Non-fullerene Small Molecular Acceptors Research Articles

Spirobifluorene-cored wide bandgap non-fullerene small molecular acceptor with 3D structure for organic solar cells

In this paper, a wide bandgap small molecule (SF(BR)4) based on spirobifluorene (SF) core, which is flanked with 2,1,3-benzothiadiazole and end-capped with 3-ethylrhodanine groups is synthesized and investigated as a non-fullerene acceptor in solution-processed organic solar cells (OSCs). The SF(BR)4 shows a wide optical bandgap of 2.10 eV and great advantages with respect to the low bandgap donor material poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)2,2′; 5′,2″; 5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD), such as well energy-level matching and complementary absorption. Furthermore, the three dimensional (3D) molecular geometry of SF(BR)4 helps to suppress the strong intermolecular aggregation in PffBT4T-2OD:SF(BR)4 blend, leading to efficient non-fullerene OSCs. The resultant devices show a power conversion efficiency (PCE) of 6.15%. To our best knowledge, the PCE is among the highest reported to date for 3D non-fullerene acceptors except perylene diimides derivatives.

The progress of non-fullerene small molecular acceptors for high efficiency polymer solar cells

In the past few decades, polymer solar cells (PSCs) have gained great progress. And fullerene PSCs occupy a dominant position in the early stage of the development of PSCs. Recently, non-fullerene PSCs have gained more attention because of their advantages, such as good absorption in the visible and near infra-red (NIR) region, high tunability of energy levels, better complementary absorption spectra with donors. Thus, numerous efforts have focused on the synthesis and modulation of small molecular acceptor (SMAs). And lots of non-fullerene PSCs have achieved higher power conversion efficiencies than fullerene PSCs. This short review focuses on the progress of non-fullerene small-molecule acceptors for high efficiency PSCs in recent years. And it would provide new ideas for designing high efficiency PSCs.

Enhancing the Performance of Organic Solar Cells by Hierarchically Supramolecular Self-Assembly of Fused-Ring Electron Acceptors

Three novel non-fullerene small molecular acceptors ITOIC, ITOIC-F, and ITOIC-2F were designed and synthesized with easy chemistry. The concept of supramolecular chemistry was successfully used in the molecular design, which includes noncovalently conformational locking (via intrasupramolecular interaction) to enhance the planarity of backbone and electrostatic interaction (intersupramolecular interaction) to enhance the π–π stacking of terminal groups. Fluorination can further strengthen the intersupramolecular electrostatic interaction of terminal groups. As expected, the designed acceptors exhibited excellent device performance when blended with polymer donor PBDB-T. In comparison with the parent acceptor molecule DC-IDT2T reported in the literature with a power conversion efficiency (PCE) of 3.93%, ITOIC with a planar structure exhibited a PCE of 8.87% and ITOIC-2F with a planar structure and enhanced electrostatic interaction showed a quite impressive PCE of 12.17%. Our result demonstrates the import...

Silaindacenodithiophene‐Based Fused‐Ring Non‐Fullerene Electron Acceptor for Efficient Polymer Solar Cells

AbstractIn this work, a new A‐D‐A type nonfullerene small molecular acceptor SiIDT‐IC, with a fused‐ring silaindacenodithiophene (SiIDT) as D unit and 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (INCN) as the end A unit, was design and synthesized. The SiIDT‐IC film shows absorption peak and edge at 695 and 733 nm, respectively. The HOMO and LUMO of SiIDT‐IC are of −5.47 and −3.78 eV, respectively. Compared with carbon‐bridging, the Si‐bridging can result in an upper‐lying LUMO level of an acceptor, which is benefit to achieve a higher open‐circuit voltage in polymer solar cells (PSCs). Complementary absorption and suitable energy level alignment between SiIDT‐IC and wide bandgap polymer donor PBDB‐T were found. For the PBDB‐T:SiIDT‐IC based inverted PSCs, a D/A ratio of 1: 1 was optimal to achieve a power conversion efficiency (PCE) of 7.27%. With thermal annealing (TA) of the blend film, a higher PCE of 8.16% could be realized due to increasing of both short‐circuit current density and fill factor. After the TA treatment, hole and electron mobilities were elevated to 3.42 × 10−4 and 1.02 × 10−4 cm2·V−1·s−1, respectively. The results suggest that the SiIDT, a Si‐bridged fused ring, is a valuable D unit to construct efficient nonfullerene acceptors for PSCs.

