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The development of polymer solar cells (PSCs) for the donor materials based on benzo[1,2-b:4,5-b′]dithiophene (BDT) has significantly boosted the power conversion efficiency (PCE). However, the PCE of polymer donor materials for benzo[1,2-b:4,5-b′]difuran (BDF)-based lags far behind that of their BDT analogs. To further explore efficient copolymers based on BDF units, a two-dimensional (2D) side-chain strategy was proposed to investigate the atom-changing effects on the copolymer donors for the properties of electron and optical. In this study, we designed and synthesized three new BDF-based copolymer donor materials, named PBDF-C, PBDF-O, and PBDF-S. Owing to the balanced charge transport and favorable phase separation of PBDF-S:Y6, a high PCE of 13.4%, a short-circuit current (Jsc) of 25.48 mA cm−2, an open-circuit voltage (Voc) of 0.721 V, and a fill factor (FF) of 72.6% was obtained. This research demonstrates that the BDF building block has great potential for constructing conjugated copolymer donors for high-performance PSCs and that 2D side-chain modification is a facile approach for designing high-performance BDF-based copolymer materials.

Limited by the relatively low short-circuit current density (Jsc) and fill factor (FF), the efficiency of all-polymer solar cells (all-PSCs) is still inferior to the small molecular acceptor (SMA) counterpart-based devices. Herein, we fabricate efficient ternary all-PSCs by introducing a fluorinated small molecular acceptor-derived polymer acceptor named PYF-T-oto broaden the absorption of the all-polymer blend and improve the molecular crystallinity. PYF-T-owas found to have a good compatibility with the host PY-DT acceptor, thus contributing to form a homogeneous mixed phase in the active layer. Consequently, the ternary all-PSC exhibits simultaneously improved Jscand FF than those of the binary all-PSC, resulting in an impressive efficiency of 18.15% (certified as 17.73%). Moreover, the large-area (1 cm2) and eco-friendly ternary all-PSCs (processed with o-xylene) were fabricated, and high efficiencies of 16.43% and 17.67% were obtained, respectively, showing the great potential of all-PSCs for future organic photovoltaic applications.

Currently, morphology optimization of all-polymer solar cells (all-PSCs) strongly depends on the use of solvent additives, which are usually highly toxic and harmful to the environment and human health. Here, we report a green and volatile solid additive, 2-methoxynaphthalene (2-MN). It was found that the incorporation of 2-MN into a PM6:PY-DT blend can effectively manipulate the aggregations of PM6 and PY-DT during film depositing and thermal annealing processes and results in highly ordered molecular packing and favorable phase-separated morphology. Consequently, a record-high efficiency of 17.32% is achieved for the PM6:PY-DT device. Moreover, 2-MN-processed all-PSCs were fabricated by using non-halogenated solvent. High efficiencies of 17.03% and 16.67% are obtained for all-PSCs fabricated under nitrogen atmosphere and ambient conditions, respectively. Our work shows that the utilization of 2-MN as a green and solid additive is a simple and feasible strategy to optimize the morphology and sheds new light on eco-friendly fabrication and application of all-PSCs.

The solvent additive strategy has been widely utilized to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). However, the residual solvent additive in the active layer tends to induce a gradual morphology degradation and further influences the long-term stability of OSCs. Here, a solid additive, 1,4-diiodobenzene (DIB), was introduced to fabricate efficient OSCs. We found that the treatment of DIB can lead to optimized morphology to form a bicontinuous network with intensified intermolecular packing in the donor and acceptor phases. Notably, DIB can be easily removed from the active layer viaa simple alcohol washing process and no further post-thermal annealing is needed, which is desirable for large-scale manufacturing of OSCs. As a result, high efficiencies of 17.47% for PM6:Y6 and 18.13% (certified as 17.7%) for PM6:BTP-eC9 binary OSCs are achieved, which are among the highest efficiencies reported for binary OSCs thus far. Moreover, OSCs fabricated with DIB also exhibit superior stability compared with the as-cast and traditional solvent additive processed devices. Additionally, DIB was successfully applied in different active layers, manifesting its general applicability. This work provides a feasible approach to enhance both the efficiency and stability of OSCs.

