Printed Organic Thin Film Transistors, Photodiodes, and Phototransistors for Sensing and Imaging
Light and image sensors are ubiquitous and used in a wide variety of applications ranging from consumer products to healthcare and industrial applications. The signal-to-noise ratio (SNR) from a photodetector element increases with larger photoactive area, which is costly to scale up using silicon wafers and wafer-based microfabrication. On the other hand, the performance of solution-processed photodetectors and transistors is advancing considerably. It is proposed that the printability of these devices on plastic substrates can enable low-cost areal scaling for high SNR light and image sensors.
This thesis advances the performance of printed organic thin film transistor (OTFT), pho- todiode (OPD), and phototransistor (OPT) devices optimized for light and image sensing applications by developing novel printing techniques and creating new device architectures. An overview is first given on the essential figures of merit for each of these devices and the state of the art in solution-processed image sensors. A novel surface energy-patterned doc- tor blade coating technique is presented to fabricate OTFTs on flexible substrates over large areas. Using this technique, OTFTs with average mobility and on-off ratios of 0.6 cm^(2)/Vs and 10^(5) are achieved, which is competitive with amorphous silicon TFTs. High performance OPDs are also fabricated using doctor blade coating and screen printing. These printing processes give high device yield and good controllability of photodetector performance, enabling an average specific detectivity of 3.45×10^(13) cm·Hz^(0.5)·W^(-1) that is higher than silicon photo- diodes (10^(12-13)). Finally, organic charge-coupled devices (OCCDs) and a novel OPT device architecture based on an organic heterojunction between a donor-acceptor bulk heterojunction blend and a high mobility semiconductor that allows for a wide absorption spectrum and fast charge transport are discussed. The OPT devices not only exhibit high transistor and photodetector performance, but are also able to integrate photogenerated charge at video frame rates up to 100 frames per second with external quantum efficiencies above 100%. Applications of these devices include screen printed OTFT backplanes, large-area OPDs for pulse oximeter applications, and OPT-based image sensors.