Showing papers by "Junwu Chen published in 2021"

A ligand-free direct heteroarylation approach for benzodithiophenedione-based simple small molecular acceptors toward high efficiency polymer solar cells

Abstract: The development of nonfullerene acceptors has accelerated the improvement in the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) in the past five years. However, synthetic accessibility without organostannanes is of great importance in the commercial application of PSCs. In this work, a ligand-free direct heteroarylation approach was introduced to realize high efficiency simple small molecular nonfullerene acceptors. More specifically, two simple A–D–A′–D–A type nonfullerene acceptors BDDEH-4F and BDDBO-4F with benzodithiophenedione (BDD) as the core moiety were synthesized in three steps without any hazardous organostannanes or any ligands. Despite the different lengths of side chains on the BDD, the simple fused-ring acceptors BDDEH-4F and BDDBO-4F exhibit very similar optical properties and energy levels. With PM6 as the donor, BDDEH-4F with 2-ethylhexyl side chains exhibits a much higher PCE of 12.59% and larger short-circuit current density (Jsc) of 22.57 mA cm−2 because the more balanced carrier mobilities and suitable phase separation result in favorable morphology compared to that of BDDBO-4F with longer alkyl side chains (PCE of 9.80%). Our results indicate that facile direct heteroarylation is an attractive approach to achieve high performance cost-effective nonfullerene solar cells.

Blade-coated organic solar cells from non-halogenated solvent offer 17% efficiency

Abstract: 1Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Siyuan Laboratory, Department of Physics, Jinan University, Guangzhou 510632, China 2Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China 3State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China 4Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China

Binary non-fullerene-based polymer solar cells with a 430 nm thick active layer showing 15.39% efficiency and 73.38% fill factor

Abstract: Thickness-sensitive fill factor (FF) is usually encountered by organic active layers with a thickness of several hundred nanometers, which significantly deteriorate the photovoltaic performance of thick-film polymer solar cells (PSCs). Here, paring of high-hole mobility (μh) polymer donor Si25 with benzotriazole-fused Y-series non-fullerene acceptor Y14 is proposed to overcome this challenge. Relative to the electron mobility (μe) of 8.17 × 10−4 cm2 V−1 s−1 for the Y14 pristine film, much higher μe values up to 3.79 × 10−3 cm2 V−1 s−1 were demonstrated using Si25:Y14 binary blend films, achieving μh/μe of 2.31–3.56. The Si25-induced closer packing of Y14 molecules was observed with the blend film. The high and fairly balanced charge transport-enabled PSCs with 320–600 nm thick active layers to show a low FF decay from 74.69% to 67.46%. Power conversion efficiencies (PCEs) of 15.39% and 15.03% were achieved for 430 nm and 600 nm thick active layers, respectively. The device performances can supply a wide processing window with high efficiency. Delightedly, green solvent o-xylene cast active layers of 400 nm and 530 nm thickness exhibited PCEs of 14.43% and 14.25%, respectively. This study indicates that high μh polymers and benzotriazole-fused Y-series non-fullerene acceptors are promising candidates to overcome the thickness sensitivity of FF.

Introducing Siloxane-Terminated Side Chains in Small Molecular Donors for All-Small-Molecule Organic Solar Cells: Modulated Molecular Orientation and Enhanced Efficiency.

Abstract: In this work, three small molecular donors (SMDs) S35, S35-1Si, and S35-2Si, with 3,5-difluorophenyl-substituted benzodithiophene as the central 2-dimensional unit to combine different numbers of siloxane-terminated side chain, were synthesized for all-small-molecule organic solar cells (ASM-OSCs). The three SMDs showed comparable film absorption peaks at 570 nm and optical band gaps of 1.8 eV. Relative to S35 and S35-1Si with symmetric alkyl side chains and asymmetric side chains on the central unit, respectively, the S35-2Si carrying two symmetric siloxane-terminated side chains displayed largely elevated melting and crystalline temperatures, lowered surface energy, and modulated molecular orientation. The three SMDs possessed edge-on dominated molecular orientations of their neat films; however, a big difference was found for their blend films with nonfullerene acceptor Y6. The S35:Y6 and S35-1Si:Y6 blends exhibited edge-on and face-on bimodal orientations but the S35-2Si:Y6 blend showed pure face-on orientation, indicating quite different donor:acceptor intermolecular interactions. Some large domains existed in the S35:Y6 and S35-1Si:Y6 blends, but could be suppressed by the S35-2Si:Y6 blend, leading to a more balanced charge transport. In ASM-OSCs, the two S35:Y6 and S35-1Si:Y6 active layers showed comparable power conversion efficiencies (PCE) of ∼12% but a much higher efficiency of 13.50% could be achieved with the S35-2Si:Y6 active layer. Our results suggest that the siloxane-terminated side chain is promising to regulate crystalline ability of a SMD, paving a way for high performance ASM-OSCs.

Cross-Linkable and Alcohol-Soluble Pyridine-Incorporated Polyfluorene Derivative as a Cathode Interface Layer for High-Efficiency and Stable Organic Solar Cells.

