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A Fused Ring Electron Acceptor with Decacyclic Core Enables over 13.5% Efficiency for Organic Solar Cells

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... [52][53][54] To narrow the bandgaps of NFAs, one can enhance the electron-donating ability of the donor core using the following approaches (shown in Figure 2): 1) Extending the effective π-conjugation length. [21,[55][56][57][58] For example, by extending the donor core from a five-fused-ring (e.g., IDTIC) to a seven-fused-ring (e.g., ITIC), the bandgap can be reduced from 1.70 to 1.59 eV; [21,59] by extending the donor core from naphthalene-based IHIC2 to a naphthodithiophene-based IOIC2 and then to a fused decacyclic donor core based IDCIC, the absorption onset shows a redshift from 745 to 801 nm and to 853 nm, and the maximum extinction coefficient increases from 1.6 × 10 5 to 1.8 × 10 5 m −1 cm −1 and to 3.3 × 10 5 m −1 cm −1 . As a result, the J SC of the corresponding devices increases from 16.1 to 19.7 mA cm −2 and to 21.98 mA cm −2 . ...
... As a result, the J SC of the corresponding devices increases from 16.1 to 19.7 mA cm −2 and to 21.98 mA cm −2 . [55][56] 2) Introducing electron-donating groups. [60][61][62] For example, after introducing alkoxythiophene to IDTIC, the bandgap of IEICO is reduced to 1.34 eV. ...
... Post-Treatment Process: For specific donor/acceptor blends, various strategies of the post-treatment process for the morphology optimization have been reported, including the selection of processing solvent and additive, thermal annealing (TA), solvent annealing, and post solvent treatment for post-film morphology control. [86,87,[122][123][124] We will emphasize the posttreatment process, for example, the additive, [56,123,[125][126][127] ternary strategy, [80,83,[128][129][130][131] and interfacial modification [132][133][134][135] here, which are the most used strategies in OSCs in recent years. ...
Article
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The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non‐fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single‐junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs.
... The slightly red-shifted absorption in DIO-added blend further supports this assignment [24]. It has been reported that PBnDT-FTAZ also shows more J-like behavior when it is blended with non-fullerene acceptor and DIO is added [35]. The observed strong J-aggregation leads to a stronger intrachain exciton coupling, planarization of the polymer backbone, enhanced crystallinity, and a higher hole mobility [24,36]. ...
... The slightly redshifted absorption in DIO-added blend further supports this assignment [24]. It has been reported that PBnDT-FTAZ also shows more J-like behavior when it is blended with nonfullerene acceptor and DIO is added [35]. The observed strong J-aggregation leads to a stronger intrachain exciton coupling, planarization of the polymer backbone, enhanced crystallinity, and a higher hole mobility [24,36]. ...
Article
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Optimization of charge generation in polymer blends is crucial for the fabrication of highly efficient polymer solar cells. While the impacts of the polymer chemical structure, energy alignment, and interface on charge generation have been well studied, not much is known about the impact of polymer aggregation on charge generation. Here, we studied the impact of aggregation on charge generation using transient absorption spectroscopy, neutron scattering, and atomic force microscopy. Our measurements indicate that the 1,8-diiodooctane additive can change the aggregation behavior of poly(benzodithiophene-alt-dithienyl difluorobenzotriazole (PBnDT-FTAZ) and phenyl-C61-butyric acid methyl ester (PCBM)polymer blends and impact the charge generation process. Our observations show that the charge generation can be optimized by tuning the aggregation in polymer blends, which can be beneficial for the design of highly efficient fullerene-based organic photovoltaic devices.
... Throughout the development of OSCs, it has been widely accepted that a combination of polymeric donor and SM acceptor as the photoactive layer is the most successful and prominent strategy for OSCs , Zhao et al., 2017b, He et al., 2018, Zhang et al., 2018b, Cui et al., 2020, Luo et al., 2020. Such combinations have propelled the efficiency performance of OSCs to over 15-18% range , Cui et al., 2020, Luo et al., 2020. ...
... Addition of solvent additive to the photoactive layer solution is also crucial to optimize the final photoactive layer's morphology. Studies had shown that additives could induce a desirable photoactive layer's morphology by affecting the degree of phase separation and the molecular orientation of donor and acceptor domains(He et al., 2018). The type of additive is normally chosen based on the fact that additive should be partly miscible in the processing solvent but is free to solvate donor, acceptor, neither or both(McDowell et al., 2018).Among the available choices, 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) are two commonly used additives for photoactive blend layer.Zhou et al. ...
Article
Renewable energy sources are promising long-term solution to solve the energy supply crisis due to the excessive use of non-renewable fossil fuels. One of the options is solar energy, which can be harvested directly from sunlight using photovoltaic (PV) technology. In recent years, organic solar cells (OSCs) as the building blocks of organic PV technology have emerged in the PV field, enabling the realization of environmental-friendly and low-cost PV technology. However, issues related to efficiency performance still posed a major setback to commercialization of OSCs. In view of this, this study is conducted to present comprehensive understandings on how OSCs’ performance in terms of optical, electrical, morphological and mechanical properties can be improved through device engineering strategy (interface and electrode engineering strategy). In addition, the potential applications of OSCs achieved via device engineering strategy are also being explored. In summary, the studies conducted can be divided into three main parts. The first part focuses on improving OSCs’ performance through interface engineering strategy for the realization of high-performing OSCs. Interface engineering on sol-gel zinc oxide (ZnO) electron-transporting layer (ETL) was conducted by introducing additional oxadiazole-based electron-transporting material called 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) between ZnO ETL and photoactive layer. The significance of incorporating PBD on ZnO was demonstrated by investigating the change in optical, electrical and morphological properties of pristine ZnO ETL. The findings shown that additional PBD layer could improve pristine ZnO film’s conductivity, create better energy level alignment with the photoactive layer, smoothen ZnO film’s morphology and improve ZnO film’s hydrophobicity. All those factors crucially influenced the charge extraction, transport and recombination processes in OSCs, which were conducive for the enhancement in photovoltaic performance of ZnO/PBD-based device. In fact, through interface engineering strategy, inverted OSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T donor) and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-M acceptor) could demonstrate ~7% increment in the photovoltaic performance from 10.8% (ZnO-based device) to 11.6% (ZnO/PBD-based device). The second part focuses on improving OSCs’ performance through electrode engineering strategy for the realization of high-performing flexible OSCs. Electrode engineering on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) electrode was conducted by utilizing polyhydroxy compound dopant and gentle acid post-treatment method, specifically xylitol dopant and methanesulfonic acid (MSA) treatment. The significance of xylitol dopant and MSA treatment on PEDOT:PSS electrode was demonstrated by investigating the change in optical, electrical, morphological and mechanical properties of pristine PEDOT:PSS electrode. The findings shown that both doping and acid treatment on PEDOT:PSS electrode could improve the optical transparency of electrode, enhance electrode’s conductivity and modify electrode’s morphology. In addition, such treatment could also provide electrode a stronger adhesion ability with the substrate, which were effective for improving the mechanical stability of electrode against extreme mechanical deformation. All those factors promoted the realization of high-performing flexible OSCs based on PEDOT:PSS electrode. In fact, through electrode engineering strategy, conventional OSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T-2F/PM6 donor) and 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2",3’':4’,5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (BTP-4F/Y6 acceptor) could demonstrate an excellent photovoltaic performance of 14.2% with remarkable mechanical robustness against bending and folding. The last part focuses on the application of device engineering, specifically electrode engineering as a continuation of study from the previous part. The desirable mechanical and optical properties of the engineered PEDOT:PSS could make PEDOT:PSS a great candidate for usage in foldable-flexible semi-transparent OSCs (FST-OSCs). FST-OSCs were fabricated similarly using engineered PEDOT:PSS electrode and PBDB-T-2F:Y6 photoactive layer system. As a result, high-performing FST-OSCs with over 10% efficiency and 21% average visible light transmittance, as well as excellent mechanical stability were obtained. The potential of such FST-OSCs for greenhouse application was investigated by incorporating them as part of roofs in the simulated greenhouse. Comparisons between plants grown under direct sunlight with FST-OSCs roof and those under direct sunlight yielded remarkably similar results in terms of branch sturdiness and hypertrophic leaves, proving the significance of electrode engineering strategy in realizing high-performing FST-OSCs for practical greenhouse applications.
... BDT-based polymers have been widely employed in high-performance D-A copolymer derivatives that have demonstrated high charge mobilities [27] and high solar cell power conversion efficiency. [28][29][30][31] For example, the current record efficiency solar cell employs D18 that has a functionalized BDT donor moiety. [10] We applied DMA to study the thermal transitions in a systematic set of polymers as DMA is significantly more sensitive than commonly used DSC, [18,32,33] providing deeper insight into the relaxation behavior. ...
