Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells. Nature Materials.

Carbon dots (CDs) are the new fellow of carbon family having a size less than 10 nm and attracted much attention of researchers since the last decade because of their unique characteristics, such as inexpensive and facile synthesis methods, easy surface modification, excellent photoluminescence, outstanding water solubility, and low toxicity. Due to these unique characteristics, CDs have been extensively applied in different kind of scientific disciplines. For example in the photocatalytic reactions, drug-gene delivery system, in vitro and in vivo bioimaging, chemical and biological sensing as well as photodynamic and photothermal therapies. Mainly two types of methods are available in the literature to synthesize CDs: the top-down approach, which refers to breaking down a more massive carbon structure into nanoscale particles; the bottom-up approach, which refers to the synthesis of CDs from smaller carbon units (small organic molecules). Many review articles are available in the literature regarding the synthesis and applications of CDs. However, there is no such review article describing the synthesis and complete application of CDs derived from small organic molecules together. In this review, we have summarized the progress of research on CDs regarding its synthesis from small organic molecules (bottom-up approach) via hydrothermal/solvothermal treatment, microwave irradiation, ultrasonic treatment, and thermal decomposition techniques as well as applications in the field of bioimaging, drug/gene delivery system, fluorescence-based sensing, photocatalytic reactions, photo-dynamic therapy (PDT) and photo-thermal (PTT) therapy based on the available literature. Finally, the challenges and future direction of CDs are discussed.

Glass transition temperature is a critical parameter for achieving favorable and thermally stable bulk heterojunction morphology as it determines the kinetics of molecular organization of polymeric semiconducting materials. This study presents a sensitive method of precisely determining the glass transition temperature (T g) of conjugated polymers and polymer–PCBM blends using dynamic mechanical thermal analysis (DMTA). The method presented here is very versatile in which polymer or polymer–molecule films are reinforced using a woven glass fiber and utilizes only 5–10 mg of the material. This makes the method superior to differential scanning calorimetry (DSC) for determining the thermal properties of conjugated polymers. The effects of PCBM loading, solvents, and additive on the T g of polymer–PCBM blends and on the miscibility of different phases are investigated using the novel DMTA method. For the P3HT:PC61BM system, two different thermal transitions were found corresponding to P3HT-rich and PCBM-rich phases when cast using CHCl3, while chlorobenzene was found to result in a single T g for the blend which was between those of the pure components, indicating greater miscibility when cast from chlorobenzene. On the other hand, miscibility of PCBM in TQ1 was found to be relatively low, and two thermal transitions were found for all TQ1:PCBM blends. The total PCBM content or the solvent used was found to have little influence on the resultant PCBM miscibility in TQ1. T g of a range of other polymers as measured using DMTA is also reported to prove the versatility of this technique.

The frontier molecular energy levels of organic semiconductors are decisive for their fundamental function and efficiency in optoelectronics. However, the precise determination of these energy levels and their variation when using different techniques makes it hard to compare and establish design rules. In this work, the energy levels of 33 organic semiconductors via cyclic voltammetry (CV), density functional theory, ultraviolet photoelectron spectroscopy, and low‐energy inverse photoelectron spectroscopy are determined. Solar cells are fabricated to obtain key device parameters and relate them to the significant differences in the energy levels and offsets obtained from different methods. In contrast to CV, the photovoltaic gap measured using photoelectron spectroscopy (PES) correlates well with the experimental device VOC. It is demonstrated that high‐performing systems such as PM6:Y6 and WF3F:Y6, which are previously reported to have negligible ionization energy (IE) offsets (ΔIE), possess sizable ΔIE of ≈0.5 eV, determined by PES. Using various D–A blends, it is demonstrated that ΔIE plays a key role in charge generation. In contrast to earlier reports, it is shown that a vanishing ΔIE is detrimental to device performance. Overall, these findings establish a solid base for reliably evaluating material energetics and interpreting property–performance relationships in organic solar cells.