Project Report

The organic light emitting diode display market reached approximately $1.25 billion in 2010 with expected sales exceeding $2.2 billion by 2014. Organic light emitting diode lighting applications are expected to exhibit similar market shares in the near future and, by 2030, adoption of organic light emitting diode lighting systems in place of conventional lighting (e.g. fluorescent, incandescent, high intensity discharge lamps) is predicted to save between 1.39 and 1.77 quads of energy per annum. To put this savings into perspective, one quad of energy is approximately equal to the annual per capita energy consumption of 2.9 million Americans or, alternatively, it is equivalent to 167 million barrels of oil. In 2010, one quad of energy used to generate electricity for lighting cost approximately $8.3 billion. Naturally, by 2030 the cost per unit of electricity will be higher, thus monetary savings, as a fraction of gross domestic product, will increase beyond those of equivalent energy savings today. However, in order for such savings to be realized, improvements in light emitting diode efficiency and lifetime must occur. Modern organic light emitting diodes typically rely upon a multilayer device architecture that facilitates the injection of holes and electrons from a cathode on one face of the device and an anode on the opposing face respectively. The opposite charge carriers then recombine in the center of the device to emit light after moving through conductive transport layers; however, the conductivity of the opposing transport layers must be reasonably well matched to ensure that the holes and electrons meet near the center of the device. If either charge carrier moves substantially faster than the other does, charge recombination will occur in the wrong region of the device, leading to poor efficiency or even a total lack of light emission. This project has demonstrated a new method for controlling charge carrier mobility. Molecules which easily accept and transport charges were synthetically attached to polymers at precisely defined distance intervals, which were varied within a set of polymers. Since the charge hopping rate for conduction of a charge between any two adjacent pendant molecules strongly depends on the distance between them, polymers with the smallest distance intervals exhibited the highest charge mobilities. Moreover, the mobilities were tunable over a range of three orders of magnitude. The concept demonstrated here may also be applied to charge transporting molecules other than the ones used in this study, thereby allowing independent optimization of several parameters important to organic light emitting diode design and potential improvement of device efficiency and performance.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Aitken Brian S
United States
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