Article
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The thermomechanical behavior of polymer semiconductors plays an important role in the processing, morphology, and stability of organic electronic devices. However, donor–acceptor‐based copolymers exhibit complex thermal relaxation behavior that is not well understood. This study uses dynamic mechanical analysis (DMA) to probe thermal relaxations of a systematic set of polymers based around the benzodithiophene (BDT) moiety. The loss tangent curves are resolved by fitting Gaussian functions to assign and distinguish different relaxations. Three prominent transitions are observed that correspond to: i) localized relaxations driven primarily by the side chains (γ ), ii) relaxations along the polymer backbone (β ), and iii) relaxations associated with aggregates (α ). The side chains are found to play a clear role in dictating Tγ, and that mixing the side chain chemistry of the monomer to include alkyl and oligo(ethylene glycol) moieties results in splitting the γ ‐relaxation. The β relaxations are shown to be associated with backbone elements along with the monomer. In addition, through processing, it is shown that the α‐relaxation is due to aggregate formation. Finally, it is demonstrated that the thermal relaxation behavior correlates well with the stress–strain behavior of the polymers, including hysteresis and permanent set in cyclically stretched films. The molecular origin of the complex relaxation behavior in donor‐acceptor copolymers is described. Through studying a systematic set of polymers, thermal relaxations can be ascribed to side chains, features along the backbone, and aggregates. The results also indicate the lack of a glass transition. It is then demonstrated that these relaxations provide a basis for designing polymer semiconductors for stretchable applications.
... Common solvent additives employed in NFA-PSCs are 1,8-diiodooctane (DIO), diphenyl ether (DPE), and 1-chloronaphthalene (1-CN). [15][16][17] These high boiling point additives, due to their preferential affinity for one of the components, can effectively increase the ordering of the materials to improve the morphology and, hence, the PCE of NFA-PSC devices. [15] However, after the fabrication of the active layer, these solvent additives cannot be fully removed in the thin film. ...
Article
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Simultaneously improving efficiency and stability is critical for the commercial application of non‐fullerene acceptor polymer solar cells (NFA‐PSCs). Multifunctional solid additives have been considered as a potential route to tune the morphology of the active layer and optimize performance. In this work, photoinitiator bifunctional bis‐benzophenone (BP‐BP) is used as a solid additive, replacing solvent additives, in the PBDB‐T:ITIC NFA system. With the addition of BP‐BP, the intermolecular π–π stacking of PBDB‐T and morphology is improved, leading to more balanced carrier transport and more effective exciton dissociation. Devices fabricated with BP‐BP show a power conversion efficiency (PCE) of 11.89%, with enhanced short‐circuit current (Jsc), and fill factor (FF). Devices optimized with BP‐BP show excellent reproducibility, insensitivity to thickness, and an improved thermal stability under atmospheric conditions without encapsulation. This work provides a new strategy for the application of solid additives in NFA‐PSCs.
... Therefore, using IDTT-like fused-ring cores with various side chains on the sp 3 -hybridized bridging carbon atoms, many ADA-type nonfullerene acceptors have been demonstrated with excellent PCEs of over 12%. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] However, the sp 3 carbon-bonded side chains could also affect the close p-p stacking of the acceptor materials, which in turn limits their carrier transport. Therefore, researchers started to search for other molecular design strategies toward efficient nonfullerene acceptors. ...
Article
A “face-on” molecular orientation is essential for photovoltaic materials with efficient vertical carrier transport, but understanding how the molecular structures control their orientations remains challenging. Based on a ladder-type fused-ring core without sp³-hybridized bridging atoms, novel acceptor-donor-acceptor (A-D-A or ADA)-type nonfullerene acceptors (M3 and M32) are developed, and their molecular orientation behaviors are successfully controlled by the neighboring side chains. With linear n-cetyl chains on the nitrogen atoms, the acceptor (M32) tends to adopt an “edge-on” molecular orientation, whereas with bulky branched 2-hexyldecyl chains, the acceptor (M3) has a preferred “face-on” orientation. Blended with a donor polymer of PM6, M3 shows optimal phase separation and dramatically improved electron transport, consequently leading to a much higher device performance than M32. Further optimization of the M3-based devices yields an outstanding efficiency of 16.66%. The strategy of molecular orientation control proposed here will inspire many other innovative designs and syntheses of high-performance nonfullerene acceptors.
... So far, EQE EL was improved to 10 −4 -10 −5 in many non-fullerene acceptor-based OSCs, thus affording smaller ΔE 3 ≈ 0.23-0.29 eV. [45][46][47][48][49] Recently, it is reported that EQE EL of some highly efficient OSCs based on non-fullerene acceptor has achieved > 10 −4 and thus the ΔE 3 is reduced to ≈0.2 eV. [43] Nevertheless, it is still larger than that in inorganic solar cells, [50,51] as shown in Figure 2 (e.g., ΔE 3 is only 27 meV in high-quality GaAs solar cells). ...
Article
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Impressive short‐circuit current density and fill factor have been achieved simultaneously in single‐junction organic solar cells (OSCs) with the emergence of high‐performance non‐fullerene acceptors. However, the power conversion efficiencies (PCEs) of OSCs still lag behind those of inorganic and perovskite solar cells, mainly due to the modest open‐circuit voltage (VOC) imposed by relatively large energy loss (Eloss). Generally, Eloss in solar cells can be divided into three parts. Among them, ΔE1 is inevitable for all photovoltaic cells and depends on the optical bandgap of solar cells, while radiative recombination energy loss, ΔE2, in OSCs can approach the negligible value via finely matching donor with acceptor material in the blend. The relatively large non‐radiative recombination energy loss, ΔE3, becomes the main barrier to further reduce Eloss and thus enhance PCE in non‐fullerene acceptor‐based OSCs. In this review, the recent studies and achievements about ΔE3 in non‐fullerene acceptor‐based OSCs have been summarized from the aspects of material design, morphology manipulation, ternary strategy, mechanism, and theoretical study. It is hoped that this review helps to get a deep understanding and boost the advance of ΔE3 study in OSCs. With the emergence of high‐performance non‐fullerene acceptors, significant enhancement in power conversion efficiencies (PCEs) of organic solar cells (OSCs) has been achieved. However, the modest open‐circuit voltage imposed by relatively large non‐radiative recombination energy loss (ΔE3) limits further improvement of PCEs. This review summarizes the recent advance in ΔE3 of OSCs from material design, morphology manipulation, ternary strategy, and mechanism.
... 5- 15 Most attention has been paid to the modification of the inner aromatic rings of the central cores. Since the first report of ITIC, the innermost benzene ring has been replaced with naphthalene, [16][17][18] anthracene, 19, 20 pyrene, 8,21 chrysene, 22 thieno [3,2-b]thiophene (TT) 9, 23, 24 and naphthodithiophene; 14,25,26 cyclopentadienyl has been replaced with the pyrrole ring. [27][28][29] Selecting suitable outermost aromatic rings is of particular importance, because the direct electronic communication between the outermost aromatic rings and termini exerts a strong impact on the optoelectronic properties of FREAs. ...
Article
Selecting suitable outermost aromatic rings of the central cores is of particular importance for design of fused-ring electron acceptors (FREAs), because the direct electronic communication between the outermost aromatic rings and termini exerts a strong impact on the optoelectronic properties of FREAs. In most cases, the outermost rings of the FREA cores are thiophene. This work reported the first example of using pyrrole as the outermost rings of the core. Fused hexacyclic electron acceptor, P6IC, using pyrrole in place of the often-used thiophene as the outermost rings of the central core was synthesized. Compared with its structural analogue F6IC with thiophene as the outermost rings, P6IC exhibits a remarkably upshifted HOMO energy level (P6IC: −5.43 eV, F6IC: −5.71 eV), a slightly upshifted LUMO energy level (P6IC: −3.94 eV, F6IC: −4.00 eV), 54 nm redshifted absorption, a narrower bandgap (P6IC: 1.30 eV, F6IC: 1.37 eV), and an enhanced mobility (P6IC: 8.8 × 10–4 cm2 V–1 s–1, F6IC: 7.4 × 10–4 cm2 V–1 s–1). Organic photovoltaic cells using PTB7-Th/P6IC as photoactive layer exhibit an efficiency of 12.2%, far surpassing that based on PTB7-Th/F6IC active layer (5.57%). The semitransparent devices using PTB7-Th/P6IC as active layer yield an efficiency of 10.2% with an average visible transmittance (AVT) of 17.0%, far surpassing that based on PTB7-Th/F6IC (5.26% with an AVT of 18.4%).
... The past few years evidences a rapid development of organic solar cells (OSCs) employing non-fullerene electron acceptors (NFAs), [1][2][3][4][5] with the highest power conversion efficiency (PCE) of over 18% has been reported. [6,7] Such rapid progress is mainly due to the development of new materials [8][9][10][11][12] and the continuous optimization of device structure and nanoscale morphology of the photoactive layer. ...
