Non-fullerene Small Molecular Acceptors Research Articles

Nonfullerene Small Molecular Acceptor Acting as a Solid Additive Enables Highly Efficient Pseudo-Bilayer All-Polymer Solar Cells

Nonfullerene Small Molecular Acceptor Acting as a Solid Additive Enables Highly Efficient Pseudo-Bilayer All-Polymer Solar Cells

Chalcogenopheno[3,2-b]pyrrole-Containing Donor-Acceptor-Donor Organic Semiconducting Small Molecules for Organic Field-Effect Transistors.

A group of chalcogenopheno[3,2-b]pyrroles, including thieno[3,2-b]pyrrole (TP), furo[3,2-b]pyrrole (FP), and selenopheno[3,2-b]pyrrole (SeP), and thieno[3,2-b]thiophene (TT) electron-donating units were coupled with a thiophene-flanked diketopyrrolo[3,4-c]pyrrole (ThDPP) acceptor to generate four donor-acceptor-donor (D-A-D) semiconducting small molecules (ThDPP-TT, ThDPP-FP, ThDPP-TP, and ThDPP-SeP). This study systematically investigated the differences between chalcogenopheno[3,2-b]pyrroles and TT. From the characterizations, chalcogenopheno[3,2-b]pyrrole-containing molecules showed lower band gaps and binding-energy cold crystallization behavior. The enthalpies of cold crystallization were correlated with the weight of the chalcogen in ThDPP-FP, ThDPP-TP, and ThDPP-SeP, which were evaluated as intermolecular chalcogen-bond interactions between chalcogen and pyrrole nitrogen in chalcogenopheno[3,2-b]pyrroles. A stronger chalcogen bond interaction resulted in stronger self-aggregation in thin films with thermal treatment, which resulted in a polycrystalline structure in chalcogenopheno[3,2-b]pyrrole-containing molecules. For the application in an organic field-effect transistor, all four molecules showed good performance with the highest hole mobilities as 6.33 × 10-3 cm2 V-1 s-1 for ThDPP-TT, 2.08 × 10-2 cm2 V-1 s-1 for ThDPP-FP, 1.87 × 10-2 cm2 V-1 s-1 for ThDPP-TP, and 6.32 × 10-3 cm2 V-1 s-1 for ThDPP-SeP, and the change of mobility is well correlated to the root-mean-square roughness of the thin films. Overall, all the chalcogenopheno[3,2-b]pyrrole-containing molecules showed lower band gaps, polymorphism, and better charge transport properties compared to TT-containing molecules, which motivates replacing TT with chalcogenopheno[3,2-b]pyrroles in conjugated polymers, non-fullerene small molecular acceptors, and narrow-band-gap donors.

Effect of terminal fluorine substitution of a nonfullerene small molecular acceptor on the thermal stability of organic solar cells

End group fluorine substitution of ITIC can improve the device efficiency but reduces the thermal stability of the device.

Reducing Voltage Losses of Organic Solar Cells against Energetics Modifications by Thermal Stress.

Voltage losses are one of the main obstacles for further improvement in the power conversion efficiency of organic solar cells. In this work, we investigate the effect of thermal stress on voltage losses in various material systems by multiple spectroscopic measurements on both devices and thin films. The energetics of nonfullerene small molecules are more readily altered under thermal stress compared to all-polymer and fullerene-based systems, thereby strongly affecting open-circuit voltage. These energetics variations correlate with the glass transition of respective materials. While nonfullerene small molecular acceptor systems exhibit both dynamic and static disorders which can be restrained in annealed films, all-polymeric systems exhibit dominated static disorders, which are also stable against thermal stress. The much higher voltage losses in fullerene-based systems compared to the other two counterparts are mainly due to the losses from device band gap to charge transfer states and the high nonradiative recombination.

Exploration of the Intriguing Photovoltaic Behavior for Fused Indacenodithiophene-Based A-D-A Conjugated Systems: A DFT Model Study.

