Influence of stacked Ge islands on the dark current–voltage characteristics and the conversion efficiency of the solar cells

The dark current–voltage (J–V) characteristics and the conversion efficiency of the solar cells with embedded stacked Ge islands in the intrinsic layer were investigated. These islands were grown by molecular beam epitaxy on Si substrates. We used a two-diode model to analyze the dark J–V characteristics of Ge island solar cells. Results showed that the minority carrier diffusion and the recombination current components increase as a function of the stacked Ge island layers. This increase of the minority carrier diffusion current was due to an increase of the intrinsic carrier density as a function of the number of stacked layers. Similarly, the increase in the recombination current components was due to the enormous recombination of carriers in the intrinsic region as the number of stacked layer increases. This phenomenon could lead to a decrease of the open circuit voltage, Voc. The decrease of Voc should be overcompensated by the increase of photocurrent, due to the presence of stacked Ge islands with higher absorption coefficient, in order to attain an optimum value of the conversion efficiency.