Tami Janene Faircloth

Experimental demonstration of increased organic light emitting device output via volumetric light scattering

A set of experimental measurements of scattering films and organic light emitting devices (OLEDs) is presented. We measure the reflectance, transmission, and emission characteristics of scattering media and OLED devices separately, and use this data as input into a simple radiative transfer model to predict the effect of light scattering on OLED light output. We find quantitative agreement between the radiative transfer model predictions and experimental results. We find that the introduction of volumetric scattering mechanisms increases the output of OLEDs by as much as 40%, which corresponds to over 70% of the light within a typical glass substrate being coupled to air.

High performance organic light-emitting diodes fabricated via a vacuum-free lamination process

We demonstrate high performance organic light-emitting diodes (OLEDs) fabricated via a vacuum-free, direct lamination process. The OLEDs were made by laminating an anode component to a separately engineered cathode component using a roll laminator. We further present a solution-based chemical n-doping strategy to enable efficient electron injection from an inert cathode into polymeric organic semiconductors. The n-doping strategy is demonstrated by chemically reducing a conjugated light-emitting polymer with an alkali metal in an organic solvent. The metal reduced conjugated polymer, when employed as an electron injection layer, yields laminated OLEDs with efficiency comparable to conventionally fabricated devices utilizing a vacuum-deposited, reactive metal cathode. These designs and techniques should enable applications such as lighting where extremely low cost device fabrication is required.

Light extraction from OLEDs using volumetric light scattering

One of the limitations on OLED performance is the optical extraction efficiency, ηex, which is the ratio of light generated within the device to light emitted into the ambient. Ideally ηex is equal to unity. Typical estimates for this efficiency factor in OLEDs range between 0.17-0.5. We present a simple radiative transport model that quantifies the effect of volumetric light scattering on light output in terms of a small set of readily measured parameters. Our methodology is sufficiently general to parameterize and describe many of the light extraction schemes found in the literature. We will present a set of model calculations using parameters typical of many OLEDs, and show that the introduction of light scattering sites within the otherwise transparent substrate can increase light extraction efficiencies by at least a factor of 1.4. We also present experimental data to validate our analysis and demonstrate a high level of agreement between model and experiment.

Low-cost organic light-emitting devices for general illumination

Organic light-emitting devices (OLEDs) have shown great promise for general lighting applications. Over the past several years, tremendous progress has been made in improving performance attributes such as light quality, efficacy and lifetime of OLEDs. However, achieving the low cost manufacturing potential of OLEDs, another stringent requirement to enable lighting applications, has so far not been well addressed and explored. Here, we describe a vacuum-free, direct lamination process that could reduce OLED manufacturing costs substantially below what is currently possible. With this technique, OLEDs can be made by laminating an anode component to a separately engineered cathode component using a roll laminator. When coupled with a solution-based chemical n-doping strategy to enable efficient electron injection from an inert cathode into polymeric organic semiconductors, the lamination technique is able to produce high performance OLEDs with efficiency comparable to conventionally fabricated devices utilizing a vacuum-deposited, reactive metal cathode.

A Rapid Evaluation Method to Assess Organic Film Uniformity in Roll-to-Roll Manufactured OLEDs.

In order to enable low cost roll-to-roll or sheet-processing of organic light-emitting diode (OLED) devices, completely new deposition methods for both polymer and smallmolecule layers are being developed in place of the classic semiconductor manufacturing methods. In evaluating the utility of such methods, it is advantageous to have a robust and fast method to measure the thickness uniformity of the deposited organic layers. Non-uniformities at all spatial length scales from sub-mm to several cm can occur and so need to be understood as a function of the relevant parameters for each deposition method. Here we demonstrate a simple and fast method to quantify non-uniformities in thin films over arbitrarily large length scales. Our method utilizes the color of light reflected from the coated substrate and its variation with polymer layer thickness. This concept of color change is well known, and is due to constructive interference of light of particular wavelengths related to polymer layer thickness and optical constants. In our modification, a digital camera is used to capture images of the coated substrates, and hue is extracted from the image data files. We show that hue can be linearly correlated with polymer thickness. We demonstrate this for polymer based OLEDs using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and a light-emitting polymer (LEP) deposited on transparent substrate. The correlations were successfully used for 40-140nm PEDOT:PSS layers and 20-110nm LEP layers over length scales greater than 1 inch. The method sensitivity is estimated to be better than 5 nm. We show examples of non-uniformity analysis and how it relates to OLED performance.