Bandgap states in solution-processed semiconductors

Abstract

The field of plastic electronics has opened up a new material set with which to produce microelectronics including metal oxides, polymers and small molecules. These materials are versatile in properties and processing techniques, already outmatching amorphous silicon. Thin film transistors (TFTs) produced from these materials state mobility as the highest figure of merit; while ignoring the effect that trap states on charge transport. The two are inextricably linked though as regularly observed in the gate dependence of measured field effect mobilities. This thesis presents an in-depth analysis of bandgap trap states within low-temperature, and solution-processed, high performance phototransistors and low voltage TFTs. This thesis first discusses the Grünewald bandgap analysis method used to calculate semiconductor bandgap states from a single TFT measurement. The second section demonstrates solution-processed, low temperature (≤ 200 °C) dyed-sensitized thin-film phototransistors consisting of indium oxide (In2O3) and the organic dye D102. Devices exhibit an ultrahigh photosensitivity of 10^6 and responsivity of 2x10^3 A/W. Bandgap analysis identified photoinduced n-doping of the channel as the likely mechanism. The final section presents a direct comparison of four high-k dielectric layers used as insulators within In2O3 based TFTs. An identical low-temperature (≤ 200 °C), solution-processed route was used to produce aluminium oxide (AlOx), hafnium oxide (HfOx), yttrium oxide (YOx), and zirconium oxide (ZrOx) films. The usage of AlOx, HfOx & ZrOx dielectric layers resulted in functioning TFTs with on/off ratios of 10^5, operating below 3 V. The In2O3 mobility exhibited a dielectric dependence with average values of 2.0, 6.4, and 18.7 cm2/Vs for AlOx, HfOx and ZrOx respectively. The bandgap analysis was used to eliminate trap states as a possible cause for this dielectric dependent mobility. In conclusion, bandgap analysis of current semiconducting materials greatly improves the understanding of potential candidates for future high performance solution processed microelectronics.