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Charge transport measurement using the Metal-Insulator-Semiconductor Charge Extraction by Linearly Increasing Voltage (MIS-CELIV) technique is a promising method for determining charge mobility in organic semiconductors because of its ability to study electron and hole mobilities independently. However, MIS-CELIV measurements have a number of parameters that can potentially affect the calculated mobility. There are only a few reports on MIS-CELIV being used to determine the charge mobility for materials typically used in organic light-emitting diodes (OLEDs), and the impact of each of the MIS-CELIV experimental parameters on the mobility is presently unknown. We find that the pulse duration, injection time, maximum voltage, offset voltage, and external load resistance have different levels of influence on the calculated mobility. Using the hole transporting OLED host material, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), we show that having an injection time sufficient to fully charge the insulator layer, a pulse duration comparable to the transit time, and an external circuit time constant much smaller than the transit time is required to give a mobility relevant to an OLED. The optimized MIS-CELIV parameters led to the measurement having a similar current density and electric field to that of an operational OLED. Under these conditions, the hole mobility of TCTA was determined to be 2.90 ± 0.07 × 10−4 cm2 V−1 s−1, which is similar to that measured using time-of-flight techniques. Using inappropriate experimental parameters could lead to an underestimation of the mobility by an order of magnitude. Simulations of the MIS-CELIV measurements verified the effect the different parameters played in determining the charge mobility. [ABSTRACT FROM AUTHOR]

Tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA) is a commonly used host material for organic light‐emitting diodes (OLEDs). TCTA has poor solubility and hence to improve solubility the authors have synthesized first‐ and second‐generation TCTA‐based dendrimers with n‐propyl surface groups. These dendrimeric TCTA hosts have improved solubility and similar photophysical properties to TCTA. The hole mobility of solution‐processed TCTA is found to be an order of magnitude less than films formed by evaporation and similar to that of the two dendronized TCTAs (mobilities in the order of 10−5 cm2 V−1 s−1). The hole mobility in the films is found to decrease when a green emissive first‐generation light‐emitting dendrimer is blended with the host materials due to charge trapping on the emissive guest. In the case of the second‐generation host, there is evidence of a degree of phase separation in the film. Organic light‐emitting diodes are fabricated at guest concentrations corresponding to the maximum observed photoluminescence quantum yield (PLQY) and lowest hole mobility. It is found that the external quantum efficiency (EQE) does not always scale with the PLQY, leading to devices that have EQEs higher than expected based on the PLQY and outcoupling of light from a bottom emitting device. [ABSTRACT FROM AUTHOR]

Phosphorescent emissive materials in organic light‐emitting diodes (OLEDs) manufactured using evaporation are usually blended with host materials at a concentration of 3–15 wt% to avoid concentration quenching of the luminescence. Here, experimental measurements of hole mobility and photoluminescence are related to the atomic level morphology of films created using atomistic nonequilibrium molecular dynamics simulations mimicking the evaporation process with similar guest concentrations as those used in operational test devices. For blends of fac‐tris[2‐phenylpyridinato‐C2,N]iridium(III) [Ir(ppy)3] in tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA), it is found that clustering of the Ir(ppy)3 (surface of the molecules within ≈0.4 nm) in the simulated films is directly relatable to the experimentally‐measured hole mobility. Films containing 1–10 wt% of Ir(ppy)3 in TCTA have a mobility of up to two orders of magnitude lower (≈10−6 cm2 V−1 s−1) than the neat TCTA film, which is consistent with the Ir(ppy)3 molecules acting as hole traps due to their smaller ionization potential. Comparison of the simulated film morphologies with the measured photoluminescence properties shows that for luminescence quenching to occur, the Ir(ppy)3 molecules have to have their ligands partially overlapping. Thus, the results show that the effect of guest interactions on charge transport and luminescence are markedly different for OLED light‐emitting layers. [ABSTRACT FROM AUTHOR]

