(a) Schematic illustration of a typical DCM curve for a bilayer device. There are three states in the DCM curve: (i) depletion, (ii) intermediate, and (iii) accumulation. and are defined by the intersection of the extended lines from each state. The amount of accumulated charges at the interface is proportional to the gray area. (b) Schematic illustrations of charge location in the bilayer device in each state.
Schematic illustration of the sample structure and experimental setup for (a) C-V and C-f, (b) DCM, and (c) KP measurements. For C-V and C-f measurements, a cross-bar structure was employed. The width of the top and bottom electrodes was 2 mm and the area of the organic films was mm2. For DCM, an island-type top electrode with a diameter of 1.5 mm was used. The broken lines in (a) and (b) indicate where the cross-section for the profiles was taken. Tip of KP with a diameter of 5 mm was located about 0.5 mm above the film surface.
Chemical structures of the molecules used in this study. Here, 1-5 are polar, whereas 6 and 7 are nonpolar molecules.
The surface potential of the Alq3 and α-NPD films as a function of film thickness. The Alq3 film was deposited on an α-NPD film on an ITO substrate. The α-NPD/Alq3 interface is located at a film thickness of 40 nm. The surface potential of the Alq3 film grows linearly with a slope of 48 mV/nm, although a nonlinear region appears within several nanometers from the interface (bottom inset). The potential jump at the Alq3/α-NPD interface (Δ) suggests the presence of an interface dipole. The α-NPD film also shows weak GSP behavior with a slope of ca. 5.3 mV/nm (top inset).
Surface potential of various films on an α-NPD layer.
(a) A typical DCM curve for the ITO/α-NPD/Alq3/Al device at a sweep rate of 1 V/s. (b) Schematic illustrations of an energy diagram at each bias range.
(a) and (b) DCM curves of the ITO/α-NPD/Alq3/Al device for various film thicknesses. The α-NPD and Alq3 film thicknesses are fixed at 40 nm (a) and 60 nm (b), respectively. The current density at the accumulation state depends only on the Alq3 film thickness, indicating that the injected charges are holes. The threshold voltage of the actual current is independent of the combinations of the film thickness, whereas shifts to the negative side with increasing Alq3 film thickness. Thus, the polarity of interface charge is negative.
Thickness dependence of determined from the DCM curves for the bilayer devices.
(a) Typical DCM curve for the ITO/α-NPD/UGH2/Al device at a sweep rate of 1 V/s. (b) Capacitance-voltage and actual current-voltage curves derived from the DCM curve.
Comparison between the KP and DCM results. (a) The electric field formed in the second layer determined by the KP measurement and DCM. (b) Interface charge density estimated from the GSP slope and DCM curves.
(a) Typical C-V curve for the BCP and UGH2 devices at 10 Hz and 300 K. (b) Typical C-f curves for the BCP and UGH2 devices. was −2 and 2 V for the BCP device and 0 and 2 V for the UGH2 device (corresponding to before and after hole injection, respectively). The measurement temperature was 300 K for both devices.
(a) Schematic illustration of the bilayer device with charge spreading along the interface. The broken line (red) indicates the expanded area of capacitance, whereas the inner line (green) shows the cross-bar area. (b) Schematic illustration of the bilayer device in which the polar molecule is used as the second layer. The charge dispersion along the interface is suppressed owing to the microscopic potential fluctuation caused by the dipole moment.
Interface properties of OLED-related materials. The dipole moment (p) was calculated by gaussian 03 with a basis set of B3LYP/6-31 G(d). is the highest occupied molecular orbital energy offset directly measured through ultraviolet photoelectron spectroscopy (UPS).39 The films were deposited on the α-NPD film unless explicitly stated. The relative dielectric constant () was determined from the capacitance measurement. (GSP) is derived from Eq.(3) and (DCM) is estimated by integrating DCM curves [Eq. (2)]. (‡: Ref. 40, †: α-NPD on Alq3 , 31 *: on ITO,: The surface potential slightly depends on the film thickness (but not proportionally) probably owing to the charge transfer; however, the slope was estimated from the best-fit line in the same manner as for the other films./: not applicable).
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