Many researchers are engaged nowadays in developing efficient photovoltaic materials to accomplish the demand of modern technology. Nonfullerene small molecular acceptors (NF-SMAs) show potential photovoltaic performance, accelerating the development of organic solar cells (OSCs). Herein, the first theoretical designing of a series of indacenodithiophene-based (IDIC1–IDIC6) acceptor chromophores was done by structural tailoring with various well-known acceptors from the recently synthesized IDICR molecule. For the selection of the best level of density functional theory (DFT), various functionals such as B3LYP, M06-2X, CAM-B3LYP, and ωB97XD with the 6-311G(d,p) basis set were used for the UV–visible analysis of IDICR. Consequently, UV–visible results revealed that an interesting agreement was found between experimental and DFT-based values at the B3LYP level. Therefore, quantum chemical investigations were executed at the B3LYP/6-311G(d,p) level to evaluate the photovoltaic and optoelectronic properties. Structural tailoring with various acceptors resulted in a narrowing of the energy gap (2.245–2.070 eV) with broader absorption spectra (750.919–660.544 nm). An effective transfer of charge toward lowest unoccupied molecular orbitals (LUMOs) from highest occupied molecular orbitals (HOMOs) was studied, which played a crucial role in conducting materials. Further, open circuit voltage (Voc) analysis was performed with respect to HOMOPBDB-T–LUMOACCEPTOR, and all of the derivatives exhibited a comparable value of voltage with that of the parent chromophore. Lower reorganization energies in titled chromophores for holes and electrons were examined, which indicated the higher rate of mobility of charges. Interestingly, all of the designed chromophores exhibited a preferable optoelectronic response compared to the reference molecule. Therefore, this computed framework demonstrates that conceptualized chromophores are preferable and might be used to build high-performance organic solar cells in the future.

A Dual Functional Polymer Interlayer Enables Near‐Infrared Absorbing Organic Photoanodes for Solar Water Oxidation

AbstractOrganic photovoltaic devices employing bulk heterojunctions (BHJs) of polymer donors and small molecular nonfullerene acceptors have recently demonstrated high performance, with strong visible and near‐infrared absorption and low energy losses. Such junctions are promising candidates for solar‐driven water splitting; however, the poor underwater stability of the small molecular acceptor in such devices has limited their viability to date. Here, a stable and efficient organic photoanode is demonstrated for water oxidation based upon a Y6:PM6 BHJ with a further dual functional PM6 layer transferred from water, and Au/NiFe electrocatalyst top layers. The additional PM6 layer functions to 1) increase operational stability and 2) suppress recombination losses between the BHJ and electrocatalyst layers. These BHJ/PM6 based photoanodes exhibit a photocurrent density of 4.0 mA cm−2 at 1.23 V versus the reversible hydrogen electrode and promising operational stability compared to the anode without a PM6 layer, maintaining a photocurrent ≥ 2 mA cm−2 over 1 h. Employing these photoanodes, solar water oxidation under near‐infrared irradiation is demonstrated with an incident photon‐to‐current efficiency up to 25% at 770 nm illumination.

Near-Infrared Nonfullerene Acceptors Based on 4H-Cyclopenta[1,2-b:5,4-b']dithiophene for Organic Solar Cells and Organic Field-Effect Transistors.

The development of nonfullerene small molecular acceptors (NF-SMAs) has dominated the improvement of efficiencies for organic solar cells and the near-infrared (NIR) absorption is the primary feature of NF-SMAs compared with fullerene derivatives. In this article, a series of acceptor-donor-acceptor-structured NF-SMAs (named CPICs) containing 4H-cyclopenta[1,2-b : 5,4-b']dithiophene (CPDT) electron donor and F-substituted 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC) as electron acceptor were designed and synthesized. With the increase of CPDT units, the elongated conjugations broadened the absorption range of the acceptors and tuned their energy levels sequentially. Therefore, their charge-transporting polarities switched from electron-only type to bipolar mode in organic field-effect transistors. Moreover, these changes also influenced the voltages, current densities, and eventual PCEs of their corresponding cells. When blending with PBDB-T, a champion efficiency of 10.01% was achieved in CPIC-2 based cells. This work demonstrated the importance of absorptions, suitable energy levels and charge transports in improving the efficiencies of organic solar cells.

Impact of fluorination on photovoltaic performance in high thermo- and photo-stability perylene diimide-based nonfullerene small molecular acceptors

