Electroabsorption spectroscopy on tris-(8-hydroxyquinoline) aluminum-based light emitting diodes

Abstract

We determined the electric-field distribution in organic light emitting diode LED structures fabricated with 4,4′-bis [N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD) as hole-transport material and tris-(8-hydroxyquinoline) aluminum (Alq) as electron-transport and emissive material. The electric-field distribution was obtained from an investigation of the linear and the nonlinear Stark effect of the materials when employed in organic LEDs using electroabsorption spectroscopy. We measured the electric-field distribution as a function of the applied voltage in the forward and in the reverse direction. Whilst the average electric fields in the α-NPD and the Alq layers are equal in the reverse direction, the field in the Alq layer is considerably larger than that in the α-NPD layer in the forward direction, and the factor by which these fields differ changes with increasing voltage, in particular in the vicinity of the turn-on voltage. We discuss the electric-field distribution in terms of the charge injection and charge transport in devices as well as the possibility of charge accumulation at the α-NPD/Alq interface.

Electromodulated doping of the hole transport layer in a small molecule organic light-emitting diode

“…This would suggest that even in reverse bias, the electric field strength in the α-NPD layer is roughly an order of magnitude weaker than that in the Alq 3 layer; an effect that can only be attributed to the incorporation of an HIL. The minima at λ = 520 nm was observed by neither Rohlfing et al 29 nor Martin et al 30 and thus is a direct consequence of incorporation of a hole injection layer. Figure 3(a) shows the dependence of the Alq 3 signal, measured at a fixed wavelength of 460 nm as the dc bias was swept from -5 to +2 V with a reference bias amplitude of 0.5 V rms.…”

“…1(b) for comparison. 29 This work was performed by Rohlfing et al on a device with a 100 nm thick layer of α-NPD and under a field of 21 kV/cm. The EA spectrum of α-NPD has a maxima at λ = 410 nm and does not contribute for λ > 450 nm.…”

“…13,21,[23][24][25][26][27][28][29][30] This technique relies upon a change in absorption in response to an electric field. The field causes a Stark shift of energy levels, resulting in an EA spectrum proportional to the square of the electric field and the imaginary part of the third order dc Kerr nonlinear susceptibility, χ (3) (-ω;0,0,ω).…”

“…27 Although EA spectroscopy has been used primarily to study polymer OLEDs, there have been two studies of bi-layer OLEDs that with an α-NPD hole transport layer and a tris(quinolin-8-olato) aluminum (III) (Alq 3 ) electron transport and emission layer. Rohlfing et al 29 concluded that the average electric fields in the α-NPD and the Alq 3 layers are similar in reverse bias but the field in the Alq 3 layer is considerably larger than that in the α-NPD layer in forward bias. These effect were attribute to hole accumulation at the α-NPD/Alq 3 interface.…”

Relation between the built-in voltage in organic light-emitting diodes and the zero-field voltage as measured by electroabsorption

“…The photon energy dependence of the effect has been used to resolve the electric field in distinct semiconducting layers within multilayer OLEDs. [22][23][24][25] Furthermore, the photon energy and modulation frequency dependence has been used to distinguish the Stark-like effect mentioned above from a contribution due to charge-induced absorption. [15][16][17][18][24][25][26] The voltage V 0,EA at which the relative change in reflection in a single-layer device vanishes is commonly viewed as a direct measure of V bi .…”