Phosphorescent Organic Light-emitting Devices to Sense Contact Stresses
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Phosphorescent Organic Light-emitting Devices to Sense Contact Stresses X. A. Cao and Y. Q. Zhang Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, USA ABSTRACT We studied the electrical and optical responses of organic light-emitting devices (OLEDs) with green and red phosphorescent dyes doped in a polymer matrix to compressive stresses. The green OLED converted stresses as low as 6.8 kPa into measurable and reversible changes in both current density and electroluminescence (EL) intensity. The current showed a nearly linear characteristic response with sensitivity up to 205 μA kPa-1, whereas the EL intensity decreased by over three orders of magnitude at 107 kPa. In contrast, stress-induced modulations in current and light intensity were noticeable in the red OLED only above 160 kPa. The discrepancy has been attributed to different rates of stress-enhanced back exciton energy transfer between guest and host molecules, which quenches the EL of the green OLED, but has a much smaller impact on the performance of the red OLED. It is expected that similar green phosphorescent OLEDs built on large curved surfaces may directly image stress distributions and sense touch on a par with a human finger. INTRODUCTION Many robots have the dexterity required to perform some of the tasks that we take for granted, but replication of the full manipulative capabilities of the human hand is still years away [1,2]. To improve the manipulative capabilities of robotic hands, more sophisticated tactile sensors are needed, which can be mounted on a curved surface and sense a distribution of contact stresses as low as 10 kPa at a high spatial resolution of about 40 µm over a contact area of ~1 cm2 [2]. Many technologies have been explored for tactile sensing, including conducting elastomers [3], piezoresistive materials [4,5], and MEMS systems [6]. However, their use in dexterous hands is hampered by: (i) poor spatial resolution lagging by one order of magnitude compared to a human finger; and (ii) strain-induced nonuniform background signals in devices built on curved surfaces due to the lack of flexibility. Organic materials are flexible and sensitive to external stresses. Under compression, their resistivity may change dramatically due to reduced intermolecular distance and increased orbital overlap which lead to higher rates of electron transfer between neighboring molecules [7,8]. This sensitive piezoresistive response to deformation makes them suitable for stress sensing applications [9,10]. In addition, many small-molecule organic materials have superior phosphorescent properties, and have been used to develop high-efficiency organic light-emitting devices (OLEDs) [11-14]. The luminescent efficiency of OLEDs is largely dependent upon carrier tunneling and energy transfer processes, whose rates are strong functions of the intermolecular distance [11-14]. Therefore, organic thin film structures may respond to applied forces with changes in current density as well as luminescent e
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