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Abstract
The current increase in the demand for renewable energies has led to a fast growth of solar cells mass production over the past few years. Even though solar photovoltaic is currently the fastest growing renewable energy market, the cost per watt figure of solar cells is still high compared to conventional energy sources.
To decrease the cost per watt ratio of solar cells two basic approaches can be undertaken: the first one is to decrease the cost of the devices, using cheaper deposition techniques and materials; the other is to increase the efficiency of the cells, keeping the costs below an acceptable limit.
The hot carrier solar cell is a promising third generation photovoltaic device which, consenting collection of highly energetic photogenerated carriers, allows energy conversion efficiencies up to 60%. The efficiency gain is realized minimizing the losses due to poor conversion efficiency of photons with energy above the bandgap of the absorber. This represents the main energy loss mechanism in conventional solar cells, accounting for about 40 percent of the total losses. The two main building blocks of a hot carrier solar cell are: the absorber, were electrons and holes are photogenerated, and the energy selective contacts, which allow extraction of carriers to the external circuit in a narrow range of energies.
In this thesis several theoretical and experimental aspects regarding the design and the realization of a hot carrier solar cell are discussed in details. Limiting efficiencies of the device have been calculated using a complex theoretical model. A maximum efficiency of 43% has been calculated considering a 1000 times concentrated radiation for a hot carrier solar cell with an indium nitride absorber.
The velocity of carrier cooling in III-V compound semiconductors has been investigated using time resolved photoluminescence experiments. Hot carrier cooling transients of gallium arsenide, indium phosphide and indium nitride samples have been studied, confirming that the hot phonon effect has a major role for hot carriers relaxation and that the velocity of the cooling process is strictly related to material quality.
In addition, the possibility of realizing energy selective contacts based on an all-silicon structure is studied in details. Structures consisting of a single layer of silicon quantum dots in a silicon dioxide matrix have been deposited and characterized in order to investigate their potential to be utilized as selective energy contacts for hot carrier solar cells.