Acceptor Y6 Research Articles

The optimization of π-bridge for trialkylsilyl substituted D-π-A photovoltaic polymers

Polymers with donor-π-acceptor (D-π-A) structure are widely used in organic solar cells (OSCs) because of their tunable optoelectronic properties. With the distinct 2D-conjugated structure and the s*(Si)–p*(C) bond interaction of the trialkylsilyl substitution, the electron-donating unit of 4,8-bis(5-(tripropylsilyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophenes (BDTT-Si) shows large potential to construct promising photovoltaic polymers. However, the studies of π-bridge in BDTT-Si-based polymers are not sufficient, and structure-properties relationship is still unclear. Here, we designed and synthesized three polymers PE7a, PE7b and PE7c, with BDTT-Si and difluorinated benzo[d] [1,2,3]triazole (BTA) as the D unit and A unit, respectively. By using thieno[3,2-b]thiophene (TT), TT with hexyl (C6-TT) and TT with undecyl (C11-TT) as the π-bridge, the solubility, crystallinity and optoelectronic properties of three polymers can be fine-tuned. When blended with the non-fullerene acceptor Y6, PE7a-c show different photovoltaic performance. The polymer PE7c realized the highest power conversion efficiency (PCE) of 11.4%, obviously higher than that (9.5%) of J71 with thiophene as π-bridge. The results indicate that the introduction of TT π-bridge with suitable alkyl chain into BDTT-Si containing polymer is an effective strategy to improve the photovoltaic performance.

Modulating Crystallinity and Miscibility via Side‐chain Variation Enable High Performance All‐Small‐Molecule Organic Solar Cells

Main observation and conclusionSide‐chain engineering as one of the most important molecular design strategies has been widely used to improve photovoltaic efficiency of active layer materials. Herein, a series of acceptor‐π‐donor‐π‐acceptor typed small molecule (SM)‐donors (SL1, SL2, SL3, and SL4), on the basis of high‐performance SM‐donor BTTzR (SL1) with thiazolo[5,4‐d]thiazole as the π‐bridging units and 3‐butylrhodanine as the terminal electron‐withdrawing groups, were designed and synthesized to study the effect of the side‐chain substitutions of BDT‐T on the photovoltaic performance. The investigation shows that the side‐chain engineering has no obvious effect on the molecular absorption spectrum and energy levels but significantly influences on the molecular orientation and packing, and the compatibility with the acceptor Y6. Among these SM‐donors neat films, SL1 and SL3 with two mixed branched and straight alkyl chains exhibit stronger crystallization in the face‐on direction, and SL4 with two shorter alkyl chains achieves more compact packing. In the Y6‐based blend films, three SM‐donors (SL1, SL3, and SL4) with double alkyl chains on thienyl of BDT‐T have much better compatibility compared to SL2 with single long alkyl chain, while their compatibility increases with the increase of alkyl chain length. The stronger crystallization, moderate molecular packing, and better blend compatibility of SL1 offer higher Jsc of 23.2 mA·cm–2 and fill factor (FF) of 0.68 in the Y6‐based all‐small‐molecule organic solar cells (all‐SM‐OSCs). Ultimately, the SL1:Y6‐based devices achieved a promising power conversion efficiency of 13.9%, which is much higher than that of 11.5% from the SL2:Y6‐based devices (Jsc = 21.5 mA·cm–2 and FF = 0.60). This work indicates that modulating the side chain of SM‐donors is a promising strategy to obtain efficient all‐SM‐OSCs.

Synergistic Engineering of Substituents and Backbones on Donor Polymers: Toward Terpolymer Design of High-Performance Polymer Solar Cells.

