Packing Principles for Donor–Acceptor Oligomers from Analysis of Single Crystals

D–A conjugated molecules are complicated in both their molecular and their packing structures. In this perspective, we summarize more than 40 crystal lattices of conjugated oligomers to identify the morphological influence of each building block on the D–A molecules. These lattice structures reveal not only the packing preferences of the conjugated oligomers but also the conformational disorder in the lattices. The presence of this disorder in slowly grown crystals implies that attaining total long-range conformational order is challenging for D–A oligomers, which are structurally complicated and readily distorted and which have building blocks of incommensurate packing dimensions. In optoelectronic applications, a decreased duration of processing can prevent ordering and trap the thin films of D–A oligomers from becoming crystalline phases. Although D–A oligomers conform to packing principles in the formation of a single crystal, their phase behaviors in the formation of active thin films are much more difficult to comprehend. Continuous advances in methods of characterization are still strongly required for the steps of attaining a true structure–property relation of D–A oligomers in active films for optoelectronic applications

Stepwise Structural Evolution of a DTS‑F₂BT Oligomer and Influence of Structural Disorder on Organic Field Effect Transistors and Organic Photovoltaic Performance

An A–D–A oligomer, DTS­(F₂BT)₂, was synthesized; its structural evolution was studied with DSC, POM, 2D-WAXD, and in-situ GI-XRD. The structural evolution of DTS­(F₂BT)₂ is stepwise and kinetically slow. Both rapid drying and the presence of PC₇₁BM trapped DTS­(F₂BT)₂ in a less ordered nematic (N) phase. PDMS-assisted crystallization enabled a pristine DTS­(F₂BT)₂ thin film to attain a more ordered equilibrium phase, and enhanced the OFET mobility of DTS­(F₂BT)₂. In OPV devices, DIO additive drove the DTS­(F₂BT)₂ domains in the DTS­(F₂BT)₂:PC₇₁BM blended film from the N phase toward the equilibrium phase, and resulted in enhanced OPV performances. These results reveal the slow ordering process of the A–D–A oligomer, and the importance of monitoring the degree of structural evolution of the active thin films in organic optoelectronics

Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode

We report the synthesis and characterization of alloyed Sn–Pb methylammonium mixed-halide perovskites (CH₃NH₃Sn_<i>y</i>Pb_1–<i>y</i>I_3–<i>x</i>Cl_<i>x</i>) to extend light harvesting toward the near-infrared region for carbon-based mesoscopic solar cells free of organic hole-transport layers. The proportions of Sn in perovskites are well-controlled by mixing tin chloride (SnCl₂) and lead iodide (PbI₂) in varied stoichiometric ratios (<i>y</i> = 0–1). SnCl₂ plays a key role in modifying the lattice structure of the perovskite, showing anomalous optical and optoelectronic properties; upon increasing the concentration of SnCl₂, the variation of the band gap and band energy differed from those of the SnI₂ precursor. The CH₃NH₃Sn_<i>y</i>Pb_1–<i>y</i>I_3–<i>x</i>Cl_<i>x</i> devices showed enhanced photovoltaic performance upon increasing the proportion of SnCl₂ until <i>y</i> = 0.75, consistent with the corresponding potential energy levels. The photovoltaic performance was further improved upon adding 30 mol % tin fluoride (SnF₂) with device configuration FTO/TiO₂/Al₂O₃/NiO/C, producing the best power conversion efficiency, 5.13%, with great reproducibility and intrinsic stability