ring small-molecule electron acceptors, IDTC16-IC, IDTC16-Th, and IDTC16-4F, were designed and synthesized by introducing indacenodithiophene (IDT) as the electron-donating core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC), fluorinated IC, and a thiophene-based unit as the electron-withdrawing end group. Here, instead of the commonly used n-hexyl or n-hexylphenyl side chains, n-hexadecyl peripheral substituents were employed at the IDT core to study the influence of alkyl groups on photovoltaic performance of the nonfullerene acceptors. The introduction of flexible n-hexadecyl group endowed the three acceptors with excellent solubility in common organic solvents. All the three acceptors presented strong absorption ranging from 450 nm to 720 nm in solution with high molar extinction coefficients. As a result, the as-cast organic solar cells (OSCs) based on IDTC16-IC and the wide bandgap polymer donor PM6 exhibited a power conversion efficiency (PCE) of 5.12%. The OSCs based on PM6:IDTC16-Th and PM6:IDTC16-4F showed much better photovoltaic performance with PCEs of 8.76% and 8.55%, respectively. The PCE values were improved to 5.89%, 9.09%, and 9.42% for the PM6:IDTC16-IC, PM6:IDTC16-Th, and PM6:IDTC16-4F OSCs, respectively, with the addition of the solvent additive 1,8-diiodooctane. These findings demonstrate that the combination of alkyl chains at the fused rings and fluorination or aromatic structure change of the terminal groups leads to greatly enhanced photovoltaic performance of nonfullerene acceptors through improving the photophysical, molecular orbital, and film morphological properties.     

Retarding the Crystallization of a Nonfullerene Electron Acceptor for High‐Performance Polymer Solar Cells

Developing a fundamental understanding of the molecular order within the photoactive layer, and the influence therein of solution casting conditions, is a key factor in obtaining high power conversation efficiency (PCE) polymer solar cells. Herein, the molecular order in PBDB‐T:INPIC‐4F nonfullerene solar cells is tuned by control of the molecular organization time during film casting, and the crucial role of retarding the crystallization of INPIC‐4F in achieving high performance is demonstrated. When PBDB‐T:INPIC‐4F is cast with the presence of solvent vapor to prolong the organization time, INPIC‐4F molecules form spherulites with a polycrystalline structure, resulting in large phase separation and device efficiency below 10%. On the contrary, casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure, and encourages face‐on π?π stacking of INPIC‐4F. This molecular transformation of INPIC‐4F significantly enhances the absorption ability of INPIC‐4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport, leading to the achievement of a maximum PCE of 13.1%. This work provides a rational guide for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.     

High-Efficiency Organic Solar Cells Enabled by Non-Fullerene Acceptors with Benzimidazole as the Central Core

Electron-deficient central core plays a crucial role in the construction of efficient Y-series non-fullerene acceptors (NFAs). Here, fused-ring benzimidazole (BIm) served as a central core for the first time to yield a new NFA named MZ-1 and its structural analogue named MZ-2, which is obtained by replacing the methyl group on the 2C position of BIm in MZ-1 with trifluoromethyl group. Compared with MZ-1, MZ-2 shows obviously blue-shifted absorption and lowers the highest occupied molecular orbital (HOMO) energy level that is more matched to that of polymer donor PM6. Benefiting from the more efficient charge transport and favorable microphase separation morphology of the active layer, the acceptor MZ-2-based device affords an excellent power conversion efficiency (PCE) of 17.31% along with a high open-circuit voltage (V_oc) of 0.903 V, a short-circuit current density (J_sc) of 26.32 mA cm⁻² and a fill factor (FF) of 72.83%, which is remarkably superior to that of MZ-1-based devices with PCE of 10.70%. This study offers valuable insight into the design of acceptors to enrich Y series NFAs for high-performance organic solar cells (OSCs).     

Enhancing Field‐Effect Mobility of Conjugated Polymers Through Rational Design of Branched Side Chains

The design of polymer semiconductors possessing effective π–π intermolecular interactions coupled with good solution processability remains a challenge. Structure‐property relationships associated with side chain structure, π–π intermolecular interactions, polymer solubility, and charge carrier transport are reported for a donor–acceptor(1)‐donor–acceptor(2) polymer: 5‐Decylheptadecyl (5‐DH), 2‐tetradecyl (2‐DT), and linear n‐octadecyl (OD) chains are substituted onto a polymer backbone consisting of terthiophene units (T) between two different electron acceptors, benzothiadiazole (B), and diketopyrrolopyrrole (D), pTBTD, to afford pTBTD‐5DH, pTBTD‐2DT, and pTBTD‐OD, respectively. In the 5‐DH side chain, the branching position is remote from the polymer backbone, whereas it is proximal in 2‐DT. This study demonstrates that incorporation of branched side chains where the branching position is remote from the polymer backbone merges the advantages of improved solubility from branched units with effective π–π intermolecular interactions normally associated with linear chains on conjugated polymers. pTBTD‐5DH exhibits superior qualities with respect to the degree of polymerization, solution processability, π–π interchain stacking, and charge carrier transport relative to the other analogs. pTBTD‐5DH exhibits a field‐effect hole mobility of up to 2.95 cm² V^–1 s^–1, a factor of 3–7 times that achieved with pBDT6‐DT and pBDT6‐OD.     

Efficient Ternary Organic Solar Cells Enabled by the Integration of Nonfullerene and Fullerene Acceptors with a Broad Composition Tolerance

The ternary structure that combines fullerene and nonfullerene acceptors in a photoactive layer is demonstrated as an effective approach for boosting the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, highly efficient ternary OSCs comprising a wide‐bandgap polymer donor (PBT1‐C), a narrow‐bandgap nonfullerene acceptor (IT‐2F), and a typical fullerene derivative (PC₇₁BM) are reported. It is found that the addition of PC₇₁BM into the PBT1‐C:IT‐2F blend not only increases the device efficiency up to 12.2%, but also improves the ambient stability of the OSCs. Detailed investigations indicate that the improvement in photovoltaic performance benefits from synergistic effects of increased photon‐harvesting, enhanced charge separation and transport, suppressed trap‐assisted recombination, and optimized film morphology. Moreover, it is noticed that such a ternary system exhibits excellent tolerance to the PC₇₁BM component, for which PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of PBT1‐C:IT‐2F binary reference device.