Efficiency analysis for multi-junction PV hetero-structures

Our life is based on production and utilization of energy. We burn down a fossil fuel to heat and light up our homes, prepare food, and for quick travels. However, the fossil fuel resources are limited and energy production technologies affect our global climate and the ecological state of our environment. The most promising energy source which enables us to replace the existing fuel source is the Sun. In average, the Earth's surface obtains about 1.2×10¹⁷ W of solar power. Thus, our main concern now is the utilization of this energy in the most efficient way. The key solution for this problem is the photovoltaic effect which converts the solar irradiation into electricity. Two main factors prevent us presently for a wide application in the photovoltaic effect, i.e. conversion of solar energy to electricity: a. the cost of semiconductor materials, b. the low efficiency of the existing solar cells. These factors lead us to a photovoltaic device solution in the shape of laminated hetero-junction semiconductor structure. However, plurality of thin film semiconductor materials and methods of combination requires a preliminary evaluation of these materials, preparation methods and proper matching of their physical properties. Moreover, such evaluation methods should predict the basic parameters of the prospective photovoltaic devices, e.g. their short circuit current, open circuit voltage, and their efficiency. Taking into account the economical factors of PV devices through a theoretical evaluation leads to a necessity in concentrating systems. Sun light concentration can drastically reduce the cost of photovoltaic devices. However, high concentration affects on the heating component in the semiconductor device. Therefore, the PV evaluation method should enable estimation of optimal concentration techniques. In this chapter we describe an evaluation approach for the estimation of PV -photovoltaic device efficiency, and methods for semiconductor materials selection. This evaluation method is based on rouge estimation of the charged carriers generation rate, for each step in constructing a multi-junction system. This method was approved for the evaluation of heterojunction systems based on ZnO-ZnSe-Si-Ge structure, and is compatible for various solar cell structures. Methods of concentration optimization will be considered as well.