Design of terpolymers via copolymerization has emerged as a potential strategy for expanding the family of high-performing donor polymers and boosting the photovoltaic performance of non-fullerene polymer solar cells (PSCs). Herein, double-ester-substituted thiophenes and thienothiophenes are incorporated as third building blocks into the donor polymer PBDB-TF, developing two groups of terpolymers with donor-acceptor 1-donor-acceptor 2 (D-A1-D-A2)-type backbones. An optimum 10% concentration of double-ester-substituted thiophene units in PBDB-TF-T10 downshifts the molecular energy and increases the dielectric constant, and delivers proper miscibility and nanostructure in blends with the high-performing acceptor Y6. These characteristics are designed to synergistically enhance the photovoltage, photocurrent, and efficiency of PSCs. The resulting power conversion efficiency (PCE) of 16.4% surpasses the conventional PBDB-TF/Y6 PSCs, and it is among the best-performing PSCs based on PBDB-TF-derived terpolymers. Gratifyingly, PBDB-TF-T10 does not show significant batch-to-batch variation and it retains high PCEs above 16% in a broad range of molecular weights. This work introduces a facile strategy to easily synthesize terpolymers in combination with Y6 for the attainment of high-performing and reproducible non-fullerene PSCs.

Efficient Electron Transport Layer Free Small-Molecule Organic Solar Cells with Superior Device Stability.

Electron transport layers (ETLs) placed between the electrodes and a photoactive layer can enhance the performance of organic solar cells but also impose limitations. Most ETLs are ultrathin films, and their deposition can disturb the morphology of the photoactive layers, complicate device fabrication, raise cost, and also affect device stability. To fully overcome such drawbacks, efficient organic solar cells that operate without an ETL are preferred. In this study, a new small-molecule electron donor (H31) based on a thiophene-substituted benzodithiophene core unit with trialkylsilyl side chains is designed and synthesized. Blending H31 with the electron acceptor Y6 gives solar cells with power conversion efficiencies exceeding 13% with and without 2,9-bis[3-(dimethyloxidoamino)propyl]anthra[2,1,9-def:6,5,10-d'e'f ']diisoquinoline-1,3,8,10(2H,9H)-tetrone (PDINO) as the ETL. The ETL-free cells deliver a superior shelf life compared to devices with an ETL. Small-molecule donor-acceptor blends thus provide interesting perspectives for achieving efficient, reproducible, and stable device architectures without electrode interlayers.

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Summary Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC).

15.3% Efficiency All-Small-Molecule Organic Solar Cells Achieved by a Locally Asymmetric F, Cl Disubstitution Strategy.

Single junction binary all‐small‐molecule (ASM) organic solar cells (OSCs) with power conversion efficiency (PCE) beyond 14% are achieved by using non‐fullerene acceptor Y6 as the electron acceptor, but still lag behind that of polymer OSCs. Herein, an asymmetric Y6‐like acceptor, BTP‐FCl‐FCl, is designed and synthesized to match the recently reported high performance small molecule donor BTR‐Cl, and a record efficiency of 15.3% for single‐junction binary ASM OSCs is achieved. BTP‐FCl‐FCl features a F,Cl disubstitution on the same end group affording locally asymmetric structures, and so has a lower total dipole moment, larger average electronic static potential, and lower distribution disorder than those of the globally asymmetric isomer BTP‐2F‐2Cl, resulting in improved charge generation and extraction. In addition, BTP‐FCl‐FCl based active layer presents more favorable domain size and finer phase separation contributing to the faster charge extraction, longer charge carrier lifetime, and much lower recombination rate. Therefore, compared with BTP‐2F‐2Cl, BTP‐FCl‐FCl based devices provide better performance with FF enhanced from 71.41% to 75.36% and J sc increased from 22.35 to 24.58 mA cm−2, leading to a higher PCE of 15.3%. The locally asymmetric F, Cl disubstitution on the same end group is a new strategy to achieve high performance ASM OSCs.

