Enhanced optical absorbance of epitaxial emitter silicon solar cells with a back germanium epilayer

Abstract

Direct germanium (Ge) epilayer growth on silicon (Si) solar cells offers the potential of improving the power conversion efficiency (PCE) as a result of enhancement in optical absorbance, particularly at longer wavelength. The realization of this potential depends critically on the competing effects between the short circuit current density (JSC) gain due to the improved infrared absorption and the reduction in open circuit voltage (VOC) due to smaller bandgap in the Ge layer, as well as imperfection at the Ge -Si interface. This paper investigates the feasibility of growing a thin Ge epilayer, by rapid thermal chemical vapor deposition (RTCVD), on the backside of a Si solar cell with epitaxial emitter. Experimental results of the solar cell with such configuration yield a remarkably high JSC of 35.1 mA/cm 2 , VOC of 445 mV, and a respectable PCE of 4.6% under one sun and AM 1.5G illumination with no surface texturization or antireflective coating. A high fill factor (FF) could be realized in the cell with further optimization in surface passivation, contact resistance and reduction of misfit dislocations at the Ge -Si interface.

Low temperature silicon-based epitaxy for solar cells applications

“…Inside the reactor, the substrates were heated in-situ in ultra pure H 2 at 1100°C to remove the native surface oxides. From a previous study, [58] an open-circuit voltage (V oc ) degradation of the solar cell was observed when the back Ge epilayer was grown after epitaxial growth of the n-Si emitter. To rectify this issue, the growth sequence of Si and Ge epilayers was reversed.…”