Experiment and Modeling of Conversion of Substrate-Waveguided Modes to Surface-Emitted Light by Substrate Patterning
- PDF / 211,582 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 81 Downloads / 197 Views
Experiment and Modeling of Conversion of Substrate-Waveguided Modes to Surface-Emitted Light by Substrate Patterning Min-Hao M. Lu, Conor F. Madigan, and J. C. Sturm Center for Photonic and Optoelectronic Materials, Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, U.S.A. ABSTRACT To predict the optical power that could be harvested from light emission that is waveguided in the substrate of organic light emitting devices (OLEDs), a quantitative quantum mechanical model of the light emitted into the waveguided modes has been developed. The model was used to compute the exact distribution of energy in external, substrate and ITO/organic modes as a function of the distance of the emission zone from the cathode. The results are compared to the classical ray optics model and to experiments in two-layer OLED devices. Classical ray optics is found to substantially over-predict the light in waveguided modes. INTRODUCTION OLEDs have received enormous interest because of their promise for cheaper and more efficient flat panel displays. A large amount of light is trapped in the substrate due to total internal reflection; therefore, substrate patterning can be used to increase external coupling efficiency [1-3]. The exact distribution of optical energy in all of the waveguide modes has not been calculated previously, except for by classical ray optics. The goal of this paper is to develop such a quantitatively accurate model of such waveguided light and verify it with experiments. The radiative modes can be classified into external, substrate and ITO/organic modes (Figure 1a). External modes are those with angle to the surface normal in the organic layer less than the critical angle between air and Alq3, θc1 = sin-1 nair/nalq; substrate modes are those with angle between θc1 and the critical angle between glass and Alq3, θc2 = sin-1 nglass/nalq; and θff
I
Air
Lens
Substrate
II
ITO
With lens
III
Organic
Substrate Emitting center
Cathode
Without lens
OLED
a.
b.
Figure 1. a. Three radiative modes in OLEDs: I. External modes, II. Substrate modes, and III. ITO/organic modes. B. Attaching a lens to the back side of OLED converts some light in substrate modes into external modes [3].
Q3.7.1
ITO/organic modes are those with angle greater than θc2. Substrate modes are emitted through the edges of the substrate, whereas the ITO/organic modes are heavily attenuated, and does not emit through the edge [5]. According to classical ray optics the amount of energy emitted into the external, substrate and ITO/organic modes are 18.9%, 34.2% and 46.9%, respectively, for refractive indices of nalq = 1.71, nglass = 1.51, and nair = 1. By laminating lens arrays on the backside of the OLEDs, some of the light waveguided in the substrate is allowed to emit externally (Figure 1b). For example, Figure 3a shows the farfield emission intensity with and without a lens attached to an OLED made with a single layer poly-(N-vinylcarbazole) (PVK) blend [3]. In one case, the OLED was 2.3 mm below the center of cur
Data Loading...
