In comparison to exciton diffusion, they can be expected to be in the nanometre range, at least for typical disordered organic films 23, 24, 25. Charge carrier minority diffusion lengths have remained unexplored in OSC materials until now. Majority carrier diffusion length in fullerenes has been estimated to be on the centimetre scale, raising interesting questions about carrier diffusion physics in OSCs 21, 22. Most studies have addressed exciton diffusion, which dominates owing to the weak dielectric screening in organic compounds 19, 20. However, organic bipolar junction transistors (OBJTs) have not yet been realized, mainly because they rely on minority carrier diffusion through a thin and precisely doped base layer. Although they have disadvantages with regard to miniaturization and process integration, bipolar transistors possess substantially higher operational speeds than comparable field-effect devices 18. However, other factors, such as contact resistance and overlap capacitances, often limit further improvement of operational frequencies 16, 17.Ī device that offers both low capacitance and contact resistance is the bipolar junction transistor. Reducing the length of transistor channels is an effective strategy for improving the operational speed of the device, as shown both in FET 13, 14 and other device concepts such as organic permeable-base transistors 11, 15. The substantially lower charge carrier mobility in organic semiconductors (OSCs) compared with their inorganic counterparts is a limitation to the performance of organic transistors. Nevertheless, they are still restricted to the low-to-medium megahertz range, which does not allow broad application 12, 13, 14.
Organic field-effect transistors (FET) were first reported in 1986 and have shown impressive improvements in the past two decades 4, 5, 6, 7, 8, 9, 10, 11. Our results open the door to new device concepts of high-performance organic electronics with ever faster switching speeds. These bipolar transistors also give insight into the minority carrier diffusion length-a key parameter in organic semiconductors. Here we present organic bipolar transistors with outstanding device performance: a previously undescribed vertical architecture and highly crystalline organic rubrene thin films yield devices with high differential amplification (more than 100) and superior high-frequency performance over conventional devices. The potential of organic electronics can be leveraged only if the performance of organic transistors is improved markedly. Among materials systems suitable for thin-film electronics, organic semiconductors are of particular interest their low cost, biocompatible carbon-based materials and deposition by simple techniques such as evaporation or printing enable organic semiconductor devices to be used for ubiquitous electronics, such as those used on or in the human body or on clothing and packages 1, 2, 3. Devices made using thin-film semiconductors have attracted much interest recently owing to new application possibilities.
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