Optoelectrical Switching of Nonfullerene Acceptor Y6 and BPQD‐Based Bulk Heterojunction Memory Device through Photoelectric Effect

AbstractThe as‐prepared BPDQs:Y6:PVP blends, in which Y6 acts as nonfullerene acceptor, black phosphorus quantum dots (BPQDs) serve as donor, and polyvinylpyrrolidone (PVP) as a polymer matrix, can respond to both the electrical and optical stimuli. Using these blends as the active layer, the first bulk heterojunction device with the functions of organic photovoltaics and information storage, Al/BPQDs:Y6:PVP/indium tin oxide, is fabricated. When the applied voltages vary from 0 to −0.8 V, this device exhibits both the photoinduced resistive state changes and volatile photoresponse characteristic in the broadband visible region. The light illumination gives rise to the significantly decrease of the device resistance. Furthermore, it is also found that, when the applied voltages are changed from 0 to ± 3 V, this device shows a typical nonvolatile rewritable memory performance in the dark, with an ON/OFF current ratio of 8 × 103, a switching‐on voltage of −1.68 V, and a smaller switching bias window. Upon illumination with different wavelength light, both the switching‐on voltage and the ON/OFF current ratio of the device are found to be greatly decreased. This work can be expected to open a way to the integration of information storage, modulating and demodulating functions, photovoltaic effect, and photoelectric detection in an optoelectronic device.

High performance tandem organic solar cells via a strongly infrared-absorbing narrow bandgap acceptor

Tandem organic solar cells are based on the device structure monolithically connecting two solar cells to broaden overall absorption spectrum and utilize the photon energy more efficiently. Herein, we demonstrate a simple strategy of inserting a double bond between the central core and end groups of the small molecule acceptor Y6 to extend its conjugation length and absorption range. As a result, a new narrow bandgap acceptor BTPV-4F was synthesized with an optical bandgap of 1.21 eV. The single-junction devices based on BTPV-4F as acceptor achieved a power conversion efficiency of over 13.4% with a high short-circuit current density of 28.9 mA cm−2. With adopting BTPV-4F as the rear cell acceptor material, the resulting tandem devices reached a high power conversion efficiency of over 16.4% with good photostability. The results indicate that BTPV-4F is an efficient infrared-absorbing narrow bandgap acceptor and has great potential to be applied into tandem organic solar cells.

Simple (thienylmethylene)oxindole‐based polymer materials as donors for efficient non‐fullerene polymer solar cells

AbstractAlthough the significant progress in the power conversion efficiencies (PCEs) of non‐fullerene polymer solar cells (PSCs) has proved their commercial potential, the development of efficient photovoltaic materials from easily available raw materials and simple chemical synthesis is still lagging. In this work, three copolymers donors (PTEI‐H, PTEI‐F, and PTEI‐Cl) based on easily synthesized (Z)‐3‐(thiophen‐2‐yl‐methylene)‐indolin‐2‐one (TEI) as electron‐deficient unit were synthesized and characterized. As compared to the polymer PTEI‐H, the fluorinated and chlorinated polymers (PTEI‐F and PTEI‐Cl) show broader band gaps and deeper HOMO energy levels. However, after blending with small molecule acceptor Y6, the as‐cast PTEI‐H devices exhibit a PCE of 8.27%, outperforming those of the PTEI‐F devices (7.22%) and PTEI‐Cl devices (6.50%). The relatively high photovoltaic performance of the PTEI‐H devices are associated with their increased short‐circuit current density and fill factor values, resulting from the efficient exciton dissociation and charge generation, balanced charge transport properties, and reduced recombination loss. In consideration of the merits of the structural simplicity and synthetic accessibility of TEI unit, we believe it is a potential electron‐deficient moiety in developing high‐performance polymer donors for possible mass‐production of non‐fullerene PSCs.

