Chlorinated Oligomers with Regulate Planarity Achieving Superior Photovoltaic Performance.

Chlorine substitution, as an effective and low-cost modification strategy, has been applied in the design of donor and acceptor structures in organic solar cells. We synthesized a series of chlorinated dimerized acceptors to investigate the effect of chlorine numbers and locations on the photovoltaic properties. The results show that the planarity and morphology of DYV-γ-2Cl are greatly improved due to the appropriate numbers and positions of the substituted chlorine atoms. Therefore, the device based on PM6:DYV-γ-2Cl achieves a superior power conversion efficiency (PCE) of 15.54% among the three oligomeric acceptors with optimized molecular planarity and film morphology. This work demonstrated the positive effect of suitable numbers and the substitution positions of chlorines on the molecular arrangement and photovoltaic properties of the corresponding dimerized acceptors.

Dimerized Acceptors with Conjugate-Break Linker Enable Highly Efficient and Mechanically Robust Organic Solar Cells.

Designing new acceptors is critical for intrinsically stretchable organic solar cells (IS-OSCs) with high efficiency and mechanical robustness. However, nearly all stretchable polymer acceptors exhibit limited efficiency and high-performance small molecular acceptors are very brittle. In this regard, we select thienylene-alkane-thienylene (TAT) as the conjugate-break linker and synthesize four dimerized acceptors by the regulation of connecting sites and halogen substitutions. It is found that the connecting sites and halogen substitutions considerably impact the overall electronic structures, aggregation behaviors, and charge transport properties. Benefiting from the optimization of the molecular structure, the dimerized acceptor exhibits rational phase separation within the blend films, which significantly facilitates exciton dissociation while effectively suppressing charge recombination processes. Consequently, FDY-m-TAT-based rigid OSCs render the highest power conversion efficiency (PCE) of 18.07 % among reported acceptors containing conjugate-break linker. Most importantly, FDY-m-TAT-based IS-OSCs achieve high PCE (14.29 %) and remarkable stretchability (crack-onset strain [COS]=18.23 %), significantly surpassing Y6-based counterpart (PCE=12.80 % and COS=8.50 %). To sum up, these findings demonstrate that dimerized acceptors containing conjugate-break linkers have immense potential in developing highly efficient and mechanically robust OSCs.

Rational molecular and device design enables organic solar cells approaching 20% efficiency.

For organic solar cells to be competitive, the light-absorbing molecules should simultaneously satisfy multiple key requirements, including weak-absorption charge transfer state, high dielectric constant, suitable surface energy, proper crystallinity, etc. However, the systematic design rule in molecules to achieve the abovementioned goals is rarely studied. In this work, guided by theoretical calculation, we present a rational design of non-fullerene acceptor o-BTP-eC9, with distinct photoelectric properties compared to benchmark BTP-eC9. o-BTP-eC9 based device has uplifted charge transfer state, therefore significantly reducing the energy loss by 41 meV and showing excellent power conversion efficiency of 18.7%. Moreover, the new guest acceptor o-BTP-eC9 has excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, which enables an efficiency of 19.9% (19.5% certified) in PM6:BTP-C9:o-BTP-eC9 based ternary system with enhanced operational stability.

The Balance Effect of π-π Electronic Coupling on NIR-II Emission and Photodynamic Properties of Highly Hydrophobic Conjugated Photosensitizers.

Deep NIR organic phototheranostic molecules generally have large π-conjugation structures and show highly hydrophobic properties, thus, forming strong π-π stacking in the aqueous medium, which will affect the phototheranostic performance. Herein, an end-group strategy is developed to lift the performance of NIR-II emitting photosensitizers. Extensive characterizations reveal that the hydrogen-bonding interactions of the hydroxyl end group can induce a more intense π-π electronic coupling than the chlorination-mediated intermolecular forces. The results disclose that π-π stacking will lower fluorescence quantum yield but significantly benefit the photodynamic therapy (PDT) efficiency. Accordingly, an asymmetrically substituted derivative (BTIC-δOH-2Cl) is developed, which shows balanced phototheranostic properties with excellent PDT efficiency (14.6 folds of ICG) and high NIR-II fluorescence yield (2.27%). It proves the validity of the end-group strategy on controlling the π-π interactions and rational tuning the performance of NIR-II organic phototheranostic agents.

