Abstract
Multiband solar cells are one type of third generation photovoltaic devices in which an increase of the power conversion efficiency is achieved through the absorption of low energy photons while preserving a large band gap that determines the open circuit voltage. The ability to absorb photons from different parts of the solar spectrum originates from the presence of an intermediate energy band located within the band gap of the material. This intermediate band, acting as a stepping stone allows the absorption of low energy photons to transfer electrons from the valence band to the conduction band by a sequential two photons absorption process. It has been demonstrated that highly mismatched alloys offer a potential to be used as a model material system for practical realization of multiband solar cells. Dilute nitride GaAs1-xNx highly mismatched alloy with low mole fraction of N is a prototypical multiband semiconductor with a well-defined intermediate band. Currently, we are using chemical beam epitaxy to synthesize dilute nitride highly mismatched alloys. The materials are characterized by a variety of structural and optical methods to optimize their properties for multiband photovoltaic devices.
Highlights
Recent attempts to increase the efficiency of solar cells beyond the Shockley and Queisser limit [1] have resulted in many novel device concepts
The high power conversion efficiency of multiband solar cells (MSC) arises from the increased photocurrent through absorption of low energy photons by the intermediate band (IB) while preserving a large band gap that determines the open circuit voltage
A chemical beam epitaxy system has been used to grow multiband solar cells based on a p/n junction of GaAs1-xNx sandwiched between p and n blocking layers of AlxGa1-xAs
Summary
Recent attempts to increase the efficiency of solar cells beyond the Shockley and Queisser limit [1] have resulted in many novel device concepts. An alloy with an N mole fraction in the range xN=0.02 to xN=0.03 has the optimum band gap energies for an IB device [12].
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