Tuning the molecular ordering of COi8DFIC from flat-on and edge-on lamellae to Hand J-type p-p stacking results in broadened absorption spectrum and fine phase separation with the electron donor PTB7-Th, which promotes efficient exciton dissociation at the donor/acceptor interface together with enhanced and balanced carrier mobility, and leads to an unprecedented PCE of 13.8% of singlejunction, binary PTB7-Th:COi8DFIC solar cell.

particular, bismuth (Bi) is an appealing anode candidate for SIBs because of its high theoretical volumetric capacity and favorable sodium alloy potential. [20][21][22][23] Bismuth, the last element of the pnictogens (i.e., group VA), is a well-known low-cost, nontoxic, and environment-friendly metal with a high electronic conductivity and chemical properties similar to those of its lighter homologs phosphorus and antimony. [24][25][26] In terms of appearance and structure, bismuth closely resembles graphite: it is gray and flaky with a puckered layered crystal structure composed of sixmembered rings [26] ( Figure S1, Supporting Information). Its interlayer spacing along the c-axis is 3.979 Å, which remarkably facilitates the diffusion of lithium (1.52 Å) and sodium (2.04 Å) ions, [26,27] rendering it a very promising anode material for rechargeable batteries. Regrettably, bismuth generally shows a large capacity decay [28] due to severe pulverization caused by a large volume change of 352% during sodiation and desodiation processes [29] (Table S1, Supporting Information). More importantly, the specific expansion mechanism of bismuth during battery operation has not yet been clarified. Thus, unraveling the charge storage mechanism and proposing a rational design for sophisticated nanostructures to accommodate volume expansion and optimize the performance of bismuth-based SIBs is relevant and urgent.Here, we discover that sodium storage in bismuth proceeds through sodium-ion insertion followed by reversible crystalline-phase evolution, which results in a large anisotropic Bismuth is a promising anode material for state-of-the-art rechargeable batteries due to its high theoretical volumetric capacity and relatively low working potential. However, its charge storage mechanism is unclear, hindering further improvement of the cell performance. Here, using in situ transmission electron microscopy and X-ray diffraction techniques as well as theoretical analysis, it is found that a large anisotropic volume expansion of 142% occurs along the z-axis largely due to the alloy reaction during sodiation, significantly reducing the electrochemical performance of bismuth electrodes. To address this problem, ultrathin few-layer bismuthene with a large aspect ratio is rationally synthesized, and can relieve the expansion strain along the z-axis. A free-standing bismuthene/graphene composite electrode with tunable thickness achieves a strikingly stable and high areal sodium storage capacity of 12.1 mAh cm −2 , which greatly exceeds that of most reported electrode materials. The clarification of the charge storage mechanism and the superior areal capacity achieved should facilitate the development of bismuth-based high-performance anodes for practical electrochemical energy-storage applications.

Alloying anodes are promising high-capacity electrode materials for K-ion batteries (KIBs). However,K IBs based on alloying anodes suffer from rapid capacity decay due to the instability of Km etal and large volume expansion of alloying anodes.Herein, the effects of salts and solvents on the cycling stability of KIBs based on atypical alloying anode such as amorphous red phosphorus (RP) are investigated, and the potassium bis(fluorosulfonyl)imide (KFSI) salt-based carbonate electrolyte is versatile to achieve simultaneous stabilization of Kmetal and RP electrodes for highly stable KIBs.This saltsolvent complex with amoderate solvation energy can alleviate side reactions between Kmetal and the electrolyte and facilitate K + ion diffusion/desolvation. Moreover,robust SEI layers that form on Km etal and RP electrodes can suppress Kd endrite growth and resist RP volume change.T his strategy of electrolyte regulation can be applicable to other alloying anodes for high-performance KIBs.

Developing a fundamental understanding of the molecular order within the photoactive layer, and the influence therein of solution casting conditions, is a key factor in obtaining high power conversation efficiency (PCE) polymer solar cells. Herein, the molecular order in PBDB‐T:INPIC‐4F nonfullerene solar cells is tuned by control of the molecular organization time during film casting, and the crucial role of retarding the crystallization of INPIC‐4F in achieving high performance is demonstrated. When PBDB‐T:INPIC‐4F is cast with the presence of solvent vapor to prolong the organization time, INPIC‐4F molecules form spherulites with a polycrystalline structure, resulting in large phase separation and device efficiency below 10%. On the contrary, casting the film on a hot substrate is effective in suppressing the formation of the polycrystalline structure, and encourages face‐on π−π stacking of INPIC‐4F. This molecular transformation of INPIC‐4F significantly enhances the absorption ability of INPIC‐4F at long wavelengths and facilitates a fine phase separation to support efficient exciton dissociation and balanced charge transport, leading to the achievement of a maximum PCE of 13.1%. This work provides a rational guide for optimizing nonfullerene polymer solar cells consisting of highly crystallizable small molecular electron acceptors.

Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties, and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced Voc but increased Jsc and FF, and therefore the ultimate influence to efficiency depends on the compensation of Voc loss and gains of Jsc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic bandgaps and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, with the conversion of ITIC to IT-4F reduces structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger p-p stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection towards anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encourage the intermixing of donor/acceptor for efficient charge generation.