Lanthanide tetrafluorobenzoates as emitters for OLEDs: New approach for host selection

Brightly luminescent and highly soluble lanthanide tetrafluorobenzoates, as well as their mixed ligand complexes, were synthesized and thoroughly characterized. The low charge carrier mobility hampered their use in OLED, but this problem was overcome by a thoughtful selection of host material. The organic molecules acted both as ligands in the complex and as the host material, leading to zero increase in the Stokes shift.     

用BPhen作基质的铕配合物OLED发光机制研究

用具有良好电子传输/空穴阻挡性能的BPhen(4,7-diphenyl-1,10-phenanthroline)作基质,Eu(DBM)3pyzphen(pyzphen=pyrazino-[2,3-f][1,10]-phenanthroline,DBM=Dibenzoylmethane)作发射材料,成功制得了高效率、高亮度的有机电致发光器件OLED.器件的最大外量子效率为2.5%,最大电流效率为5.3 cd/A,最大亮度为1 320 cd/m2.在亮度为200和1 000 cd/m2时,器件的色坐标分别为(0.66,0.33)和(0.65,0.34).深入研究了该器件的发光机制,发现在电致发光(EL)过程中,载流子直接被Eu(DBM)3pyzphen陷获是主要的发光机制,同时在BPhen与Eu(DBM)3pyzphen间还存在着有效的能量传递.     

Improved efficiency and stability of inverted polymer solar cells with a solution-processed BPhen interlayer and polystyrene beads

We demonstrate improved power conversion efficiency (PCE) and strongly enhanced stability of inverted organic solar cells (OSCs) with Cs halides by solution casting BPhen (4,7-di(phenyl)-1,10-phenanthroline) on the halide layer and ∼100 nm polystyrene beads (PSB) on the blank side of the OSC’s substrate. The PCE of ITO/CsCl/P3HT:PCBM/MoO₃/Al (where P3HT is poly 3-hexylthiophene and PCBM is [6,6]-phenyl-C₆₀-butyric acid methyl ester) improves by up to 46%, from 2.5% to ∼3.7%, by adding a solution-processed BPhen layer between the CsCl and the active layer. For such cells with CsI (PCE ∼3.3–3.4%) the increase was only 6–9%, to 3.5–3.7%. The PCE of cells devoid of the halides but with BPhen was ∼3.3%. The cells were optimized by varying the BPhen concentration in a chlorobenzene solution. The results are consistent with reduced charge recombination at the ITO interface in the presence of the hole blocking BPhen interlayer. The use of hole blocking BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), as a substitute for BPhen, also showed an enhancement (though lower due to its lower electron mobility), verifying the effect of these materials as hole blocking interlayers. Interestingly, the stability of such non-encapsulated devices with CsCl/BPhen or CsI/BPhen improved significantly. For example, the PCE of unencapsulated cells with CsCl/BPhen kept in the dark under ambient conditions dropped by less than 2% after more than 3 weeks; the PCE of similar cells devoid of the BPhen layer dropped by ∼60% during the same period. The PCE of the cell with CsCl/BPhen dropped by ∼16% after 2 months. High humidity, as expected, resulted in faster deterioration in cell performance. The PCE, however, was restored to within ∼10% of the original value for 2 week old cells by solution–application of a PSB layer on the blank side of the cell’s glass substrate. These beads direct and scatter the light to enhance absorption in the active layer. The results demonstrate that a simple approach such as casting a film of ∼100 nm diameter PSB from an aqueous suspension on the blank side of the OSC substrate can improve long-term performance, and that spin coating BPhen is a low-cost and easy approach to reduce charge recombination at the cathode in inverted structures for increased PCE and stability.