Electrical characteristics of amorphous Si:H/crystalline Si0.3Ge0.7 heterojunction solar cells grown on compositionally graded buffer layers

We examined the influence of the processing temperature on hydrogenated amorphous Si/crystalline Si0.3Ge0.7 heterojunction solar cells with a bandgap of 1.0 eV, which were grown on Si(001) by solid-source molecular beam epitaxy. Stepwise, compositionally graded buffer Si1−xGex layers were applied to prepare Si0.3Ge0.7 films with low dislocation density. We studied the influence of the annealing temperature applied at the buffer layer on the diode characteristics. The diode factor was minimized to 1.45 at 800 °C, and the saturation current density monotonically decreased from 7.5×10−4 A/cm2 for an annealing temperature of 650 °C to 5.0×10−5 A/cm2 for that of 900 °C, as a result of the promotion of dislocation annihilation. Consequently, the Si0.3Ge0.7 heterojunction solar cell showed a maximum conversion efficiency of 1.50% when the annealing temperature was 850 °C. We also studied the influence of the growth temperature of the p-Si0.3Ge0.7 active layer (absorber) on the cell performance. The short circuit current density increased from 16.03 mA/cm2 for 520 °C to 20.95 mA/cm2 for 600 °C. This result corresponded to an improvement of quantum efficiency response in the longer wavelength region due to the reduction of crystal defects caused by oxygen contamination. However, the Si0.3Ge0.7 active layer processed at a higher temperature showed an accumulation of threading dislocations and roughening of the surface, resulting in the degradation of both the open circuit voltage and the fill factor.