Fused-ring Electron Acceptors Research Articles

14.55% efficiency PBDB-T ternary organic solar cells enabled by two alloy-forming acceptors featuring distinct structural orders

Ternary organic solar cells (OSCs) have demonstrated great potential in boosting the power conversion efficiencies (PCEs) of single-junction devices. Screening suitable third component is crucial but challenging for different host binary OSCs. We report herein an efficient ternary strategy in achieving highly efficient PBDB-T based devices by incorporating two alloy-forming fused-ring electron acceptors (FREAs) with distinct structural orders. INPIC-Si as the third component showcases a relatively narrower bandgap and higher energy levels than the host acceptor IDTO-T-4F. Featuring good compatibility and alloy-forming blends, two FREAs generate Förster resonance energy transfer from IDTO-T-4F to INPIC-Si, facilitating efficient exciton dissociation and charge carrier transport. Importantly, the low crystalline INPIC-Si decreases the strong aggregation of PBDB‐T:IDTO-T-4F blends for optimal phase-separation and structural order. Consequently, the best PBDB‐T:IDTO-T-4F:INPIC-Si ternary OSCs deliver a champion PCE of 14.55% with simultaneously improved device parameters over binary ones. To the best of our knowledge, this performance is among the highest for PBDB-T-based ternary OSCs in literature. Our work demonstrates the incorporation of a narrow bandgap FREA with low structural order into heavily aggregated binary blends can optimize the blend morphology for reduced energy loss and significantly improved photovoltaic performance in ternary OSCs.

Elucidating the Key Role of the Cyano (−C≡N) Group to Construct Environmentally Friendly Fused-Ring Electron Acceptors

In recent years, advances in nonfullerene acceptors, especially fused-ring electron acceptors (FREAs), have enabled the power conversion efficiencies of organic solar cells to exceed 16%. FREAs typ...

Nonfullerene acceptors from thieno[3,2-b]thiophene-fused naphthalene donor core with six-member-ring connection for efficient organic solar cells

Comprehensive design ideas on the fused-ring donor-core in state-of-the-art acceptor-donor-acceptor (A-D-A) nonfullerene acceptors (NFAs) are still of great importance for regulating the electron push-pull effect for the sake of optimal light-harvesting, frontier molecular orbital levels, and finally their photovoltaic properties. Herein, thieno[3,2-b]thiophenes were fused in bay-area of naphthalene via six-member-ring connection, resulting in the formation of dihydropyrenobisthieno[3,2-b]thiophene based octacyclic ladder-type donor core, which was flanked by two 1,1-dicyanomethylene-3-indanone (IC) acceptor motifs with and without 5,6-diflourination, namely PTT-IC and PTT-2FIC, respectively, as novel efficient A-D-A fused-ring electron acceptors (FREAs). Compared with PTT-IC, fluorinated PTT-2FIC possesses narrower optical bandgap of 1.48 eV, better π-π stacking, and its PBDB-T:PTT-2FIC blend film exhibited better morphology, and better hole and electron mobility. As a result, nonfullerene solar cells using PBDB-T:PTT-2FIC as the active layer achieved a decent PCE of 10.40%, with an open-circuit voltage (VOC) of 0.87 V, a fill factor (FF) of 0.65, and a much higher short-circuit current (JSC) of 18.26 mA/cm2. Meanwhile, the PBDB-T:PTT-IC cells delivered a lower JSC of 12.58 mA/cm2 but a higher VOC of 0.99 V, thus resulting in a PCE of 7.39% due to its wider optical bandgap of 1.58 eV and higher LUMO energy level. These results demonstrated that NFAs based on fused-ring donor core from fusing thieno[3,2-b]thiophenes with naphthalene via six-member-ring connection are promising for organic photovoltaic applications.

A Fully Non-fused Ring Acceptor with Planar Backbone and Near-IR Absorption for High Performance Polymer Solar Cells.

Fused-ring electron acceptors have made significant progress in recent years, while the development of fully non-fused ring acceptors has been unsatisfactory. Here, two fully non-fused ring acceptors, o-4TBC-2F and m-4TBC-2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o-4TBC-2F formed planar backbones, while m-4TBC-2F displayed a twisted backbone. Additionally, the o-4TBC-2F film showed a markedly red-shifted absorption after thermal annealing, which indicated the formation of J-aggregates. For fabrication of organic solar cells (OSCs), PBDB-T was used as a donor and blended with the two acceptors. The o-4TBC-2F-based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o-4TBC-2F gave a PCE of 10.26 %, which was much higher than those based on m-4TBC-2F at 2.63 %, and it is one of the highest reported PCE values for fully non-fused ring electron acceptors.

