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Technology of semiconductor heterostructures for hot carrier solar cells

Solar energy is generated at the Sun in form of heat. Thermodynamics proves that the most efficient heat engine is based on Carnot process. In this process, it is necessary that gas molecules have continuous distribution of energy. Therefore, any attempt to introduce an energy gap leads to drastic decrease of maximum achievable efficiency. However usual solar cells are based on semiconductors having a band gap.

There are very few approaches in the contemporary photovoltaics, which would avoid utilization of the energy of only charge carriers, just separated by one or several discrete band gaps. An alternative approach of “hot carrier” solar cells is an attempt to build a heat engine based on hot electron gas instead of the hot molecular gas. Electrons in a solid, heated by the Sun, expand in the structure, doing work against the electric force.

While the idea is now about 30 years old, very few implementations are known so far. The reason is obviously unusual requirements on the materials system, which the conventional semiconductor technology fails to meet. The aim of the current project is twofold: extending the existing theoretical ideas towards readily implementable device structures and extending the existing semiconductor technology towards the properties, required by hot carrier solar cells. At the current stage, a special double heterojunction device structure is identified being capable of hot carrier utilization. On the other hand, electrochemical epitaxy of PbSe on InP was successful to provide structures, showing photocurrent due to interaction of electrons originating from optical excitation at an energy of 0,66 eV with hot electrons from simultaneous optical excitation at 2,4 eV.