Suling Zhao

Causes of efficiency roll-off in phosphorescent organic light emitting devices: Triplet-triplet annihilation versus triplet-polaron quenching

Delayed electroluminescence measurements are used to probe and differentiate between triplet-triplet-annihilation (TTA) and triplet-polaron-quenching (TPQ) processes and their correlation with efficiency roll-off in fac-tris(2-phenylpyridine) iridium-based phosphorescent organic light emitting devices. Investigations on devices employing 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl) triphenylamine, two widely used host materials, show that the efficiency roll-off is primarily due to TPQ processes. Guest-guest TTA, on the other hand, is found to play no major role, contrary to speculations, especially at low guest concentrations. Evidence of host-host TTA in certain cases, and its possible contribution to exciton quenching in the case of devices with CBP host, is also reported.

Shape-Controlled Synthesis of All-Inorganic CsPbBr3 Perovskite Nanocrystals with Bright Blue Emission

We developed a colloidal synthesis of CsPbBr3 perovskite nanocrystals (NCs) at a relative low temperature (90 °C) for the bright blue emission which differs from the original green emission (∼510 nm) of CsPbBr3 nanocubes as reported previously. Shapes of the obtained CsPbBr3 NCs can be systematically engineered into single and lamellar-structured 0D quantum dots, as well as face-to-face stacking 2D nanoplatelets and flat-lying 2D nanosheets via tuning the amounts of oleic acid (OA) and oleylamine (OM). They exhibit sharp excitonic PL emissions at 453, 472, 449, and 452 nm, respectively. The large blue shift relative to the emission of CsPbBr3 bulk crystal can be ascribed to the strong quantum confinement effects of these various nanoshapes. PL decay lifetimes are measured, ranging from several to tens of nanoseconds, which infers the higher ratio of exciton radiative recombination to the nonradiative trappers in the obtained CsPbBr3 NCs. These shape-controlled CsPbBr3 perovskite NCs with the bright blue emission will be widely used in optoelectronic applications, especially in blue LEDs which still lag behind compared to the better developed red and green LEDs.

Influence of evaporation conditions of Alq3 on the performance of organic light emitting diodes

The influence of evaporation conditions of organic semiconductor material tris(8-hydroxyquinoline)aluminium (Alq3) on the performance of organic light emitting diodes (OLEDs) is reported. In the process of organic material thermal evaporation, the chamber was dark or had white light from a 100 W filament lamp. The devices prepared in the dark show higher emission intensity and efficiency compared with the ones prepared in white light under the same driving voltage. Atomic force microscopy measurements show that surface morphology and phase of Alq3 thin films are quite different for the previous cases. The different evaporation conditions are found to have direct effects on the electrical and luminance performance. The Alq3 thin films prepared in the dark as active emission layers of OLEDs show higher intensity and efficiency.

The Upconversion Luminescence of Er3+/Yb3+/Nd3+ Triply-Doped β-NaYF4 Nanocrystals under 808-nm Excitation

In this paper, Nd3+–Yb3+–Er3+-doped β-NaYF4 nanocrystals with different Nd3+ concentrations are synthesized, and the luminescence properties of the upconversion nanoparticles (UCNPs) have been studied under 808-nm excitation for sensitive biological applications. The upconversion luminescence spectra of NaYF4 nanoparticles with different dopants under 808-nm excitation proves that the Nd3+ ion can absorb the photons effectively, and the Yb3+ ion can play the role of an energy-transfer bridging ion between the Nd3+ ion and Er3+ ion. To investigate the effect of the Nd3+ ion, the decay curves of the 4S3/2 → 4I15/2 transition at 540 nm are measured and analyzed. The NaYF4: 20% Yb3+, 2% Er3+, 0.5% Nd3+ nanocrystals have the highest emission intensity among all samples under 808-nm excitation. The UC (upconversion) mechanism under 808-nm excitation is discussed in terms of the experimental results.

An Azulene‐Containing Low Bandgap Small Molecule for Organic Photovoltaics with High Open‐Circuit Voltage

A simple azulene-containing squaraine dye (AzUSQ) showing bandgap of 1.38 eV and hole mobility up to 1.25×10(-4)  cm(2)  V(-1)  s(-1) was synthesized. With its low bandgap, an organic photovoltaic (OPV) device based on it has been made that exhibits an impressive open-circuit voltages (Voc ) of 0.80 V. Hence, azulene might be a promising structural unit to construct OPV materials with simultaneous low bandgap, high hole mobility and high Voc .