Fluorinations play the momentous impact in tuning the optical absorption, energy levels, aggregation and microstructurally morphology. Two acceptor-donor-acceptor (A-D-A) non-fullerene small molecular acceptors (SMAs), namely, 2T-(PDI-HD)2 and F2T-(PDI-HD)2, by incorporating two fluorine atoms into the central 2,2′-bithiophene linked core, were synthesized. The resultant PDI-based SMAs had excellent thermo- and photo-stability, wide absorption ranging from 300 to 700 nm, and ELUMOs of −3.71 to −3.84 eV. It was demonstrated that the enhanced molar absorption coefficient, elevated ELUMO, slightly strengthened film aggregation and lowered miscibility with donor PTB7-Th were observed after incorporating fluorine atom onto the SMAs. Further surface electrostatic potential (ESP) calculation elucidated that the fluorination could increase molecular polarity and maximize the ESP difference between PTB7-Th and SMAs, facilitating exciton dissociation and the charge transport. Accordingly, an 39.01% improved PCE from 1.82% to 2.53% was yielded in PTB7-Th:F2T-(PDI-HD)2 based device, mainly benefitting from 26.17% enhanced JSC and 22.82% elevated FF even if the adverse condition of decreased VOC. This improvement was profited from the increased absorption coefficient, enhanced electron mobility as well as favorable microstructurally morphology. The results demonstrated that inserting fluorine atoms into the central linked core was an effective strategy to improve the device performance.

A Synergistic Strategy of Manipulating the Number of Selenophene Units and Dissymmetric Central Core of Small Molecular Acceptors Enables Polymer Solar Cells with 17.5 % Efficiency

AbstractA dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A‐DA′D‐A type non‐fullerene small molecular acceptors (NF‐SMAs) with different numbers of selenophene. From S‐YSS‐Cl to A‐WSSe‐Cl and to S‐WSeSe‐Cl, a gradually red‐shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A‐WSSe‐Cl and S‐WSeSe‐Cl exhibit stronger and tighter intermolecular π–π stacking interactions, extra S⋅⋅⋅N non‐covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge‐transfer networks in comparison with thiophene‐based S‐YSS‐Cl. The dissymmetric A‐WSSe‐Cl‐based device has a PCE of 17.51 %, which is the highest value for selenophene‐based NF‐SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve Jsc and FF without sacrificing Voc.

Carbazole core derived dyes: New non-fullerene acceptor for all small-molecule organic solar cells with very high open-circuit voltage of 1.12 V

We have design and synthesized two similar A-π-D-π-A structured V-shaped small molecular non-fullerene acceptors named as BYG3 and BYG4 for BHJ-OSCs applications. The molecular architecture of BYG3 and BYG4 consists of an electron rich carbazole as central core, benzothiadiazole as π-spacer and finally it is end caped with naphthamide unit via ethyne linker. The major difference among BYG3 and BYG4 non-fullerene acceptors is fluorine atom substitution at benzothiadiazole unit and the effects of fluorine atom on its optical, electrochemical and photovoltaic properties were well discussed. The optoelectronic properties and band gap of BYG3 and BYG4 acceptors were well synchronized to small molecule donor SMD. Therefore, all small molecule BHJ-OSCs device were constructed using BYG3 and BYG4 as acceptors and small molecule (SMD) as donor results power conversion efficiency (PCE) of 8.09% and 8.57% with high open circuit voltages (Voc) of 1.12 V and 1.05 V respectively. Also, BYG4 based OSCs devices were displayed high values of short-circuit current density (Jsc) and fil factor (FF) are higher for than that for BYG3, may be related to the more effective exciton dissociation and charge transfer in the SMD:BYG4 active layer than that for SMD:BYG3 counterpart. However, the larger value of Voc for the BYG3 based OSCs than BYG4 counterpart may be related to the up-shifted LUMO energy level of BYG3.

Planar Organic Bilayer Heterojunctions Fabricated on Water with Ultrafast Donor‐to‐Acceptor Charge Transfer

Herein, planar heterojunctions comprising a nonfullerene small molecular acceptor (NFA) and a polymer donor are demonstrated by transferring polymer films on a water surface on top of NFA layers. So far, most solution‐processed layer‐by‐layer architectures have been reported as sequentially deposited bulk heterojunctions or pseudo‐bilayers because mixed regions at the donor/acceptor interface are inevitable in these methods. By virtue of the unique properties of conjugated polymers such as hydrophobicity and spontaneous film formation on a water surface, the fabrication of NFA/polymer bilayer nanostructures is clearly demonstrated by dramatically simplified methods. These bilayers are successfully rendered into bilayer organic solar cells achieving a power conversion efficiency of up to 7.47%. This reflects that these bilayers have appropriate morphological and optoelectrical properties to be operated as photoactive layers in photovoltaic devices. Further, ultrafast charge transfer from the polymer donor to the NFA and fast carrier mobility are investigated by transient‐absorption spectroscopy and photoinduced charge‐extraction measurements. Fast carrier dynamics are observed, which are essential for the efficient harvest of excitons in photovoltaic devices. It is believed that the formation of planar heterojunctions on water can offer technical diversity for the fabrication methods of the photovoltaic devices.

