Colloidal Quantum Dots and J-Aggregating Cyanine Dyes for Infrared Photodetection

Abstract

The emergence of nanostructured semiconducting materials enables new approaches toward the realization of photodetectors that operate in the technologically important near- and short-wave infrared (NIR and SWIR) spectral ranges. In particular, organic semiconductors and colloidal quantum dots (QDs) possess numerous advantages over conventional crystalline semiconductors including highly tunable optical and electronic characteristics, the prospect for low-temperature processing, and compatibility with inexpensive and exible substrates. Photodetectors based on such materials may lead to low-cost IR focal plane arrays for night-vision imaging as well as a multitude of novel applications in biological and chemical sensing. This thesis describes the development and detailed characterization of several photodetectors that incorporate colloidal QD and organic semiconductor thin films as active layers. The electronic properties of PbS QDs are thoroughly investigated in a field effect transistor (FET) geometry and several QD-based photoconductive structures exhibiting SWIR photosensitivity are demonstrated. We describe a novel QD-sensitized lateral heterojunction photoconductor in which the functions of optical absorption and charge transport are dissociated into different physical layers that can be independently optimized. This structure is advantageous because it obviates the need for aggressive chemical treatments to the QD film that may compromise the quality of QD surface passivation. Photovoltaic device architectures are addressed, noting their advantages of being operable without an external applied bias and at fast response speeds. We present detailed experimental and theoretical characterization of a photovoltaic structure that is sensitized at NIR wavelengths by a J-aggregating cyanine dye. The high absorption coefficient of the J-aggregate film, combined with the use of a reflective anode and optical spacer layer, enables an external quantum efficiency (EQE) of \(16.1\pm0.1\%(\lambda = 756 nm)\) to be achieved at zero-bias in a device that incorporates an \(8.1\pm0.3 nm\)-thick dye film. The merits and drawbacks of the various device architectures and nanostructured material systems are discussed and the outlook for nanostructured photodetectors that exhibit infrared sensitivity is presented.