An Optimized Fibril Network Morphology Enables High-Efficiency and Ambient-Stable Polymer Solar Cells.

Morphological stability is crucially important for the long-term stability of polymer solar cells (PSCs). Many high-efficiency PSCs suffer from metastable morphology, resulting in severe device degradation. Here, a series of copolymers is developed by manipulating the content of chlorinated benzodithiophene-4,8-dione (T1-Cl) via a random copolymerization approach. It is found that all the copolymers can self-assemble into a fibril nanostructure in films. By altering the T1-Cl content, the polymer crystallinity and fibril width can be effectively controlled. When blended with several nonfullerene acceptors, such as TTPTT-4F, O-INIC3, EH-INIC3, and Y6, the optimized fibril interpenetrating morphology can not only favor charge transport, but also inhibit the unfavorable molecular diffusion and aggregation in active layers, leading to excellent morphological stability. The work demonstrates the importance of optimization of fibril network morphology in realizing high-efficiency and ambient-stable PSCs, and also provides new insights into the effect of chemical structure on the fibril network morphology and photovoltaic performance of PSCs.

A General Approach for Lab-to-Manufacturing Translation on Flexible Organic Solar Cells.

The blossoming of organic solar cells (OSCs) has triggered enormous commercial applications, due to their high-efficiency, light weight, and flexibility. However, the lab-to-manufacturing translation of the praisable performance from lab-scale devices to industrial-scale modules is still the Achilles' heel of OSCs. In fact, it is urgent to explore the mechanism of morphological evolution in the bulk heterojunction (BHJ) with different coating/printing methods. Here, a general approach to upscale flexible organic photovoltaics to module scale without obvious efficiency loss is demonstrated. The shear impulse during the coating/printing process is first applied to control the morphology evolution of the BHJ layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot-die printing and spin-coating is detected. Compelling results of morphological evolution, molecular stacking, and coarse-grained molecular simulation verify the validity of the impulse translation. Accordingly, the efficiency of flexible devices via slot-die printing achieves 9.10% for PTB7-Th:PC71 BM and 9.77% for PBDB-T:ITIC based on 1.04 cm2 . Furthermore, 15 cm2 flexible modules with effective efficiency up to 7.58% (PTB7-Th:PC71 BM) and 8.90% (PBDB-T:ITIC) are demonstrated with satisfying mechanical flexibility and operating stability. More importantly, this work outlines the shear impulse translation for organic printing electronics.

An EPDM/MVQ polymer blend based magnetorheological elastomer with good thermostability and mechanical performance.

Magnetorheological elastomers (MREs) with outstanding magnetic-control properties are highly desirable for applications such as vibration attenuation, smart sensing, and soft robots. However, the low strength and thermolability of these materials still restrict their application in attenuating the vibration of large-scale devices. In this paper, we prepared an MRE based on ethylene-propylene-diene monomer (EPDM)/methylvinyl silicone rubber (MVQ) polymer blends. The resulting MRE showed good thermostability and mechanical properties. Good interfacial interaction and particle dispersion were achieved by modifying the surface of carbonyl iron powder (CIP) with silica coating by the sol-gel method. The compatibility between the EPDM and MVQ was promoted using silane coupling agents. Moreover, the resulting MRE had high mechanical strength and elongation at break. The dynamic viscoelastic properties of the MRE were tested using a rheometer. The influences of frequency, strain, matrices, temperature, and magnetic fields were discussed comprehensively, and relevant physical mechanisms were proposed. Finally, thermal aging tests were performed to evaluate the heat resistance of the MRE. Analytical results showed that the resulting MRE could be significantly applied to reduce the vibration of large devices because of its excellent mechanical properties and thermostability.

Morphology Control Enables Efficient Ternary Organic Solar Cells.

Ternary organic solar cells are promising alternatives to the binary counterpart due to their potential in achieving high performance. Although a growing number of ternary organic solar cells are recently reported, less effort is devoted to morphology control. Here, ternary organic solar cells are fabricated using a wide-bandgap polymer PBT1-C as the donor, a crystalline fused-ring electron acceptor ITIC-2Cl, and an amorphous fullerene derivative indene-C60 bisadduct (ICBA) as the acceptor. It is found that ICBA can disturb π-π interactions of the crystalline ITIC-2Cl molecules in ternary blends and then help to form more uniform morphology. As a result, incorporation of 20% ICBA in the PBT1-C:ITIC-2Cl blend enables efficient charge dissociation, negligible bimolecular recombination, and balanced charge carrier mobilities. An impressive power conversion efficiency (PCE) of 13.4%, with a high fill factor (FF) of 76.8%, is eventually achieved, which represents one of the highest PCEs reported so far for organic solar cells. The results manifest that the adoption of amorphous fullerene acceptor is an effective approach to optimizing the ternary blend morphology and thereby increases the solar cell performance.

Vertical Stratification Engineering for Organic Bulk-Heterojunction Devices.

