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

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 (J_SC) gain due to the improved infrared absorption and the reduction in open circuit voltage (V_OC) 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 J_SC of 35.1 mA/cm², V_OC 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.