Effect of Side-Chain Modification on the Active Layer Morphology and Photovoltaic Performance of Liquid Crystalline Molecular Materials

A polymer library enables the rapid identification of a highly scalable and efficient donor material for organic solar cells

The dramatic improvement of the PCE (power conversion efficiency) of organic photovoltaic devices in the past few years has been driven by the development of new polymer donor materials and non-fullerene acceptors (NFAs). In the design of such materials synthetic scalability is often not considered, and hence complicated synthetic protocols are typical for high-performing materials. Here we report an approach to readily introduce a variety of solubilizing groups into a benzo[c][1,2,5]thiadiazole acceptor comonomer. This allowed for the ready preparation of a library of eleven donor polymers of varying side chains and comonomers, which facilitated a rapid screening of properties and photovoltaic device performance. Donor FO6-T emerged as the optimal material, exhibiting good solubility in chlorinated and non-chlorinated solvents and achieving 15.4% PCE with L8BO as the acceptor (15.2% with Y6) and good device stability. FO6-T was readily prepared on the gram scale, and synthetic complexity (SC) analysis highlighted FO6-T as an attractive donor polymer for potential large scale applications.

Organic Photovoltaics Utilizing Small-Molecule Donors and Y-Series Nonfullerene Acceptors

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure-property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

Progress and Future Potential of All-Small-Molecule Organic Solar Cells Based on the Benzodithiophene Donor Material

Organic solar cells have obtained a prodigious amount of attention in photovoltaic research due to their unique features of light weight, low cost, eco-friendliness, and semitransparency. A rising trend in this field is the development of all-small-molecules organic solar cells (ASM-OSCs) due to their merits of excellent batch-to-batch reproducibility, well-defined structures, and simple purification. Among the numerous organic photovoltaic (OPV) materials, benzodithiophene (BDT)-based small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking the 17% efficiency barrier in single-junction OPV devices, indicating the significant potential of this class of materials in commercial photovoltaic applications. This review specially focuses on up-to-date information about improvements in BDT-based ASM-OSCs since 2011 and provides an outlook on the most significant challenges that remain in the field. We believe there will be more exciting BDT-based photovoltaic materials and devices developed in the near future.

Selenophene-Containing Small-Molecule Donor with a Medium Band Gap Enables High-Efficiency Ternary Organic Solar Cells

To construct efficient donor:donor:acceptor (D1:D2:A)-type ternary devices, two new selenophene-containing small-molecule (SM) donors named FSBTSeEHR and FSBTSeHR have been designed and synthesized that show broader and red-shifted absorption spectra than the thiophene analogues. With the introduction of SM donors into the D18:CH-6F host system, enhanced light harvesting and charge transport were achieved, benefiting from more complementary absorptions and cascaded energy levels. Furthermore, the doping of the SM donor could effectively modulate the micromorphology and enable a more suitable phase separation size in the active layer. After systematic optimization, the FSBTSeEHR-based ternary organic solar cell (TOSC) exhibited an excellent power conversion efficiency (PCE) of 18.02% with a high open-circuit voltage (Voc) of 0.905 V, short-circuit current density (Jsc) of 26.41 mA cm-2, and fill factor (FF) of 0.754. By contrast, the FSBTSeHR counterpart showed a superior efficiency of 18.55% due to the higher Jsc (26.91 mA cm-2) and FF (0.761). The outstanding PCEs of D1:D2:A-type TOSCs based on our SM donors, FSBTSeEHR and FSBTSeHR, are significantly higher than those of the corresponding binary host system (16.86%) and among the highest values reported to date. This work demonstrates that D1:D2:A-type TOSCs have tremendous potential to achieve superior performances through elaborate design of the SM donor guest and reasonable combination of D and A ingredients.

Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

Crystallinity has played a major role in organic solar cells (OSCs). In small molecule (SM) bulk-heterojunction (BHJ) OSCs, the crystallinity and crystalline packing of SM donors have been shown to have a dramatic impact on the formation of an optimum microstructure leading to high-power conversion efficiency (PCE). Herein we describe how crystallinity differs from polymers to SMs, and how the packing habits of SMs (particularly donors) in active layers of BHJ devices can be described as following two different main modes: a single crystal-like and a liquid crystal-like packing type. This notion is reviewed from a chronological perspective, emphasising milestone donor structures and studies focusing on the crystallinity in SM-BHJ OSCs. This review intends to demonstrate that a shift towards a liquid crystalline-like packing can be identified throughout the history of SM-BHJ, and that this shift can be associated with an increase in overall PCE.