A Narrow-Bandgap n-Type Polymer with an Acceptor-Acceptor Backbone Enabling Efficient All-Polymer Solar Cells.

Narrow-bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow-bandgap polymer acceptor L14, featuring an acceptor-acceptor (A-A) type backbone, is synthesized by copolymerizing a dibrominated fused-ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A-A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low-lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open-circuit voltage (Voc ), which is attributed to a small nonradiative recombination loss (Eloss,nr ) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and Voc , an excellent efficiency of 14.3% is achieved, which is among the highest values for all-polymer solar cells (all-PSCs). The results demonstrate the superiority of narrow-bandgap A-A type polymers for improving all-PSC performance and pave a way toward developing high-performance polymer acceptors for all-PSCs.

2D Side‐Chain Engineered Asymmetric Acceptors Enabling Over 14% Efficiency and 75% Fill Factor Stable Organic Solar Cells

AbstractThe charge transport and morphology of active layers are key considerations for device performance and stability in organic solar cells (OSCs). Such properties can be fine‐tuned via elaborate molecular design of fused‐ring electron acceptors (FREAs), especially conjugation extension and side chain engineering. In this work, N‐functionalized conjugation is explored in the design of high‐efficient asymmetric FREAs. The twisting of N‐conjugated side chains from backbone endows three FREAs with similar energy levels and light absorptions (≈850 nm edge). Their blends with PBDB‐T exhibit high charge carrier mobility and ordered phase separation. Excitingly, IPT2F‐TT based OSCs yield a champion power conversion efficiency (PCE) of 14.02% with a fill factor (FF) of 75.06%, outperforming PBDB‐T devices with IPT2F‐Th (12.52%, 71.20%), IPT2F‐Ph (13.13%, 72.11%), and octylated IPT‐2F (13.70%, 71.50%). The PCE over 14% and FF over 75% are among the highest values for 2D FREAs OSCs reported to date. More importantly, outstanding thermal stability and light soaking stability are observed with PCE over 12% maintained after thermal or light aging for 100 h. This work demonstrates N‐conjugated FREAs design as an effective strategy to simultaneously improve the photovoltaic performance and device stability for the OSCs.

Noncovalently Fused-Ring Electron Acceptors with C2v Symmetry for Regulating the Morphology of Organic Solar Cells.

Four noncovalently fused-ring electron acceptors p-DOC6-2F, o-DOC6-2F, o-DOC8-2F, and o-DOC2C6-2F have been designed and synthesized. p-DOC6-2F and o-DOC6-2F have the same molecular backbone but different molecular shapes and symmetries. p-DOC6-2F has an S-shaped molecular backbone and C2h symmetry, whereas o-DOC6-2F possesses a U-shaped molecular backbone and C2v symmetry. The molecular shape and symmetry can influence the dipole moment, solubility, optical absorption, energy level, molecular packing, and film morphology. Compared with the corresponding p-DOC6-2F, o-DOC6-2F exhibits better solubility, a wider band gap, and a larger dipole moment. When blended with the donor polymer PBDB-T, the C2v symmetric o-DOC6-2F can form an appropriate active layer morphology, whereas the C2h symmetric p-DOC6-2F forms oversized domains. Organic solar cells (OSCs) based on p-DOC6-2F, o-DOC6-2F, o-DOC8-2F, and o-DOC2C6-2F obtained power conversion efficiencies of 9.23, 11.87, 11.23, and 10.80%, respectively. The result reveals that the molecular symmetry can facilely regulate the performance of OSCs.

Ultrafast and Long-Range Exciton Migration through Anisotropic Coulombic Coupling in the Textured Films of Fused-Ring Electron Acceptors.

