Electrolyte-Gated Organic Thin-Film Transistors
نویسنده
چکیده
There has been a remarkable progress in the development of organic electronic materials since the discovery of conducting polymers more than three decades ago. Many of these materials can be processed from solution, in the form as inks. This allows for using traditional high-volume printing techniques for manufacturing of organic electronic devices on various flexible surfaces at low cost. Many of the envisioned applications will use printed batteries, organic solar cells or electromagnetic coupling for powering. This requires that the included devices are power efficient and can operate at low voltages. This thesis is focused on organic thin-film transistors that employ electrolytes as gate insulators. The high capacitance of the electrolyte layers allows the transistors to operate at very low voltages, at only 1 V. Polyanion-gated pchannel transistors and polycation-gated n-channel transistors are demonstrated. The mobile ions in the respective polyelectrolyte are attracted towards the gate electrode during transistor operation, while the polymer ions create a stable interface with the charged semiconductor channel. This suppresses electrochemical doping of the semiconductor bulk, which enables the transistors to fully operate in the field-effect mode. As a result, the transistors display relatively fast switching (! 100 "s). Interestingly, the switching speed of the transistors saturates as the channel length is reduced. This deviation from the downscaling rule is explained by that the ionic relaxation in the electrolyte limits the channel formation rather than the electronic transport in the semiconductor. Moreover, both unipolar and complementary integrated circuits based on polyelectrolyte-gated transistors are demonstrated. The complementary circuits operate at supply voltages down to 0.2 V, have a static power consumption of less than 2.5 nW per gate and display signal propagation delays down to 0.26 ms per stage. Hence, polyelectrolyte-gated circuits hold great promise for printed electronics applications driven by low-voltage and low-capacity power sources. Populärvetenskaplig Sammanfattning I slutet av 1970-talet fann man att det var möjligt att göra vissa typer av polymerer (plaster) elektriskt ledande. Denna upptäckt lade grunden till ett helt nytt forskningsområde, i gränslandet mellan fysik och kemi, kallat organisk elektronik. Efter många år av forskning och utveckling är det idag möjligt att tillverka en mängd olika elektroniska komponenter, som t.ex. transistorer, lysdioder och solceller, av organiska material. Ett exempel på en produkt som redan tagit sig ut på marknaden är bildskärmar baserade på organiska lysdioder (OLED). En fördel med organiska material är att de ofta kan lösas upp i lösningsmedel, vilket gör det möjligt att använda traditionella tryckmetoder för masstillverkning av elektroniska komponenter och kretsar på flexibla substrat till en mycket låg kostnad. Många av de tilltänkta produkterna kommer att använda sig av tryckta batterier eller solceller som spänningskällor. De ingående elektroniska komponenterna, t.ex. organiska transistorer, bör därför kunna drivas med låga spänningar och vara strömsnåla. Den här avhandlingen är fokuserad på organiska tunnfilmstransistorer (TFT) i vilka det isolerande skiktet mellan gate-elektroden och halvledaren utgörs av en jonledande elektrolyt. Elektrolytskiktet, mellan gate och halvledare, erbjuder extremt hög kapacitans vilket gör det möjligt att använda väldigt låga spänningar (~1 V) för att driva denna komponent. En risk med att använda elektrolyter i organiska transistorer är att joner från elektrolyten kan tränga in i den organiska halvledaren och göra transistorn svår att styra. Detta problem har jag lyckats undvika genom att använda polyelektrolyter, material där en av jonerna representeras av en polymer. Både p-kanalsoch n-kanalstransistorer kan tillverkas med polyelektrolyter som isolatormaterial. Det har gjort det möjligt att också tillverka tryckbara, snabba och strömsnåla logiska kretsar med hjälp av komplementär kretsdesign. Dessa transistorer är väl lämpade för att användas inom tryckt elektronik.
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