Metal Modified Ge-Se Glass Films and Their Potential for Nanodipole Junctionless Photovoltaics

In this project, we studied Bi-Ge-Se and Zn-Ge-Se chalcogenide glass films as potential materials for novel, Junctionless nanodipole photovoltaic (PV) applications. The principle of operation of junctionless nanodipole PV devices is based on the use of nanocrystals, with uncompensated electric dipole moments, which are dispersed in a photoconductive host medium. The electric field of these nanodipoles serves to separate the photogenerated electron-hole pairs and thus generate photocurrent.

Bulk samples of these chalcogenide materials were prepared using a conventional melt quench technique, and thin films were then deposited by a thermal evaporation technique under high vacuum condition from the prepared bulk samples. The thin films were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X-Ray diffraction (XRD) and Raman spectroscopy.

Both types of glasses are based on the Ge-Se binary glass system. The as-deposited thin films prepared were amorphous, or glassy, in nature and had chemical compositions that were close to what is considered intermediate-phase Ge-Se glass with additions of small amounts of Bi or Zn. The glass transitions and the crystallization tendency of both the bulk materials and the thin films were studied by DSC. In an effort to induce the formation of a crystalline, and preferably, a nano-crystalline phase, in the glass, glass samples were annealed at several different temperatures in the vicinity of their measured glass transition temperatures. The results from our XRD and Raman spectroscopy measurements indicated that upon annealing of Bi-Ge-Se films at 210°C, nanocrystals of Bi₂Se₃ are formed in the Bi-Ge-Se glass. At higher temperatures, c-GeSe and c-GeSe₂ formed as well and the glass material crystallizes too much for the purposes of nanodipole PV. The Bi₂Se₃ crystallites formed by disturbing the Ge-Se and Se chain structure are observed in the Raman spectra of the films. Thus we are confident we were able to obtain Bi₂Se₃ nanocrystals embedded in the Ge-Se glass matrix.

The results from our electrical measurements and specifically open-circuit voltage (V_oc) measurements upon illumination, of both as-deposited and annealed Bi-Ge-Se films can be interpreted in terms of the formation of nanodipole composite and this indirectly confirms the structural characterization results.

In the case of Zn-Ge-Se glasses, we have found that partial crystallization was difficult or impossible to induce, and no photovoltage could be measured.

The main result of these initial studies is that chalcogenide-glass-based systems, such as Bi-Ge-Se, provide a combination of material parameters and processing flexibility, which are attractive for the implementation of novel nanodipole PV devices. This work is expected to stimulate further research on the topic and lead to significant findings, and potentially, to the development of new technological solutions.

This thesis consists of two main parts. In the first part, Chapter 3, a detailed literature review is provided on the Bi-Ge-Se and Zn-Ge-Se glass systems, together with several other chalcogenide systems, which would also be of interest to such applications. In the second main part, Chapter 5, we present and discuss our experimental results. In addition, the thesis has a short introductory chapter, and a chapter that briefly reviews the experimental techniques used. The last chapter gives the conclusions from our work.