Article
Full-text available
Fused-ring non-fullerene electron acceptors (NFAs) boost the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Asymmetric and halogenated NFAs have drawn increasing attention in recent years due to their unique optoelectronic properties. Starting from the symmetric NFA ITCC-M, this work systematically designs and synthesizes an asymmetric counterpart ITCC-M-2F, halogenated counterpart ITCC-Cl, and asymmetric and halogenated counterpart IDTT-Cl-2F. Among these NFAs, IDTT-Cl-2F shows the shallowest lowest unoccupied molecular orbital energy level, broader absorption range, and the tightest molecular packing. As a result, when blended with the donor PBDB-T-2Cl, IDTT-Cl-2F-based OSCs yield the highest PCE of 13.3% with an open-circuit voltage of 0.96 V, short-circuit current of 19.20 mA cm–2, and fill factor of 71.1%, which is the highest PCE of OSCs employing 2-(2-chloro-6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene) malononitrile (ClIC) unit terminated NFA. The results demonstrate the synergistic effect of asymmetry and halogenation toward tuning of the optoelectronic properties of NFAs for high performance OSCs.
... More recently with the advent of fused ring electron acceptors, FTAZ-based solar cells have achieved over 13% efficiency. 27,28 A summary of some of the structural variations of the TAZ core we have reported is shown in Figure S9, along with the solar cell efficiencies published with PCBM. 24,25,29−36 While we acknowledge that pairing many of these polymers with fused ring electron acceptors (such as ITIC 37 and Y6 38 ) can result in higher performance, this would increase the number of variables and make distilling meaningful structure−property relationships across publications difficult. ...
Article
With the recent remarkable advances in the efficiency of organic solar cells, the need to distill key structure–property relationships for semiconducting materials cannot be understated. The fundamental design criteria based on these structure–property relationships will help realize low-cost, scalable, and high-efficiency materials. In this study, we systematically explore the impact of a variety of functional groups, including nitrogen heteroatoms, fluorine substituents, and cyano groups, on benzotriazole (TAZ)-based acceptor moieties that are incorporated into the conjugated polymers. Specifically, a pyridine heterocycle was used to replace the benzene unit of TAZ, leading to the PyTAZ polymer, and a cyano substituent was added to the benzene of the TAZ unit, resulting in the CNTAZ polymer. The PyTAZ polymer suffers from low mobility and poor exciton harvesting, driven by large and excessively pure domains when blended with PCBM. The inclusion of fluorine substituents, placed strategically along the polymer backbone, can mitigate these issues, as shown with 4FT–PyTAZ. However, when this same approach is used for the cyano-functionalized polymer (CNTAZ), the resulting polymer (4FT–CNTAZ) is overfunctionalized and suffers from impure domains and recombination issues. The cyano group has a larger impact on the TAZ core compared to the nitrogen heteroatom due to the strong electron-withdrawing strength of the cyano group. Because of this, further functionalization of the cyano-based polymers has less fruitful impact on the polymer properties and results in deterioration of the solar cell efficiency. Overall, this work highlights some of the benefits, thresholds, and limitations for functionalization of conjugated polymers for organic solar cells.
... The similar phenomenon can be observed in the previous researches based on BHJ OSCs. 43,44 Semitransparent Organic Solar Cells. Since the donor and the acceptor were separately deposited, simply increasing the thickness of IEICO-4F should have a tiny influence on visible light transmission spectra. ...
... Thieno [3,2- IDCIC with fused-10-ring core was developed, where the outmost thiophene rings in IOIC2 were replaced with TT units. 180 The FTAZ:IDCIC-based OSCs yielded a PCE of 13.58% due to the higher energy levels and red-shifted absorption spectrum than IOIC2. ...
Article
The research in advanced functional polymers is being driven by the fast-growing demand of new functional materials for revolutionary technologies. Polymers can be endowed with functions by the use of some special preparation methods or by introducing functional groups or fillers into materials. These functions are either possessed intrinsically by materials or actuated by external stimuli. In this review, we present an overview of the recent developments in research hotspots of functional polymers, involving polymerization methodologies, luminescent polymers, photovoltaic polymers, other electronic and optical polymers (including low-k polyimide and second-order nonlinear optical polymers), bio-related polymers (especially those for biomedical applications), supramolecular polymers, stimuli-responsive polymers, shape memory polymers, separation polymer membranes, energy storage polymers, and covalent organic framework polymers. The concepts, design strategies at molecular level, preparation methods, classifications, properties, potential applications, and the recent progress of such polymers are summarized. Challenges and future perspectives of each type of functional polymers are also addressed, making research efforts on the design and fabrication of functional polymers for serving the increasing needs of new materials.
... When ITIC was blended with wide bandgap polymer donors, the corresponding PSCs afforded exciting PCEs of 9-12% [16][17][18]. Since RESEARCH ARTICLE Ma et al. 1887 then, many ITIC-derived nonfullerene acceptors have been developed to fine-tune the energy levels and morphology thereby leading to further improved PCEs approaching ∼13-15% [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. ...
Article
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Nonfullerene acceptors are being investigated for use in polymer solar cells (PSCs), with their advantages of extending the absorption range, reducing the energy loss and therefore enhancing the power conversion efficiency (PCE). However, to further boost the PCE, mobilities of these nonfullerene acceptors should be improved. For nonfullerene acceptors, the π–π stacking distance between cofacially stacked molecules significantly affects their mobility. Here, we demonstrate a strategy to increase the mobility of heteroheptacene-based nonfullerene acceptors by reducing their π–π stacking distances via control over the bulkiness of lateral side chains. Incorporation of 2-butyloctyl substituents into the nonfullerene acceptor (M36) leads to an increased mobility with a reduced π–π stacking distance of 3.45 Å. Consequently, M36 affords an enhanced PCE of 16%, which is the highest among all acceptor-donor-acceptor-type nonfullerene acceptors to date. This strategy of control over the bulkiness of side chains on nonfullerene acceptors should aid the development of more efficient PSCs.
Article
Near infrared (NIR) acceptors are the key components for the construction of efficient organic photovoltaics (OPVs) and organic photodetectors (OPDs). Herein, a near-infrared acceptor named 6TSe-OFIC incorporating selenium heterocycles has been designed, which shows an absorption onset around 1000 nm with a low optical bandgap of 1.29 eV. OPV device based on PCE10: 6TSe-OFIC demonstrates a power conversion efficiency (PCE) of 13.18% with a high and broad EQE response. The semi-transparent OPV device based on PCE10: 6TSe-OFIC shows a PCE of 10.06% with AVT 20%. With the same active layer, the OPD devices achieve the responsivities of 0.54 A W⁻¹ at ∼850 nm. The results highlight the opportunities to careful design of the NIR molecules for efficient OPVs and OPDs.
Article
An acceptor-donor-acceptor (A-D-A) type small molecule acceptor ITOT-4Cl incorporating a dithienopyran (DTP) fused backbone, has been designed and synthesized. With the strong electron-donating capability of the backbone D unit, ITOT-4Cl shows a narrow band gap of 1.28 eV, and has an efficient absorption in near-infrared region up to 968 nm. The photovoltaic devices based on PBDB-T:ITOT-4Cl give a power conversion efficiency (PCE) of 12.50%, with a high short circuit current density (Jsc) of 22.89 mA cm⁻². With F–Br as the third component, the ternary organic solar cells device shows an enhanced PCE of 14.29% with an improved Jsc of 24.40 mA cm⁻², which is among the best results for oxygen heterocycle fused nonfullerene acceptor based devices.
Article
A new acceptor-donor-acceptor (A-D-A) small molecule acceptor named FCO-2F, is designed and synthesized based on the previous acceptor F-H. By inserting oxygen atom into the backbone of F-H and fluorination on the end group, FCO-2F shows a much red-shifted absorption compared with F-H, and also a wider absorption up to 830 nm. Using polymer PM6 as the donor, the organic solar cell (OSC) devices based on FCO-2F offer a power conversion efficiencies (PCEs) of 13.36%, with much improved short circuit current (Jsc) of 20.90 mA cm-2.
Article
Inspired by the encouraging properties of Ge-fused heterocyclic dithienogermole (DTG) in optoelectronic applications, we here report two narrow-bandgap acceptor–donor′–donor–donor′–acceptor (A–D′–D–D′–A)-type isomeric nonfullerene acceptors based on DTG (DTG-IW with inward-facing side chains vs DTG-OW with outward-facing side chains) for use in organic solar cells (OSCs). The introduction of the inward-facing side chains into the backbone results in extremely confined face-on crystallites in the solid state, as verified by grazing-incidence wide-angle X-ray scattering measurements. This result is attributed mainly to a better power conversion efficiency (PCE) of 9.16% in the OSC based on a blend of DTG-IW with a narrow-bandgap PTB7-Th donor polymer, as compared with the corresponding DTG-OW-based one. Furthermore, the appealing feature of the blend mixing of the narrow-bandgap donor and acceptor pair is that it enables the construction of a green-tinted efficient semitransparent OSC with a PCE of 6.19% and transmittance of 50.4% in the green wavelength region through incorporation of the recently formulated semitransparent Ag/Sb2O3/Ag electrode. Overall, in addition to providing useful perspectives into the side-chain engineering of nonfullerene acceptors, this work highlights that OSC based on the A–D′–D–D′–A-type DTG is a promising narrow-bandgap acceptor for further improvement of the performance of semitransparent OSCs.
