Optoelectronic properties of sputtered germanium absorbers used in ultrathin film photovoltaics

The extensive glass facades wrapping the exterior of many large buildings around the world are commonly equipped with solar protection glazing to enhance the building energy efficiency. Thereby, the peak cooling energy demand is reduced by thin-film coatings that absorb and/or reflect parts of the incident solar radiation. These films are deposited using vast inline sputter coaters. To unlock the untapped electricity potential of the facades, we want to enable the glass industry to also integrate photovoltaic functionality into their glazing products using the existing production lines. To enable this, it is essential to develop a semitransparent fully sputtered solar cell. In this paper, we show that magnetron sputtered ultrathin germanium absorber layers can be used in thin-film solar cells. We compare the optoelectronic properties and microstructure of sputter coated germanium absorber layers with reference layers that have been deposited using the usual plasma-enhanced chemical vapor deposition. By systematically varying sputter mode, power, gas pressure, and substrate temperature, we identify process conditions that yield sputtered germanium layers with properties comparable to those produced by PECVD. Furthermore, a sputtered germanium layer fabricated using the optimized conditions is incorporated into a solar cell, achieving a power conversion efficiency of 2.8%, a short-circuit current of 21.6 mA/cm2, and an open-circuit voltage of 449 mV. These results show that we have established a sputter process that produces high-quality germanium absorbers suitable for thin-film photovoltaics, which represents a significant step toward the realization of fully sputtered semitransparent solar cells for window applications.