Abstract: Device performance and commercialization of organic solar cells (OSCs) are strongly influenced by the characteristics of the interface layers. Cross-linked polymer interface layers with solvent-resistant properties are very compatible with large-area solution-processing methods of OSCs and may be beneficial to the environmental stability of OSCs due to the viscoelastic and cross-linked characteristics of the cross-linked polymer. In this work, a novel cross-linkable and alcohol-soluble pyridine-incorporated polyfluorene derivative, denoted as PFOPy, is synthesized and used as a cathode interface layer (CIL) in OSCs. For PFOPy, the pendant epoxy group can be effectively cross linked through cationic polymerization under thermal treatment and the pendant pyridine group can offer good alcohol solubility. Optical absorption tests of PFOPy films before/after washing by chloroform demonstrate the excellent solvent-resistance property for the cross-linked PFOPy film. Compared with the typical ZnO CIL, the cross-linked PFOPy CIL can also substantially reduce ITO's work function and form a better interface contact with the active layer. Utilizing an inverted device structure and a typical active layer of PM6:Y6, ZnO-based OSCs display an optimal power conversion efficiency (PCE) of 15.83% while PFOPy-based OSCs exhibit superior photovoltaic performance with an optimal PCE of 16.20%. Moreover, ZnO-based and PFOPy-based OSCs separately maintain 89% and 90% of the corresponding initial PCE after 12 h of illumination, indicating similarly excellent photostability. More importantly, after 26 complete thermal cycles, ZnO-based OSCs only maintain 81% of the initial PCE while PFOPy-based OSCs retain 92% of the initial PCE and exhibit obviously better thermal cycling stability, indicating that the cross-linked PFOPy CIL should offer stronger interface robustness against thermal cycling stress due to the viscoelastic and cross-linked characteristics of PFOPy. The impressive results indicate that the cross-linked PFOPy CIL would be a very promising CIL in OSCs.

An unprecedented quinoid–donor–acceptor strategy to boost the carrier mobilities of semiconducting polymers for organic field-effect transistors

Abstract: Quinoidal–aromatic conjugated polymers hold great application potential in organic field-effect transistors (OFETs). However, the development of high mobility quinoidal–aromatic conjugated polymers still lags behind the more popular donor–acceptor (D–A) conjugated polymers, mainly owing to the lack of a rational design strategy and efficient building block. Herein, a novel quinoid–donor–acceptor (Q–D–A) strategy is demonstrated to modulate the energy-level and boost the charge carrier transport mobility of conjugated polymers as opposed to the D–A system. On the basis of this strategy, a quinoidal–aromatic conjugated polymer, namely PAQM-BT, is designed and synthesized. With the combined use of quinoid, donor and acceptor units in the backbone, the resulting Q–D–A polymer PAQM-BT displays the narrowest bandgap with the deepest-lying lowest unoccupied molecular orbital (LUMO) energy level, highest backbone coplanarity, enhanced thin-film crystallinity and smallest effective hole mass in comparison with the corresponding D–A polymer PT3B1 and quinoid–donor (Q–D) polymer PAQM-3T. Benefitting from the more effective intra- and inter-chain charge transport, as corroborated by experiment and theoretical calculations, OFET devices based on PAQM-BT exhibit a highly boosted hole mobility of up to 5.10 cm2 V−1 s−1, which is one and four orders of magnitude higher than that of PAQM-3T and PT3B1, respectively, and is among the highest for quinoidal–aromatic conjugated polymers. The potent Q–D–A strategy not only allows the energy level to be modulated but also leads to effective charge carrier transport, opening up possibilities to the development of high mobility quinoidal–aromatic conjugated polymers based on a variety of quinoids, donors, and acceptors.

Replacing alkyl side chain of non-fullerene acceptor with siloxane-terminated side chain enables lower surface energy towards optimizing bulk-heterojunction morphology and high photovoltaic performance

Abstract: Towards a good control of the morphology of bulk-heterojunction (BHJ) active layers for polymer solar cells (PSCs), selecting an appropriate side chain for a polymer donor and a nonfullerene acceptor (NFA) is very crucial. In this work, two novel NFAs i-IE-4F and i-IESi-4F comprising alkyl and siloxane-terminated side chains on the central indacenodithiophene (IDT) core, respectively, were synthesized. Attaching the siloxane-terminated side chain in i-IESi-4F affords surface energy (γ) of 33.32 mN/m, much lower than that of 39.83 mN/m for i-IE-4F, supplying a big chance to tune miscibility with a polymer donor. Two fluorobenzotriazole-based polymer donors J52 and PBZ-2Si bearing alkyl and siloxane-terminated side chains, respectively, show γ values of 36.08 and 33.10 mN/m, respectively. The estimated Flory-Huggins interaction parameters (χD, A) indicate that the i-IESi-4F is more miscible than i-IE-4F in pairing with J52 or PBZ-2Si. The resulting i-IESi-4F-based blend films exhibits low film roughness and accompanies obviously improved BHJ uniformity. In PSCs, the J52:i-IESi-4F and PBZ-2Si:i-IESi-4F active layers display power conversion efficiencies (PCEs) of 12.67% and 14.54%, respectively, all remarkably higher than PCEs ≼ 7.34% of the i-IE-4F-based active layers. Interestingly, the PBZ-2Si:i-IESi-4F active layer, a donor:acceptor blend system comprising siloxane-terminated side chains (DSi:ASi matching) with the highest BHJ miscibility due to the combinatory effect of the side chains, shows the highest efficiency, as supported by efficient exciton dissociation, the lowest bimolecular recombination, and the optimal charge transports. Our results demonstrate that attaching siloxane-terminated side chains in NFAs, as a side chain engineering, has big potential in lowering its surface energy towards fine control of BHJ morphology and leading to a better donor:acceptor blend system.