High-Performance Additive-/Post-Treatment-Free Nonfullerene Polymer Solar Cells via Tuning Molecular Weight of Conjugated Polymers.

In recent years, rapid advances are achieved in polymer solar cells (PSCs) using nonfullerene small molecular acceptors. However, no research disclosing the influence of molecular weight (Mn ) of conjugated polymer on the nonfullerene device performance is reported. In this work, a series of polymers with different Mn s are synthesized to systematically investigate the connection between Mn and performance of nonfullerene devices for the first time. It is found that the device performance improves substantially as the Mn increases from 12 to 38 kDa and a power conversion efficiency (PCE) as high as 10.5% is realized. It has to be noted this PCE is achieved without using any additives and post-treatments, which is among the top efficiencies of additive- and post-treatment-free PSCs. Most importantly, the variation trend of the optimal active layer thickness and morphology is significantly different from the device with fullerene as acceptor. The findings clarify the effect of Mn on the performance of nonfullerene PSCs, which would benefit further efficiency improvement of nonfullerene PSCs.

Absorptive Behaviors and Photovoltaic Performance Enhancements of Alkoxy-Phenyl Modified Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptors

Nonfullerene (NF) small molecular acceptors are very attractive for further improving the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) to overcome the limited absorptive region and fixed-energy-level drawbacks of fullerene-based electronic acceptors (PC61BM and PC71BM). The acceptor–donor–acceptor (A-D-A)-type oligomers (ITIC) containing an electron-rich core (four hexyl-phenyl-substituted indacenodithieno[3,2-b]thiophene) as a donor motif sealed with 2-(3-oxo-2,3-dihydroinden-1-ylidene)-malononitrile as an acceptor motif has been intensively investigated, because of its excellent absorptive and photovoltaic properties. Side-chain modifications have been proven to be an effective approach to modulate the energy levels and absorptive behaviors of conjugated polymers, as well as conjugated small molecules. Through the introduction of various side-chain and end groups, a series of promisingly modified ITIC-based small molecules have been synthesized and well-studied. Herein, we reported...

N-Annulated perylene diimide derivatives as non-fullerene acceptors for solution-processed solar cells with an open-circuit voltage of up to 1.14 V

Three different non-fullerene small molecular acceptors containing N-annulated perylene diimide, named di-PNR, TPA-PNR and EDOT-PNR, were successfully designed and synthesized for photovoltaic applications.

Reducing the confinement of PBDB-T to ITIC to improve the crystallinity of PBDB-T/ITIC blends

The ordered aggregation of non-fullerene small molecular acceptors (SMAs) plays a key role in determining the charge transport and bimolecular recombination in polymer/SMA solar cells.

Influence of Donor Polymer on the Molecular Ordering of Small Molecular Acceptors in Nonfullerene Polymer Solar Cells

AbstractNonfullerene polymer solar cells (PSCs) based on polymer donors and nonfullerene small molecular acceptors (SMAs) have recently attracted considerable attention. Although much of the progress is driven by the development of novel SMAs, the donor polymer also plays an important role in achieving efficient nonfullerene PSCs. However, it is far from clear how the polymer donor choice influences the morphology and performance of the SMAs and the nonfullerene blends. In addition, it is challenging to carry out quantitative analysis of the morphology of polymer:SMA blends, due to the low material contrast and overlapping scattering features of the π–π stacking between the two organic components. Here, a series of nonfullerene blends is studied based on ITIC‐Th blended with five different donor polymers. Through quantitative morphology analysis, the (010) coherence length of the SMA is characterized and a positive correlation between the coherence length of the SMA and the device fill factor (FF) is established. The study reveals that the donor polymer can significantly change the molecular ordering of the SMA and thus improve the electron mobility and domain purity of the blend, which has an overall positive effect that leads to the enhanced device FF for nonfullerene PSCs.

