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 x 10(6) 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. (C) 2008 Elsevier B.V. All rights reserved.