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Abstract

Systematic side-chain engineering on nonacyclic acceptor-donor-acceptor (A-D-A) nonfullerene acceptors, NNFA[n, m], was carried out. “n” and “m” stand for the number of alkyl carbon atoms in the in-plane and out-of-plane side chains, respectively. Five acceptors, NNFA[0, 6], NNFA[6, 3], NNFA[6, 6], NNFA[12, 3] and NNFA[12, 6], were prepared and applied in organic solar cells by blending with a wide-bandgap copolymer donor (FTAZ). The alkyl chains substantially affect NNFAs’ solubility and photovoltaic performance. The solubility varies from 23 mg/mL to 226 mg/mL in chloroform when changing the total alkyl carbons (2n + 4m). If the total alkyl carbons are equal, NNFA with longer out-of-plane alkyl chains (higher “m”) shows higher solubility than that with longer in-plane alkyl chains (higher “n”). NNFA[6, 6] and NNFA[12, 3] with medium solubility (100 mg/mL) present suitable miscibility with FTAZ, and afford more favorable morphology and higher device performance than other NNFAs. FTAZ:NNFA[12, 3] solar cells gave the highest power conversion efficiency of 10.81%.

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... In 2015, the report of acceptor-donor-acceptor (ADA) small molecule acceptor ITIC (Fig. 1a) opened a new era for organic solar cells (OSCs). 1 Compared with traditional fullerene acceptors, the ADA acceptors show many advantages like strong visible to near-infrared (NIR) light-harvesting capability, easily adjustable chemical structures and energy levels, small energy loss and good morphological stability. [2][3][4][5][6][7][8][9][10][11][12][13] Much higher power conversion efficiencies (PCEs) have been achieved by ADA-acceptor-based solar cells. [14][15][16][17][18] Recent years, developing asymmetric ADA acceptors has attracted considerable research interests. ...
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... However, it delivered only 6.84% PCE in fullerene cells due to low J sc . In this work, we investigated the performance of PDTPO-BDTT in NFA cells by blending it with IT-4F, 7 NNFA-4F, 8 and CO i 8DFIC, 9 respectively. All PDTPO-BDTT:NFA solar cells gave broad external quantum efficiency (EQE) spectra and high J sc . ...
Article
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Article
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Article
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Article
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Article
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Article
Three nonfullerene acceptors CO5DFIC, CO5DFIC-OT and CO5DFIC-ST were developed. CO5DFIC-OT and CO5DFIC-ST have alkoxythiophene and alkylthiothiophene -bridges, respectively, and they show higher LUMO and enhanced light-harvesting capability than CO5DFIC without -bridges. CO5DFIC-OT and CO5DFIC-ST solar cells gave higher open-circuit voltage, short-circuit current density and power conversion efficiency than CO5DFIC cells.
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Organic photovoltaics (OPVs) represent one of the potential candidates of next-generation solar cells for converting the green and sustainable solar energy into electrical power. An OPV cell utilizes a blend of electron donor (D) and electron acceptor (A) materials as the photo-active layer, where the photogenerated excitons are separated into mobile electrons and holes. Relative to the fullerene acceptors, nonfullerene small-molecule acceptors (NF-SMAs) have several advantages such as the synthesis-facile chemical modifications and straightforward tunability in the absorptivity, spectral coverage, optical band gap, and frontier molecular orbitals. In recent 3 years, the progress in design and synthesis of the fused-ring NF-SMAs with perpendicular side-chains on the electron-rich core, and again, on the design and synthesis of the wide/medium/low band gap polymer donors have led to realizations of over 13% power conversion efficiencies (PCEs). The rapid advances requires timely review articles. In this review article, we will focus on this type of fused-ring NF-SMAs reported in the past 3 years, with sepcial attention on their molecular structure design and structure-property relationship.
Article
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Over the past three years, a particularly exciting and active area of research within the field of organic photovoltaics has been the use of non-fullerene acceptors (NFAs). Compared with fullerene acceptors, NFAs possess significant advantages including tunability of bandgaps, energy levels, planarity and crystallinity. To date, NFA solar cells have not only achieved impressive power conversion efficiencies of ~13–14%, but have also shown excellent stability compared with traditional fullerene acceptor solar cells. This Review highlights recent progress on single-junction and tandem NFA solar cells and research directions to achieve even higher efficiencies of 15–20% using NFA-based organic photovoltaics are also proposed. This Review describes how non-fullerene electron acceptor materials are bringing improvements in the power conversion efficiency and stability of organic solar cells.
Article
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Molecular engineering of nonfullerene acceptors (NFAs) plays a vital role in the development of organic photovoltaics. Oxygen as an electron donating atom is incorporated into the NFA system as alkoxyl forms at central, terminal, or central conjugated moieties due to the tunability at structural conformation, solubility, electron donating ability, absorption, energy levels, etc. In this work, a novel dipyran-based ladder-type building block (Ph-DTDP), which possesses two oxygen atoms in the conjugated skeleton, is designed and facilely synthesized. It is applied as the donor core for the acceptor–donor–acceptor-type NFA design and such functionalized-O efficiently enhances the electron donating ability, lowers the band gap, redshifts and extends the absorption spectra. In addition, the π-bridge effects are considered as well. Photovoltaic performances are systematically investigated and a high power conversion efficiency of 9.21% can be afforded with an energy loss of 0.57 eV. Meanwhile, the morphologies as well as the carrier mobilities of the blend films are studied to assist further understanding of the structure–property relationships. Overall, the study in this work provides a new promising ladder-type dipyran building block and brings in a novel way to use oxygen in NFA molecular structure design.
Article
Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications.
Article
A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600-950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 105 m-1 cm-1 , low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10-3 cm2 V-1 s-1 . Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.
Article
Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.
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