Thieno[3,2-b]pyrrolo-Fused Pentacyclic Benzotriazole-Based Acceptor for Efficient Organic Photovoltaics

A novel nonfullerene small molecular acceptor (BZIC) based on a ladder-type thieno[3,2-b]pyrrolo-fused pentacyclic benzotriazole core (dithieno[3,2-b]pyrrolobenzotriazole, BZTP) and end-capped with 1,1-dicyanomethylene-3-indanone (INCN) has been first reported in this work. Through introducing multifused benzotriazole and INCN, BZIC could maintain a high-lying lowest unoccupied molecular orbital (LUMO) energy level of -3.88 eV. Moreover, BZIC shows a low optical bandgap of 1.45 eV with broad and efficient absorption band from 600 to 850 nm due to increased π-π interactions by the covalently locking thiophene and benzotriazole units. A power conversion efficiency of 6.30% is delivered using BZIC as nonfullerene acceptor and our recently synthesized hexafluoroquinoxaline-based polymer HFQx-T as donor. This is the first time to synthesize mutifused benzotriazole-based molecules as nonfullerene electron acceptor up to date. The preliminary results demonstrate that the mutifused benzotriazole derivatives hold great potential for efficient photovoltaics.

Recent development of perylene diimide-based small molecular non-fullerene acceptors in organic solar cells

Recently, perylene diimide (PDI) derivatives were attractive as the electron-deficient acceptor materials in non-fullerene organic solar cells since Tang first used a single PDI compound as the n-type semiconductor to fabricate photovoltaic devices in 1986, which achieved a power conversion efficiency of 1%. Beside the monomeric PDIs, the linear and three dimensional (3D) PDI-based small molecular acceptors have also made great achievements with the power conversion efficiencies over 9.0% in single-junction polymer solar cells, and over 10.0% in tandem solar cells. The excellent device performance can be realized by forming suitable twisted structure, developing suitable donor materials and optimizing device technologies. In this review, we summarize the recent development of PDI-based small molecular non-fullerene acceptors in non-fullerene organic solar cells, including molecular design strategies and structure-property relationships.

Non-fullerene small molecular acceptors with a carbazole core for organic solar cells with high open-circuit voltage

Two novel carbazole-cored small molecules CzC6C8 and CzC8 bearing different side chains and two 3-ethylrhodanine end-capping groups were synthesized and used as electron acceptors in organic solar cells (OSCs). Devices based on these two acceptors showed a high open circuit voltage (Voc) about 1 V. Due to more closely molecular stacking and improved film morphology, CzC8 bearing a linear alkyl side chain showed a better photovoltaic performance than CzC6C8 bearing a branched one. CzC8 based OSCs can still exhibit a good performance even though the ΔELUMO and ΔEHOMO between donor and acceptor are both small. Three polymer donors (PBT, PTB7-Th, and PTFBDT) with a similar LUMO level but various HOMO levels were tested and they all showed good PCE values larger than 4.6%. Among them, PTB7-Th:CzC8 based devices afforded the highest PCE of 4.91% and PTFBDT:CzC8 based ones gave the highest Voc of 1.16 V.

