Flexible Electronics Based on Solution Processable Organic Semiconductors and Colloidal Semiconductor Nanocrystals

Solution-processable semiconductors hold great potential for the large-area, low-cost fabrication of flexible electronics. Recent advances in flexible electronics have introduced new functional devices such as light-weight displays and conformal sensors. However, key challenges remain to develop flexible devices from emerging materials that use simple fabrication processes and have high-performance. In this thesis, we first use a solution-processable organic semiconductor to build field-effect transistors on large-area plastic with mobility of 0.1 cm^2/Vs. Combined with passive components, we are able to build voltage amplifiers to capture few mV amplitude bio-signals. This work provides a proof of concept on applying solution processable materials in flexible circuits. In the second part of the thesis, we introduce colloidal CdSe nanocrystals (NCs) as solution-processable "inks" of semiconductor thin film devices. By strongly coupling and doping the CdSe NC thin films, we demonstrate high-performance, flexible nanocrystal field-effect transistors (NC-FETs) with mobility greater than 20 cm^2/Vs under 2V supply. Using these NC-FETs as building blocks, we demonstrate the first flexible nanocrystal integrated circuits (NCICs) with switching speed of 600 µsec. To design reliable integrated circuits with low-noise, we characterize the flicker noise amplitude and origin. We find the figure of merit for noise, the Hooge parameter, to be 3 x 10^-2 for CdSe NC-FETs, comparable to other emerging solution processable organic semiconductors and promising for low-noise circuit applications.As most of NCs are reactive and their devices tend to degrade in air, we develop processes that allow manipulation of the NCs in ambient atmosphere without compromising device performance. These processes open up opportunities for NC-based devices to be fabricated over large area using photolithography. By scaling the devices and reducing device parasitics, we are able to fabricate hundreds of NC-FETs on wafer-scale substrates and integrate them as circuits. We demonstrate voltage amplifiers with bandwidths of a few kHz and ring-oscillators with a stage delay of 3 µsec. We also show functional NCICs NOR and NAND logic. This thesis demonstrates the use of colloidal NCs to realize flexible, large-area circuits and the feasibility of more advanced analog and digital NCICs built on flexible substrates for various applications.