Charge and Exciton Dynamics in Organic Optoelectronic Devices

Abstract

Organic optoelectronics use carbon-based molecules to interface between light and electrical signals. The operation of these devices is determined by the dynamic behaviors of their charges and excited states. For example, organic light-emitting diodes use injected electrical charges to form excited states that, in turn, emit light. Organic photovoltaics and photodetectors operate by the reverse process. Understanding the dynamics of charges and excited states is crucial to designing high performance devices.

The first part of this thesis focuses on understanding charge and exciton dynamics in organic light emitting devices. First, charge balance and exciton confinement in blue-emitting phosphorescent organic light-emitting diodes are studied using sensitizer methods and an analytical model based on drift-diffusion transport. We find that triplet excitons leak into the hole transporting layer at high current densities and improve device performance by incorporating a high triplet energy blocking layer to prevent such leakage. The impact of changes in charge balance and exciton confinement on the lifetime of blue phosphorescent organic light emitting diodes is also investigated. We find that that contribution of loss of charge balance is negligible, and that increased exciton leakage is responsible for less than 4% of luminance loss. The understanding gained in these studies is then applied to the design of a highly reliable stacked white-emitting device for solid state lighting. These devices employ red-emitting blocking layers as well as highly stable, low voltage charge generation layers. A five-stack device achieves 2780 K coordinated color temperature with a high color rendering index of 89 and 80±20 krs lifetime (T70, 1000 cd/m2).

The second part focuses on charge diffusion in organic heterostructures laterally, i.e, in plane with the thin film. Because of the low charge mobilities of organic semiconductors, organic devices are typically thin with negligible lateral charge transport. We show that charge can be transported laterally across centimeters in certain organic heterostructures. This phenomenon arises from the combination of a trap-free, high diffusivity channel material and energetic confinement of carriers that prevents rapid recombination. The confining energy barrier arises from a polarization shift of the acceptor material when blended with a highly dipolar donor. Lateral transport heterostructures are then used to develop the first organic charge-coupled devices. We observe clear charge-coupled transport of photogenerated charge packets in a linear four-pixel shift register. Calculations indicate that millisecond readout times are possible using many-pixel organic charge-coupled sensors, and strategies for the improvement of these devices are discussed.