Article
Adding additive is one of effective strategies to fine tune active layer morphology and improve performance of organic solar cells. In this work, a binary additive of 1,8-diiodooctane (DIO) and 2,6-dimethoxynaphthalene (DMON) to optimize the morphology of PBDB-T:TTC8-O1-4F-based devices are reported. With the binary additive, a power conversion efficiency (PCE) of 13.22% was achieved, which is higher than those of devices using DIO (12.05%) or DMON (11.19%) individually. Comparison studies demonstrate that DIO can induce the acceptor TTC8-O1-4F to form ordered packing, while DMON can inhibit excessive aggregation of donor and acceptor. With the synergistic effect of these two additives, PBDB-T:TTC8-O1-4F blend film with DIO & DMON exhibit a suitable phase separation and crystallite size, leading to a high short-circuit current density (Jsc) of 23.04 mA·cm-2 and fill factor (FF) of 0.703 and thus improved PCE.
Article
Here, a pair of A1–D–A2–D–A1 unfused ring core‐based nonfullerene small molecule acceptors (NF‐SMAs), BO2FIDT‐4Cl and BT2FIDT‐4Cl is synthesized, which possess the same terminals (A1) and indacenodithiophene unit (D), coupling with different fluorinated electron‐deficient central unit (difluorobenzoxadiazole or difluorobenzothiadiazole) (A2). BT2FIDT‐4Cl exhibits a slightly smaller optical bandgap of 1.56 eV, upshifted highest occupied molecular orbital energy levels, much higher electron mobility, and slightly enhanced molecular packing order in neat thin films than that of BO2FIDT‐4Cl. The polymer solar cells (PSCs) based on BT2FIDT‐4Cl:PM7 yield the best power conversion efficiency (PCE) of 12.5% with a Voc of 0.97 V, which is higher than that of BO2FIDT‐4Cl‐based devices (PCE of 10.4%). The results demonstrate that the subtle modification of A2 unit would result in lower trap‐assisted recombination, more favorable morphology features, and more balanced electron and hole mobility in the PM7:BT2FIDT‐4Cl blend films. It is worth mentioning that the PCE of 12.5% is the highest value in nonfused ring NF‐SMA‐based binary PSCs with high Voc over 0.90 V. These results suggest that appropriate modulation of the quinoid electron‐deficient central unit is an effective approach to construct highly efficient unfused ring NF‐SMAs to boost PCE and Voc simultaneously. An effective intramolecular locking strategy is designed by introducing the central electron‐deficient quinoid to unfused ring A1–D–A2–D–A1‐type nonfullerene small molecule acceptors (NF‐SMAs). The polymer solar cells (PSCs) based on BT2FIDT‐4Cl with difluorobenzothiadiazole central unit show a power conversion efficiency (PCE) of 12.5% with Voc of near 1 V. This is the best result for nonfused ring NF‐SMAs with electron‐deficient A2 unit in binary PSCs.
Article
NC-FIC nonfullerene acceptor, using a didodecylnaphthodithiophene (NDT) embedded octacyclic ladder-type donor NC, had been described previously. In this work, a new cross-shaped TC-FIC nonfullerene acceptor is designed by substitution of the NDT unit in the NC skeleton with a 2-dimensional didodecyltetrathienonaphthalene (TTN) unit. The 2-dimensional TC π-system with two vertically fused thiophenes strengthens electron-donating ability, promotes π-electron delocalization and enhances intramolecular donor-acceptor charge transfer and intermolecular π-π interactions. As a result, TC-FIC shows more red-shifted and widened absorption compared to the NC-FIC counterpart. The PBDB-T:TC-FIC: device exhibits a higher efficiency of 11.6% which greatly outperformed the corresponding PBDB-T:NC-FIC device with an efficiency of 7.52%. The dramatic 54% enhancement in efficiency is ascribed to the improved light-harvesting ability and enhanced charge mobilities which are benefited from the central 2-dimensional TTN moiety. By incorporating PC71BM to improve absorption and electron transport, the ternary-blend PBDB-T:TC-FIC:PC71BM device (1:1:1 in wt%) achieves a highest PCE of 13.49% with an enhanced Jsc of 23.80 mA/cm2, an FF of 64.4%, and a Voc of 0.88 V.
Article
In comparison with symmetric small molecule acceptors (SMAs), their asymmetric counterparts have received much less attention. Organic solar cells (OSCs) based on asymmetric non-fullerene SMAs previously reported show relatively low power conversion efficiency (PCE < 15%) and lower short circuit current densities (Jsc < 20 mA cm-2), although they exhibit larger dipole moment and stronger intermolecular interactions. Here we reported a new asymmetric A-DA’D-A type SMA, namely Y22, containing an asymmetric hexacyclic fused donor-acceptor-donor (DA’D) type central framework and end-capped with two electron-deficient 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile moieties. Y22 shows good solubility and a broad near-infrared light absorption. The effects of asymmetric structure on the physicochemical and photovoltaic performance were systematically investigated. Y22-based OSCs were fabricated by using PM6, QX2 and J71 as polymer donors, all exhibiting high PCEs over 10%. Notably, the optimal PM6:Y22-based device shows a high PCE of 15.4% and a high Jsc of 24.37 mA cm-2, which are among the best values reported for asymmetric SMA based OSCs, attributed to the fine nanofiber morphology after thermal annealing and solvent additive. This work demonstrates that the asymmetric fused structure with electron-deficient-core is a promising building block to build high performance SMAs towards efficient photovoltaic applications.
Article
Given that the ring-fusion strategy can effectively tail the electrical, optical, and structural properties of π-conjugated materials, we have herein developed Y-series non-fullerene acceptors with a dual acceptor–donor–dual acceptor-type structure, bearing non-fused bare bithiophene (Y–Th2), hexagonal ring-fused bithiophene (Y-BDT), or pentagonal ring-fused bithiophene (Y-CDT) central donor units. Several characterization techniques were applied to perform a comparative investigation of their optical and physical properties and frontier energy levels with respect to the different ring-fusion of the central units. Moreover, for the cascade energy level alignment, the synthesized acceptors were employed as a third component in the PM6:Y6 that served as a binary host platform. The introduction of the optimal amount of Y–Th2 or Y-BDT into the host system improved the device performance. Specifically, Y–Th2 exhibited the best power conversion efficiency (PCE) (16.01%) along with improved photovoltaic parameters, whereas the addition of Y-CDT impaired the PCE. Moreover, the optimized Y–Th2-based ternary organic solar cell achieved a PCE of 22.72% under indoor illumination at 1000 lux. Thus, the in-depth structural, morphological, and electrical analyses not only established a structure–property correlation, but also provided better design criteria for the guest-oriented non-fullerene acceptors for ternary photovoltaic applications, especially for Y6-containing systems.
Article
Although much progress has been made for organic photovoltaics (OPVs), the active layer material design is generally based on a trial and error approach. It is still a challenge to rationally design active layer materials to further improve OPV performance. Herein, guided by a semi-empirical model we have proposed, two new small molecule acceptors, named F-2F and FO-2F, have been designed and synthesized based on an acceptor F-H. With difluoro substituted end group, F-2F shows red-shifted absorption than F-H, but still far from the range required in the semi-empirical model. Thus, with a subtle molecular optimization by inserting oxygen atom into the backbone of F-2F, FO-2F has been designed, which exhibits much red-shifted absorption, close to the preferred absorption range of the semi-empirical model. Blending with donor polymer PM6, OPV device based on FO-2F achieved an impressive PCE of 15.05% with a Voc of 0.878 V, a Jsc of 22.26 mA cm-2 and a notable FF of 0.77. Both the Voc and Jsc are among the predicted range of the model. These results renders molecule FO-2F a new acceptor example which could demonstrates over 15% PCE only observed almost entirely for Y6 series.