The exciton migration mechanism in organic photovoltaic devices is still an ambiguity owing to the insufficient understanding of molecular arrangement on a microscopic scale. Herein, we reveal the relationship between the molecular stacking modes and exciton migration for a representative fused-ring electron acceptor, namely, ITIC. The precise molecular stacking patterns are extracted, and directional Coulombic couplings are calculated based on the information of a single-crystal structure, which proves the anisotropic character for exciton motion. The theoretical analysis results indicate ultrafast exciton migration along the head-to-tail stacking directions with maximum migration length of 330 nm in the finite lifetime of 1 ns. Experimentally, the exciton diffusion length is determined to be 183 nm by exciton-exciton annihilation measurement. This work reveals head-to-tail type intermolecular stacking induces strong anisotropic Coulombic coupling, leading to the ultrafast and long-range exciton migration in nonfullerene systems.

Fused-ring electron acceptors in China

Fused-ring electron acceptors in China

Insight into the effects of alkoxy side chain position in nonfullerene electron acceptors on the morphological stability of organic solar cells

The stable morphology of active layers plays important roles in the applications of organic solar cells. Four fused-ring electron acceptors were synthesized by introduction alkoxy side chains on the π-bridge for IDTP-P-O, on the sp3 carbon of central core for IDTP-O-C, both on the two positions for IDTP-O-O, and reference compound (IDTP-P-C). The results indicate that the presence of alkoxy side chains on the sp3 carbon will retain the large negative electrostatic potential (ESP, ≤-22.79 kcal/mol) of oxygen atoms, impact little on the molecular geometry, absorption, and energy level. However, the introduction of alkoxy side chains on the π-bridge will trigger better backbone planarity, red-shifted absorption, higher HOMO energy level and much less negative ESP (≥−4.58 kcal/mol) due to the additional conjugated effect and intramolecular non-bonded S⋯O interaction. The studies of DSC, AFM, TEM and organic solar cells suggest that the introduction of alkoxy side chains on the sp3 carbon of central core will improve the morphological stability.

Non-fullerene electron acceptors constructed by four strong electron-withdrawing end groups: Potential to improve the photoelectric performance of organic solar cells by theoretical investigations

Recent advances in non-fullerene acceptors have enabled the power conversion efficiencies of organic solar cells (OSCs) to exceed 16%. Fused-ring electron acceptors (FREAs) are some of the most widely investigated non-fullerene acceptors. Classical FREAs, containing two strong electron-withdrawing end groups, are typically constructed using linear architecture. Meanwhile, the development of 2D FREAs is rarely studied. Therefore, we proposed a series of novel 2D FREAs, namely, 2D-IC, 2D-ICT, 2D-ICTS and 2D-ICTSS, which were composed of four strong electron-withdrawing end groups linked by a 2D fused-ring core to form a rotational C2h-symmetric structure. Through theoretical calculations we found that as the degree of conjugation increased, the spectrum red shifted, the integrated absorption intensity was enhanced, and the hole-electron Coulomb attraction decreased. Compared with linear molecule ITOIC-2F, which had the same end groups, 2D-ICTSS exhibited two significant absorption peaks at red and near-infrared regions. In addition, its integrated absorption intensity was 1.8 times that of ITOIC-2F. Simultaneously, the hole-electron Coulomb attraction of 2D-ICTSS was smaller than that of ITOIC-2F. When alkyl chains were introduced, the electron mobility of 2D-ICTSS was estimated to be 1.98 times that of ITOIC-2F. Given that numerous critical factors (including absorption intensity, exciton separation, and electron mobility) of 2D-ICTSS were superior to those of ITOIC-2F, we propose that this rotational C2h-symmetric molecule is highly suitable as a high-performance electron acceptor. This molecule is expected to be an important candidate for the development of next-generation FREAs for high-performance OSCs.

Comparison of Fused-Ring Electron Acceptors with One- and Multidimensional Conformations.

Three fused-ring electron acceptors (FXIC-1, FXIC-2, and FXIC-3) were designed and synthesized. This FXIC series has similar electron-rich central units and the same electron-poor termini. Due to the different steric structures of fluorene, bifluorenylidene, and spirobifluorene, FXIC-1 is a one-dimensional (1D) crystal, while FXIC-2 and FXIC-3 are multidimensional (MD) amorphous materials. The conformations of the FXIC series have a slight impact on their absorption and energy levels. FXIC-1 has higher electron mobility than FXIC-2 and FXIC-3. When blending with different polymer donors (PTB7-Th, J71, and PM7), the FXIC-1-based organic solar cells have efficiencies higher than those of the FXIC-2/FXIC-3-based cells. Meanwhile, the ternary-blend cells based on PTB7-Th:F8IC with FXIC-1, FXIC-2, and FXIC-3 show similar efficiencies, which are all better than those of the binary-blend devices.