Marked effects of indolyl vs. indolinyl substituent on solid-state structure, carrier mobility and photovoltaic efficiency of asymmetrical squaraine dyes

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm−3 for 1a but is 1.187 g cm−3 for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC71BM blended film is about 7 times higher than that of 1b:PC71BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.

Electroplex emission from bi-layer blue emitting organic materials

Electroluminescence (EL) and photoluminescence (PL) spectra of an electron donor, an (poly(N-vinylcarbazole) (PVK))/electron acceptor, and a (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD)) bi-layer solid film are analysed. The EL emission peak has an apparent red-shift with the increase of driving voltage. There maybe exist an electroplex emission between the PVK and PBD interface under high electric field strength. According to their energy level, the electroplex emission peak should locate at 460 nm. There are great spectra overlapping between PVK emission and electroplex emission, and the ratio of electroplex emission intensity to exciton emission intensity (Ielectroplex/Iexciton) increases from 0.38 at 10 to 0.81 at 16 V. Therefore the measured emission peaks continuously shift from 410 nm at 10 V to 445 nm at 16 V.

The Effects of Improved Photoelectric Properties of PEDOT:PSS by Two-Step Treatments on the Performance of Polymer Solar Cells Based on PTB7-Th:PC71BM

UNLABELLED In this paper, we present a smart two-step treated method to simultaneously improve the work function, conductivity, and transmittance of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) ( PEDOT PSS). With the two-step treated PEDOT PSS, the short-circuit current density of polymer solar cells (PSCs) based on PTB7-Th:PC71BM increases from 13.41 to 22.10 mA/cm(2), and the power conversion efficiency increases from 7.35% to 9.82% with 33% improvement. The underlying mechanisms on performance improvement of PSCs can be summarized as follows: (1) two-step treated PEDOT PSS with the improved work function and the conductivity, which contributes significantly to the charge collection of PSCs; and (2) two-step treated PEDOT PSS with higher transmittance, which is of benefit to the light absorption of the active layer in PSCs.

N,N-Diarylamino end-capping as a new strategy for simultaneously enhancing open-circuit voltage, short-circuit current density and fill factor in small molecule organic solar cells

A series of new asymmetrical squaraine derivatives bearing N,N-diarylamino substituents as end-capping groups, namely ASQAr-1–6, were designed and synthesized. In comparison with the reference compound ASQB bearing a N,N-diisobutylamino end-capper, all the six target compounds exhibit improved thermal stability, red-shifted and broadened absorption bands as well as lower HOMO and LUMO energy levels. Despite the hole mobility of most of the compounds still being lower than that of ASQB, solution-processed bulk-heterojunction small molecule organic solar cells (BHJ-SMOSCs) using ASQAr-1–6 as electron donor materials all show drastically higher power conversion efficiency (PCE, 3.08–3.69%) than that of the ASQB-based reference device (PCE = 1.54%). The much enhanced photovoltaic performance of BHJ-SMOSCs based-on ASQAr-1–6 could be attributed to the simultaneously enhanced open-circuit voltage (Voc, 0.81–0.87 V vs. 0.75 V), short-circuit current density (Jsc, 8.07–9.06 mA cm−2 vs. 5.40 mA cm−2), and fill factor (FF, 0.45–0.47 vs. 0.38) relative to those of the reference ASQB-based device.

Exciplex emission and decay of co-deposited 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine:tris-[3-(3-pyridyl)mesityl]borane organic light-emitting devices with different electron transporting layer thicknesses

Highly efficient fluorescence organic light-emitting diodes (OLEDs) based on the mixed 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine:tris-[3-(3-pyridyl)mesityl]borane (1:1) system are reported. The electroluminescence due to the exciplex emission is red shifted when the thickness of the electron-transporting layer increases. The prepared OLEDs achieve a low turn-on voltage of 2.1 V, a high current efficiency of 36.79 cd/A, and a very high luminescence of 17 100 cd/m2, as well as a low efficiency roll-off. The current efficiency of the optimized OLED is maintained at more than 28.33 cd/A up to 10 000 cd m−2. The detailed recombination mechanism of the prepared OLEDs is investigated by the transient electroluminescence method. It is concluded that there are no contributions from trapped charges and annihilations of triplet-triplet excitons to the detected electroluminescence.