Triphenylamine-Derived Solid-State Emissive Carbon Dots for Multicolor High-Efficiency Electroluminescent Light-Emitting Diodes.

Two kinds of triphenylamine-derived solid-state emissive carbon dots (CDs) with orange and yellow color are facilely synthesized through solvothermal treatment, taking advantage of the nonplanar structure and good carrier mobility of triphenylamine unit. Theoretical calculations show that the triphenylamine structure could greatly inhibit the direct π-π stacking of aromatic skeletons and enhance the fluorescence properties of CDs in aggregation state. By adopting the CDs as single emissive layer, high-performance orange-color and green-color electroluminescent light-emitting diodes (LEDs) are successfully fabricated, with maximum brightness of 9450/4236 cd m-2 , high current efficiency of 1.57/2.34 cd A-1 and low turn-on voltage of 3.1/3.6 eV are respectively achieved. Significantly, white-color LED device is further prepared. This work provides a universal platform for the construction of novel solid-state emissive CDs with significant applications in photoelectric device.

Gram-Scale Room-Temperature Synthesis of Solid-State Fluorescent Carbon Nanodots for Bright Electroluminescent Light Emitting Diodes.

The reaction conditions of high temperature and high pressure will introduce structural defects, high energy consumption, and security risks, severely hindering the industrial application of organic carbon nanodots (CDs). Moreover, the aggregation caused quenching effect also fundamentally limits the CDs based electroluminescent light emitting diodes (LEDs). Herein, for the first time, a rapid one-step room temperature synthetic strategy is introduced to prepare highly emissive solid-state-fluorescent CDs (RT-CDs). A strong oxidizing agent, potassium periodate (KIO4 ), is adopted as a catalyst to facilitate the cyclization of o-phenylenediamine and 4-dimethylamino phenol in aqueous solution at room temperature for only 5 min. The resultant organic molecule, 2-(dimethylamino) phenazine, will self-assemble kinetically to generate supramolecular-structure CDs during crystallization. The elaborately arranged supramolecular structure (J aggregates) endows CDs with intense solid-state-fluorescence. Density functional theory (DFT) calculation shows that the excited state of RT-CDs exhibits charge transfer characteristic owing to the unique donor-Π-acceptor structure. A high-performance monochrome RT-CDs based electroluminescent LEDs (2967 cd m-2 and 1.38 cd A-1 ) were fabricated via systematic optimizations of device engineering. This work provides a concrete and feasible avenue for the rapid and massive preparation of CDs, advancing the commercialization of CDs based optoelectronic devices.

Self-Aggregated Light-Trapping Nanodots for Highly Efficient Organic Solar Cells.

The typical thickness of the photoactive layer in organic solar cells (OSCs) is around 100 nm, which limits the absorption efficiency of the incident light and the power conversion efficiency (PCE) of OSCs. Therefore, light-trapping schemes to reduce the optical losses in the thin photoactive layers are critically important for efficient OSCs. Herein, light-trapping and electron-collection dual-functional small organic molecules, N,N,N',N'-tetraphenyloxalamide (TPEA) and N,N,N',N'-tetraphenylmalonamide (TPMA), are designed and synthesized by a one-step acylation reaction. Driven by strong intermolecular force, TPEA and TPMA tend to self-aggregate into hemispherical light-trapping nanodots on the photoactive layer, resulting in enhanced light harvesting. Meanwhile, TPEA and TPMA demonstrate high electron mobility and excellent electron-collection ability.  Compared with the device without cathode buffer layer (CBL, PCE = 14.09%), PM6:BTP-eC9 based OSCs with TPEA and TPMA light-trapping CBLs demonstrate greatly enhanced PCE of 16.21% and 17.85%, respectively. Furthermore, a record PCE of 19.02% can be achieved for PM6:BTP-eC9:PC71 BM based ternary OSC with TPMA light-trapping CBL. Moreover, TPMA exhibits a low synthesis cost of only 0.61 $ g-1 with high yield. These findings could open a window for the rational design of multifunctional CBLs for efficient and stable OSCs.

p-Type Carbon Dots for Effective Surface Optimization for Near-Record-Efficiency CsPbI2 Br Solar Cells.

Interface modification to minimize charge recombination and trapping for efficient charge transport is crucial for the performance of perovskite solar cells (PSCs). Herein, functionalized p-type blue carbon dots (B-CDs) are ventured as an interface passivation layer to enhance the efficiency and long-term stability of all-inorganic CsPbI2 Br PSCs. It is found that first the blue carbon dots with abundant NH, CN, CO, and CO functional groups effectively passivate defects by reacting with I- and Pb2+ ions in the perovskite through hydrogen and coordinative bonds. Second, the p-type B-CDs modifiers form a P-N junction with the n-type perovskite to provide efficient pathways for hole transfer and electron blocking. Third, the B-CDs increase the hydrophobicity of the perovskite film to improve the stability of CsPbI2 Br PSCs. With the above advantages, the CsPbI2 Br PSC with B-CDs modification shows an efficiency as high as 16.76%, one of the highest for its type. In addition, the modification renders significant improvement of air and light stability, with 95.33% of the initial PCE retained after storage in the ambient environment for 1000 h. This work demonstrates the great potential of B-CDs application in perovskite-based optoelectronic devices.