The energy offset, considered as the driving force for charge transfer between organic molecules, has significant effects on both charge separation and charge recombination in organic solar cells. Herein, we designed material systems with gradually shifting energy offsets, including both positive and negative values. Time-resolved spectroscopy was used to monitor the charge dynamics within the bulk heterojunction. It is striking to find that there is still charge transfer and charge generation when the energy offset reached −0.10 eV (ultraviolet photoelectron spectroscopy data). This work not only indicates the feasibility of the free carrier generation and the following charge separation under the condition of a negative offset but also elucidates the relationship between the charge transfer and the energy offset in the case of polymer chlorination.

Benzo[1,2-b:4,5-b′]dithiophene (BDT) has been widely used to construct donor-acceptor (D-A) copolymers in organic solar cells (OSCs). However, benzo[1,2-b:4,5-b′]difuran (BDF), an analogue of BDT, has received less attention than BDT. The photovoltaic performance of BDF copolymers has lagged behind that of BDT copolymers. Here, we designed and synthesized two BDF copolymers, PBF1-C and PBF1-C-2Cl. PBF1-C-2Cl, which is composed of BDF and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione connected by a chlorinated thiophene π-bridge, displays a low-lying highest occupied molecular orbital energy level, which helps in yielding a high open-circuit voltage (Voc) in OSCs. As a result, when blended with Y6, PBF1-C-2Cl-based devices showed a high Vocof 0.83 V and a power conversion efficiency (PCE) of 13.10%. To the best of our knowledge, the PCE of 13.10% is among the highest efficiency values for OSCs based on BDF copolymers.

Research on asymmetric A–D–A structured non-fullerene acceptors has lagged far behind the development of symmetric counterpart. In this contribution, by simply replacing one sulfur atom in indacenodithiophene unit with a selenium atom, an asymmetric building block SePT and a corresponding asymmetric non-fullerene acceptor SePT-IN have been developed. Asymmetric SePT-IN achieved a high efficiency of 10.20% in organic solar cells when blended with PBT1-C, much higher than that of symmetric TPT-IN counterpart (8.91%). Our results demonstrated an effective heteroatom substitution strategy to develop asymmetric A–D–A structured non-fullerene acceptors.

A bicontinuous network formed spontaneously upon film preparation is highly desirable for bulk-heterojunction (BHJ) organic solar cells (OSCs). Many donor-acceptor (D-A) type conjugated polymers can self-assemble into polymer fibrils in the solid state and such fibril-assembly can construct the morphological framework by forming a network structure, inducing the formation of ideal BHJ morphology. Our recent works have revealed that the fibril network strategy (FNS) can control the blend morphology in fullerene, non-fullerene and ternary OSCs. It has been shown that the formation of fibril network can optimize phase separation scale and ensure efficient exciton dissociation and charge carriers transport, thus leading to impressive power conversion efficiencies (PCEs) and high fill factor (FF) values. We believe that FNS will provide a promising approach for the optimization of active layer morphology and the improvement of photovoltaic performance, and further promote the commercialization of OSCs.

π-Bridges play a crucially important role in determining the optical absorption, bandgap and energy levels of donor-π-acceptor-π type conjugated polymers. In this contribution, a new polymer donor named PBT1-C-2Cl is synthesized by inserting the chlorine atoms onto π-bridges of the polymer PBT1-C. The introduction of chlorine atoms enables PBT1-C-2Cl with a lower highest occupied molecular orbital energy level and enhanced crystallinity than those of PBT1-C due to the high electronegativity and big atomic radius of chloride atom. When PBT1-C-2Cl is blended with the non-fullerene acceptor IT-4F, a high power conversion efficiency (PCE) of 12.7%, an open-circuit voltage (Voc) of 0.85 V and an impressive fill factor (FF) of 76.0% are achieved, which are all higher than those of PBT1-C:IT-4F based devices (PCE = 10.9%, Voc= 0.79 V and FF = 70.5%). The enhanced charge dissociation, efficient charge transport and reduced charge recombination account for the improved photovoltaic performance of PBT1-C-2Cl: IT-4F devices. The results indicate that inserting chlorine atoms onto π-bridges is a rational strategy for the synthesis of high-performance donor-π-acceptor-π type conjugated polymer donors.