Delicately Controlled Polymer Orientation for High-Performance Non-Fullerene Solar Cells with Halogen-Free Solvent Processing.

Abstract: Molecular orientation in polymer solar cells (PSCs) is a critical subject of investigation that promotes the quality of bulk heterojunction morphology and power conversion efficiency (PCE). Herein, the intrinsic polymer orientation transition can be found upon delicate control over the branching point position of the irregular alkoxy side chain in difluoroquinoxaline-thiophene-based conjugated polymers. Three polymers with branching points at the third, fourth, and fifth positions away from the backbone were synthesized and abbreviated as PHT3, PHT4, and PHT5, respectively. Temperature-dependent absorption behavior manifests the polymer aggregation ability in the order of PHT3 < PHT4 < PHT5. Surprisingly, the polymer orientation transition from typical face-on to edge-on emerged between PHT4 and PHT5, as evidenced by X-ray-scattering analysis. The enhanced face-on crystallinity of PHT4 endowed the o-xylene-processed PHT4:IT-4Cl-based devices with the highest PCE of 13.40%. For PHT5 with stronger aggregation, the related o-xylene-processed PSCs still showed a good PCE of 12.66%. Our results demonstrate that a delicate polymer orientation transition could be realized through a precisely controlled strategy of the side chain, yielding green-solvent-processed high-performance PSCs.

Dithienobenzoxadiazole-based wide bandgap donor polymers with strong aggregation properties for the preparation of efficient as-cast non-fullerene polymer solar cells processed using a non-halogenated solvent

Abstract: The efficient as-cast polymer solar cells processed using a non-halogenated solvent are suitable for roll-to-roll printing. In this study, three dithienobenzoxadiazole-based wide bandgap polymers PBOffDT, PBOTT and PBOTVT, which have different backbone structures, were designed and synthesized to reveal the relationship between the backbone structure and photovoltaic performance. The backbone structure had a great influence on the energy level, absorption behavior, aggregation and packing behavior of the polymers. PBOffDT exhibited the deepest HOMO level, the strongest aggregation and the most ordered crystallinity among the three polymers. IT-4Cl was used as the acceptor to fabricate PSC devices, and power conversion efficiencies (PCEs) of 10.56, 8.25 and 3.12% were obtained for PBOffDT, PBOTT and PBOTVT based as-cast devices using non-halogenated 1,3,5-trimethylbenzene (1,3,5-TMB) as the solvent, respectively. PBOffDT:IT-4Cl based devices showed the highest open circuit voltage (Voc), which can be ascribed to the deepest HOMO level of PBOffDT. Meanwhile, as revealed by the surface energy measurement, PBOffDT had suitable compatibility with IT-4Cl, and thus a favorable morphology can be achieved leading to efficient exciton dissociation, suppressed recombination and balanced charge transport for the PBOffDT:IT-4Cl devices. Our studies showed that the enhancement and retention of polymer crystallinity can be valuable to enhance the photovoltaic performance of the DTfBO-based polymer in non-halogenated solvent processed as-cast devices.

Hydrogen Evolution Prediction for Alternating Conjugated Copolymers Enabled by Machine Learning with Multidimension Fragmentation Descriptors.

Abstract: Hydrogen evolution by alternating conjugated copolymers has attracted much attention in recent years. To study alternating copolymers with data-driven strategies, two types of multidimension fragmentation descriptors (MDFD), structure-based MDFD (SMDFD), and electronic property-based MDFD (EPMDFD), have been developed with machine learning (ML) algorithms for the first time. The superiority of SMDFD-based models has been demonstrated by the highly accurate and universal predictions of electronic properties. Moreover, EPMDFD-based, experimental-parameter-free ML models were developed for the prediction of the hydrogen evolution reaction, displaying excellent accuracy (real-test accuracy = 0.91). The combination of explainable ML approaches and first-principles calculations was employed to explore photocatalytic dynamics, revealing the importance of electron delocalization in the excited state. Virtual designing of high-performance candidates can also be achieved. Our work illustrates the huge potential of ML-based material design in the field of polymeric photocatalysts toward high-performance photocatalysis.