Fullerene as an additive for increasing the efficiency of organic solar cells to more than 17%

In this work, we introduced a fullerene acceptor (PC71BM) into the binary photo-active layer based on a polymer donor (PM6) and a non-fullerene small molecular acceptor (BTP-BO-4Cl), and as a consequence, the ternary organic solar cells realized a high-power conversion efficiency of 17.39% compared to 16.65% in binary solar cells. The performance enhancement was found to be due to the optimized morphology and hence balanced hole and electron mobilities, which is responsible for the suppressed charge recombination and hence high photocurrent in solar cells. In addition, PC71BM shows the complementary absorption with PM6 and BTP-BO-4Cl, which can broaden the absorption range of the photo-active layer and hence more photons from the sunlight can be utilized. Besides, PC71BM shows the cascade energy level alignment between PM6 and BTP-BO-4Cl, which is helpful for charge transfer from donor to acceptor. All these merits explain the high performance in ternary solar cells, and also demonstrate that ternary photovoltaics adopting non-fullerene acceptor with the fullerene acceptor as small amount of additive is an efficient strategy to gain high performing organic solar cells.

Mobility Relaxation of Holes and Electrons in Polymer:Fullerene and Polymer : Non-Fullerene Acceptor Solar Cells.

A non-fullerene small molecular acceptor (NFA) is a prominent molecule that shows moderate electron mobility and a narrow bandgap complementary to middle-bandgap p-type conjugated polymers, which leads to great improvement in the performance of organic photovoltaic (OPV) cells. However, little is known about the relaxation of charge carriers, which is key to efficient charge transport. Simultaneous time-of-flight (TOF) and time-resolved microwave conductivity (TRMC) measurements have been carried out on benzodithiophene-based polymer (PBDB-T):soluble C70 -fullerere (PCBM) and PBDB-T:NFA (ITIC or Y6) blends, as benchmark systems. In addition to the conventional TOF mobilities, relaxation of the hole and electron mobility are evaluated by TRMC under an external electric field. PBDB-T : ITIC exhibits much faster relaxation than PBDB-T : PCBM, whereas that in PBDB-T : Y6 is moderate. This is consistent with the energetic disorder estimated from the photoabsorption onset. Interestingly, the slower relaxation of the electrons compared to the holes in PBDB-T : Y6 is in line with the preferred normal device structure. Our work deepens the understanding of the energetics of polymer : NFA blends and offers a basis for achieving efficient NFA properties.

Synthesis and photovoltaic performance of a non-fullerene acceptor comprising siloxane-terminated alkoxyl side chain

As an effective molecular modification strategy, side chain engineering has been widely used in promoting the photovoltaic performance of non-fullerene acceptors. Herein, a novel non-fullerene small molecular acceptor i-IEOSi-4F comprising siloxane-terminated alkoxyl side chain was successfully designed and synthesized. The molecule shows an optical band gap of 1.53 eV, with large extinction coefficient of 2.36 × 105 M−1 cm−1 in solution. Two fluorobenzotriazole based polymers J52 and PBZ-2Si with the same backbone units but different side chains were employed as the donor to construct the active layers that all can demonstrate suitable energy levels and complementary absorptions with i-IEOSi-4F. Relative to J52 only bearing alkyl side chain, PBZ-2Si with siloxane-terminated side chain could induce more balanced carrier transports and more favorable morphology, leading to a higher power conversion efficiency (PCE) of 12.66% with a good fill factor of 71.45%. The efficiency is 21% higher than that of 10.46% for the J52 based devices. Our results not only indicate that siloxane-terminated alkoxyl side chain is valuable for efficient non-fullerene acceptors, but also demonstrate that siloxane-terminated side chain on both polymer donor and small molecular acceptor is a useful combination to realize more efficient polymer solar cells.