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The composition profiles of a series of model polystyrene/fullerene bilayers are measured, before, during and after thermal annealing, using in situ neutron reflectometry. In combination with grazing-incidence X-ray diffraction measurements, these experiments, which quantify layer compositions as a function of molecular weight using changes in both scattering length density and layer thickness, extend and corroborate recent measurements on ex situ annealed samples and demonstrate that the composition profiles rapidly formed in these systems correspond to two co-existing liquid–liquid phases in thermodynamic equilibrium. The measurements also demonstrate a clear and systematic onset temperature for diffusion of the fullerenes into the PS layer that correlates with the known glass-transition temperatures of both the polymer (as a function of molecular weight) and the fullerene, revealing that the molecular mobility of the fullerenes in these systems is controlled by the intrinsic mobility of the fullerenes themselves and the ability of the polymer to plasticise the fullerenes at the interface. Over the temperature range investigated (up to 145 °C), measurements of equilibrated composition profiles as a function of temperature (during gradual cooling) reveal no significant changes in composition profile, other than those associated with the known thermal expansion/contraction of polystyrene thin-films.
Article
Introducing intramolecular noncovalent interactions is an effective method for enlarging the conjugated area of the donor core in nonfullerene acceptors (NFAs) to improve the photovoltaic performance. In this study, a novel NFA (NTO-4F) based on a para-substituted naphtho[1,2-b:5,6-b’]dithiophene core with alkoxy side chains was designed and synthesized, which has a featured oxygen atom at the side chains to facilitate an intramolecular noncovalent S---O interaction with thiophene at the core. NTO-4F exhibits a broad and strong absorption, suitable energy levels, and appropriate crystallinity. As a result, when NTO-4F is blended with the wide bandgap polymer, PM6, as the active layer, the optimized polymer solar cells (PSCs) achieve a power conversion efficiency (PCE) of 11.5% with a high open-circuit voltage (Voc) of 0.99 V, and a simultaneous short-circuit current density (Jsc) of 19.1 mA cm−2. The solar cells also exhibit a low energy loss of 0.56 eV. Furthermore, PM6:NTO-4F-based devices show an excellent tolerance to temperature upon thermal annealing (TA) treatment and obtain over 10% efficiency for all devices under TA treatment at 140–200 °C.
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Electron-acceptor small-molecules possessing a long exciton lifetime and a narrow energy band gap, opposing the energy gap law, are highly desirable for high-performance organic photovoltaics (OPVs) by realizing their efficient light-harvesting ability (LH), exciton diffusion (ED), and charge transfer (CT). Toward this goal, we designed an acceptor–donor–acceptor (A–D–A) type nonfullerene acceptor (NFA), TACIC, having an electron-donating, self-assembling two-dimensional (2D) nanographene unit, thienoazacoronene, at the center with electron-withdrawing groups at both ends. The TACIC film exhibited a narrow band gap (1.59 eV) with excellent LH. Surprisingly, the TACIC film showed an extremely long exciton lifetime (1.59 ns), suppressing undesirable nonradiative decay by its unique self-assembling behavior. When combined with a conjugated polymer donor, PBDB-T, slow ED and CT were observed (60 ps) with the excitation of TACIC owing to the large TACIC domain sizes. Nevertheless, the unusually high efficiencies of ED and CT (96% in total) were achieved by the long TACIC exciton lifetime. Additionally, unusual energy transfer (EnT) from the excited PBDB-T to TACIC was seen, demonstrating its dual LH role. The OPV device with PBDB-T and TACIC showed a high incident photon-to-current efficiency (IPCE) exceeding 70% at up to 710 nm and a power conversion efficiency of ∼10%. This result will open up avenues for a rational strategy of OPVs where LH, ED, and CT from the acceptor side as well as LH, EnT, ED, and CT from the donor side can be better designed by using 2D nanographene as a promising building block for high-performance A–D–A type NFAs.
Article
Developing simple-structure and high-efficiency non-fullerene small molecule acceptors (NF-SMAs) are crucial for application of organic solar cells (OSCs). To deeply understand the effect of electron-withdrawing central building block on photo-electronic properties and better design simple-structure NF-SMAs, in this work, a novel simple SMA of BTz1 with an A (π-A′-A″)2 framework was synthesized and characterized, in which a unfused-ring weak acceptor (A) of bithiazole and two adjacent strong acceptors of A′-A″ are used as molecular central and terminal units, respectively. And the UV absorption, energy level, crystallinity, mobility, morphology and photovoltaic per-formance of BTz1 were studied systematically. It is found that BTz1 exhibits similar molecular conformations and photophysical properties, but a low HOMO and high LUMO energy level in comparison with its analogue of TTz1 based on thiazolothiazole center. Furthermore, significant different photovoltaic properties are observed in the BTz1 and TTz1 based PSCs using polymer J71 as donor in an inverted device structure. The BTz1-based OSCs exhibit an ascending open-circuit voltage (Voc) of 0.96 V, but TTz1-based devices display a significantly increasing power conversion efficiency (PCE) of 8.77%. The better film morphology of photoactive layer makes the TTz1-based OSCs exhibit higher PCE than the BTz1-based OSCs. This work indicates that tuning electron-withdrawing central structure is crucial to construct simple-structure and high-efficiency NF-SMAs.
Article
Three new asymmetric 9,9′-bifluorenylidene-based derivatives, 2,7-dibutoxyl-3′,6′-bis(5-methylenemalononitrile-3-octylthiophen-2-yl)-9,9′-bifluorenylidene (BF-TDCN2), 2,7-dibutoxyl-3′,6′-bis(5-(methylene-indene-1,3-dione)-3-octylthiophen-2-yl)-9,9′-bifluorenylidene (BF-TID2) and 2,7-dibutoxyl-3′,6′-bis(5-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile)-3-octylthiophen-2-yl)-9,9′-bifluorenylidene (BF-TDCI2), were successfully synthesized by grafting different electron-withdrawing groups (malononitrile (DCN), 1H-indene-1,3(2H)-dione (ID) and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (DCI)), which were used as the electron acceptors for organic photovoltaic cells. By changing the electron-withdrawing ability of the terminal group, the molecular energy level and band gap can be easily adjusted. The optical bandgaps of the three compounds in the thin films decreased with increasing the electron-withdrawing ability of the terminal group. Besides, the lateral chains of alkoxy groups located at the asymmetric end also play a certain influence on the solubility, molecular aggregation and the miscibility with polymer donor. Among these electron acceptors, the photovoltaic cell fabricated PBDB-T:BF-TDCI2 exhibited a maximum power conversion efficiency of 4.85% with an open-circuit voltage of 0.88 V and a low energy loss of 0.62 eV. By investigating different processing processes, the results showed that the power conversion efficiency can be improved by 20% with simple solvent annealing treatment. Through further study on the morphology and photophysical properties of the active layers, it was found that the processed device had better phase separation size and morphology, which was favorable to enhancing the intermolecular interaction, thus improving exciton separation and charge transfer in the active layer.
Article
A new heterocyclic aromatic structure, thieno[3,2-c]quinolin-4(5H)-one (TQO), was designed and synthesized as an electron-accepting building block for donor-acceptor-type copolymers. TQO was copolymerized with 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDT) to give PBDT-TQO. Thieno[3,2-c]isoquinolin-5(4H)-one (TIQO), an isomeric structure of TQO, was also synthesized to give PBDT-TIQO. The absorption regions of both polymers cover 300–600 nm, complementary to those of well-known non-fullerene acceptors (ITIC or IT-4F). PBDT-TQO exhibited a slightly wider bandgap and a lower highest occupied molecular orbital energy level than PBDT-TIQO, owing to the different intra-molecular charge transfer characteristics. Photovoltaic devices fabricated using PBDT-TQO and PBDT-TIQO donors with a fullerene acceptor (PC71BM) showed the maximum power conversion efficiency (PCE) of 3.02 and 3.92%, respectively, while those using the non-fullerene acceptors (ITIC or IT-4F) showed much higher PCEs. The device using PBDT-TQO:IT-4F showed the highest PCE of 7.62%, with a short-circuit current of 14.44 mA cm-2, an open circuit voltage of 0.90 V, and a fill factor of 58.92%.
Article
Recent advances in material design for organic solar cells (OSCs) are primarily focused on developing near-infrared non-fullerene acceptors, typically A-DA′D-A type acceptors (where A abbreviates an electron-withdrawing moiety and D, an electron-donor moiety), to achieve high external quantum efficiency while maintaining low voltage loss. However, the charge transport is still constrained by unfavorable molecular conformations, resulting in high energetic disorder and limiting the device performance. Here, a facile design strategy is reported by introducing the “wing” (alkyl chains) at the terminal of the DA′D central core of the A-DA′D-A type acceptor to achieve a favorable and ordered molecular orientation and therefore facilitate charge carrier transport. Benefitting from the reduced disorder, the electron mobilities could be significantly enhanced for the “wing”-containing molecules. By carefully changing the length of alkyl chains, the mobility of acceptor has been tuned to match with that of donor, leading to a minimized charge imbalance factor and a high fill factor (FF). We further provide useful design strategies for highly efficient OSCs with high FF.
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Researchers are endeavoring to decode the fundamental reasons for the non-fullerene acceptor, Y6, to deliver high-performance organic solar cells. In this manuscript, we tackle this problem from the molecular packing...
