Photovoltaic Performance of Ge-Subcell Evaluated Directly in Ge-Based Triple-Junction Solar Cells

A diffused germanium (Ge) p-n junction as a Ge-subcell was fabricated by using an overall Ge-based triple-junction structure to evaluate its photovoltaic properties. The key processes in the proposed Ge-subcell include: (i) patterning a contact window with 5% area of the Ge-subcell to form top electrodes and (ii) semiconductor layers grown for the InGaP top cell and the (In) GaAs middle cell are remained atop of the Ge-subcell. Thus, our Ge-subcell has a duplicate of absorption spectrum of the Ge bottom cell in the triple-junction solar cell. Important photovoltaic parameters extracted showed that the photo-generated current density is similar to 26 mA/cm(2) being independent of the testing temperature. Besides, open-circuit voltages linearly decrease with increasing temperature and hence the Ge-subcell cannot act as a practical solar cell when temperature is higher than similar to 140 degrees C. Furthermore, maximum conversion efficiency (eta(max)) at 30 degrees C is 3.22% for the intrinsic Ge-subcell. A corresponding temperature coefficient is similar to-0.037%/degrees C. Cell temperature of our Ge-subcell under a 100 mW/cm(2) simulator increases quickly to saturate at similar to 48 degrees C. This causes reduction of more than 0.8% in eta(max). (C) 2016 The Electrochemical Society. All rights reserved.