Asymmetrically noncovalently fused-ring acceptor for high-efficiency organic solar cells with reduced voltage loss and excellent thermal stability

Abstract Simultaneously broadening the spectral response and reducing the energy loss are challenging tasks in the material design of organic solar cells (OSCs). Herein, a novel asymmetrically noncovalently fused-ring electron acceptor (NFEA) with unilateral alkylthio-substituted thiophene π-bridge, namely IDST-4F, is synthesized. IDST-4F exhibits a broader absorption, higher-lying energy levels, larger dipole moments and suppressed crystallinity than its symmetric counterpart (ID-4F) without the π-bridge. Compared to the devices of PM6:ID-4F, the optimized PM6:IDST-4F-based devices display simultaneously enhanced current density and photovoltage, resulting in an excellent power conversion efficiency (PCE) of 14.3%, which is the highest value among the OSCs based on NFEAs reported in the literature to date. More importantly, the PM6:IDST-4F-based OSCs possess excellent thermal stability with 82% of the initial PCE after thermal treatment at 150 °C for 1200 min. In summary, this study indicates that asymmetrically NFEAs are promising to achieve high efficiency with excellent thermal stability.

Enhancing Performance of Fused-Ring Electron Acceptor Using Pyrrole Instead of Thiophene.

Selecting suitable outermost aromatic rings of the central cores is of particular importance for the 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 highest occupied molecular orbital energy level (P6IC: -5.43 eV, F6IC: -5.71 eV), a slightly upshifted lowest unoccupied molecular orbital energy level (P6IC: -3.94 eV, F6IC: -4.00 eV), 54 nm red-shifted absorption, a narrower band gap (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 a 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 the active layer yield 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%).

Designing alkoxy-induced based high performance near infrared sensitive small molecule acceptors for organic solar cells

Scientist are dedicated to design and synthesize efficient photovoltaic materials to overcome the energy crises. In this regard, herein, we have designed four new small acceptor molecules namely ( A1, A2, A3 and A4 ) for better performance in organic solar cells. These molecules consist Alkoxy-Induced Naphtho-dithiophene core unit flanked with 2,2-ethylidene-5,6-dicyano-3-oxo-2,3-dihydroinden-1-ylidene-malononitrile ( A1 ), methyl-2-cyano-2,2-ethylidene-5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene-acetate ( A2 ), 5,2-ethylidene-5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene-3-methyl-2-thioxothiazolidin-4-one ( A3 ) and 2,5-ethylidene-6-oxo-5,6-dihydrocyclopenta-thiophen-4-ylidene-malononitrile ( A4 ) end-capped acceptor groups. Their optical, electrical and geometries have been compared with reported molecule R. Frontier molecular orbital diagram reveal excellent charge transfer rate, The electron density is shifted from donor to acceptor unit. Among all, A1 exhibits the highest absorption in the visible region (λ max ) at 798 nm in chloroform solvent. The maximum open circuit voltage (2.08 V) is observed for A3 when blended with PTB7-Th donor polymer. All studied molecules have high charge mobilities due to lower reorganization energy values with respect to model molecule R . A1 has the highest electron mobility among all molecules due to lower value of reorganization energy which is 0.0034. Furthermore, all designed molecules show good Solubilities in organic solvent. A2 exhibit high value of dipole moment which reveal good solubilities in fabrication process. We have designed four ( A1, A2, A3 and A4 ) fused ring electron acceptor for their use in OSCs. These molecules contain Alkoxy-Induced Naphtho-dithiophene core unit attached with different acceptor groups to enhance PCE of OSCs. All newly designed molecule have better performance as compared to R . Additionally, A1 exhibits strong absorption in visible region in both gas phase (λ max = 724.1 nm) and solvent (λ max = 798.4 nm) and exhibit lowest energy band gap (1.92 eV). • A1 shows the highest absorption maximum (λ max ) 798.4 nm in chloroform solvent. • The Designed molecules are blended with well-known donor polymer PTB7-Th. • All designed molecules have lower binding energy as compared to reference molecule R. • All newly designed molecules have excellent opto-electronic properties with respect to R.