A small molecular acceptor based on dithienocyclopentaindenefluorene core for efficient fullerene-free polymer solar cells

In recent years, acceptor–donor–acceptor (A–D–A) type of small molecules have become very promising non-fullerene acceptors (NFAs) to overcome the energy levels and absorption drawbacks of the fullerene-based acceptors in polymer solar cells (PSCs). A non-fullerene small-molecular acceptor named DTIF-IC with dithienocyclopentaindenefluorene (DTIF) as the electron-donating core sealed with 2-(3-oxo-indane-1-ylidene)-malononitrile as the electron-withdrawing groups was developed. The molecule exhibits strong absorption in the range from 550 to 720 nm and relatively high-lying lowest unoccupied molecular orbital (LUMO) energy level (−3.86 eV). Due to its up-shifted LUMO energy levels, high and balanced charge mobility and the homogeneous nanophase separations, the inverted polymer solar cells blending with the donor polymer poly[[2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]-[3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7-Th) and DTIF-IC deliver a best power conversion efficiency (PCE) of 7.84% with relatively high open circuit voltage (Voc) of 0.95 V by thermal annealing treatment, which demonstrated that the DTIF-type small molecule could be the promising electron acceptor for high-performance polymer solar cells with a high Voc.

Efficient As‐Cast Polymer Solar Cells with High and Stabilized Fill Factor

Molecular ordering and miscibility of donor and acceptor materials play critical roles in developing high‐performance as‐cast polymer solar cells (PSCs). In this work, a highly crystalline nonfullerene small molecular acceptor, namely, C8‐IT‐4F, based on alkylated indacenodithieno[3,2‐b]thiophene as the aromatic core and 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile moieties as end groups, is selected and synthesized. The π–π stacking distance in C8‐IT‐4F film can be tuned from 3.88 Å to a more compact (3.48 Å) state by a film‐formation process and the confinement induced by the preaggregated polymer donor PM7, leading to broadened absorption and fine phase separation in the blend film. The optimal morphology with a framework of preaggregated polymer donor, J‐type face‐on π–π stacked acceptor, and appropriate donor/acceptor miscibility facilitates charge generation and transport and reduces charge recombination. As a result, the best PSC based on the as‐cast PM7:C8‐IT‐4F blend film exhibits power conversion efficiency of 14.3%, with an open‐circuit voltage of 0.82 V, a short‐circuit current density of 22.7 mA cm−2, a fill factor of 77.1%, and good photostability with a stabilized fill factor.

Tetrabromination versus Tetrachlorination: A Molecular Terminal Engineering of Nonfluorinated Acceptors to Control Aggregation for Highly Efficient Polymer Solar Cells with Increased Voc and Higher Jsc Simultaneously

Herein, a pair of tetrahalogenated nonfullerene small molecular acceptors (NF‐SMAs) (TSeIC4Cl and TSeIC4Br) are designed and synthesized, with the same indacenodithieno[3,2‐b]selenophene central unit and two different dihalogenation terminal groups, respectively. The systematic investigation is achieved to reveal the impact of two different nondifluorinated terminal groups on the device performance of the resultant ITIC series NF‐SMAs. TSeIC4Br shows red‐shifted absorption range and higher frontier energy levels compared with that of TSeIC4Cl. Moreover, PM6:TSeIC4Br blend film exhibits more suitable phase‐separated morphology with more ordered molecular packing and improved miscibility compared with that of the tetrachlorinated counterpart. Importantly, PM6:TSeIC4Br‐based device exhibits a better power conversion efficiency (PCE) of 11.92%, with a higher open‐circuit voltage (Voc) and an enhanced short‐circuit current density (Jsc) when compared with that of PM6:TSeIC4Cl‐based device (11.13%). Furthermore, the energy loss can be reduced by replacing the disubstituents of end group from chlorine to bromine atoms. The results demonstrate that incorporation of indacenodithieno[3,2‐b]selenophene and replacement of tetrachlorination by tetrabromination on the end group contributes to elevating the Jsc, reduce energy loss, and enhancing the PCE for its relevant ITIC series device simultaneously, which may give a new avenue for achieving high‐performance multihalogenated ITIC series NF‐SMAs.

Significantly Increasing the Power Conversion Efficiency by Controlling the Orientation of Nonfullerene Small Molecular Acceptors via Side Chain Engineering

