n-Type silicon quantum dots and p-type crystalline silicon heteroface solar cells

Heteroface devices have been realized by depositing phosphorus-doped silicon (Si) quantum dots (QDs) (n-type) on a p-type crystalline silicon substrate. To compare the quantum confinement effect, different sizes (3, 4, 5, and 8±1 nm) of Si QD were fabricated, whose optical energy bandgaps are in the ranges of 1.3–1.65 eV. The electrical and photovoltaic properties of heterojunction devices were characterized by illuminated and dark I–V measurements, C–V measurements, and spectral response measurements. The diodes showed a good rectification ratio of 5×106 for 4 nm Si QDs at the bias voltage of ±1.0 V at 298 K. The ideality factor and junction built-in potential deduced from current–voltage (I–V) and capacitance–voltage (C–V) plots are 1.86 and 0.847 V for 3 nm QD device, respectively. From the illuminated IV characteristics, the open circuit voltages were 556, 540, 512, and 470 mV for mean QD diameters 3, 4, 5, and 8±1 nm, respectively. Temperature-dependant dark I–V measurements suggest that the carrier transport in the devices is controlled by recombination in the space-charge region. This study indicates the silicon QDs can be good candidates for all-silicon tandem solar cells.