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In this paper, ternary polymer solar cells (PSCs) consisting of two blend donors (FTAZ and PTB7-Th) and one acceptor (N3) with similar highest occupied molecular orbital (HOMO) energy levels, different absorption peaks and cascading lowest unoccupied molecular orbital (LUMO) energy levels are built. The ternary PSCs achieve higher photovoltaic performance and better thermal stability compared to the FTAZ:N3 binary PSCs. The A medium wavelength absorption FTAZ polymers and long wavelength absorption PTB7-Th polymers as the blend donor, and near-infrared wavelength absorption N3 as the acceptor. The optimize ternary PSCs exhibit a maximum power conversion efficiency of 15.65%, with an open-circuit voltage of 0.852 V, a short-circuit current of 25.25 mA cm⁻², and an FF of 72.5%. This performance of device is attributed to more efficient exciton dissociation and suppression of charge recombination within ternary films. The ternary PSCs also show good crystallinity and surface morphology at room temperature and after thermal annealing for 20 h at 70 °C, which is attribute to the better thermal stability of the ternary PSCs.
Article
Two isomeric fluorene-based heteroundecenes of bis(thienocyclopenthieno[3,2-b]thieno) fluorene (BT2T-F) and bis(dithieno[3,2-b:2’,3’-d]thiophene)cyclopentafluorene (B3T-F) have been designed and synthesized. The side chains of 4-hexylphenyl anchor on the 5th and 8th positions in B3T-F while on the 4th and 9th positions in BT2T-F, in which the former is closer to the center of the fused ring. The corresponding acceptor-donor-acceptor (A-D-A) type small molecule acceptors (SMAs) of BT2T-FIC and B3T-FIC were prepared by linking BT2T-F and B3T-F as fused ring donor units with the acceptor unit of 2-(5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC-2F), respectively. B3T-FIC presents a superior crystallinity with intense face-on π-π stacking in its neat film while BT2T-FIC is more disordered. When blended with PBDB-T-2Cl as a polymer donor, the optimized PBDB-T-2Cl:BT2T-FIC device exhibits an averaged power conversion efficiency (PCE) of 10.56% while only 7.53% in the PBDB-T-2Cl:B3T-FIC device. The improved short-circuit current (Jsc) and fill factor (FF) of the PBDB-T-2Cl:BT2T-FIC device are the main contribution of its higher performance, which is attributed to its more efficient and balanced charge transport and better carrier recombination suppression. Given that BT2T-FIC blend and B3T-FIC blend films both take a preferential face-on orientated π-π stacking with comparable distances, the suitable SMA domain size obtained in the BT2T-FIC blend could account for its more efficient photovoltaic performance. These results highlight the importance of side-chain strategy in developing efficient SMAs with huge fused ring cores.
Article
Precise determination of structural organization of semi-conducting polymers is of paramount importance for the further development of these materials in organic electronic technologies. Yet, prior characterization of some of the best-performing materials for transistor and photovoltaic applications, which are based on polymers with rigid backbones, often resulted in conundrums in which X-ray scattering and microscopy yielded seemingly contradicting results. Here we solve the paradox by introducing a new structural model, i.e., semi-paracrystalline organization. The model establishes that the microstructure of these materials relies on a dense array of small paracrystalline domains embedded in a more disordered matrix. Thus, the overall structural order relies on two parameters: the novel concept of degree of paracrystallinity (i.e., paracrystalline volume/mass fraction, introduced here for the first time) and the lattice distortion parameter of paracrystalline domains (g-parameter from X-ray scattering). Structural parameters of the model are correlated with long-range charge carrier transport, revealing that charge transport in semi-paracrystalline materials is particularly sensitive to the interconnection of paracrystalline domains.
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For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (ECT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an ECT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.
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Solution processible organic solar cells (OSCs) attract much attention as one of the most promising candidates for sustainable energetic techniques over the past two decades. So far, the power conversion...
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Most polymer donors developed so far for high-performance non-fullerene OSCs are designed in planar molecular geometries containing BDT units. In this work, two D−A conjugated polymers with wide band gap,...
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Considering the special functions of fused-ring aromatic building blocks and Si-atom in high-performance donor–acceptor-conjugated materials at the same time, herein the synthesis of a novel fused-ring tricyclic heterocycle, triisopropylsilyl-substituted benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (iBDD-Si), an isomer of well-known benzo[1,2-c:4,5-c′]dithiophene-4,8-dione is presented. The iBDD-Si-based copolymer series (PM6, PM6-5Si, PM6-10Si, and PM6-15Si) is synthesized via Stille polymerization, revealing fine-tuned optical and electrochemical properties, and molecular packing with varying iBDD-Si contents in the backbone. Organic solar cells are fabricated by pairing the copolymer donors with nonfullerene acceptor N3 and characterized. High power conversion efficiency of more than 17% is achieved using the PM6-5Si-based solar cell, which is attributed to the balanced charge transport, enhanced charge generation/dissociation kinetics, and minimized total energy and recombination losses. It is demonstrated that iBDD-Si is a promising backbone toolbox for various high-performance conjugated materials.
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Solar energy conversion has nowadays attracted research interests of the community, wherein conjugated polymers (CPs) become a class of workhorses on photon-to-electron and photon-to-fuel conversion studies. In recent years, exciting breakthroughs have been made in these multi-interdisciplinary fields, with the assistance of the intrinsic flexibilities on tuning optoelectronic, mechanical and structural properties of CPs. In this review, we summarize the recent notable development of CPs in polymer solar cells, perovskite solar cells, and photocatalysts, wherein CPs function well as light-capture and conversion components for polymer solar cells and photocatalysts as well as charge extraction materials for perovskite solar cells. By analyzing the principles, status, and structural-properties of these areas, we outline the design strategies and perspectives of CPs for further advancing solar energy conversions.
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A comprehensive study on the relationship between the structure and the photophysical and photovoltaic properties of acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (NFAs) is of pivotal importance to obtain design guidelines for high-performance organic photovoltaics (OPVs). In this study, we synthesized a D unit, NTT, in which the central benzene core, indacenodithieno[3,2-b]thiophene (IT), is replaced by naphthalene. Notably, NTTIC, an A-D-A type NFA with NTT as the D unit, showed a longer singlet exciton lifetime in the film than the IT-based benchmark NFA, ITIC. When paired with a conjugated polymer donor, PBDB-T, the NTTIC-based OPV device exhibited a higher power conversion efficiency (PCE = 9.95%) than the ITIC-based device (9.71%). Furthermore, due to the larger domain size of NTTIC in PBDB-T:NTTIC, the exciton diffusion (ED) at the donor/acceptor interface and the charge transfer (CT) were slower (14 ps in total) than those of PBDB-T:ITIC (7 ps in total). Nevertheless, the total efficiency of the ED and CT in PBDB-T:NTTIC was similar to that in PBDB-T:ITIC (95%) owing to the longer singlet exciton lifetime of the NTTIC film. These results demonstrate the high potential of naphthalene-cored D units of A-D-A type NFAs to achieve a long singlet exciton lifetime and a resultant high PCE in NFA-based OPVs.
Article
Novel A-D-A type non-fullerene acceptors (NFAs) with chlorinated non-conjugated thienyl groups are designed and synthesized for the first time. The effects of chlorinated non-conjugated side thienyl groups on the absorption spectra, energy levels, photovoltaic performance, energy losses are studied. With similar absorption spectra, chlorinated NFA MAL2-sCl exhibited a higher power conversion efficiency compared with the non-chlorinated analogue MAL1. In addition, the chlorinated NFAs exhibit suppressed non-radiative energy loss. This research paves a new way to modify the photovoltaic performance and reduce the non-radiative energy loss.
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Two FREAs, IUIC-O and IUIC-T, based on an undecacyclic core were developed. IUIC-T having higher extinction coefficient, affords aligned energy levels with PBDB-T, finer nanoscale morphology and more orderly molecular...
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Significant breakthroughs have been achieved in bulk-heterojunction (BHJ)-based OSCs with power conversion efficiencies (PCE) of over 17% by the development of new cores of non-fullerene acceptors, and the isomers which are often produced in the synthesis of cores. Here, two BDT (benzo[1,2-b:4,5-b′]dithiophene)-based acceptors, which are isomers with different orientations of the thiophene rings on the backbone are reported. Different molecular configurations and electron cloud distribution of the isomers were developed by theoretical calculations, which indicate completely different optical properties and various electrochemical energy levels of the isomers. Grazing-incidence wide-angle X-ray scattering (GIWAXS) tests showed that the two isomers have different crystalline structures. The OSC devices developed from each of the two isomers exhibit significant differences in electron/hole mobilities and packing modes, leading to clearly distinct filling factors and short-circuit currents, affecting the PCEs of final devices. Our studies of core isomerism provide some new insights into the configurations of backbones and can assist the molecular design of new n-type electron acceptors.