High-efficiency organic solar cells (OSCs) can be produced through optimization of component molecular design, coupled with interfacial engineering and control of active layer morphology. However, vertical stratification of the bulk-heterojunction (BHJ), a spontaneous activity that occurs during the drying process, remains an intricate problem yet to be solved. Routes toward regulating the vertical separation profile and evaluating the effects on the final device should be explored to further enhance the performance of OSCs. Herein, we establish a connection between the material surface energy, absorption, and vertical stratification, which can then be linked to photovoltaic conversion characteristics. Through assessing the performance of temporary, artificial vertically stratified layers created by the sequential casting of the individual components to form a multilayered structure, optimal vertical stratification can be achieved. Adjusting the surface energy offset between the substrate results in donor and acceptor stabilization of that stratified layer. Further, a trade-off between the photocurrent generated in the visible region and the amount of donor or acceptor in close proximity to the electrode was observed. Modification of the substrate surface energy was achieved using self-assembled small molecules (SASM), which, in turn, directly impacted the polymer donor to acceptor ratio at the interface. Using three different donor polymers in conjunction with two alternative acceptors in an inverted organic solar cell architecture, the concentration of polymer donor molecules at the ITO (indium tin oxide)/BHJ interface could be increased relative to the acceptor. Appropriate selection of SASM facilitated a synchronized enhancement in external quantum efficiency and power conversion efficiencies over 10.5%.

Improved Glass Transition Temperature towards Thermal Stability via Thiols Solvent Additive versus DIO in Polymer Solar Cells.

The halogen-free solvent additive, 1,4-butanedithiol (BT) has been incorporated into PTB7-Th:PC71 BM, leading to higher power conversion efficiency (PCE) value as well as substantially enhanced thermal stability, as compared with the traditional 1,8-diiodooctane (DIO) additive. More importantly, the improved thermal stability after processing with BT contributes to a higher glass transition temperature (Tg ) of PTB7-Th, as determined by dynamic mechanical analysis. After thermal annealing at 130 °C in nitrogen atmosphere for 30 min, the PCE of the specimen processed with BT reduces from 9.3% to 7.1%, approaching to 80% of its original value. In contrast, the PCE of specimens processed with DIO seriously depresses from 8.3% to 3.8%. These findings demonstrate that smart utilization of low-boiling-point solvent additive is an effective and practical strategy to overcome thermal instability of organic solar cells via enhancing the Tg of donor polymer.

Butanedithiol Solvent Additive Extracting Fullerenes from Donor Phase To Improve Performance and Photostability in Polymer Solar Cells.

In this work, we demonstrated that the excited poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2,6-diyl)] (PTB7-Th) will be degraded by [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) or photolysis fragment of 1,8-diiodooctane (DIO) in the presence of oxygen and under irradiation of red light. From the previous reports, the fragment of DIO may be involved in the reaction directly. Our work indicates the PC71BM is not directly involved in the reaction, but is acting as a catalyst to promote the reaction of excited donors with oxygen. Thus, PTB7-Th urgently needs a kind of nonresidual iodine-free additive to replace DIO and remove the fullerene from the donor phase at the same time. Taking into consideration PC71BM solubility and boiling point difference between solvent additives and host solvents, 1,4-butanedithiol solvent was selected to fabricate PTB7-Th:PC71BM-based solar cells achieving a best power conversion efficiency (PCE) of 10.2% (8.5% for PTB7:PC71BM). Iodine-free butanedithiol can not only avoid excited polymer reacting with the photolysis fragment of DIO but also suppress the degradation of the excited PTB7-Th caused by synergistic effect between the fullerene and oxygen via extracting the free/trapped PC71BM from the donor phase. Eventually, the film prepared with 1,4-butanedithiol shows higher stability than the film prepared without any additives and much better than the film with DIO in macro-/micromorphology, light absorption, and device performance.

Surface treatment by binary solvents induces the crystallization of a small molecular donor for enhanced photovoltaic performance.

The surface treatment of the active layer with binary solvents composed of methanol (MeOH) and 1-chloronaphthalene (CN), was demonstrated to effectively improve the power conversion efficiency (PCE) from 2.4% to 6.5% for p-DTS(FBTTh2)2:PC71BM based small molecular solar cells. The optical properties and morphology of the p-DTS(FBTTh2)2:PC71BM films were carefully investigated. The results indicate that treatment with MeOH:CN binary solvents could significantly enhance the absorption of the active layer, due to the formation of more p-DTS(FBTTh2)2 nanofibrils associated with higher crystallinity as revealed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The two-dimensional grazing incidence wide-angle X-ray scattering (GIWAXS) results further demonstrate that the molecular packing of p-DTS(FBTTh2)2 molecules could be strongly enhanced after treatment with the binary solvents. In contrast, pristine methanol shows no significant influence on the crystalline structure, phase separation or the photovoltaic properties of the p-DTS(FBTTh2)2:PC71BM system, showing that the CN solvent plays the main role in inducing the crystallization of p-DTS(FBTTh2)2 molecules.

Fusing three perylenebisimide branches and a truxene core into a star-shaped chromophore with strong two-photon excited fluorescence and high photostability.

A novel star-shaped chromophore, Tr-PBI, was constructed by fusing three perylenebisimide branches and a truxene core. Tr-PBI exhibits high photostability and excellent two-photon properties: the maximum of δ(TPA) is 11,000 GM at 990 nm and fluorescence quantum efficiency Φ is 0.40 in THF.

Fused perylenebisimide-carbazole: new ladder chromophores with enhanced third-order nonlinear optical activities.

A series of ladder chromophores featuring planar structures of fused perylenebisimide and carbazole have been efficiently synthesized via photocyclization under sun light. Compared to N,N'-bis(3-pentyl) perylenebisimide (PBI-1), they show remarkably enhanced nonlinear properties.