Over 15% efficiency all-small-molecule organic solar cells enabled by a C-shaped small molecule donor with tailorable asymmetric backbone

Abstract In the past few years, compared with the rapid development of polymer solar cells (PSCs), the research progress of all-small-molecule organic solar cells (ASM-OSCs) lags far behind. Herein, two kinds of C-shaped SM donors named FBD-S3 and TBD-S4, bearing subtly tailorable asymmetric central core, are firstly reported. Both the two SMs show broad as well as matched absorption and energy level with classical acceptor Y6. The introduction of asymmetric core endows SM with Y6-similar bending skeleton and a unique network assembly in which the SM donor can easily interpenetrate into neighboring Y6 via strong intermolecular interaction and facile stacking direction. These features would hinder rapid aggregation and prevent to form large domains of single component in blends, resulting in an extraordinary power conversion efficiency of 15.10% in TBD-S4:Y6-based device, which is one of the highest values for ASM-OSCs. In addition, our comparative study demonstrates that although the donor material TBD-S4 exhibits reduced crystallinity, such a molecule matches better with acceptor Y6 and works more efficiently for ASM-OSCs. This work indicates that conformation tailorable by asymmetric central core is a novel and powerful method to design SM donor, which open a new avenue for highly efficient ASM-OSCs.

Molecular dispersion enhances photovoltaic efficiency and thermal stability in quasi-bilayer organic solar cells

In comparison to widely adopted bulk heterojunction (BHJ) structures for organic solar cells (OSC), exploiting the sequential deposition to form planar heterojunction (PHJ) structures enables to realize the favorable vertical phase separation to facilitate charge extraction and reduce charge recombination in OSCs. However, effective tunings on the power conversion efficiency (PCE) in PHJ-OSCs are still restrained by the currently available methods. Based on a polymeric donor PBDBT-2F (PBDBT= Poly [[4,8-bis [5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo [1,2- b :4,5- b ′]dithiophene-2,6-diyl]-2,5-thiophenediyl [5,7-bis (2-ethylhexyl)-4,8-dioxo-4 H ,8 H -benzo [1,2- c :4,5- c ′]dithiophene-1,3-diyl]-2,5-thiophenediyl]) and a non-fullerene (NF) acceptor Y6, we proposed a strategy to improve the properties of photovoltaic performances in PHJ-based OSCs through dilute dispersions of the PBDBT-2F donor into the acceptor-dominant phase with the sequential film deposition. With the control of donor dispersions, the charge transport balance in the PHJ-OSCs is improved, leading to the expedited photocarrier sweep-out with reduced bimolecular charge recombination. As a result, a PCE of 15.4% is achieved in the PHJ-OSCs. Importantly, the PHJ solar cells with donor dispersions exhibit better thermal stability than corresponding BHJ devices, which is related to the better film morphology robustness and less affected charge sweep-out during the thermal aging.

Enhanced and Balanced Charge Transport Boosting Ternary Solar Cells Over 17% Efficiency.

Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, an OSC with a ternary architecture featuring a highly crystalline molecular donor DRTB-T-C4 as a third component to the host binary system consisting of a polymer donor PM6 and a nonfullerene acceptor Y6 is reported. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor (FF) of 0.813 and yielding an impressive PCE of 17.13%. The heterojunctions are designed using so-called pinning energies to promote exciton separation and reduce recombination loss. In addition, the preferential location of DRTB-T-C4 at the interface between PM6 and Y6 plays an important role in optimizing the morphology of the active layer.

Fibril Network Strategy Enables High‐Performance Semitransparent Organic Solar Cells

AbstractThe development of semitransparent organic solar cells (ST‐OSCs) represents a significant step toward the commercialization of OSCs. However, the trade‐off between power conversion efficiency (PCE) and average visible transmittance (AVT) restricts further improvements of ST‐OSCs. Herein, it is demonstrated that a fibril network strategy can enable ST‐OSCs with a high PCE and AVT simultaneously. A wide‐bandgap polymer PBT1‐C‐2Cl that can self‐assemble into a fibril nanostructure is used as the donor and a near‐infrared small molecule Y6 is adopted as the acceptor. It is found that a tiny amount of PBT1‐C‐2Cl in the blend can form a high speed pathway for hole transport due to the well distributed fibril nanostructure, which increases the transmittance in the visible region. Meanwhile, the acceptor Y6 guarantees sufficient light absorption. Using this strategy, the optimized ST‐OSCs yield a high PCE of 9.1% with an AVT of over 40% and significant light utilization efficiency of 3.65% at donor/acceptor ratio of 0.25:1. This work demonstrates a simple and effective approach to realizing high PCE and AVT of ST‐OSCs simultaneously.

