Date of Award

Summer 2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical/Computer Engineering

Committee Director

Sylvain Marsillac

Committee Member

Hani E. Elsayed-Ali

Committee Member

Gon Namkoong

Committee Member

Gene Hou

Abstract

The global demand for renewable energy is growing rapidly. Increasing the global share of alternative sources of energy would not only bring environmental benefits, but also enhance overall energy security by diversifying energy supply. Many technology options exist nowadays to harvest the power of the sun, a sustainable energy source, and generate electricity directly from this source via the photovoltaic effect. Among them, Cu(In,Ga)Se2 (CIGS) has gained significant momentum as a possible high efficiency and low cost thin film solar cell material. The capacity to scale up any photovoltaic technology is one of the criteria that will determine its long term viability. In the case of CIGS, many manufacturers are showing the way for GW-scale production capacity. However, as CIGS technology continues to increase its share of the market, the scarcity and high price of indium will potentially affect its ability to compete with other technologies. One way to avoid this bottleneck is to reduce the importance of indium in the fabrication of the cell simply by reducing its thickness without significant efficiency loss. Reducing the thickness of CIGS thin film will not only save the material but will also lower the production time and the power needed to produce the cell.

The material properties of Cu(In,Ga)Se2 thin films are different with deposition process. Many different methods to deposit Cu(In,Ga)Se2 thin film have been tried until now but Cu(In,Ga)Se2 thin films prepared by co-evaporation of elemental sources are the most successful due to the control over the sequence of evaporation of individual material. However, the co-evaporation process is a complex process and, depending on the individual sources and substrate temperature, the thin films are grown with different characteristics. The characteristics of these thin films changes with the change in the atomic percentage (at.%) of Cu, In, Ga and Se, which depends on the evaporation conditions. Among different co-evaporation techniques to grow Cu(In,Ga)Se2, 1-stage, 2-stage and 3-stage co-evaporation processes are the most successful processes.

Co-evaporation process is the best technique for highly efficient CIGS solar cell but this process needs a precise control of the elemental composition in the vapor flux in order to achieve high quality material, which is not easy to obtain due to the low sticking coefficient of Selenium. One of the major concerns in Cu(In,Ga)Se2 thin film solar cell fabrication that affect significantly the cell performance during deposition is stoichiometry. CIGS with low Se samples exhibited very low Cu content, additional chalcopyrite phases, very small grain size, and poor solar cell performance. So it is very important to find the minimal selenium flux to obtain high quality Cu(In,Ga)Se2 thin film.

During the deposition of ultrathin Cu(In,Ga)Se2 film with 1-stage, 2-stage and 3-stage co-evaporation processes, real time spectroscopic ellipsometry (RTSE) is implemented to study the material properties as well as to monitor the process. The deposition processed for individual layer of CIGS device are optimized to enhance the efficiency of the CIGS solar cell.

DOI

10.25777/dsrd-mh49

ISBN

9781339126418

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