Vinylene- and ethynylene-bridged perylene diimide dimers as nonfullerene acceptors for polymer solar cells

Vinylene (V)- and ethynylene (E)-bridged perylene diimide dimers (PDI-V and PDI-E) were designed, synthesized and used as nonfullerene acceptors for polymer solar cells. Our researches revealed that the linkage between two PDI units has a great impact on the molecular geometry, the optical properties, the blend film morphology, the molecular packing orientation, and the photovoltaic performance. Computational calculations via density functional theory (DFT) showed that PDI-E and PDI-V possessed planar and twisted geometric structures, respectively. TEM investigations showed that PTB7-Th:PDI-V based blend film exhibited a uniform morphology with small domain size and PTB7-Th:PDI-E based one showed apparent phase separation with large domain size. GIWAXS results revealed that the PDI-V can influence PTB7-Th to take on a face-on orientation, which is beneficial for vertical charge transport to increase Jsc. A PCE of 4.51% with a Voc of 0.76 V, a Jsc of 10.03 mA cm−2, and an FF of 0.59 was obtained for PSCs based on PTB7-Th:PDI-V, which is almost two times higher than that of PTB7-Th:PDI-E based devices, which showed a PCE of 2.66%, a Voc of 0.66 V, a Jsc of 7.33 mA cm−2, and an FF of 0.55. These results help to gain deeper insight into the design of new nonfullerene small molecular acceptors for high efficiency PSCs.

Non-fullerene small molecular acceptors based on dithienocyclopentafluorene and dithienocyclopentacarbazole cores for polymer solar cells

Recently, acceptor-donor-acceptor (A-D-A) type of small molecules based on indacenodithieno[3,2-b]thiophene (IDTT) or indacenodithiophene (IDT) as the electron-donating cores were very promising non-fullerene (NF) acceptors to overcome the energy levels and absorption drawbacks of the fullerene-based acceptors in polymer solar cells (PSCs). However, few attempts have been done to develop novel electron-donating cores to construct highly efficient A-D-A acceptors in PSCs. We design and prepare two novel non-fullerene small molecular acceptors (DTF-IC and DTC-IC) based on dithienocyclopentafluorene and dithienocyclopentacarbazole as the electron-donating cores sealed with 2-(3-oxo-indane-1-ylidene)-malononitrile as the electron-withdrawing groups. The two molecules exhibit relatively high lowest unoccupied molecular orbital (LUMO) energy levels and strong absorption in the region from 550 to 740 nm. Relative to the molecule DTF-IC possessing dithienocyclopentafluorene core, the dithienocyclopentacarbazole based acceptor DTC-IC shows slightly red-shifted absorption spectrum, enhanced extinction coefficients, better crystallinity and improved electron mobility. The polymer solar cells based on these two NF acceptors exhibit power conversion efficiencies of 7% when blended with the narrow bandgap polymer poly({4,8-bis[(5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th) in inverted solar cells.

Small molecular non-fullerene acceptors based on naphthalenediimide and benzoisoquinoline-dione functionalities for efficient bulk-heterojunction devices

Through the conjunction of naphthalenediimide and benzoisoquinoline-dione functionalities, two novel, solution-processable non-fullerene electron acceptors, 2,7-dioctyl-4,9-bis(2-octyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (coded as NDI-N1) and 2,7-dioctyl-4,9-bis(5-(2-octyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)thiophen-2-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (coded as NDI-N2), were designed, synthesized, and used as active components for bulk-heterojunction photovoltaic devices. The new chromophores were based on the acceptor–acceptor–acceptor module where naphthalenediimide unit served as the central acceptor flanked by terminal benzoisoquinoline-dione acceptor functionality. The optoelectronic and photovoltaic properties of NDI-N1 and NDI-N2 were directly compared. Solution-processable bulk-heterojunction devices were fabricated using NDI-N1 and NDI-N2 as non-fullerene electron acceptor materials. Studies on the photovoltaic properties revealed that the best poly(3-hexylthiophene) (P3HT): NDI-N2-based device showed an impressive enhanced power conversion efficiency of 4.04%, around 40% increase with respect to the efficiency of the best NDI-N1-based device (2.91%). It is justifiable to mention that the device outcome reported herein provides strong support and incentive for the current research strategy, and that the conjoint use of potential acceptor building blocks, such as naphthalenediimide and benzoisoquinoline-dione, can indeed generate efficient non-fullerene acceptors.

