A Rapid Evaluation Method to Assess Organic Film Uniformity in Roll-to-Roll Manufactured OLEDs
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A rapid evaluation method to assess organic film uniformity in roll-to-roll manufactured OLEDs Svetlana Rogojevic, Tami Faircloth, Maria M. Otero, James C. Grande, Robert W. Tait, Joseph Shiang, and Anil R. Duggal GE Global Research Center Niskayuna, NY 12309, U.S.A. ABSTRACT 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(3,4ethylenedioxythiophene) 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. INTRODUCTION In recent years we have seen a significant push in the video display community, and, more recently, in the lighting community, to develop mechanically flexible, thin organic light emitting diode (OLED) devices. This effort is driven by the opportunity to develop unique new applications (for example roll-up displays and light-emitting wallpaper) and to reduce the manufacturing cost of such new and already existing products. In order to achieve acceptable market price, it is realized that low cost roll-to-roll or sheetprocessing methods must be developed in place of the classic semiconductor fabrication methods currently used. For example, thin organic layers that constitute the heart of an OLED device, have typically been deposited either by spin-coating (in case of polymers) or vacuum evaporation (in case of small molecule materials), which usually results in smooth and uniform films. At present, completely new deposition methods for both polymer and small-molecule devices are being developed, such as ink-jet printing, gravure, flexographic, offset
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