Science and applications of infrared semiconductor nanocrystals

Abstract

In this work we study several applications of semiconductor nanocrystals (NCs) with infrared band gaps. In the first half, we explore the physics of two systems with applications in NC based photovoltaics. The physics of mixed films of CdTe and CdSe NCs is studied in chapter 2 as a model for NC based bulk heterojunction photovoltaics. We demonstrate that the presence of an active electron trap on the CdTe dramatically reduces the electron mobility in mixed films. The trapping state is linked to oxidation of the CdTe NCs. A cadmium oleate treatment is shown to reduced the oxidation rate. In chapter 3, we present a method to switch the carrier type of InAs NCs deposited in a thin film from p-type to n-type by the addition of cadmium. This provides a stable pre-deposition technique to control the NC carrier type and is a step towards pn homojunction based NC devices. We discuss the role that surface passivation and substitution doping may play in determining the carrier type. The second half explores the use of NCs for photodetector applications. Chapter 4 presents our efforts to move from a single pixel, proof of principle PbS NC infrared detector to a large area infrared imaging camera. A method to control the resistivity of the NC film through oxidation and re-treatment with ethanedithiol is presented. This allows for integration of our NC film with existing read out technology. The noise spectrum is shown to be dominated by 1/f noise and the dependence of the noise on the bias and channel length is determined. The detectivity is found to be determined by the carrier lifetime and dark current carrier density. In chapter 5, we demonstrate efficient UV-IR dual band detectors based on luminescent down conversion. In this design, NCs absorb UV light and re-emit the light in the infrared band of an InGaAs detector. The high quantum yields of infrared nanocrystals and unique absorption profile are shown to provide a significant advantage over organic dyes. The bandwidth of the detectors is measured and the effect of the down conversion layer on the spatial resolution is characterized.