Highly efficient green TADF organic light-emitting diodes by simultaneously manipulating hole and electron transport.

This work demonstrates effective performance improvement by simultaneous manipulating of the hole injection and electron transport layers for (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) based green thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs). A 3 wt% sorbitol doped PEDOT:PSS layer results in the highest maximum current efficiency (CEmax) of 28.28 cd A-1 and external quantum efficiency (EQE) of 17.04%. Single carrier devices denote that hole mobility gradually rises with the sorbitol ratio. The electroluminescence mainly originates from the emission of 4CzIPN. Atomic force microscopy images imply that 3 wt% sorbitol doped PEDOT:PSS film includes the largest PEDOT aggregate, which contributes to a higher electric conductivity thus the better performance of 3 wt% sorbitol doped device. Also the 4CzIPN ratio in the emissive layer was optimized, and 4 wt%-4CzIPN in CBP achieves the highest EQE of 20.99% and CEmax of 34.99 cd A-1. The EL spectrum is independent of the luminous angle at a low 4CzIPN ratio but becomes more sensitive to the luminous angle at a high 4CzIPN ratio. Finally, we find out that the TADF OLED performance is very sensitive to TPBi thickness ranging from 20 nm to 65 nm, and 40 nm of TPBi achieves a CEmax up to 64.10 cd A-1 and an excellent EQE of 25.14%, ascribing from its more balanced carrier transport.

Influence of deposition substrate temperature on the morphology and molecular orientation of chloroaluminum phthalocyanine films as well the performance of organic photovoltaic cells.

The dependence of the morphology of neat chloroaluminum phthalocyanine (ClAlPc) films on substrate temperature (Tsub) during deposition is investigated by variable angle spectroscopic ellipsometry (VASE), x-ray diffraction (XRD), and atomic force microscopy (AFM) to obtain detailed information about the molecular orientation, phase separation, and crystallinity. AFM images indicate that both grain size and root mean square (RMS) roughness noticeably increase with Tsub both in neat and blend films. Increasing Tsub from room temperature to 420 K increases the horizontal orientation of the ClAlPc molecules with an increase of the mean molecular tilt angle from 60.13° (300 K) to 65.86° (420 K). The UV-vis absorption band of the corresponding films increases and the peak wavelength slightly red shifts with the Tsub increase. XRD patterns show a clear diffraction peak at Tsub over 390 K, implying the π-stacking of interconnected ClAlPc molecules at high Tsub. Planar and bulk heterojunction (BHJ) photovoltaic cells containing pristine ClAlPc films and ClAlPc:C60 blend films fabricated at Tsub of 390 K show increases in the power conversion efficiency (ηPCE) of 28% (ηPCE = 3.12%) and 36% (ηPCE = 3.58%), respectively, relative to devices as-deposited at room temperature. The maximum short circuit current in BHJs is obtained at 390 K in the Tsub range from 300 K to 450 K.