Optimization Requirements of Efficient Polythiophene:Nonfullerene Organic Solar Cells

Summary Polythiophene (PT) and its derivatives have attracted long-standing attention in the organic photovoltaic community for their low cost and high scalability of synthesis. However, due to the lack of rational guidelines in controlling morphology and matching materials, the power conversion efficiencies (PCEs) based on PTs reported so far are generally below 10%. Here, we establish the first-ever relationship between miscibility, morphology, and device performance of binary blends, based on various nonfullerene acceptors (ITIC-Th1, ITIC, IT4F, IDIC, and Y6) and a PT derivative named PDCBT-Cl by scattering and calorimetric characterizations. Benefiting from a properly quenched mixed phase, PDCBT-Cl:ITIC-Th1 system shows the best efficiency of over 12%. Conversely, the blend of PDCBT-Cl and the star acceptor Y6 remained in a homogeneous state due to their high miscibility, resulting in abysmal performance with PCE of 0.5%. Specific guidelines are also proposed to remediate the performance of PDCBT-Cl:Y6, which are crucial for advancing their practical applications.

Shorter alkyl chain in thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors – Yield efficient non-fullerene polymer solar cells

Typically, conjugated polymers are composed of conjugated backbones and alkyl side chains. In this contribution, a cost-effective strategy of tailoring the length of alkyl side chain is utilized to design high-performing thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors PBDT-BiTPD(Cχ) (χ = 48, 52, 56), in which χ represents the alkyl side chain length in term of the total carbon number. A combination of light absorption, device, and morphology examinations make clear that the shorter alkyl side chains yield (i) higher crystallinity and more predominant face-on crystallite orientation in their neat and BHJ blend films, (ii) higher charge mobilities (6.7 × 10−4 cm2 V−1 s−1 for C48 vs. 3.2 × 10−4 cm2 V−1 s−1 for C56), and negligible charge recombination, consequently, (iii) significantly improved fill-factor (FF) and short current (JSC), while almost the same open circuit voltage (VOC) of ca. 0.82 V in their corresponding BHJ devices. In parallel, as alkyl side chain lengths decrease from C56 to C48, power conversion efficiencies (PCEs) increased from 7.8% for C56 to 11.1% for C52, and further to 14.1% for C48 in their BHJ solar cells made with a narrow bandgap non-fullerene acceptor Y6. This systematic study declares that shortening the side chain, if providing appropriate solubility in device solution processing solvents, is of essential significance for developing high-performing polymer donors and further improving device photovoltaic performance.

Synergistic Effects of Polymer Donor Backbone Fluorination and Nitrogenation Translate into Efficient Non-Fullerene Bulk-Heterojunction Polymer Solar Cells.

State-of-the-art non-fullerene bulk-heterojunction (BHJ) polymer solar cells outperform the more extensively studied polymer-fullerene BHJ solar cells in terms of efficiency, thermal-, and photostability. Considering the strong light absorption in the near-infrared region (600-1000 nm) for most of the efficient acceptors, the exploration of high-performing large band gap (LBG) polymer donors with complementary optical absorption ranging from 400 to 700 nm remains critical. In this work, the strategy of concurrently incorporating fluorine (-F) and unsaturated nitrogen (-N) substituents along the polymer backbones is used to develop the LBG polymer donor PB[N][F]. Results show that the F- and N-substituted polymer donor PB[N][F] realizes up to 14.4% efficiency in BHJ photovoltaic devices when paired with a benchmark molecule acceptor Y6, which largely outperforms the analogues PB with an efficiency of only 3.6% and PB[N] with an efficiency of 11.8%. Systematic examinations show that synergistic effects of polymer backbone fluorination and nitrogenation can significantly increase ionization potential values, improve charge transport, and reduce bimolecular recombination and trap-assisted recombination in the PB[N][F]:Y6 BHJ system. Importantly, our study shows that the F- and N-substituted conjugated polymers are promising electron-donor materials for solution-processed non-fullerene BHJ solar cells.

