MOVPE Growth and Study of III-V Multi-Junction Structures for Advanced Photovoltaic Applications

Abstract

The energy question is one of the main problem of modern society and is particularly urgent because of the drawbacks of fossil fuel exploitation, which provide 80% of the total energy we currently consume. The only realistic and far-seeing solution to current energy crisis is represented by the employment of renewable energy sources, assisted by global energy conservation policies. Photovoltaic represents one of the most interesting renewable technologies, since it’s the only one that can convert solar energy directly into electricity, without the use of any moving parts. Solar radiation, moreover, is abundant, inexhaustible and diffuse all over the world. Since the 1950s, when the first silicon solar cell was produced, three generations of devices were conceived with the purpose of improving the production cost/conversion efficiency ($/W) ratio to become market-competitive. In particular, the third generation is based on innovative devices that mean to exceed the theoretical efficiency limit for single junction p-n solar cells, by reducing their main energy loss mechanisms. This PhD thesis deals with the study of two types of third generation structures based on multiple band gaps. The first structure is based on a InGaP p-i-n junction with an intrinsic region consisting of 30 periods of 8 nm thick GaAs quantum wells (QW) and 12 nm thick InGaP barriers, conceived to be part of a quantum well solar cell (QWSC). This heterostructure was grown by a low pressure MOVPE reactor, with the employment of liquid alternative metalorganic precursors for the group V elements, terbutylarsine (TBAs) and terbutylphosphine (TBP). In particular, it was investigated the light response of the structure by an accurate photoelectric spectroscopy (PES) study: both the photocurrent (PC) and photovoltage (PV) signals were detected by a standard lock-in technique, as a function of the wavelength, at different sample temperatures and for different frequencies of the exciting light, modulated by a chopper. A second type of photovoltaic structure was designed and realized, consisting in a relatively simple monolithic GaAs-based tandem structure grown on GaSb substrates. By taking advantage of the high temperature of the growth process (T=600-650° C), the deposition of a highly Zn doped GaAs layer enabled the Zn diffusion into the Te-doped (n-type) GaSb substrate, forming a buried GaSb p-n homojunction. By depositing additional GaAs layers with appropriate doping levels, a tunnel and a top junction were stacked to obtain the final tandem structure. The originality of the proposal is related both to the method employed to activate the Zn diffusion in GaSb, and also to the assessment of the GaAs-on-GaSb epitaxial growth. The possibility to realize a tandem cell by properly modulating the doping of the same compound (GaAs), thus making the fabrication process very simple, is the main advantage of this structure.