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Multi-modal imaging-guided synergistic photodynamic therapy (PDT) and photothermal therapy (PTT) show great benefits in cancer treatment. The phototheranostics with near-infrared (NIR) absorption, high reactive oxygen species (ROS) generation and photothermal conversion efficiency is highly desirable. Here, we designed and synthesized an “acceptor-donor-acceptor” (A-D-A) structured molecule named IDCIC, with strong absorbance in the NIR region. We further prepared IDCIC into water-soluble nanoparticles (NPs) by wrapping with 1,2-distearoylsn-glycero-3-phosphoethanolamine-N-[(polyethylene glycol)-2000]-amine (DSPE-PEG2000-NH2). The obtained IDCIC NPs show an NIR absorption peak at 760 nm and an NIR-II fluorescence spectrum peak at ∼1000 nm with a fluorescent quantum yield of 1.2%, enabling them excellent photoacoustic and NIR-II fluorescent imaging capabilities. Moreover, IDCIC NPs could simultaneously generate singlet oxygen (with a quantum yield of 9.1%), hydroxyl radicals (·OH) and heat (with a photothermal conversion efficiency of 78.9%) under 808-nm laser irradiation. Based on the above-mentioned properties, IDCIC NPs were used for dual-modal imagingguided synergistic photodynamic/photothermal therapy of cancer.
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Nonfullerene acceptors (NFAs) have played an important role in the development of organic solar cells. However, the optical absorption of most NFAs is limited within 600–900 nm, prohibiting further improvement of short-circuit current density (Jsc). To alleviate this problem, a fused-ring π-core BzS was designed by combining weakly electron-withdrawing benzotriazole (Bz) and strongly electron-donating selenophene together. Besides, the length of N-alkyl chain on the Bz moiety was engineered to tune the morphology, affording two NFAs mBzS-4F and EHBzS-4F. Both NFAs possess an absorption edge approaching 1000 nm, as resulted from the enhanced intramolecular charge transfer in conjunction with efficient intra- and intermolecular interactions. Binary photovoltaic devices based on PM6:mBzS-4F showed a power conversion efficiency of 17.02% with a very high Jsc of 27.72 mA/cm2 and a low energy loss of 0.446 eV. This work provides a strategy for future design of efficient NIR-responsive materials.
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Ternary strategy is a promising approach to broaden the photoresponse of polymer solar cells (PSCs) by adopting combinatory photoactive blends. However, it could lead to a more complicated situation in manipulating the bulk morphology. Achieving an ideal morphology that enhances the charge transport and light absorption simultaneously is an essential avenue to promote the device performance. Herein, two polymers with different lengths of side groups (P1 is based on phenyl side group and P2 is based on biphenyl side group) are adopted in the dual‐acceptor ternary systems to evaluate the relationship between conjugated side group and crystalline behavior in the ternary system. The P1 ternary system delivers a greatly improved power conversion efficiency (PCE) of 13.06%, which could be attributed to the intense and broad photoresponse and improved charge transport originating from the improved crystallinity. Inversely, the P2 ternary device only exhibits a poor PCE of 8.97%, where the decreased device performance could mainly be ascribed to the disturbed molecular stacking of the components originating from the overlong conjugated side group. The results demonstrate a conjugated side group could greatly determine the device performance by tuning the crystallinity of components in ternary systems. Side group effect in ternary polymer solar cells is studied by adopting polymers with different side groups. With appropriate side group modification, high power conversion efficiency (PCE) over 13% is realized, which could mainly be attributed to the broadened photoresponse and optimized molecular stacking. The results demonstrate that side group plays a crucial role in determining the molecular stacking of ternary heterojunction.
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A-D-A small molecule acceptors (SMAs) have attracted much attention, due to their merits of strong absorption, low bandgap, relatively high electron mobility and proper miscibility with donors. Benefitting from the rapid development of A-D-A SMAs, the efficiency of organic solar cells (OSCs) has been improved significantly, which excites the OSC community. In this review, we collect the reported high-efficient A-D-A SMAs according to the ring number of ladder-type donor core in the center, and summarize the evolution process of molecular structures and their photovoltaic parameters, aimed to afford an overview of A-D-A SMAs and help readers to get a deep understanding of them.
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Fluorine‐contained polymers, which have been widely used in highly efficient polymer solar cells (PSCs), are rather costly due to their complicated synthesis and low yields in the preparation of components. Here, the feasibility of replacing the critical fluorine substituents in high‐performance photovoltaic polymer donors with chlorine is demonstrated, and two polymeric donors, PBDB‐T‐2F and PBDB‐T‐2Cl, are synthesized and compared in parallel. The synthesis of PBDB‐T‐2Cl is much simpler than that of PBDB‐T‐2F. The two polymers have very similar optoelectronic and morphological properties, except the chlorinated polymer possess lower molecular energy levels than the fluorinated one. As a result, the PBDB‐T‐2Cl‐based PSCs exhibit higher open circuit voltage (Voc) than the PBDB‐T‐2F‐based devices, leading to an outstanding power conversion efficiency of over 14%. This work establishes a more economical design paradigm of replacing fluorine with chlorine for preparing highly efficient polymer donors.
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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.
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Non-fullerene acceptors (NFAs) are currently a major focus of research in the development of bulk-heterojunction organic solar cells (OSCs). In contrast to the widely used fullerene acceptors (FAs), the optical properties and electronic energy levels of NFAs can be readily tuned. NFA-based OSCs can also achieve greater thermal stability and photochemical stability, as well as longer device lifetimes, than their FA-based counterparts. Historically, the performance of NFA OSCs has lagged behind that of fullerene devices. However, recent developments have led to a rapid increase in power conversion efficiencies for NFA OSCs, with values now exceeding 13%, demonstrating the viability of using NFAs to replace FAs in next-generation high-performance OSCs. This Review discusses the important work that has led to this remarkable progress, focusing on the two most promising NFA classes to date: rylene diimide-based materials and materials based on fused aromatic cores with strong electron-accepting end groups. The key structure–property relationships, donor–acceptor matching criteria and aspects of device physics are discussed. Finally, we consider the remaining challenges and promising future directions for the NFA OSCs field.
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A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
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A nonfullerene electron acceptor (IEIC) based on indaceno[1,2-b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile was designed and synthesized. IEIC exhibited good thermal stability, strong absorption in the 500–750 nm region with an extinction coefficient of 1.1 × 105 M−1 cm−1 at 672 nm, deep LUMO energy level (−3.82 eV) close to those of fullerenes, and a relatively high electron mobility of 2.1 × 10−4 cm2 V−1 s−1. Fullerene-free polymer solar cells (PSCs) based on the blends of the IEIC acceptor and a low-bandgap polymer donor PTB7-TH, using a perylene diimide derivative as a cathode interlayer, showed power conversion efficiencies (PCEs) of up to 6.31%, which is among the best PCEs reported for fullerene-free PSCs.
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The solution-processable small molecules based on carbazole or fluorene containing rhodanine dyes at both ends were synthesized and introduced as acceptors in organic photovoltaic cells. The high energy levels of their lowest unoccupied molecular orbitals resulted in a power conversion efficiency of 3.08% and an open circuit voltage of up to 1.03 V.
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Tremendous progress has been made on the design and processing of new active and interfacial materials to enable organic photovoltaics to achieve high power conversion efficiencies of >10%. In this Feature Article the development of functional fullerenes as (1) acceptors, (2) electron selective layers, and (3) morphology stabilizers for bulk heterojunction polymer solar cells is reviewed. In addition to the standard PCBM based acceptors, a wide variety of newly developed fullerene-derived molecules have appeared in the past few years and started to show very encouraging photovoltaic performance when they were blended with low bandgap conjugated polymers. New fullerene derivatives with proper molecular design can also serve as electron selective interfacial materials and morphology stabilizers for the bulk heterojunction layer, which are essential to improve the interfacial property and long term stability of polymer solar cells. Although there still are many challenges ahead before practical polymer solar cells will arrive in the market place, the research in functional fullerenes deserves to have more attention in order to expedite this development process.
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As the performance in terms of power conversion efficiency and operational stability for polymer and organic solar cells is rapidly approaching the key 10-10 targets (10 efficiency and 10 years of stability) the quest for efficient, scalable, and rational processing methods has begun. The 10-10 targets are being approached through consistent laboratory research efforts, which coupled with early commercial efforts have resulted in a fast moving research field and the dawning of a new industry. We review the roll-to-roll processing techniques required to bring the magnificent 10-10 targets into reality, using quick methods with low environmental impact and low cost. We also highlight some new targets related to processing speed, materials, and environmental impact.
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A structure-device performance correlation in bulk heterojunction solar cells for new indandione-derived small molecule electron acceptors, FEHIDT and F8IDT, is presented. Devices based on the former exhibit higher power conversion efficiency (2.4%) and higher open circuit voltage, a finding consistent with reduced intermolecular interactions.