We report on the realization of charge selective CELIV (charge extraction by linearly increasing voltage) via the selective extraction of each charge carrier species in a SiO 2 inserted MIS (metal-insulator-semiconductor) structure. The experiments are performed in a typical active layer of P3HT (poly (3-hexylthiophene)):PCBM (phenyl C 61 -butyric acid methyl ester) with a composition ratio of 1:1. The presence of double CELIV peaks indicates the involvement of deep trap states in the charge transport process. Furthermore the difference in excitation intensity dependence between photo-excited holes and electrons suggests a difference in population probabilities of electrons and holes for shallow and deep trap states. Charge selective CELIV experiments also show that the simultaneous extraction of holes and electrons is not a simple linear superposition of the cases where they are extracted separately, indicating that a fraction of photo-excited charges populating shallow trap states can escape from the device. [ABSTRACT FROM AUTHOR]

Solution-processed monochrome and white organic light-emitting diodes (OLEDs) that have a light-emitting layer composed of a 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and (5-terphenyl-1,3-phenylene)bis(diphenylphosphine oxide) (POPH) exciplex host and soluble blue, green and/or red phosphorescent dendrimers have been fabricated and characterised. The OLED performance characteristics were found to be scan dependent, with the first scans having large external quantum efficiencies (EQEs) at low luminance, with subsequent scans showing stable performance. The monochrome (blue, green and red) films were found to have high photoluminescence quantum yields, relatively balanced hole and electron mobilities, and the OLEDs had stable EQEs of between 9 and 12% at 100 cd m−2 over multiple scans. The blue, green and red emissive materials were blended with the exciplex host, with their ratio tuned to achieve white emission. The optimal blend ratio provided white OLEDs that had EQEs of 11.7% and 10.6% at 100 cd m−2 and 1000 cd m−2, respectively. The best balance of 1931 Commission Internationale d'Eclairage (CIE) coordinates (0.40,0.40), Colour Rendering Index (70), and Delta uv (0.003) was achieved for an OLED with the light-emitting layer containing the blue, green and red dendrimers in a ratio of 20.0:0.4:0.8 wt%. A feature of the white OLEDs was the stability of the CIE coordinates, with a change of only (0.013,0.005) between a luminance of 100 cd m−2 and 4000 cd m−2. [Display omitted] • A common exciplex host composed for solution processed blue, green, red and white emissive OLEDs. • Monochrome light-emitting dendrimer:exciplex host blend films have relatively balanced hole and electron mobilities. • Monochrome OLEDs with external quantum efficiencies of up to 22.5%. • White emitting OLEDs external quantum efficiencies of 10.6% at 1000 cd m−2 • White OLEDs with stable CIE coordinates across a broad range of luminance. [ABSTRACT FROM AUTHOR]

Fac -tris[2-phenylpyridinato- C 2 , N ]iridium(III) [Ir(ppy) 3 ] is a well-known phosphorescent material for organic light-emitting diodes (OLEDs) and while uniform thin films can be formed using evaporation under high vacuum, it is not sufficiently soluble to enable it to be processed from solution. Ir(ppy) 3 -cored dendrimers with solubilizing surface groups can be processed from solution to form good quality neat or guest:host blends, even when the host has relatively poor solubility. In this manuscript, we report the effect of adding different solubilizing surface groups, namely 2-ethylhexyloxy, n -propyl or t -butyl to first generation dendrons attached to the Ir(ppy) 3 core on the optoelectronic properties of neat and blend films with tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The different dendrons were found to have minimal effect on the photoluminescence spectra and efficiency of energy transfer from the host to the guest in the blend films. The hole mobility of neat films of the solution-processed Ir(ppy) 3 -cored dendrimer films was around 10−6 cm2 V−1 s−1, which was an order of magnitude less than Ir(ppy) 3 films formed by evaporation. Blending the dendrimers with TCTA at a concentration of 11 mol% (the concentration of the highest film photoluminescence quantum yield) led to an order of magnitude decrease in the hole mobility compared to the neat films. However, despite the relatively low mobilities, simple two-layer OLEDs composed of a blend light-emitting layer and an electron transporting layer were found to reach a maximum EQE of 11.1% at a luminance of 1000 cd m−2 for a film with a PLQY of 60%. [Display omitted] • Ir(ppy)3-cored dendrimers with solubilising surface groups are solution processed. • Good quality guest:host blend films formed with poorly soluble TCTA host. • Dendron type was found to have minimal effect on the host to guest energy transfer. • Charge mobility in dendrimer:TCTA blends less than in neat film. • Simple two-layer OLEDs had maximum EQEs of 11.1% at a luminance of 1000 cd m-2. [ABSTRACT FROM AUTHOR]