Three S,N‐heteroacene nonfullenere small molecular acceptors (NF‐SMAs) with different branching positions on side chains named SNBDT1‐F, SNBDT2‐F, and SNBDT3‐F are synthesized to understand the relationship among branching positions, molecular orientations of NF‐SMAs, and photovoltaic performance of nonfullerene organic solar cells (OSCs). When the branching position is moved close to the conjugated backbone, the proportion of face‐on orientation increases gradually from 4% (SNBDT3‐F), 16% (SNBDT2‐F), to 44% (SNBDT1‐F), suggesting that the molecular orientation changes from edge‐on to face‐on, which has a positive influence on exciton dissociation and charge transport. As a result, the device based on PBDB‐T:SNBDT1‐F shows the highest exciton dissociation probability and carrier mobilities due to the highest proportion of face‐on orientation of SNBDT1‐F. Therefore, the highest power conversion efficiency (PCE) of 12.70% with a high short‐circuit current (Jsc) of 20.97 mA cm−2 and a fill factor of 70.4% is obtained for SNBDT1‐F‐based device, which is much higher than PCEs of SNBDT2‐F‐based (6.57%) and SNBDT3‐F‐based devices (5.87%). Overall, these results provide deep insight into the relationship among the branching positions, molecular orientation, and photovoltaic performance in nonfullerene OSCs, which is important for designing new NF‐SMAs with face‐on orientation and promoting the development of nonfullerene OSCs.

Designing indenothiophene-based acceptor materials with efficient photovoltaic parameters for fullerene-free organic solar cells.

Non-fullerene small molecular acceptors (NFSMAs) exhibit promising photovoltaic performance which promoted the rapid progress of organic solar cells (OSCs). In this study, an attempt is done to explore indenothiophene-based high-performance small molecular electron acceptors for organic solar cells. We have designed five acceptor molecules (M1-M5) with strong donor moiety indenothiophene linked to five different end-capped group acceptor moieties: diflouro-2-methylene-3-oxo-2,3-dihydroindene-1-ylidene)malononitrile (A1), 1-(dicyanomethylene)-2-methylene-3-oxo-2,3-dihydro-1H-indene-5,6-dicarbonitrile (A2), methyl-6-cyano-3-(dicyanomethylene)-2-methylene-1-oxo-2,3-dihydro-1H-indene-5-carboylate (A3), 2-(6-cyano-5-fluoro-2-methylene-3-oxo-2,3 dihydro-1H-indene-1-ylidene)malononitrile (A4), and (Z)-methyl 3-(benzo [c][1,2,5]thiadiazol-4-yl)-2-cyanoacrylate (A5) respectively. The structure-property relationship was studied and effects of structural modification on the optoelectronic properties of these acceptors (M1-M5) were determined systematically by comparing it with reference molecule R, which is recently reported as excellent non-fullerene-based small acceptor molecule. Among all designed molecules, M5 is proven as a suitable candidate for organic solar cell applications due to better photovoltaic properties including narrow HOMO-LUMO energy gap (2.11 eV), smallest electron mobility (λe= 0.0038eV), highest λmax values (702.82 nm in gas) and (663.09 nm in chloroform solvent) and highest open-circuit voltage (Voc = 1.49V) with respect to HOMOPTB7-Th-LUMOacceptor. Our results indicate that introducing more end-capped electron-accepting units is a simple and effective alternative strategy for the design of promising NFSMAs. This theoretical framework also proves that the conceptualized NFSMAs are superior and thus are recommended for the future construction of high-performance organic solar cell devices. Graphical abstract.

Low-Bandgap n-Type Polymer Based on a Fused-DAD-Type Heptacyclic Ring for All-Polymer Solar Cell Application with a Power Conversion Efficiency of 10.7.

An n-type polymer (A701) is designed and synthesized with an alternative A'-DAD-A'-D' backbone, where 1,1-dicyanomethylene-3-indanone (IC), dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (TPBT), and benzodithiophene (BDT) are used as A', DAD, and D' units, respectively. A701 shows enhanced light absorption with a narrow bandgap of 1.42 eV and a high absorption coefficient of 6.85 × 104 cm-1 at 780 nm. It displays an uplifted LUMO (the lowest unoccupied molecular orbital) level of -3.80 eV. By introducing a high point solvent additive of 1,8-diiodooctane (DIO), all-polymer solar cells (all-PSCs) based on the PBDB-T:A701 blend exhibit efficient exciton dissociation, enhanced charge transport, and decreased bimolecular recombination. Thus, a high open-circuit voltage (VOC) of 0.92 V, a short-circuit current (JSC) of 18.27 mA cm-2, and a fill factor (FF) of 0.64 are attained, affording an impressive power conversion efficiency (PCE) of 10.70%. The low voltage loss of 0.50 V and high efficiency of 10.7% are among the top values for all-PSCs. Our results indicate that the fused DAD-type heptacyclic ring can be utilized to construct not only nonfullerene small molecular acceptors but also promising polymer acceptors.