Highly-Efficient 2D Nonfullerene Acceptors Enabled by Subtle Molecular Tailoring Engineering.

The conjugate expansion of nonfullerene acceptors is considered to be a promising approach for improving organic photovoltaic performance because of its function in tuning morphological structure and molecular stacking behavior. In this work, two nonfullerene acceptors are designed and synthesized using a 2D π-conjugate expansion strategy, thus enabling the construction of highly-efficient organic solar cells (OSCs). Compared with YB2B (incorporating dibromophenanthrene on the quinoxaline-fused core), YB2T (incorporating dibromobenzodithiophene on the quinoxaline-fused core) has red-shifted spectral absorption and better charge transport properties. Moreover, the more orderly and tightly intermolecular stacking of YB2T provides the possibility of forming a more suitable phase separation morphology in blend films. Through characterization and analysis, the YB2T-based blend film is found to have higher exciton dissociation efficiency and less charge recombination. Consequently, the power conversion efficiency (PCE) of 17.05% is achieved in YB2T-based binary OSCs, while YB2B-based devices only reached 10.94%. This study demonstrates the significance of the aromatic-ring substitution strategy for regulating the electronic structure and aggregation behavior of 2D nonfullerene acceptors, facilitating the development of devices with superior photovoltaic performance.

Trifluoromethylation in the Design and Synthesis of High-Performance Wide Bandgap Polymer Donors for Quasiplanar Heterojunction Organic Solar Cells.

New strategies for the molecular design to construct efficient electron-deficient units for D-A-type donor copolymers are urgently needed. Halogenation of electron-deficient units (A) has been shown to be the most effective strategy reported to date with which to produce high-performance donor polymers. Herein, we have constructed two different trifluoromethyl-substituted polymer donors, PBQP-CF3 and PBQ-CF3. The trifluoromethylation process typically involves complex protocols, which are not widely used in the synthesis of polymer donors. Accordingly, we have developed a single-step, one-pot synthesis of the new trifluoromethyl-substituted electron-deficient unit (A) of PBQ-CF3. The strong electron-withdrawing ability of the trifluoromethyl group ensures deeper highest occupied molecular orbital (HOMO) energy levels, and the non-covalent bonding interactions of the fluorine atoms are beneficial to the regulation of aggregation properties. Thus, both of the trifluoromethyl-substituted polymer donors obtained much higher power conversion efficiency (PCE) than PBDP-H (6.66%). PBQ-CF3 exhibits a deeper HOMO energy level, better aggregation behavior, and higher hole mobility than PBQP-CF3. PBQ-CF3-based quasiplanar heterojunction (Q-PHJ) devices therefore achieve simultaneously enhanced open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) and an impressive PCE (16.02%), which is much higher than that obtained by PBQP-CF3-based devices (12.57%). This work reveals a promising path to synthesis of the trifluoromethylation polymer donors and demonstrates that the trifluoromethylation strategy can be used to enhance the photovoltaic performance.

Phenanthroline-carbolong interface suppress chemical interactions with active layer enabling long-time stable organic solar cells.

To restrain the chemical reaction at cathode interface of organic solar cells, two cathode interfacial materials are synthesized by connecting phenanthroline with carbolong unit. Consequently, the D18:L8-BO based organic solar cell with double-phenanthroline-carbolong achieve the highest efficiency of 18.2%. Double-phenanthroline-carbolong with larger steric hindrance and stronger electron-withdrawing property confirms to suppress the interfacial reaction with norfullerene acceptor, resulting the most stable device. Double-phenanthroline-carbolong based device can sustain 80% of its initial efficiency for 2170 h in dark N2 atmosphere, 96 h under 85 oC and keep 68% initial efficiency after been illuminated for 2200 h, which are significantly better than bathocuproin based devices. Moreover, superb interfacial stability of double-phenanthroline-carbolong cathode interface enables thermal posttreatment of organic sub-cell in perovskite/organic tandem solar cells and obtained a remarkable efficiency of 21.7% with excellent thermal stability, which indicates the potentially wide application of phenanthroline-carbolong materials for stable and efficient solar device fabrications.

Chlorination-Mediated π-π Stacking Enhances the Photodynamic Properties of a NIR-II Emitting Photosensitizer with Extended Conjugation.

