Transparent Ag-Free OLED Fabricated by OVPD Using Thin Au Contacts
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Transparent Ag-Free OLED Fabricated by OVPD Using Thin Au Contacts 1
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P. Pfeiffer , D. Stümmler , S. Loginkin , M. Heuken , A. Vescan and H. Kalisch 1 2
Device Technology, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany AIXTRON SE, Dornkaulstraße 2, 52134 Herzogenrath, Germany
ABSTRACT We demonstrate Ag-free transparent OLED (TOLED) fabricated by organic vapor phase deposition (OVPD) using thin Au contacts. Three types of TOLED devices have been studied. The first one has been deposited on ITO substrates to compare thin Ag and Au films as top cathodes. A 6-fold increase in operational lifetime (LT50, 4 mA/cm²) from 27 h to 172 h can be observed when replacing Ag by Au while maintaining similar electro-optical characteristics. Furthermore, a second type of TOLED on thin Au films, replacing ITO and suppressing laterally guided modes [1], has been studied. TOLED on ITO substrates and on thin Au films exhibit very low onset voltages of 2.2 V. Both types show about 30% transparency in the VIS light region and emit orange light with a peak wavelength of 608 nm from either side with a total EQE of about 9% (measured at 1000 cd/m² in sum). The third type of TOLED was fabricated with an inverted structure, with the aim to further increase operational lifetime by burying the reactive LiF/Al electron injection layer (EIL). This will make the EIL less accessible for oxygen and moisture. Our results show difficulties in electron injection when depositing the organic stack on Al/LiF, which may be attributed to an insufficient thermal activation of the EIL. INTRODUCTION Due do their ultra-thin and bendable layers, OLED can be built on flexible substrates such as plastic [2] or metal foils [3] allowing almost any shape. Their most outstanding feature may be the potential to be transparent light sources. Transparent OLED (TOLED) are capable to become a key component for numerous applications such as head-mounted displays, displays integrated in automotive windshields and smart windows [4]. Common concepts for TOLED include either a transparent Mg:Ag [5] or a sputtered ITO [6] top cathode. A major drawback of Ag-based TOLED is their limited operational lifetime, mostly due to diffusion of Ag from the cathode into the organic layers [7]. Wang et al. [8] have shown that a thin LiF layer between Alq3 and Ag can dramatically reduce diffusion into the organic layers and therefore promises longer operational lifetimes of such devices. However, no operational lifetime studies on those devices have been conducted. Employing ITO as transparent top cathode leads to damage of the underlying organic layers due to the sputtering deposition process [9]. Such damage can be reduced by various protective layers such as PEDOT:PSS [10], a combination of pentacene and PEDOT:PSS [11] or CuPc [12]. However, any protective layer increases the operation voltage by adding a series resistance. Additionally, PEDOT:PSS is usually spin-coated onto the OLED which adds a breach of vacuum into the fabrication steps and further re
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