Using an external electric field to tune active layer morphology enabling high-efficiency organic solar cells via ambient blade coating.

The nanoscale morphology of the photoactive layer notably impacts the performance of organic solar cells (OSCs). Conventional methods to tune the morphology are typically chemical approaches that adjust the properties (such as solubility and miscibility) of the active components including donor, acceptor, and/or additive. Here, we demonstrate a completely different approach by applying an external electric field (EEF) on the active layer during the wet coating. The EEF-coating method is perfectly compatible with an ambient blade coating using environmentally friendly solvents, which are essential requirements for industrial production of OSCs. A record 18.6% efficiency is achieved using the EEF coating, which is the best value for open-air, blade-coated OSCs to date. Our findings suggest broad material applicability and attribute-enhanced performance to EEF-induced fiber formation and long-range ordering of microstructures of acceptor domains. This technique offers an effective method for producing high-performance OSCs, especially suited for industry OSC production based on open-air printing.

Desirable Uniformity and Reproducibility of Electron Transport in Single-Component Organic Solar Cells.

Despite the simplified fabrication process and desirable microstructural stability, the limited charge transport properties of block copolymers and double-cable conjugated polymers hinder the overall performance of single-component photovoltaic devices. Based on the key distinction in the donor (D)-acceptor (A) bonding patterns between single-component and bulk heterojunction (BHJ) devices, rationalizing the difference between the transport mechanisms is crucial to understanding the structure-property correlation. Herein, the barrier formed between the D-A covalent bond that hinders electron transport in a series of single-component photovoltaic devices is investigated. The electron transport in block copolymer-based devices is strongly dependent on the electric field. However, these devices demonstrate exceptional advantages with respect to the charge transport properties, involving high stability to compositional variations, improved film uniformity, and device reproducibility. This work not only illustrates the specific charge transport behavior in block copolymer-based devices but also clarifies the enormous commercial viability of large-area single-component organic solar cells (SCOSCs).

Fluorination of Terminal Groups Promoting Electron Transfer in Small Molecular Acceptors of Bulk Heterojunction Films.

The fluorination strategy is one of the most efficient and popular molecular modification methods to develop new materials for organic photovoltaic (OPV) cells. For OPV materials, it is a broad agreement that fluorination can reduce the energy level and change the morphology of active layers. To explore the effect of fluorination on small molecule acceptors, we selected two non-fullerene acceptors (NFA) based bulk heterojunction (BHJ) films, involving PM6:Y6 and PM6:Y5 as model systems. The electron mobilities of the PM6:Y5 and PM6:Y6 BHJ films are 5.76 × 10-7 cm2V-1s-1 and 5.02 × 10-5 cm2V-1s-1 from the space-charge-limited current (SCLC) measurements. Through molecular dynamics (MD) simulation, it is observed that halogen bonds can be formed between Y6 dimers, which can provide external channels for electron carrier transfer. Meanwhile, the "A-to-A" type J-aggregates are more likely to be generated between Y6 molecules, and the π-π stacking can be also enhanced, thus increasing the charge transfer rate and electron mobility between Y6 molecules.

Ruthenium(II)-catalyzed deoxygenation of ketones.

The classical Wolff-Kishner reduction plays a key role in organic synthesis to convert carbonyl functionalities into methylene groups; however, it generally requires harsh reaction conditions and a strategy with wider applications demands further development. Herein, a ruthenium-catalyzed Wolff-Kishner type reduction of ketones is developed with 31 examples under mild conditions. This strategy tolerates aryl and alkyl ketones with reactive functional groups including halogens, hydroxyls, carboxylic acid, unsaturated functional groups, and so on. The corresponding methylene products were obtained in 32% to 95% yields while using water or methanol as solvents.

Reproducibility in Time and Space-The Molecular Weight Effects of Polymeric Materials in Organic Photovoltaic Devices.

The reproducibility issue is one of the major challenges for the commercialization of large-area organic electronic devices. It involves both the device-to-device variation and opto-electronic properties in different positions of a single thin film. Herein, the molecular weight effects in polymeric semiconductors with three widely used photovoltaic donor materials P3HT, PBDB-T, and PM6 are systematically investigated. A simple but effective method is proposed to evaluate the uniformity of large-area devices by adopting the micron-level grid electrodes in organic thin films. An interesting phenomenon is observed that the device is gradually improved uniformly with the Mw range lower than 100 kg mol-1 . In neat films, both the mobility and energetic disorder values of hole carriers exhibit relatively lower coefficient of variation (cv ) in high molecular-weight systems. After blending with the electron-accepting materials, their bulk heterojunction films also enjoy more uniform hole transfer rates, fluorescence lifetimes, and power conversion efficiencies in single and different devices. This work not only proposes a facile approach to evaluate the electrical properties of large-area organic thin films, but also demonstrates the relationship between molecular weight and device reproducibility in polymer solar cells. This contribution provides a new insight into the commercial large-scale production of organic electronics.

An Aggregation-Suppressed Polymer Blending Strategy Enables High-Performance Organic and Quantum Dot Hybrid Solar Cells.

Solution-processing hybrid solar cells with organics and colloidal quantum dots (CQDs) have drawn substantial attention in the past decade. Nevertheless, hybrid solar cells based on the recently developed directly synthesized CQD inks are still unexplored. Herein, a facile polymer blending strategy is put forward to enable directly synthesized CQD/polymer hybrid solar cells with a champion efficiency of 13%, taking advantage of the conjugated polymer blends with finely optimized aggregation behaviors. The spectroscopic and electrical investigations on carrier transport and recombination indicate that polymer blends can endow fast carrier transport and less recombination over the single counterparts. Moreover, the blending strategy offers a "dilution effect" for top-notch photovoltaic polymers with excessively strong aggregation tendency, resulting in moderate feature domain size and surface roughness, which afford fast hole transport and therefore high photovoltaic performance. The effectiveness of this strategy is successfully validated using two pairs of photovoltaic polymers. Accordingly, the relationships between polymer morphology, carrier transport, and photovoltaic performance are established to advance the progress of CQD/polymer hybrid solar cells. Such progress stresses that the utilization of aggregation-suppressed polymer blends is a facile approach toward the fabrication of high-efficiency organic-inorganic hybrid solar cells.

Chemoselective Synthesis of Z-Olefins through Rh-Catalyzed Formate-Mediated 1,6-Reduction.

Z-olefins are important functional units in synthetic chemistry; their preparation has thus received considerable attention. Many prevailing methods for cis-olefination are complicated by the presence of multiple unsaturated units or electrophilic functional groups. In this study, Z-olefins are delivered through selective reduction of activated dienes using formic acid. The reaction proceeds with high regio- and stereoselectivity (typically >90:10 and >95:5, respectively) and preserves other alkenyl, alkynyl, protic, and electrophilic groups.