NIR-II-emitting photosensitizers (PSs) have attracted great research interest due to their promising clinical applications in imaging-guided photodynamic therapy (PDT). However, it is still challenging to realize highly efficient PDT on NIR-II PSs. In this work, we develop a chlorination-mediated π-π organizing strategy to improve the PDT of a PS with conjugation-extended A-D-A architecture. The significant dipole moment of the carbon-chlorine bond and the strong intermolecular interactions of chlorine atoms bring on compact π-π stacking in the chlorine-substituted PS, which facilitates energy/charge transfer and promotes the photochemical reactions of PDT. Consequently, the resultant NIR-II emitting PS exhibits a leading PDT performance with a yield of reactive oxygen species higher than that of previously reported long-wavelength PSs. These findings will enlighten the future design of NIR-II emitting PSs with enhanced PDT efficiency.

Morphological transition and transformation of 2D nanosheets by controlling the balance of π-π stacking interaction and crystalline driving forces.

Nanoscale organic two-dimensional (2D) materials of block polymers (BCPs) have attracted interest on account of their wide potential applications in a range of fields. Herein, we design a new poly(p-phenylenevinylene) (PPV) based BCP that contains a triisopropylsilyl side chain and poly (2-vinyl pyridine) (P2VP) corona, which could assemble into a series of 2D square and rectangular micelles in isopropanol. The aspect ratios and the scales of the 2D micelles can be tuned in two ways, including altering the ratios of the P2VP and PPV-TIPS blocks and their concentrations. By precisely controlling the aspect ratios, micro-scale rod-like micelles are also obtained. From in depth studies of the morphology transition from rectangular micelles to rod-like or square micelles, it is found that the BCPs initially organize into fibers and then assemble into final micelles by the combined forces of π-π interactions and the crystalline force from TIPS side chains. Based on the balance of the two interactions, 2D circle-like micelles are also achieved by heterogenous co-assembly of two kinds of polymers with different cores.

Oligomeric Acceptor: A "Two-in-One" Strategy to Bridge Small Molecules and Polymers for Stable Solar Devices.

Oligomeric acceptors are expected to combine the advantages of both highly developed small molecular and polymeric acceptors. However, organic solar cells (OSCs) based on oligomers lag far behind due to their slow development and low diversity. Here, three oligomeric acceptors were produced through oligomerization of small molecules. The dimer dBTICγ-EH achieved the best power conversion efficiencies (PCEs) of 14.48 % in bulk heterojunction devices and possessed a T80 (80 % of the initial PCE) lifetime of 1020 h under illumination, which were far better than that of small molecular and polymeric acceptors. More excitingly, it showed PCEs of 16.06 % in quasi-planar heterojunction (Q-PHJ) devices which is the highest value OSCs using oligomeric acceptors to date. These results suggest that oligomerization of small molecules is a promising strategy to achieve OSCs with optimized performance between the high efficiency and durable stability, and offer oligomeric materials a bright future in commercial applications.

Side-Chain-Tuned Molecular Packing Allows Concurrently Boosted Photoacoustic Imaging and NIR-II Fluorescence.

It is generally considered that photoacoustic imaging (PAI) and fluorescence imaging (FLI) cannot be enhanced concurrently, as they are dependent on competitive photophysical processes at the single-molecule level. Herein, we reveal that BDTR9-OC8 and BDTR9-C8, which have identical π-conjugated backbones but are substituted by side chains of different rigidity, show distinct phototheranostic properties in the aggregated state. The NIR-II FLI and PAI brightness of BDTR9-C8 nanoparticles are enhanced by 4.6 and 1.4 times compared with BDTR9-OC8 nanoparticles. Theoretical calculations and GIWAXS analysis revealed that BDTR9-C8 with rigid side chains shows a relative amorphous condensed state, which will benefit the efficient transportation of photo-generated excitons and phonons, subsequently enhancing the FLI and PAI signals. Besides, both nanoparticles exhibit excellent photothermal conversion efficiency due to their strong light-harvesting capability and are considered effective photothermal therapy materials. This work provides an illuminating strategy for material design in the future.

Hierarchical Chiral Supramolecular Nanoarchitectonics with Molecular Detection: Helical Structure Controls upon Self-Assembly and Coassembly.

