Degradation of Ru $$\left( {{\text{bpy}}} \right)_3^{2 + }$$ -based OLEDs
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Degradation of Ru(bpy)32+-based OLEDs Velda Goldberg1, Michael Kaplan1,2, Leonard Soltzberg2, Joseph Genevich1, *Rebecca Berry 1,2, *Alma Bukhari1,2, *Sherina Chan1,2, *Megan Damour1,2, *Leigh Friguglietti1,2, *Erica Gunn1,2, *Karen Ho1,2, *Ashley Johnson1,2, *Yin Yin Lin1,2, *Alisabet Lowenthal1,2, *Seiyam Suth1,2, *Regina To1,2, *Regina Yopak1,2, Jason D. Slinker3, George G. Malliaras3, Samuel FloresTorres4, and Hector D. Abruña4 1 Physics, Simmons College, Boston, Massachusetts; 2Chemistry, Simmons College, Boston, Massachusetts; 3Materials Science and Engineering, Cornell University, Ithaca, New York; 4 Chemistry and Chemical Biology, Cornell University, Ithaca, NY. *Undergraduate student authors. ABSTRACT Analysis of the possible mechanisms of degradation of Ru(bpy)32+-based OLEDs has led to the idea of quencher formation in the metalloorganic area close to the cathode. It has been suggested that the quencher results from an electrochemical process where one of the bipyridine (bpy) groups is replaced with two water molecules [1] or from reduction of Ru(bpy)32+ to Ru(bpy)30 [2]. We have tested these and other degradation ideas for Ru(bpy)32+-based OLEDs, both prepared and tested with considerable exposure to the ambient environment and using materials and procedures that emphasize cost of preparation rather than overall efficiency. In order to understand the mechanisms involved in these particular devices, we have correlated changes in the devices’ electrical and optical properties with MALDI-TOF mass spectra and UVvis absorption and fluorescence spectra.
INTRODUCTION In their simplest form OLEDs consist of a single organic semiconductor sandwiched between electrodes. Under forward bias, the anode injects holes into the highest occupied molecular orbital (HOMO) of the organic layer and the cathode injects electrons into the lowest unoccupied molecular orbital (LUMO) of the organic layer. These charges migrate in the opposite directions and when they meet, they may form an exciton, and the radiative decay of a fraction of these excitons produces light. For such a simple device to produce a high yield, the single organic layer must easily transport both electrons and holes. Since for many materials this is not the case, high efficiency OLEDs often employ multiple organic layers, which increases the cost and the processing steps. As an alternative to the use of organic semiconductors in OLEDs, ionic transition metal complexes have created much interest because of their charge transport properties. We have focused on a particular example of these materials—Ru(bpy)3(PF6)2. In this material [Ru (bpy)3]2+ is surrounded by (PF6)- ions. Under forward bias, holes are injected into the t2g orbital of the predominantly-metal (HOMO) and electrons are injected into the π* orbitals with mostly ligand-type character. In addition, the counter ions also contribute to ionic conductivity. These materials show promise because their performance is now close to that for very good OLED devices.[3] They show stability in mul
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