Effect of Nitro-Substituted Ending Groups on the Photovoltaic Properties of Nonfullerene Acceptors

tert-Butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate

Non-fullerene acceptors have recently attracted much attention as components of organic solar cells. 1H-indene-1,3(2H)-dione is a key compound for the synthesis of the end-capping component of non-fullerene acceptors. In this communication, an intermediate for the synthesis of this compound, tert-butyl (E)-3-oxo-2-(3-oxoisobenzofuran-1(3H)-ylidene)butanoate, was prepared by the reaction between phthalic anhydride and tert-butyl acetoacetate. Further treatment with sodium methoxide in methanol led to the formation of 1H-indene-1,3(2H)-dione in a high yield. The structure of the newly synthesized compound was established by means of elemental analysis, high-resolution mass spectrometry, 1H, 13C NMR, IR spectroscopy, mass spectrometry and X-ray analysis.

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.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Structural Cutting of Non-fullerene Acceptors by Chlorination: Effects of Substituent Number on Device Performance

Effects of chlorination on photovoltaic performance of organic solar cells are yet largely unclear though it is emerging as a special yet effective strategy to design highly efficient non-fullerene acceptors (NFAs). Herein, a bi-chlorine-substituted NFA with regioregularity, namely, bichlorinated dithienothiophen[3.2-b]- pyrrolobenzothiadiazole (BTP-2Cl-δ), is synthesized and compared to the non-chlorinated BTP and tetra-chlorine-substituted BTP-4Cl to study the effects of Cl number on the photovoltaic performance. From BTP to BTP-2Cl-δ and BTP-4Cl, the three molecules show gradually red-shifted absorption peaks, narrowed band gaps, and lowered highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Polymer solar cells are fabricated using PM6 as the donor and the three small molecules as the acceptors. From BTP to BTP-2Cl-δ, efficiencies (8.8 vs 15.4%) are significantly enhanced due to the better film morphology and strong crystallization of the BTP-2Cl-δ-based device, giving rise to boosted fill factors (FFs) and short-circuit current densities (J_SC's). From BTP-2Cl-δ to BTP-4Cl, although J_SC's (24.3 vs 25.0 mA cm^-2) are slightly elevated due to the higher crystallinity of BTP-4Cl, leading to improved exciton dissociation and collection efficiencies, FFs (71.1 vs 68.0%) are obviously decreased owing to the unfavorable film morphology, unbalanced hole-electron mobilities, and higher charge recombination in BTP-4Cl-based devices. As such, the efficiency of the BTP-2Cl-δ-based device (15.4%) is superior to that of the BTP-4Cl-based device (14.5%). This work elucidates a design strategy by cutting the numbers of substituent chlorine to obtain desired energy levels and crystallization with optimal performance.