A novel nonconjugated polymer named poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt) (PAMPS-Na) was designed and synthesized. The PAMPS-Na has good solubility in polar solvents, such as water, methanol, and ethanol, which can be used as the cathode buffer layer in organic solar cells (OSCs) through solution processing without damaging the underlying active layer. Moreover, it was found that PAMPS-Na can significantly decrease the Al work function when it was modified with Al. To reveal its universal application in organic photovoltaic devices, a variety of photovoltaic donor materials, including two medium-band gap polymers, a wide-band gap polymer, and a small molecule donor were employed to fabricate OSCs. Compared with OSCs with Ca/Al electrode, the devices based on PAMPS-Na/Al exhibited higher photovoltaic performance, mainly because of the increased short-circuit current. Additionally, OSCs with PAMPS-Na/Al displayed better ambient stability than devices with Ca/Al. It is also interesting to find that the performance of the devices can tolerate a wide change of PAMPS-Na’s thickness, enabling the potential for large-scale fabrication of OSCs. The results suggest that PAMPS-Na is a promising candidate as the cathode buffer layer to improve the efficiency and stability of OSCs.

Tungsten oxide as an alternative to conventional acidic PEDOT:PSS has attracted much attention in organic solar cells (OSCs). However, the vacuum-processed WO3layer and high-temperature sol–gel hydrolyzed WOXare incompatible with large-scale manufacturing of OSCs. Here, we report for the first time that a specific tungsten oxide WO2.72(W18O49) nanowire can function well as the anode buffer layer. The nw-WO2.72film exhibits a high optical transparency. The power conversion efficiency (PCE) of OSCs based on three typical polymer active layers PTB7:PC71BM, PTB7-Th:PC71BM, and PDBT-T1:PC71BM with nw-WO2.72layer were improved significantly from 7.27 to 8.23%, from 8.44 to 9.30%, and from 8.45 to 9.09%, respectively compared to devices with PEDOT:PSS. Moreover, the photovoltaic performance of OSCs based on small molecule p-DTS(FBTTh2)2:PC71BM active layer was also enhanced with the incorporation of nw-WO2.72. The enhanced performance is mainly attributed to the improved short-circuit current density (Jsc), which benefits from the oxygen vacancies and the surface apophyses for better charge extraction. Furthermore, OSCs based on nw-WO2.72show obviously improved ambient stability compared to devices with PEDOT:PSS layer. The results suggest that nw-WO2.72is a promising candidate for the anode buffer layer materials in organic solar cells.

Perylenediimide (PDI)-based materials exhibit great potential as non-fullerene acceptors in bulk-heterojunction (BHJ) organic solar cells (OSCs). Recent investigations have revealed that PDI molecules with a twisted structure could disrupt aggregation of perylene unit. Here, we present a PDI monomer via bay-substitutions with four fused naphthalene units by three-step reactions, named TN-PDI. The introduction of four fused naphthalene rings into the bay positions of PDI unit leads to a strong steric hindrance with a twist angle of 33° between the two PDI subplanes. Blended with a wide-band gap polymer donor (PDBT-T1), the TN-PDI based non-fullerene solar cells show power conversion efficiency (PCE) of 3.0%. Our results indicate that the bay-substitutions with fused aromatic substitutions could be an efficient approach to develop monomeric PDI acceptors. 近年来, 本体异质结型有机太阳能电池领域中, 苝酰亚胺类衍生物作为富勒烯类电荷受体材料的最佳替代材料而得到广泛关注. 很 多研究表明扭曲构型的分子结构设计可以明显地抑制苝酰亚胺的聚集行为. 本研究在苝酰亚胺的四个湾位同时共轭引入萘基官能团, 从 而合成一种简单结构的苝酰亚胺单分子型电荷受体材料TN-PDI. 四个共轭萘基的引入使苝酰亚胺平面骨架扭曲至33°. 分别以TN-PDI和一 种宽带隙聚合物PDBT-T1为电荷受体材料和给体材料构筑本体异质结型有机太阳能电池, 其光电转换效率可达3.0%. 研究结果表明, 四个 湾位同时共轭取代苝酰亚胺是一种制备简单高效苝酰亚胺单分子型电荷受体材料的有效方法.