Graduation Year

2004

Document Type

Thesis

Degree

M.S.E.E.

Degree Granting Department

Electrical Engineering

Major Professor

Morel, Don L.

Keywords

molybdenum, sodium, backcontact, gallium

Abstract

The goal of this research was to understand the role of the glass substrate and molybdenum (Mo) back contact on the performance of Copper Indium Gallium diselenide (CIGS) / Cadmium Sulphide (CdS) based photovoltaic devices, and to improve the performance of these devices. The CIGS absorber layer was fabricated in a 2 stage process. In this process the metal precursors were deposited at 275oC followed by a high temperature selenization step. The advantage of the 2 stage process is that it is manufacturing friendly. The first step in fabrication of solar cells is to clean the substrate which is necessary to obtain good device performance. A variety of environmentally friendly solvents were evaluated, to determine the optimal cleaning agent. At elevated temperatures of processing sodium tends to diffuse out of Soda lime glass (SLG) and enter the semiconductor. The presence of this sodium during CIGS fabrication is necessary to obtain high efficiency CIGS based solar cells.

A silicon nitride barrier layer was sputtered onto the SLG substrates, and this substrate was used to make complete devices. The CIGS absorber layer was deposited by the Type I recipe in two different vacuum systems.These devices were compared with standard devices the Si₃N₄ barrier layer, to understand the role of sodium on the devices fabricated from both of the systems. Furthermore, the influence of molybdenum processing parameters, such as thickness and rate of sputtering, on device performance were studied. The Voc of devices fabricated using the Type I process was limited to 460mV. In order to improve the Voc's a new absorber recipe (Type IV) was developed. Voc's of upto 490mV, Jsc's of upto 37.4mA/cm² and FF of 64%, were obtained. This improvement in performance was due to incorporation of gallium in the space charge region. Techniques such as I-V measurements, spectral response, SEM and EDS measurements were used to characterize the devices.

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