The morphological transformation from microspheres to helical supramolecular nanofibers with controllable handedness is achieved by the introduction of molecular chirality based on amino acid derivatives (TDAP), and the chirality of the supramolecular architectures that are achieved is nullified through the coassembly of the equivalent TDAP enantiomers. The molecular detection of achiral melamine based on the R-TDAP-COOH supramolecular system is achieved by the appearance of helicity and inversion.

17.6%-Efficient Quasiplanar Heterojunction Organic Solar Cells from a Chlorinated 3D Network Acceptor.

Bulk heterojunction (BHJ) organic solar cells (OSCs) have achieved great success because they overcome the shortcomings of short exciton diffusion distances. With the progress in material innovation and device technology, the efficiency of BHJ devices is continually being improved. For some special photovoltaic material systems, it is difficult to manipulate the miscibility and morphology of blend films, and this results in moderate, even poor device performance. Quasiplanar heterojunction (Q-PHJ) OSCs have been proposed to exploit the excellent photovoltaic properties of these materials. An OSC with BTIC-BO-4Cl has a 3D interpenetrating network structure with multiple channels that can facilitate the exciton diffusion and charge transport, and BTIC-BO-4Cl is therefore a good candidate for Q-PHJ OSCs. In this work, a D18:BTIC-BO-4Cl-based Q-PHJ device is fabricated. The exciton diffusion lengths of D18 and BTIC-BO-4Cl are in accord with the requirements of the Q-PHJ device and the efficiency of Q-PHJ device is as high as 17.60%. This study indicates that the Q-PHJ architecture can replace the BHJ architecture to produce excellent OSCs for certain unique donors and acceptors, providing an alternative approach to photovoltaic material design and device fabrication.

End-Group Modifications with Bromine and Methyl in Nonfullerene Acceptors: The Effect of Isomerism.

The development of isomeric molecules has been widely exploited in molecular structures associated with organic solar cells (OSC) and is an effective pathway to finely tune the photoelectric properties and device performance. The molecular properties of nonfullerene acceptors and the morphology of blend films can be effectively controlled by manipulating isomeric substituent positions on benzene-fused end-capping groups (EG) in acceptors. Here, three isomeric EGs were designed and synthesized which simultaneously possess an electron-withdrawing bromine and an electron-donating methyl substituent. By linking three isomeric EGs, (Br,Me), (Br,Me)-1, and (Br,Me)-2 each with the BTP-CHO core, three isomeric small-molecule acceptors (SMA) were obtained. The power conversion efficiency (PCE) of PM6:BTP-(Br,Me)-1-based OSCs is 13.43%, is much higher than that of PM6:BTP-(Br,Me)- (11.92%) and PM6:BTP-(Br,Me)-2- (11.08%) based devices. Our results show that isomeric EGs can provide strategies to tune the absorption spectra of SMAs, intramolecular charge transfer (ICT) and electron mobility of organic semiconductor device, and ultimately increase the performance of nonfullerene acceptors.

Push or Pull Electrons: Acetoxy and Carbomethoxy-Substituted Isomerisms in Organic Solar Cell Acceptors.

Isomerism is a major factor affecting the properties of materials. Herein, two isomeric acceptors based on acetoxy and methyl ester end group substituents, BTIC-OCOMe and BTIC-COOMe are reported. When blended with PBDB-TF, devices based on BTIC-OCOMe achieve an inferior (8.32%) power conversion efficiency (PCE) while the BTIC-COOMe material has a superior PCE of 13.25%. We investigated the reasons why these two devices, which differ only in the isomeric substituents on the terminal site, have such a large difference in photovoltaic performance. Our investigation conducted theoretical calculations and examined UV-vis absorption, energy levels, exciton dissociation and bimolecular recombination, mobilities tests, photoluminescence, and packing modes. It is found that the energy levels of the materials are fine-tuned, the absorption spectra are adjusted, and the energy loss is regulated. Our studies explored the reasons for the properties of materials differing, and the acetoxy and carbomethoxy substitutions provided some useful information concerning high-performance acceptor materials.

17.1 %-Efficient Eco-Compatible Organic Solar Cells from a Dissymmetric 3D Network Acceptor.

