UC San Diego

Amorphous semiconducting oxides are attractive alternatives to silicon for implementing thin-film transistors (TFTs) in large-area electronics, because of the oxides’ promising electron mobility, optical transparency in the visible spectrum, and compatibility with low-cost solution processing. Due to these advantages, it has been widely utilized as active layer of active matrix in display.

Firstly, in chapter 2, we fabricate indium zinc oxide thin film transistor by inkjet printing. To enhance the semiconductor performance and stability, indium zinc oxide (IZO) is tuned by charge-transfer molecular doping on the film surface. An air-stable, strongly reducing molecule benzyl viologen (BV) is used to induce charge-transfer doping of the indium zinc oxide semiconductor in inkjet-printed thin-film transistors. The device mobility is improved from 5.8 ±1.4 cm2(V·s)-1 in the undoped devices and reached up to 8.7 ±1.0 cm2(V·s)-1 after BV treatment. Through measurement of frequency-dependent admittance and capacitance, we quantify the density of interface states, and show that interfacial trap density is four times lower in the BV-doped transistors compared to un-doped devices.

In chapter 3, we demonstrate high-performance infrared phototransistors that uses a broadband organic bulk heterojunction (BHJ) responsive from the visible to the infrared, from 500 nm to 1400 nm. The device structure is based on a bilayer transistor channel that decouples the photogeneration and charge transport, and thus enabling independent optimization of each process. The organic layer is improved by incorporating highly polarizable camphor to increase carrier lifetime, and IZO with high electron mobility is employed for rapid charge transport. The phototransistors achieve a dynamic range of 127 dB and reach detectivity up to 51012 Jones under low light condition around 20 nW/cm2, which outperforms commercial germanium photodiodes in the spectral range below 1300 nm. The photodetector metrics are measured with respect to the temporal bandwidth, applied voltage, and incident light power. In particular, the frequency and light dependence of the phototransistor characteristics are analyzed to understand the change in photoconductive gain under different working conditions.