Stand-alone germanium solar cells, intended for application as bottom cell in mechanically stacked solar cells, have been realized applying an innovative process where the front contact is diffused through the a-Si passivation layer to establish the contact. The developed germanium solar cell process has resulted in world-class energy conversion efficiencies above 8 percent (AM1.5G). To optimize germanium cells for use in TPV applications, where an increased cell response at high wavelengths is desirable, an innovative back-side contacting mechanism has been developed. In order to increase the back surface reflection properties of the cell, laser fired contacts have been optimized and used. Using this contact a germanium thermophotovoltaic cell with an AM1.5 energy conversion efficiency of 6.3 percent has been realized, showing an improved response at high wavelengths compared to the classical germanium solar cell. Cost estimations show that by integrating this type of germanium photocells in a TPV system, an electricity cost of 1.9 epsilon ct/kWh is achievable when assuming a system efficiency of 10 percent and a photocell cost of 6000 epsilon/m(2).
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