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Following the development of the bulk heterojunction1 structure, recent years have seen a dramatic improvement in the efficiency of polymer solar cells. Maximizing the open-circuit voltage in a low-bandgap polymer is one of the critical factors towards enabling high-efficiency solar cells. Study of the relation between open-circuit voltage and the energy levels of the donor/acceptor2 in bulk heterojunction polymer solar cells has stimulated interest in modifying the open-circuit voltage by tuning the energy levels of polymers3. Here, we show that the open-circuit voltage of polymer solar cells constructed based on the structure of a low-bandgap polymer, PBDTTT4, can be tuned, step by step, using different functional groups, to achieve values as high as 0.76 V. This increased open-circuit voltage combined with a high short-circuit current density results in a polymer solar cell with a power conversion efficiency as high as 6.77%, as certified by the National Renewable Energy Laboratory.
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Three novel nonfullerene 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 intra-supramolecular interaction) to enhance the planarity of backbone and electrostatic interaction (inter-supramolecular interaction) to enhance the π-π stacking of terminal groups. Fluorination can further strengthen the inter-supramolecular electrostatic interaction of terminal groups. As expected, the designed acceptors exhibited an excellent device performance when blending with polymer donor PBDB-T. In comparison with the parent ac-ceptor molecule DC-IDT2T reported in 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 importance of comprehensive design in the development of high performance nonfullerene small molecular acceptors.
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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%.
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Organic solar cells (OSCs) have advantages like lightweight , flexibility, colorfulness and solution processability [1]. The active layer of OSCs generally contains two organic semiconductors: an electron donor and an electron acceptor. The donor and acceptor make nanoscale phase separation to allow efficient exciton dissoci-ation and also form a three-dimensional (3D) passage to rapidly transfer free charge carriers to respective electrodes [2]. However, such binary system usually shows insufficient coverage of solar irradiation spectrum due to the narrow optical absorption of organic compounds [3]. Recently, ternary OSCs containing three absorption-complementary materials (e.g., two donors and one acceptor, or one donor and two acceptors) have attracted great attention. Ternary solar cells harvest more sunlight and demonstrate better performance than binary solar cells in some cases [3]. Polymer:fullerene:nonfullerene solar cells combine the advantages of fullerene acceptors (high electron mobility) and non-fullerene acceptors (strong visible or near-infrared (NIR) absorption), and achieved over 10% power conversion efficiencies (PCEs) [4,5]. Recently, we reported a highly efficient low-bandgap nonfullerene acceptor (CO i 8DFIC) with strong NIR absorption. PTB7-Th:CO i 8DFIC (1:1) binary cells gave 26.12 mA cm À2 short-circuit current density (J sc) and 12.16% PCE [6]. Here, we report highly efficient ternary cells based on PTB7-Th, CO i 8DFIC and PC 71 BM (Fig. 1a). Fullerene improves electron transport in the active layer and enhances external quantum efficiency (EQE), leading to high J sc and fill factor (FF). A PCE of 14.08% was achieved. The absorption spectra for PTB7-Th, CO i 8DFIC and PC 71 BM films are shown in Fig. 1b. PC 71 BM absorbs short-wavelength light, which is complementary to PTB7-Th and CO i 8DFIC. The lowest unoccupied molecular orbital levels (LUMO) for PTB7-Th (À3.12 eV), PC 71 BM (À3.67 eV) and CO i 8DFIC (À3.88 eV) show a stepwise alignment (Fig. 1c), suggesting that PC 71 BM can facilitate electron transfer from PTB7-Th to CO i 8DFIC [6,7]. Solar cells with a structure of ITO/ZnO/D:A 1 :A 2 /MoO 3 /Ag were made, where D is PTB7-Th, A 1 is CO i 8DFIC and A 2 is PC 71 BM. The weight ratio between D and A 1 + A 2 was fixed to 1:1.5, while the content of A 2 in acceptors gradually increased from 0% to 100% (Table S1 online) [8]. Initially, PTB7-Th:CO i 8DFIC (1:1.5) binary cells gave a PCE of 10.48%, with an open-circuit voltage (V oc) of 0.69 V, a J sc of 23.84 mA cm À2 and a FF of 63.8%. After adding small amount of fullerene (A 2) into the blend, J sc and FF increased dramatically. When D:A 1 :A 2 ratio (w:w:w) was 1:1.05:0.45, the ternary cells gave a PCE of 14.08%, with a V oc of 0.70 V, a J sc of 28.20 mA cm À2 and a FF of 71.0%. To the best of our knowledge, this is the first report demonstrating that the PCE for organic solar cells exceeds 14%. Further increasing fullerene content, V oc slightly increased, while J sc and FF decreased, leading to reduced PCEs. PTB7-Th:PC 71 BM (1:1.5) binary cells gave a PCE of 7.36%, with a V oc of 0.75 V, a J sc of 16.21 mA cm À2 and a FF of 60.2%. The performance for ternary cells (D:A 1 :A 2 = 1:1.05:0.45) is sensitive to the active layer thickness and additive content (Tables S2, S3 online). The optimal thickness for the active layer and the optimal 1,8-diiodooctane (DIO) content are 108 nm and 1 vol%, respectively. The J-V curves and the corresponding EQE spectra for the binary and the best ternary solar cells are shown in Fig. 1d, e. Compared with PTB7-Th:CO i 8DFIC cells, the ternary cells show enhanced EQE at 300-1,050 nm, consisting with the high J sc. The integrated current densities from EQE spectra of PTB7-Th:CO i 8DFIC and the ternary cells are 22.75 and 26.92 mA cm À2 , respectively. The EQE enhancement for the ternary cells might result from enhanced light absorption and efficient generation and transport of free charge carriers. The absorption spectra for the binary and ternary blend films are shown in Fig. S1 (online). Compared with
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Naphtho[1,2-b:5,6-b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: −3.78 eV vs IHIC2: −3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10−3 cm2 V−1 s−1 vs IHIC2: 5.0 × 10−4 cm2 V−1 s−1). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.
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A kind of new fused-ring electron acceptor, IDT-OB, bearing asymmetric side chains, is synthesized for high-efficiency thick-film organic solar cells. The introduction of asymmetric side chains can increase the solubility of acceptor molecules, enable the acceptor molecules to pack closely in a dislocated way, and form favorable phase separation when blended with PBDB-T. As expected, PBDB-T:IDT-OB-based devices exhibit high and balanced hole and electron mobility and give a high power conversion efficiency (PCE) of 10.12%. More importantly, the IDT-OB-based devices are not very sensitive to the film thickness, a PCE of 9.17% can still be obtained even the thickness of active layer is up to 210 nm.
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Two novel small molecule acceptors (DTNIC6 and DTNIC8) based on a ladder-type dithienonaphthalene (DTN) building block with linear (hexyl) or branched (2-ethylhexyl) alkyl substituents are designed and synthesized. Both acceptors ex-hibit strong and broad absorption in the range from 500 to 720 nm as well as appropriate HOMO and LUMO levels. Replacing the linear hexyl chains with the branched 2-ethylhexyl chains has a large impact on the film morphology of pho-toactive layers. In the blend film based on DTNIC8 bearing the branched alkyl chains, morphology with well-defined phase separation was observed. This optimal phase morphology yields efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced charge carrier mobilities. Benefited from these factors, OSCs based on PBDB-T:DTNIC8 delivered a highest PCE of 9.03% with a high FF of 72.84%. This unprecedented high FF of 72.84% is one of the highest FF values reported for non-fullerene OSCs. Our work not only affords a promising electron acceptor for non-fullerene solar cells, but also provides a side-chain engineering strategy towards high performance OSCs.
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Low-bandgap polymers/molecules are an interesting family of semiconductor materials, and have enabled many recent exciting breakthroughs in the field of organic electronics, especially for organic photovoltaics (OPVs). Here, such a low-bandgap (1.43 eV) non-fullerene electron acceptor (BT-IC) bearing a fused 7-heterocyclic ring with absorption edge extending to the near-infrared (NIR) region was specially designed and synthesized. Benefitted from its NIR light harvesting, high performance OPVs were fabricated with medium bandgap polymers (J61 and J71) as donor, showing power conversion efficiency of 9.6% with J61 and 10.5% with J71 along with extremely low energy loss (0.56 eV for J61 and 0.53 eV for J71). Interestingly, femtosecond transient absorption spectroscopy studies on both two systems show that efficient charge generation were observed despite the fact that the highest occupied molecular orbital (HOMO)-HOMO offset (ΔEH) in the blends were as low as 0.10 eV, suggesting such a small ΔEH is not a crucial limitation in realizing high performance of NIR non-fullerene based OPVs. Our results indicated that BT-IC is an interesting NIR non-fullerene acceptor with great potential application in tandem/multi-junction, semitransparent, and ternary blend solar cells.