Characterization of organic light-emitting devices Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/7p88ck38w

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  • In this thesis steady-state (i.e. steady-state with respect to the applied voltage waveform) transient current-transient voltage [i(t)-v(t)], transient brightness-transient current [b(t)-i(t)], transient brightness-transient voltage [b(t)-v(t)], transient current [i(t)], transient brightness [b(t)], and detrapped charge analysis are introduced as novel organic light emitting device (OLED) characterization methods. These analysis methods involve measurement of the instantaneous voltage [v(t)] across, the instantaneous current [i(t)] through, and the instantaneous brightness [b(t)] from an OLED when it is subjected to a bipolar, piecewise-linear applied voltage waveform. The utility of these characterization methods is demonstrated via comparison of different types of OLEDs and polymer light emitting devices (PLEDs) and from a preliminary study of OLED aging. Some of the device parameters obtained from these characterization methods include: OLED capacitance, accumulated charge, electron transport layer (ETL) thickness, hole transport layer (HTL) thickness, OLED thickness, parallel resistance, and series resistance. A current bump observed in i(t)-v(t) curves is attributed to the removal of accumulated hole charge from the ETL/HTL interface and is only observed in heterojunctions (i.e. OLEDs), not in single-layer devices (i.e. PLEDs). Using the characterization methods developed in this thesis, two important OLED device physics conclusions are obtained: (1) Hole accumulation at the ETL/HTL interface plays an important role in establishing balanced charge injection of electrons and holes into the OLED. (2) The ETL behaves as a leaky insulator while the HTL more efficiently conducts charge and acts as a voltage-dependent resistor. A preliminary investigation of the aging properties of OLEDs is presented as further evidence of the utility of the novel characterization methods developed in this thesis. In general, aging is characterized by a softer turn on of the forward bias portions of i(t)-v(t) and b(t)-v(t) curves. Also, some aging recovery is possible if the OLEDs are subjected to a zero or reverse bias.
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