To elevate the performance of polymer solar cells (PSC) processed by non-halogenated solvents, a dissymmetric fused-ring acceptor BTIC-2Cl-γCF3 with chlorine and trifluoromethyl end groups has been designed and synthesized. X-ray crystallographic data suggests that BTIC-2Cl-γCF3 has a 3D network packing structure as a result of H- and J-aggregations between adjacent molecules, which will strengthen its charge transport as an acceptor material. When PBDB-TF was used as a donor, the toluene-processed binary device realized a high power conversion efficiency (PCE) of 16.31 %, which improved to 17.12 % when PC71ThBM was added as the third component. Its efficiency of over 17 % is currently the highest among polymer solar cells processed by non-halogenated solvents. Compared to its symmetric counterparts BTIC-4Cl and BTIC-CF3 -γ, the dissymmetric BTIC-2Cl-γCF3 integrates their merits, and has optimized the molecular aggregations with excellent storage and photo-stability, and also extending the maximum absorption peak in film to 852 nm. The devices exhibit good transparency indicating a potential utilization in semi-transparent building integrated photovoltaics (ST-BIPV).

Bromination: An Alternative Strategy for Non-Fullerene Small Molecule Acceptors.

The concept of bromination for organic solar cells has received little attention. However, the electron withdrawing ability and noncovalent interactions of bromine are similar to those of fluorine and chlorine atoms. A tetra-brominated non-fullerene acceptor, designated as BTIC-4Br, has been recently developed by introducing bromine atoms onto the end-capping group of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and displayed a high power conversion efficiency (PCE) of 12%. To further improve its photovoltaic performance, the acceptor is optimized either by introducing a longer alkyl chain to the core or by modulating the numbers of bromine substituents. After changing each end-group to a single bromine, the BTIC-2Br-m-based devices exhibit an outstanding PCE of 16.11% with an elevated open-circuit voltage of V oc = 0.88 V, one of the highest PCEs reported among brominated non-fullerene acceptors. This significant improvement can be attributed to the higher light harvesting efficiency, optimized morphology, and higher exciton quenching efficiencies of the di-brominated acceptor. These results demonstrate that the substitution of bromine onto the terminal group of non-fullerene acceptors results in high-efficiency organic semiconductors, and promotes the use of the halogen-substituted strategy for polymer solar cell applications.

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.

3D Interpenetrating Network for High-Performance Nonfullerene Acceptors via Asymmetric Chlorine Substitution.

Two chlorine-substituted isomers, ITIC-2Cl-ß and a-ITIC-2Cl, were synthesized for potential use as nonfullerene acceptors. The two molecules differ in the position of chlorine atoms, leading to symmetric (ITIC-2Cl-ß) and asymmetric (a-ITIC-2Cl) molecular configuration. In single crystals, the two molecules exhibit a completely different arrangement and stacking as derived from X-ray diffraction analysis. Whereas ITIC-2Cl-ß has a linear packing structure, a-ITIC-2Cl forms a 3D interpenetrating network structure with shorter π-π distances and better molecular planarity. Therefore, a high power conversion efficiency of >12% is obtained by a-ITIC-2Cl-based devices. It is ∼10% higher than that of ITIC-2Cl-ß-based devices due to the chlorine substituent effect. Thus the fine-tuning of the Cl-substituted position seems to be a promising strategy to construct a 3D interpenetrating charge transportation network and achieve higher performance organic solar cells (OSCs).

Isomer-free: Precise Positioning of Chlorine-Induced Interpenetrating Charge Transfer for Elevated Solar Conversion.

The influence caused by the position of the chlorine atom on end groups of two non-fullerene acceptors (ITIC-2Cl-δ and ITIC-2Cl-γ) was intensely investigated. The single-crystal structures show that ITIC-2Cl-γ has a better molecular planarity and closer π-π interaction distance. More importantly, a 3D rectangle-like interpenetrating network is formed in ITIC-2Cl-γ and is beneficial to rapid charge transfer along multiple directions, whereas only a linear stacked structure could be observed in ITIC-2Cl-δ. The two acceptor-based solar cells show power conversion efficiencies (PCEs) over 11%, higher than that of the ITIC-2Cl-m-based device (10.85%). An excellent PCE of 13.03% is obtained by the ITIC-2Cl-γ-based device. In addition, the ITIC-2Cl-γ-based device also shows the best device stability. This study indicates that chlorine positioning has a great impact on the acceptors; more importantly, the 3D network structure may be a promising strategy for non-fullerene acceptors to improve the PCE and stability of organic solar cells.