MSE Ph.D. Defense – Jaehan Jung

 Date: Friday, March 13, 2015
Location: MOSE 4100F
Time: 10:00 AM

Committee members:

Dr. Zhiqun Lin (MSE, Advisor)
Dr. Vladimir Tsukruk (MSE)
Dr. Dong Qin (MSE)
Dr. Joseph Perry (CHEM)
Dr. Elsa Reichmanis (ChBE)

Title: ORGANIC-INORGANIC NANOCOMPOSITES FOR RENEWALBE ENERGY CONVERSION DEVICES

Abstract:

Semiconductor inorganic-organic nanocomposites have been widely recognized as  promising materials for energy harversting due to the complementary advatages from organic and inorganic constituents. Recent research has witnessed rapid advances in an emerging field of directly tethering conjugated polymer (CPs) on the nanocrystals (NCs) surface to yield an intimately contacted CP-NC nanocomposite possessing a well-defined interface that markedly promotes the dispersion of NCs within the CP matrix, facilitates the photoinduced charge transfer between these two semiconductor components, and provides an effective platform for studying the interfacial charge separation and transport. However, controling the shape of nanocomposites and acquiring the intimate contact at the interface of donors and acceptors are still callenging. This study is degined to craft semiconductor organic-inorganic nanocomposite materials as photoactive layer composed of CPs as electron donor that are in intimate contact with inorganic NCs as electron accpetor for use in solar cells with improved efficiency.

This thesis aims to not only fully explore optoelectronic properties of NCs but, also craft novel semiconducting CP-grafted NC nanocomposties, which exhibit enhanced optical and electronic properties as well as solution proccessable characteristic. In all studies, cadmium selenide (CdSe) and cadmium telluride (CdTe) were employed and demonstrated as inorganic semiconducting components due to the following reasons. CdSe possesses much higher charge carrier mobility as compared to CPs and fullerene derivatives. In addition, the optoelectronic properties of CdSe (e..g, band gap; Eg = 1.72 eV) can be easily tuned simply by controlling their size owing to the quantum confinement effect. Similarly, CdTe is an appealing inorganic semiconductor for inorganic solar cells due to its optimum band gap energy (Eg = 1.45 eV) and high absorption coefficient (α = 104 cm-1),4 thereby improving the light harvesting efficiency by extending into the near-infrared (NIR) range. Cleary, incorporating these inorganic semiconductor nanocrystals in CPs may result in intriguing optoelectronic properties. For the sake of easy demonstration for optoelectronic devices, P3HT was adopted as organic component for preparing all organic-inorganic nanocomposites investigated in this thesis as it is one of most widely studied CPs and serves as electron donor in photovoltaic applications.

The optical and electronic properties of semiconducting NCs were first optimized by controlling their size, architecture, and composition. The simple strategy to achieve graded shell architecture for highly luminescent, photostable, and Stokes’ shift engineered semiconducting QDs was also explored. This thesis also presented the robust routes to crafting CP-NC nanocomposites via click coupling between two semiconducting constituents by the utilization of bifunctional short ligands. First, in order to produce NCs with desirable architecture such as quantum dots, nanorods, and tetrapods, the effects of the monomer concentration, the amount of short bifunctional ligands, and the type of surfactants on the shape of NCs were studied. In addition, solar cells were fabricated using these CP-NC nanocomposites by spin-casting. The influence of the shape and size of NCs, the types of ligands, and the weight ratio of CPs to NCs on the device performance was explored. Crafting CP-grafted NCs and their utilization in optoelectronic devices also provided insights into the use in photovoltaics as well as a variety of other applications.