The investigations of two conjugated polymers that show distinctly different photovoltaic properties in polymer solar cells

In fullerene-free polymer solar cells (PSC), the non-fullerene small molecular acceptors (NF-SM) demonstrates quite different film morphology in comparison with the conventional fullerene acceptors when blended with the same conjugated polymer donor. In this work, a NF-SM acceptor, ITIC, was introduced into two efficient binary PSCs based on fullerene to fabricate ternary solar cells. It is found that the addition of ITIC led fundamentally different results. After carefully investigated the difference on film morphology and charge carriers' mobility, the results showed that the good miscibility between ITIC and the polymer may deteriorate the favorable film morphology with appropriate phase separation and suppress the formation of continues charge transport channel. This work offered a useful consideration to produce high-performance ternary PSCs by rational selecting the donors and acceptors.

An H-shaped, small molecular non-fullerene acceptor for efficient organic solar cells with an impressive open-circuit voltage of 1.17 V

An H-shaped, non-fullerene small molecular electron acceptor displayed promising optoelectronic properties and afforded an encouraging efficiency of 5.42%.

New developments in non-fullerene small molecule acceptors for polymer solar cells

This review describes developments of non-fullerene small molecular acceptors in solar cells since 2015, including rylene imide, indacenodithiophene and diketopyrrolopyrrole.

High‐Efficiency Nonfullerene Organic Solar Cells: Critical Factors that Affect Complex Multi‐Length Scale Morphology and Device Performance

Organic solar cells (OSCs) made of donor/acceptor bulk‐heterojunction active layers have been of widespread interest in converting sunlight to electricity. Characterizing of the complex morphology at multiple length scales of polymer:nonfullerene small molecular acceptor (SMA) systems remains largely unexplored. Through detailed characterizations (hard/soft X‐ray scattering) of the record‐efficiency polymer:SMA system with a close analog, quantitative morphological parameters are related to the device performance parameters and fundamental morphology–performance relationships that explain why additive use and thermal annealing are needed for optimized performance are established. A linear correlation between the average purity variations at small length scale (≈10 nm) and photovoltaic device characteristics across all processing protocols is observed in ≈12%‐efficiency polymer:SMA systems. In addition, molecular interactions as reflected by the estimated Flory–Huggins interaction parameters are used to provide context of the room temperature morphology results. Comparison with results from annealed devices suggests that the two SMA systems compared show upper and lower critical solution temperature behavior, respectively. The in‐depth understanding of the complex multilength scale nonfullerene OSC morphology may guide the device optimization and new materials development and indicates that thermodynamic properties of materials systems should be studied in more detail to aid in designing optimized protocols efficiently.

Nonfullerene Small Molecular Acceptors with a Three-Dimensional (3D) Structure for Organic Solar Cells

Two spirobifluorene (SF)-functioned 3D nonfullerene electron acceptors—SF-OR and SF-ORCN—that use rhodanine and 2-(1,1-dicyanomethylene)rhodanine as the terminal units, respectively, were designed and synthesized. These new acceptor materials show reversible electrochemical reduction and a high optical absorption coefficient, which are critical in device operation. Electronic and structural characterizations reveal that the inclusion of cyano group on rhodanine improve the electron accepting ability at the sacrifice of structure order, since the conjugated backbone becomes less planar, which prohibits cofacial stacking. SF-OR and SF-ORCN show good photovoltaic performances when paired with a poly(3-hexylthiophene) (P3HT) donor, and the optimized devices give power conversion efficiencies of 4.66% and 4.48%, respectively. These values are higher than that of fullerene acceptor-based control devices (3.55%), which are also among the best in small molecular nonfullerene acceptor organic solar cells based on ...