A Benzo[1,2-b:4,5-c']Dithiophene-4,8-Dione-Based Polymer Donor Achieving an Efficiency Over 16.

It is of great significance to develop efficient donor polymers during the rapid development of acceptor materials for nonfullerene bulk-heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT-F, based on a strongly electron-deficient core (5,7-dibromo-2,3-bis(2-ethylhexyl)benzo[1,2-b:4,5-c']dithiophene-4,8-dione, TTDO), is developed through the design of cyclohexane-1,4-dione embedded into a thieno[3,4-b]thiophene (TT) unit. When blended with the acceptor Y6, the PBTT-F-based photovoltaic device exhibits an outstanding power conversion efficiency (PCE) of 16.1% with a very high fill factor (FF) of 77.1%. This polymer also shows high efficiency for a thick-film device, with a PCE of ≈14.2% being realized for an active layer thickness of 190 nm. In addition, the PBTT-F-based polymer solar cells also show good stability after storage for ≈700 h in a glove box, with a high PCE of ≈14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of donor polymers, and these results demonstrate that PBTT-F is a very promising donor polymer for use in polymer solar cells, providing an alternative choice for a variety of fullerene-free acceptor materials for the research community.

Understanding the Effect of the Third Component PC71BM on Nanoscale Morphology and Photovoltaic Properties of Ternary Organic Solar Cells

Herein, PC71BM is used as the third component (the second acceptor) to improve the photovoltaic performance of the organic solar cells (OSCs) based on a low‐cost polymer donor PTQ10 and a nonfullerene small‐molecule acceptor Y6. The ternary OSCs based on PTQ10:Y6:PC71BM reach a higher power conversion efficiency (PCE) of 16.07% with enhanced short‐circuit current density and a better fill factor in comparison with the binary OSCs based on PTQ10:Y6, which is ascribed to the higher electron mobility, better charge extraction, and suppressed charge recombination of the ternary PSCs. The film morphology of the OSCs is studied by grazing‐incidence wide‐angle X‐ray scattering, photo‐induced force microscopy, and transmission electron microscopy, which reveals that with the treatment of additive (0.5% CN) and thermal annealing, the phase separation of Y6 is obviously enhanced, whereas no significant changes occur for that of PTQ10 component. Besides, the addition of PC71BM slightly lowers the ratio of the face‐on orientation in the blend film and attenuate the aggregation of acceptor Y6. In addition, compared with the binary PTQ10:Y6 OSC, the ternary device based on PTQ10:Y6:PC71BM shows better device stability, demonstrating a great potential for the practical application of ternary OSCs.

16.5% efficiency ternary organic photovoltaics with two polymer donors by optimizing molecular arrangement and phase separation

Ternary organic photovoltaics (OPVs) combining two polymer donors (PM6, J71) and one non-fullerene acceptor Y6 were prepared in conventional configuration. The complementary absorption spectra of PM6 and J71 can maximize photon harvesting in ternary films that is beneficial to the enhancement of short circuit current density (JSC). The open circuit voltage (VOC) of ternary OPVs show a monotonously increased trend along with the increase of J71 content, which is attributed to reduced energy loss considering the similar HOMO energy levels of two donors. Furthermore, the optimized molecular arrangement and phase separation in ternary films result a high fill factor (FF) of 76.0%. As a result, by incorporating 10 wt% J71 in ternary films, the power conversion efficiency of ternary OPVs achieves 16.5% that is the highest values for ternary OPVs with two donors. The results indicate that ternary OPVs with two donors could achieve high performance by well-optimized photon harvesting and phase separation.

Single-crystal field-effect transistors based on a fused-ring electron acceptor with high ambipolar mobilities

A fused-ring electron acceptor Y6 was used to fabricate single-crystal organic field-effect transistors, providing high ambipolar mobilities.