Current-Induced Degradation in Polythiophene
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Current-Induced Degradation in Polythiophene Velda Goldberg, Michael Kaplan, Leonard Soltzberg, Joseph Genevich, Emily Coombs, Erica Giacomozzi, Valarie Kwasnik, Samia Naeem, Eleana Pham Simmons College, Physics and Chemistry Departments, Boston, MA 02115 George Malliaras Materials Science Department, Cornell University, Ithaca, NY 14853
ABSTRACT The three-year collaboration between Simmons College and the Cornell Center for Materials Research (CCMR) is focused on undergraduate student/faculty research in organic light emitting diodes (OLEDs). The physics of OLED devices is characterized by three major processes: charge injection, charge transfer, and light emission as a result of the electron-hole recombination. In the first year of the program our research has been related to the first two stages. OLEDs based on small molecule as well as polymeric layers have been investigated. The devices were prepared using mostly aluminum (also nickel and iron) as electrodes and PC:TPD or polythiophene as the organic layer. Electrodes of about 20 nm were formed by vacuum evaporation, and organic layers of approximately 100-200 nm were spin-coated. The currentvoltage characteristics, measured under forward and reverse bias up to 10 volts, demonstrate typical semiconductor S-shape behavior, and show variations dependent on aging, thickness of the polymer layer, and type and combination of electrodes. The results presented here specifically track the degradation of devices using polythiophene sandwiched between aluminum electrodes. The I-V curves and successive current response as a function of time and under constant voltage drive are presented along with complementary mass spectra and UV-visible and infrared absorption spectra. These measurements along with preliminary computer modeling of HOMO and LUMO energies for a series of thiophene oligomers suggest a correlation between internal changes in the polymer and variations in the electrical characteristics of the devices.
ELECTRICAL MEASUREMENTS OLED devices were fabricated on a glass substrate, subsequent to cleaning and exposure to UV/ozone to remove surface contamination. A bottom layer of 20 nm of aluminum was then deposited by vacuum evaporation. A layer of poly(3-octyl thiophene) [P3OT] was formed by spin-coating at approximately 2000 rpm. For this process, the polythiophene was dissolved in toluene at a ratio of 1:30 by weight, stirred overnight, and then gently heated at 50-60oC for 5 minutes just prior to spin coating. Finally, a top layer of 20 nm of aluminum was vacuum evaporated onto the polythiophene. By masking the top aluminum layer, seven devices, each of an active area of approximately 0.17 cm2, were created on each sample. None of the devices were encapsulated and all were stored in small individual containers under ambient conditions. I-V curves for these devices using a Keithley 2400 SourceMeter show the typical S-shape as
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