Organic thin-film transistors: A review of recent advances
by C. D. Dimitrakopoulos and D. J. Mascaro
In this paper we review recent progress in materials, fabrication processes, device designs, and applications related to organic thin-film transistors (OTFTs), with an emphasis on papers published during the last three years. Some earlier papers that played an important role in shaping the OTFT field are included, and a number of previously published review papers that cover that early period more completely are referenced. We also review in more detail related work that originated at IBM during the last four years and has led to the fabrication of high-performance organic transistors on flexible, transparent plastic substrates requiring low operating voltages.
For more than a decade now, organic thin-film transistors (OTFTs) based on conjugated polymers, oligomers, or other molecules have been envisioned as a viable alternative to more traditional, mainstream thin-film transistors (TFTs) based on inorganic materials. Because of the relatively low mobility of the organic semiconductor layers, OTFTs cannot rival the performance of field-effect transistors based on single-crystalline inorganic semiconductors, such as Si and Ge, which have charge carrier mobilities (µ) about three orders of magnitude higher . Consequently, OTFTs are not suitable for use in applications requiring very high switching speeds. However, the processing characteristics and demonstrated performance of OTFTs suggest that they can be competitive for existing or novel thin-film-transistor applications requiring large-area coverage, structural flexibility, low-temperature processing, and, especially, low cost. Such applications include switching devices for active-matrix flat-panel displays (AMFPDs) based on either liquid crystal pixels (AMLCDs)  or organic light-emitting diodes (AMOLEDDs) [3, 4]. At present, hydrogenated amorphous silicon (a-Si:H) is the most commonly used active layer in TFT backplanes of AMLCDs. The higher performance of polycrystalline silicon TFTs is usually required for well-performing AMOLEDDs, but this field is still in the development stage; improvements in the efficiency of both the OLEDs and the TFTs could change this requirement. OTFTs could also be used in active-matrix backplanes for “electronic paper” displays  based on pixels comprising either electrophoretic ink-containing microcapsules  or “twisting balls” . Other applications of OTFTs include low-end smart cards and electronic identification tags.
There are at least four ways in which a new, exploratory technology such as OTFTs can compete with or supplement a widely used, entrenched technology such as (a-Si:H) TFTs, for which many billions of dollars have already been invested:
By far surpassing the performance of the entrenched technology and offering a substantial performance advantage.
By enabling an application that is not achievable using the entrenched technology, taking advantage of one or more unique properties or processing characteristics of OTFTs. An example for Case 2 could be a flexible AMFPD fabricated on a plastic substrate. Because of the high processing temperature used in a-Si:H deposition (approximately 360°C), which is required for the fabrication of well-performing a-Si:H TFTs, it is not possible to fabricate an AMLCD based on such TFTs on a transparent plastic substrate. OTFTs, which can be processed at or close to room temperature and thus are compatible with transparent plastics, are an enabling technology which complements the entrenched technology instead of competing with it.
By significantly reducing the cost of manufacturing OTFTs as compared to mainstream TFTs while delivering similar performance.
By leveraging a potential reduced cost advantage to create a new way of using an existing application, or to change the usage pattern or user habit for an existing application, even if performance is lower than that of the entrenched technology. An example for Case 4 could be a large-area AMFPD which uses a backplane comprising OTFTs that have been fabricated using very low-cost processes compared to a-Si:H TFTs. Because of its significantly reduced cost, such a display could have a substantially reduced lifetime compared to a conventional AMFPD, since the user would be able to replace it several times over a period equal to the lifetime of a more expensive, conventional AMFPD.