Deciphering the Role of Fluorination: Morphological Manipulation Prompts Charge Separation and Reduces Carrier Recombination in All‐Small‐Molecule Photovoltaics

Fluorination has proven effective in increasing the absorption, downshifting the energy levels, and enhancing the crystallinity of high‐performance fused‐ring electron acceptors (FREAs). However, an in‐depth understanding of the effects of fluorination is still lacking, as research efforts have mainly focused on increasing the power conversion efficiency (PCE). In addition, fluorination on FREAs has rarely been reported in all‐small‐molecule organic solar cells (ASM OSCs). Herein, fluorination on FREAs is systematically studied in ASM OSCs using the popular FREA 2,2′‐((2Z,2′Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile and its fluorinated analog paired with DRCN5T, an oligothiophene donor seldom investigated in ASM OSCs to date. (Photo)physical studies are conducted on both systems and it is identified that, along with the aforementioned ones, fluorination exerts several additional effects, including the following. First, it optimizes the morphology, thereby accelerating charge separation and reducing geminate recombination charge pairs; second, it suppresses energetic disorder; and third, it prolongs the carrier lifetime and thus aids charge extraction. Consequently, the short‐circuit current density and fill factor are significantly enhanced, and in turn, the PCE yields a 36% improvement, climbing to 9.25% and rivaling that of the current state‐of‐the‐art oligothiophene‐donor/nonfullerene ASM OSCs. The insights decipher the working mechanism of ASM OSCs that use fluorinated FREAs, paving the way toward high‐performance ASM OSCs.

Regulating the Packing of Non-Fullerene Acceptors via Multiple Noncovalent Interactions for Enhancing the Performance of Organic Solar Cells.

Three noncovalently fused-ring electron acceptors (FOC6-IC, FOC6-FIC, and FOC2C6-2FIC) are synthesized. Single crystals of FOC6-IC and FOC2C6-2FIC are prepared, and structure analyses reveal that the molecular backbone can be planarized via the formation of the intramolecular noncovalent interactions. These acceptor molecules can be packed closely in the solid state via π-π stacking and static interactions between the central phenylene unit and the terminal group with a distance of 3.3-3.4 Å. Besides, multiple intermolecular noncovalent interactions can be observed in the single crystal structure of the fluorinated acceptor FOC2C6-2FIC, which help increase the crystallinity of acceptors and the charge mobility of the blends. Photovoltaic devices based on FOC2C6-2FIC give a power conversion efficiency of 12.36%, higher than 12.08% for FOC6-FIC and 10.80% for FOC6-IC.

Functionalizing triptycene to create 3D high-performance non-fullerene acceptors.

Non-fullerene acceptors have been widely investigated for organic solar cells (OSCs). In particular, fused-ring electron acceptors (FREAs), composed of two strongly electron-withdrawing end groups connected by a planar fused-ring core, have been successfully applied to develop high-performance OSCs (>16%). In this work, we proposed two novel 3D FREAs named BFT-3D and BFTT-3D, which can reduce the formation of crystalline domains and increase the interface with donors to promote exciton separation. These 3D FREAs consist of three strongly electron-withdrawing end groups linked by a central triptycene hub to form a three-bladed propeller nanostructure. In comparison with high-performance FREA (ITOIC-2F), these FREAs have stronger absorption intensity and smaller exciton binding energy. These findings demonstrated that these three-bladed propeller-shaped FREAs can absorb abundant energy from sunlight to generate excitons, easily separate excitons to free electrons and holes, and reduce the recombination of excitons. In addition, the electron mobility of BFT-3D (8.4 × 10−4 cm2 V−1 s−1) is higher than that of BFTT-3D (1.0 × 10−4 cm2 V−1 s−1), which indicated that the appropriate 3D core structure was conducive to the electron mobility of the three-bladed propeller-shaped FREAs. It can effectively improve the current density to enhance the performance of OSCs. These findings will provide new perspectives for experimental scientists to synthesize high-performance FREAs.

The regioisomeric bromination effects of fused-ring electron acceptors: modulation of the optoelectronic property and miscibility endowing the polymer solar cells with 15% efficiency

Regioisomerically brominated fused-ring electron acceptors enable a high-performance PSC with 15.03% efficiency by delicately adjusting the optoelectronic property and miscibility.

Charge separation boosts exciton diffusion in fused ring electron acceptors

The first-principles simulations of exciton diffusion in NFAs.