I don't understand this either: if the QDs are inhomogeneous in size, they may absorb different wavelengths; but how do you want to extract the carriers if he nano-particles are in solution? Please describe the set-up you envisage as 'ideal'.

 Thanks for your reply, i just thought that the reason why silicon quantum dot cannot be regarded as ideal candidate in solar cell may be the difficulty of synthesis, compared to  PbS, PbSe, etc. My set-up basically looks like a sandwiched. Say, quantum dots are sandwiched between two metal electrodes. Ohmic contact is formed at one side, another side Schottky contact. The carrier is collected at Schottky contact side. To my curiosity, there is no experimental or theoretical paper to make comparison between silicon quantum dots and other materials dots, in terms of performance in energy conversion.   

silicon quantum dots (Si-QDs) differ in a series of properties considerably from bulk silicon. Si-QDs show an increase in the energy gap with shrinking dimensions of the dot. Therefore by controlling the size it is in principle possible to shift the energy band gap of the material to the optimum energy gap (i.e. 1.6 eV) for maximum efficiency of single junction solar cells, or to stack cells with different band gap and hence different spectral response together to form tandem cells